Manhattan Beach Subsurface
Infiltration Trench and
Alternatives
Feasibility Study
FINAL
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California 92831
TEL (714) 526-7500 | FAX (714) 526-7004
www.cwecorp.com
Manhattan Beach Subsurface Infiltration
Trench and Alternatives
Feasibility Study
FINAL
Prepared for:
City of Manhattan Beach
1400 Highland Avenue
Manhattan Beach, CA 90266
TEL (310) 802-5000
Prepared by:
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California, 92831
TEL (714) 526-7500 | FAX (714) 526-7004 | www.cwecorp.com
September 2021
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Table of Contents
TABLE OF CONTENTS .................................................................................................................. i LIST OF FIGURES ...................................................................................................................... iii LIST OF TABLES ......................................................................................................................... iv ACRONYMS ................................................................................................................................ vi
1. BACKGROUND ..................................................................................................................... 1
1.1 PROJECT GOALS AND OBJECTIVES ............................................................................................... 2
2. EXISTING CONDITIONS ..................................................................................................... 3
2.1 SITE LOCATION ...................................................................................................................... 3
2.2 TOPOGRAPHY ........................................................................................................................ 5
2.3 GEOTECHNICAL ...................................................................................................................... 8
2.4 UTILITIES ........................................................................................................................... 11
3. PRELIMINARY HYDROLOGIC ANALYSIS .......................................................................... 15
3.1 APPROACH .......................................................................................................................... 15
3.2 LAND USE .......................................................................................................................... 17
3.3 RESULTS ............................................................................................................................ 19
4. SEA LEVEL RISE ANALYSIS ............................................................................................... 21
4.1 APPROACH .......................................................................................................................... 21
4.2 RESULTS ............................................................................................................................ 24
5. ALTERNATIVES ................................................................................................................. 27
5.1 INFILTRATION APPROACHES ..................................................................................................... 27
5.1.1 Subsurface Infiltration Systems.................................................................................... 27
5.1.2 Drywells .................................................................................................................... 28
5.2 BI 0286 DRAINAGE AREA (28TH STREET) ................................................................................... 29
5.2.1 26th Street Parking Facility and Polliwog Park ................................................................ 29
5.2.2 26th Street Parking Facility and Beach Infiltration ........................................................... 33
5.2.3 26th Street Parking Facility, Polliwog Park, and Beach Infiltration .................................... 34
5.2.4 Comparison ............................................................................................................... 35
5.3 BI 5452 LINE E (21ST STREET)................................................................................................ 35
5.3.1 Drywells Upstream of Beach ........................................................................................ 35
5.3.2 Beach Infiltration ........................................................................................................ 37
5.3.3 Comparison ............................................................................................................... 38
5.4 BI 5452 LINE G (27TH STREET) ............................................................................................... 39
5.4.1 Drywells Upstream of Beach ........................................................................................ 39
5.4.2 Beach Infiltration ........................................................................................................ 40
5.4.3 Comparison ............................................................................................................... 42
5.5 BI 5452 LINE I (24TH STREET) ................................................................................................ 42
5.5.1 Drywells Upstream of Beach ........................................................................................ 42
5.5.2 Beach Infiltration ........................................................................................................ 44
5.5.3 Comparison ............................................................................................................... 45 City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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5.6 CITY DRAIN NORTH (32ND STREET) ........................................................................................... 45
5.6.1 Drywells Upstream of Beach ........................................................................................ 45
5.6.2 Beach Infiltration ........................................................................................................ 47
5.6.3 Comparison ............................................................................................................... 48
5.7 FULL CAPTURE AREA (EWMP PROJECT) ..................................................................................... 48
6. PROPOSED ALTERNATIVE ................................................................................................. 51
6.1 QUANTIFIED BENEFITS ........................................................................................................... 51
6.1.1 Water Quality ............................................................................................................. 52
6.1.2 Water Supply ............................................................................................................. 52
6.1.3 Community Investment ............................................................................................... 53
6.2 DISADVANTAGED COMMUNITY BENEFITS ..................................................................................... 54
6.3 EFFECTIVENESS OF SIMILAR PROJECTS ....................................................................................... 55
6.4 NATURE-BASED SOLUTIONS .................................................................................................... 55
6.5 VECTOR CONCERNS ............................................................................................................... 56
6.6 LEGAL REQUIREMENTS ........................................................................................................... 56
7. COST AND SCHEDULE ....................................................................................................... 57
7.1 PRELIMINARY COST OPINION ................................................................................................... 57
7.2 ANTICIPATED SCHEDULE ......................................................................................................... 58
7.3 LEVERAGED FUNDING ............................................................................................................ 60
7.4 LIFE-CYCLE COST ................................................................................................................. 60
8. OPERATION, MAINTENANCE, AND MONITORING ............................................................ 62
8.1 OPERATIONS AND MAINTENANCE .............................................................................................. 62
8.2 POST-CONSTRUCTION MONITORING .......................................................................................... 64
9. ENVIRONMENTAL AND PERMITTING ............................................................................... 66
9.1 CALIFORNIA ENVIRONMENTAL QUALITY ACT ................................................................................. 66
9.2 PERMITTING REQUIREMENTS ................................................................................................... 67
9.2.1 LACFCD Connection Permit .......................................................................................... 67
9.2.2 Coastal Permit ............................................................................................................ 68
9.2.3 Los Angeles County Beaches and Harbors ..................................................................... 69
9.2.4 Los Angeles County Roads Permit ................................................................................ 69
9.2.5 Additional Permits and Coordination ............................................................................. 70
10. OUTREACH/ENGAGEMENT APPROACH.......................................................................... 71
10.1 GOALS ............................................................................................................................... 71
10.2 OUTREACH ......................................................................................................................... 72
10.3 ENGAGEMENT EVENTS ............................................................................................................ 72
11. REFERENCES .................................................................................................................. 74
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Appendices
Appendix A Geotechnical Report
Appendix B Preliminary Design Concepts
Appendix C Project Description
Appendix D Letters of Support
Appendix E Vector Minimization Plan
Appendix F O&M Manual
Appendix G Commitment Letter
Appendix H Preliminary Monitoring Plan
Appendix I LACFCD Connection Permit References
Appendix J LACBH Permit References
Appendix K LACPW Roads Permit References
List of Figures
Figure 1-1 Site Locations and Drainage Areas .................................................................................... 1 Figure 1-2 General Project Concept .................................................................................................. 2 Figure 2-1 Beach Location Map ........................................................................................................ 3 Figure 2-2 26th Street Parking Facility Location Map ........................................................................... 4 Figure 2-3 Polliwog Park Location Map .............................................................................................. 5 Figure 2-4 Beach Topographic Map ................................................................................................... 6 Figure 2-5 26th Street Parking Facility Topographic Map...................................................................... 7 Figure 2-6 Polliwog Park Topographic Map ........................................................................................ 8 Figure 2-7 Beach Boring Locations .................................................................................................... 9 Figure 2-8 26th Street Parking Facility Boring Locations ..................................................................... 10 Figure 2-9 Polliwog Park Boring Locations ....................................................................................... 11 Figure 2-10 Beach Map of Existing Wet Utilities ............................................................................... 12 Figure 2-11 26th Street Parking Facility Map of Existing Wet Utilities .................................................. 13 Figure 2-12 Polliwog Park Map of Existing Wet Utilities ..................................................................... 14 Figure 3-1 Hydrologic Soil Types by Drainage Area .......................................................................... 16 Figure 3-2 Land Use Map by Drainage Area ..................................................................................... 19 Figure 4-1 Approach to Sea-Level Rise Assessment .......................................................................... 21 Figure 4-2 Sea-Level Rise Projections (1-Meter) ............................................................................... 24 Figure 4-3 Design Constraints Considering Sea-Level Rise Projections ................................................ 25 Figure 4-4 Dune Restoration Project Site Map .................................................................................. 26 Figure 5-1 26th Street Parking Facility Drywell Concept ..................................................................... 30 Figure 5-2 Polliwog Park Subsurface Infiltration System Concept ....................................................... 31 Figure 5-3 Beach Infiltration Concept for Partial Diversion from BI 0286 ............................................ 33 Figure 5-4 Drywell Concept on BI 5452 Line E ................................................................................. 36 Figure 5-5 Beach Infiltration Concept on BI 5452 Line E ................................................................... 37 Figure 5-6 Drywell Concept on BI 5452 Line G ................................................................................. 39 City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 5-7 Beach Infiltration Concept on BI 5452 Line G ................................................................... 41 Figure 5-8 Drywell Concept on BI 5452 Line I .................................................................................. 43 Figure 5-9 Beach Infiltration Concept on BI 5452 Line I .................................................................... 44 Figure 5-10 Drywell Concept on City Drain North ............................................................................. 46 Figure 5-11 Beach Infiltration Concept on City Drain North ............................................................... 47 Figure 5-12 Full Beach Infiltration Concept ...................................................................................... 49 Figure 6-1 Proposed Alternative with Phasing .................................................................................. 51 Figure 6-2 DAC Areas in Project Vicinity .......................................................................................... 54 Figure 7-1 Project Schedule ........................................................................................................... 59 Figure 10-1 Informational Postcard ................................................................................................. 73
List of Tables
Table 2-1 Summary of Geotechnical Exploration Findings ................................................................... 9 Table 3-1 Hydrologic Modeling Data Requirements .......................................................................... 15 Table 3-2 Hydrologic Data Input by Drainage Area ........................................................................... 16 Table 3-3 Land Use and Impervious Area ........................................................................................ 18 Table 3-4 Results of Hydrologic Analysis by Drainage Area ............................................................... 20 Table 4-1 Summary of OPC’s Emissions Scenarios and Risk Aversion Projections ................................ 22 Table 4-2 Sea-Level Rise Projections in Santa Monica (OPC, 2018) .................................................... 22 Table 5-1 Advantages and Disadvantages of Subsurface Infiltration Systems ...................................... 27 Table 5-2 Advantages and Disadvantages of Drywells ...................................................................... 28 Table 5-3 Concept Characteristics for Drywells at the 26th Street Parking Facility ................................ 29 Table 5-4 Concept Characteristics for Subsurface Infiltration at Polliwog Park ..................................... 31 Table 5-5 Preliminary Cost Opinion for Drywells at 26th Street Parking Facility .................................... 32 Table 5-6 Preliminary Cost Opinion for Subsurface Infiltration at Polliwog Park ................................... 32 Table 5-7 Concept Characteristics for Partial Beach Infiltration on BI 0286 ......................................... 34 Table 5-8 Preliminary Cost Opinion for Partial Beach Infiltration on BI 0286 ....................................... 34 Table 5-9 Advantages and Disadvantages of BI 0286 Alternatives ..................................................... 35 Table 5-10 Concept Characteristics for Drywells on BI 5452 Line E .................................................... 36 Table 5-11 Preliminary Cost Opinion for Drywells on BI 5452 Line E .................................................. 37 Table 5-12 Concept Characteristics for Beach Infiltration at BI 5452 Line E ........................................ 38 Table 5-13 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line E ...................................... 38 Table 5-14 Advantages and Disadvantages of BI 5452 Line E Alternatives .......................................... 38 Table 5-15 Concept Characteristics for Drywells on BI 5452 Line G .................................................... 40 Table 5-16 Preliminary Cost Opinion for Drywells on BI 5452 Line G .................................................. 40 Table 5-17 Concept Characteristics for Beach Infiltration at BI 5452 Line G ........................................ 41 Table 5-18 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line G ...................................... 42 Table 5-19 Concept Characteristics for Drywells on BI 5452 Line I ..................................................... 43 Table 5-20 Preliminary Cost Opinion for Drywells on BI 5452 Line I ................................................... 44 Table 5-21 Concept Characteristics for Beach Infiltration at BI 5452 Line I ......................................... 45 Table 5-22 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line I ....................................... 45 Table 5-23 Concept Characteristics for Drywells on City Drain North .................................................. 46 Table 5-24 Preliminary Cost Opinion for Drywells on City Drain North ................................................ 47 Table 5-25 Concept Characteristics for Beach Infiltration at City Drain North ...................................... 48 Table 5-26 Preliminary Cost Opinion for Beach Infiltration at City Drain North .................................... 48 Table 5-27 Concept Characteristics for Full Beach Infiltration ............................................................ 49 City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Table 5-28 Preliminary Cost Opinion for Full Beach Infiltration .......................................................... 50 Table 6-1 Summary of Water Quality Benefits and Metrics ................................................................ 52 Table 6-2 Summary of Quantified Water Quality Benefits .................................................................. 52 Table 6-3 Summary of SCWP-Identified Community Investment Benefits ........................................... 53 Table 7-1 Opinion of Probable Cost for Phase 1 and Phase 2 ............................................................ 57 Table 7-2 Summary of Implementation Costs .................................................................................. 58 Table 7-3 Life-Cycle Costs .............................................................................................................. 61 Table 8-1 Summary of Anticipated O&M by Project Component ......................................................... 62 Table 8-2 Anticipated Annual Maintenance Cost ............................................................................... 64 Table 9-1 Required Environmental Studies/Process .......................................................................... 66
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Acronyms
AB Assembly Bill
ADA Americans with Disabilities Act
AF Acre-Feet
AFY Acre-Feet per Year
AIN Assessor's Identification Number
bgs Below Ground Surface
BI Bond Issue
CCC California Coastal Commission
CEQA California Environmental Quality Act
cfs Cubic Feet per Second
CoSMoS Coastal Storm Modeling System
DAC Disadvantaged Community
EWMP Enhanced Watershed Management Program
GIS Geographic Information System
FOS Factor of Safety
HDPE High-Density Polyethylene
IS Initial Study
LACBH Los Angeles County Beaches and Harbors
LACFCD Los Angeles County Flood Control District
LACPW Los Angeles County Public Works
LARIAC Los Angeles Regional Imagery Acquisition Consortium LCP Local Coastal Program
LUP Land-Use Plan
MBBG Manhattan Beach Botanical Garden
MND Mitigated Negative Declaration
MODRAT Modified Rational Method
MPN Most Probable Number
MS4 Municipal Separate Storm Sewer System
MSL Mean Sea Level
NAVD North American Vertical Datum
NOD Notice of Determination
NPDES National Pollutant Discharge Elimination System
O&M Operation and Maintenance
OCP Ocean Protection Council
RAA Reasonable Assurance Analysis
SCAG Southern California Association of Governments
SCWP Safe, Clean Water Program
SWRCB State Water Resources Control Board
TMDL Total Maximum Daily Load
WMMS Watershed Management Modeling System
USGS United States Geological Survey
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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1. Background
The City of Manhattan Beach (City) is implementing the Manhattan Beach Subsurface Infiltration Trench and Alternatives Project (Project) to identify and assess alternatives for improving runoff water quality generated within the City. The City participated with adjacent municipalities (including the Los Angeles County Flood Control District [LACFCD] and the Cities of Hermosa Beach, Redondo Beach, and Torrance) in developing the Beach Cities Enhanced Watershed Management Program (EWMP) to comply with Order No. R4-2012-0175, National Pollutant Discharge Elimination System (NPDES) Permit No. CAS004001 for Municipal Separate Storm Sewer System (MS4) Discharges within the Coastal Watersheds of Los Angeles County (MS4 Permit). The EWMP identified capital improvement projects to minimize wet-weather discharges and eliminate dry-weather discharges into the Santa Monica Bay as measures to reduce pollutant loading and thereby comply with water quality objectives for bacteria, trash, and other pollutants, as defined in Total Maximum Daily Loads (TMDLs). A beach infiltration system conceptualized to capture 1,600 acres of runoff was identified in the Beach Cities EWMP as a high priority capital project.
The Beach Cities EWMP also identified Polliwog Park as an alternative location for Project implementation.
The City later identified the 26th Street Parking Facility as another alternative site. Analyses were performed to assess the potential sites for feasibility and cost effectiveness. This Feasibility Study (Study) summarizes the analyses performed and implementation alternatives available to capture the drainage areas illustrated in Figure 1-1, which represent the area the EWMP-identified project was conceptualized to capture.
Figure 1-1 Site Locations and Drainage Areas
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1.1 Project Goals and Objectives
The Project is being implemented by the City to meet water quality goals identified in the Beaches Cities EWMP and required by the MS4 Permit. The Project will address discharges from within the City into local storm drains and ultimately the Santa Monica Bay. Figure 1-2 illustrates the general concept of the Project. The Project will reduce pollutant loading to downstream water bodies by capturing wet- and dry-weather runoff. Captured runoff will be discharged into a subsurface storage system that will facilitate infiltration.
Figure 1-2 General Project Concept
The primary goals of the Project are:
Reduce bacterial discharges from the storm drain system Reduce trash/debris discharge from the storm drain system
It is anticipated that the project will generate additional benefits, including:
Enhancing beach conditions
Reducing the potential for beach closures
Providing an enhanced environment for marine life
Creating educational and outreach opportunities for the local community
This Study aims to achieve the following goals:
Document existing conditions that impact Project design and implementation Describe Project hydrology that will influence component sizing Present concepts to be considered for implementation
Quantify benefits of preferred alternative
Provide recommendations to move forward into final design Divert urban
runoff from
storm drain
Pretreat runoff
Route runoff
to subsurface
storage
system
Infiltrate runoff
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2. Existing Conditions
The City is located within the southwestern coastal portion of Los Angeles County and is bound by El Segundo, Hawthorne, Redondo Beach, Hermosa Beach, and the Pacific Ocean. The City is a beach community with approximately 2.1 miles of beachfront and has a total land area of 3.88 square miles.
The beach, 26th Street Parking Facility, and Polliwog Park have been identified for potential implementation. This section summarizes existing conditions for each of the potential locations.
2.1 Site Location
The EWMP identified just under one mile of coastline to use for Project implementation based on the location of existing drains, as illustrated in Figure 2-1. The Strand, a local beach walking path, is located adjacent and west of beachfront homes. The Strand consists of a concrete walking path with lights, benches, landscaping, and fitness equipment. It is approximately 10-15 feet above beach level.
The Marvin Braude Bike Trail is a two-way bike path located at beach level and runs parallel to and west of The Strand. Stairs are located at every street to provide beach access. Adjacent to and west of the bike path are dunes that protect the coastline. The slope between the two paths and the dunes are covered with ice plants. Project implementation would be located beneath or further west of the dunes, within property owned by Los Angeles County Beaches and Harbors (LACBH).
Figure 2-1 Beach Location Map
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The public parking facility on 26th Street shown in Figure 2-2 is owned by Los Angeles County and used/operated by the City based on an established agreement. The parking facility is located between Manhattan Avenue and Ocean Drive and east of the Los Angeles County Fire Department Lifeguard Training Center. The public parking facility has metered parking, lights, planters with irrigation, several mature trees, and four access driveways. The facility has two parking levels and three retaining walls due to the sloped terrain towards the beach. The parking facility is associated with Assessor Identification Numbers (AINs) 4177-024-900 and 4177-024-902.
Figure 2-2 26th Street Parking Facility Location Map
Polliwog Park is a public park located on Manhattan Beach Boulevard between Peck Avenue and Redondo Avenue in the Manhattan Heights section of the City, as illustrated in Figure 2-3. The park has an approximate area of 18-acres and features baseball fields, playgrounds, amphitheater, swimming pool, lake, and concrete walking paths. The lake is a detention basin and becomes inundated with runoff during and following storm events. Several storm drains discharge into the lake and then a pump station owned and maintained by LACFCD pumps flows from the lake to the downstream drainage system. The park is in a residential area and is next to the Manhattan Beach Middle School and Manhattan Beach Preschool. The park is associated with AIN 4165-007-902, which also includes the middle school. The Manhattan Beach Unified School District owns the property and the City leases land for the park. The Manhattan Beach Botanical Garden (MBBG) is also located within Polliwog Park off Peck Avenue.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-3 Polliwog Park Location Map
2.2 Topography
The topography summarized in this section is based on elevation data collected through the Los Angeles Regional Imagery Acquisition Consortium (LARIAC), in which the City was a participant. The topography in the City slopes west towards the beach. The area directly adjacent to the beach (approximately four or more blocks) has a slope greater than 10% and is considered relatively steep. Once the Project approach is determined, site-specific topographic data will be collected to serve as the basis for design.
The topography is an important factor in determining if implementation will require a pump or can be accomplished using gravity flow systems.
The area on the beach for which infiltration is under consideration is approximately 10-20 feet above Mean Sea Level (MSL) based on North American Vertical Datum (NAVD) from 1988 and is relatively flat.
The elevation at The Strand is approximately 40 feet above MSL. The elevation at the Marvin Braude Bike Trail is approximately 30-32 feet above MSL, about 10-15 feet above the beach. The slope between the two trails is approximately 30%. Figure 2-4 illustrates the 2-foot LARIAC contours.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-4 Beach Topographic Map
The 26th Street Parking Facility ranges in elevation from 50 feet to 60 feet above MSL. This location slopes quickly towards the beach. The parking facility is divided into two levels with concrete masonry unit retaining walls. Retaining walls are located on Manhattan Avenue, Ocean Drive, and in between.
Figure 2-5 illustrates the 2-foot LARIAC contours.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-5 26th Street Parking Facility Topographic Map
Polliwog Park has hilly terrain and ranges in elevation from 70 feet to 130 feet above MSL. The park slopes in the center towards the lake, as it serves as a detention basin as described in Section 2.1. The highest point of the park is located near the southwest corner of the park at Manhattan Beach Boulevard and Peck Avenue. The terrain slopes from west to east in this location, which is why a pump station is needed to convey runoff west. The multi-purpose fields in the northwest corner of the park, near 18 th Street and Peck Avenue, are at an elevation of approximately 90 feet above MSL, which is around 20 feet higher than the adjacent baseball field and lake. The multi-purpose fields are under consideration for Project implementation, as further discussed in Section 5. Figure 2-6 illustrates the 2-foot LARIAC contours.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-6 Polliwog Park Topographic Map
2.3 Geotechnical
A geotechnical engineering field analysis was performed by Terracon Consultants, Inc. for the Project.
The resulting geotechnical engineering report, Manhattan Beach Subsurface Infiltration Trench Feasibility Study Geotechnical Engineering Report (dated July 6, 2020), is included in Appendix A and summarizes the geotechnical findings, considerations, and recommendations relevant to the Project.
During the geotechnical investigation, exploratory borings were performed to a depth approximately 10 feet greater than the anticipated infiltration systems (further described in Section 5) to determine the soil conditions. Infiltration borings were drilled to the anticipated invert of the infiltration system and used to measure
percolation rates, which were then correlated to infiltration rates. Infiltration tests were performed at the beach (on the sand and in the neighborhood just before the beach), 26th Street Parking Facility and surrounding area, and Polliwog Park. Groundwater depths were also noted when encountered. Table 2-1 summarizes the infiltration rates and groundwater depths at each site. Onsite infiltration is considered feasible, as the measured infiltration rates are higher than 0.3 inches per hour, the minimum specified in Los Angeles County for being considered feasible for infiltration. The Project locations are not within a high liquefaction potential hazard zone, which also supports onsite infiltration.
Additional soil characteristics and recommendations are included in the Geotechnical Report, while the exploration locations and site-specific discussion is included below.
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Table 2-1 Summary of Geotechnical Exploration Findings Location Test Location
(depth, ft)
Percolation Rate
(in/hr)
Infiltration Rate
(in/hr)
Groundwater
Depth (feet)
Beach
PI-1 (2 to 7 ft) >500 >200 12.5
PI-2 (0 to 5 ft) >500 155.7 Not Encountered1
PI-3 (2 to 7 ft) >500 >200 12.5
PI-4 (0 to 5 ft) >500 >200 Not Encountered1
Beach Path PI-5 (5 to 10 ft) 126.0 9.2 Not Encountered1 Polliwog Park PP-1 (10 to 16 ft) 21.6 1.0 Not Encountered1 PP-2 (10 to 16 ft) 49.7 4.0 Not Encountered1
Manhattan Ave PB-1 (10 to 16 ft) 97.2 9.6 Not Encountered1 26th Street
Parking Facility
PL-1 (25 to 31 ft) >500 >200 47.0
PL-2 (10 to 16 ft) 224.0 17.1 Not Encountered1
1 Groundwater not encountered at the test location up to the boring depth.
Ten borings were conducted on/along the beach, as illustrated in Figure 2-7. Borings BI-1 to BI-4 were drilled to depths of 11.5-16.5 feet below ground surface (bgs) and boring BI-5 was drilled to a depth of
21.5 feet bgs. Infiltration rates on the sand were much greater than what was measured on the adjacent path. Groundwater was observed in borings BI-1 and BI-3 at a depth of approximately 12.5 feet bgs while groundwater was not encountered in BI-5. Borings and percolation/infiltration tests were nearby, which is why multiple labels are shown in the figure even if two locations are not clearly differentiated.
Figure 2-7 Beach Boring Locations
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Four borings conducted at the 26th Street Parking Facility and along Manhattan Avenue are illustrated in Figure 2-8. Borings PL-1, PL-2, and BL-1 were drilled to depths of 31.5, 16.5, and 51.5 feet bgs, respectively, within the parking facility. Boring PB-1 was drilled to a depth of 16.5 feet bgs along Manhattan Avenue. Infiltration rates were found to be greater at deeper depths. Groundwater was observed in Boring BL-1 at a depth of approximately 47 feet bgs.
Figure 2-8 26th Street Parking Facility Boring Locations
Four borings were conducted within Polliwog Park, as illustrated in Figure 2-9. Borings BP-1 and BP-2 were drilled to depths of 26.5 feet bgs. Groundwater was not encountered in the borings at this site, while the historic highest groundwater level was recorded at a depth greater than 100 feet bgs.
Infiltration rates were measured to be higher at the lake and at deeper elevations. The depth of the two tests was consistent, while the surface elevation at the lake is approximately 20 feet higher at BP-1, as noted in Section 2.2. Borings and percolation/infiltration tests were nearby, which is why multiple labels are shown in the figure even if two locations are not clearly differentiated.
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Figure 2-9 Polliwog Park Boring Locations
2.4 Utilities
Utility information was provided by the City through Geographic Information System (GIS) data, as-builts, and index maps, specifically for wet utilities. Storm drain information for LACFCD storm drains was available through their online plan portal. A utility search will need to be performed using Underground Service Alert (DigAlert) to determine the existing utilities in and around the Project sites. A detailed utility search will be performed once the Project approach is determined. Dry utilities including gas lines, communication lines, and electrical conduit will be requested and mapped out during the design phase based on information collected during the DigAlert utility search. Existing utilities will be considered when determining the Project layout at each location. Diversions will be constructed on storm drains and the improvements will be located to avoid existing utilities when possible.
Existing wet utilities at and near the beach site are illustrated in Figure 2-10. Sewer pipes (10-inch and 30-inch) are located along The Strand and Marvin Braude Bike Trail. A City-owned 18-inch storm drain exists along 32nd Street and discharges onto the beach approximately 35 feet from the bike trail. The runoff from this storm drain infiltrates into the sand and eventually travels to the ocean. Four LACFCD-owned storm drains, Bond Issue (BI) 0286 and BI 5452 (Lines E, G, and I), ranging from 18-inches to 72-inches, also discharge onto the beach near the bike trail except for the 72-inch storm drain (BI 0286). BI 0286 is located along 28th Street and outlets directly into the Pacific Ocean via a concrete box culvert.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-10 Beach Map of Existing Wet Utilities
Existing wet utilities around the 26th Street Parking Facility are illustrated in Figure 2-11. A 6-inch sewer, 4-inch water, and 8-inch water pipe are aligned along the east side of Manhattan Avenue. A 6-inch and 20-inch sewer pipe are located along the north side of the 26th Street Parking Facility. A LACFCD-owned 24-inch storm drain (BI 5452, Line G) is located within 26th Street, turns north on Ocean Drive, and then discharges onto the beach from 27th Street. The runoff from this storm drain infiltrates into the sand and eventually travels to the ocean. Several water and sewer pipes are located within the streets around the 26th Street Parking Facility.
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Figure 2-11 26th Street Parking Facility Map of Existing Wet Utilities
Existing wet utilities are found within Polliwog Park, as illustrated in Figure 2-12. A 14-inch water and 18-inch sewer pipe run through the center of the park from Manhattan Beach Boulevard to 18th Street. A LACFCD-owned 96-inch storm drain (BI 5401) is aligned through the east side of the park and discharges into Polliwog Lake. Two City-owned (or private) storm drains discharge into the lake, one from the north and the other from the east. As mentioned in Section 2.1, Polliwog Lake is used as a detention basin and an LACFCD-owned pump station located in the park discharges flows downstream. The pump station force main is BI 0552, a 21-inch storm drain that originates at the south side of Polliwog Lake and then goes west on Manhattan Beach Boulevard. Several other water and sewer pipes are located within the streets around Polliwog Park including Manhattan Beach Boulevard, Peck Avenue, 18th Street, and Redondo Avenue.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Figure 2-12 Polliwog Park Map of Existing Wet Utilities
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3. Preliminary Hydrologic Analysis
A hydrologic analysis was performed to identify runoff tributary to the Project during the 85th percentile, 24-hour storm event, and the design storm event determined based on the Reasonable Assurance Analysis (RAA) results presented in the Beach Cities EWMP. The RAA was used during the EWMP development process to identify the stormwater capture/treatment necessary to meet water quality objectives. Existing land uses, calibrated using existing water quality data, were modeled through the RAA to define implementation actions needed within the watershed group. In most cases, the RAA identifies either a volume capture goal by City or watershed or a percent reduction for the priority pollutant. In most cases throughout Los Angeles County, the volume capture goals identified in an RAA were less than the 85th percentile storm event, which typically represents the largest capture volume needed to meet water quality objectives. The RAA goals throughout Los Angeles County are specific to the area and consider existing implementation efforts.
The Modified Rational Method (MODRAT) developed by LACFCD is the recommended hydrologic modeling method to maintain consistency with LACFCD hydrology. Los Angeles County Public Works (LACPW) developed a Hydrology Manual (2006) outlining the methodology used for conducting a hydrologic analysis. This section summarizes the methodology and results of the preliminary hydrologic analyses performed.
3.1 Approach
The LACPW Hydrology Manual (2006) outlines the approach for determining capture volume. Catchment area, flow path length and slope, rainfall depth, percent impervious, and soil type were used to calculate the runoff volumes and peak flow rates for each of the catchments tributary to the Project sites, as illustrated in Figure 1-1. Table 3-1 summarizes the input parameters used to perform the hydrologic analyses for each drainage area.
Table 3-1 Hydrologic Modeling Data Requirements
Required Data Description
Subarea Size Area inside the drainage boundaries
Flow Path Length Length of the conveyance between drainage area collection points Flow Path Slope Slope of the flow path
Conveyance Data Description of the flow conveyance between collection points Soil Types Soil classification identifying the hydrologic characteristics of the drainage area’s surface soils
Land Use/
Imperviousness
Classification of impervious surface area based on development types within the drainage area
Design Storm
Definition
Unique design storm based on the location and the rainfall recurrence interval being modeled
The MODRAT methodology was used to perform a preliminary hydrologic analysis in accordance with recommended methodology from the Hydrology Manual (LACPW, 2006). GIS shapefiles used for RAA modeling efforts were utilized to determine land use, which is used to find the approximate imperviousness, as described in Section 3.2. A GIS shapefile of the hydrologic soil types illustrated in the Hydrology Manual was also used to define the soil type most prominent in each drainage area (and City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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not the Project site). Most of the City is classified as soil type 10 (Oakley Fine Sand), while the coastal areas are classified as soil type 3 (Chino Silt Loam). The drainage areas extending inland (BI 0286 and Polliwog Park) overlay areas predominantly classified with soil type 10, while the coastal drainage areas are classified with soil type 3. Figure 3-1 illustrates the soil types relative to the various drainage areas.
Figure 3-1 Hydrologic Soil Types by Drainage Area
Design peak flow rates and volumes will be used for sizing the diversion structure, diversion piping, and infiltration system. The input values for each drainage area are summarized in Table 3-2.
Table 3-2 Hydrologic Data Input by Drainage Area
Drainage Area Area (ac) Flow Path
(ft)
Slope
(ft/ft)
Impervious
Percentage
Dominant
Soil Type
26th Street Parking Facility
(BI 0286) 1,518.5 19,380 0.0072 50% 10
BI 5452, Line E 20.4 1,256 0.0979 40% 3
BI 5452, Line G 18.0 2,279 0.0693 68% 3
BI 5452, Line I 22.3 1,747 0.0721 62% 3
City Drain North 48.8 1,897 0.0854 79% 3
Polliwog Park 487.6 6,295 0.0162 46% 10
The design storm event was determined based on the information included in the RAA and EWMP for the beach infiltration trench. The Beach Cities EWMP/RAA did not provide a specific capture volume or
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pollutant load reduction percentage by Project, drainage area, or agency. Instead, sizing and layout characteristics were summarized. The Beach Cities EWMP and Adaptive Management document identified peak diversion flow rates, infiltration footprint, cumulative loss rate, design storage, and infiltration system layout. The design storage was perceived to represent the storage needed if accounting for infiltration volume during the storm event, as the storage volume is extremely small compared to the peak diversion flow rates specified for each drain.
Calculations were performed for each of the drainage areas and then combined (excluding Polliwog Park) to back calculate the storm event size equivalent to the EWMP/RAA-defined characteristics. An iterative approach was used to develop the drainage area hydrograph, which was then used to size the infiltration system until the infiltration system characteristics matched those defined in the Beach Cities EWMP and Adaptive Management document. The RAA storm event was determined to be one (1) inch using the approach described above. The hydrologic analysis performed for each drainage area then used the 1-inch storm. For comparison, the 85
th percentile storm event in this area is 0.85 inches.
It is recommended that the City consider not exceeding the 85 th percentile event rainfall depth. The
results associated with the RAA and 85th percentile events are summarized below, while the RAA event was used to assess Project alternatives presented in Section 5. It is also recommended that a more detailed hydrologic analysis be performed during the design process. The preliminary hydrologic analysis summarized in this report focused on quantifying the runoff volumes anticipated based on surface characteristics, while flow routing should be used to quantify the design flow rates. The hydrologic analysis during the design process should model pipe flow throughout the drainage area, which is correlated with higher flow rates as compared to surface flow. The refined analysis during the design process may result in modifications to the infiltration footprint.
3.2 Land Use
The amount of impervious area within a drainage area is a key factor in determining the runoff volume produced from a rain event. The impervious area is related to the land use type, with industrial and commercial land uses having more impervious area and low-density residential land uses having less impervious area. The Project drainage area is comprised of several types of land uses, from developed parks to education, residential, and industrial areas based on the LACPW land use data collected by the Southern California Association of Governments (SCAG). The LACPW land data identifies an associated impervious percentage with each land use type, which was used to determine the imperviousness used for the hydrologic analysis. Table 3-3 summarizes the land uses and impervious area for each of the modeled drainage areas.
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Table 3-3 Land Use and Impervious Area
Land Use Category % Imperviousness Area (acres) Percent of Area 26th Street Parking Facility (BI 0286)
Commercial 95 133.0 9
Education 82 89.6 6
Industrial 91 20.9 1
Multi-Family Residential 80 81.1 5
Single-Family Residential 42 1,127.0 74
Vacant 10 66.9 5
Total 50
(Weighted average) 1,518.5 100%
BI 5452, Line E
Commercial 95 0.5 2
Single-Family Residential 42 18.1 89
Vacant 10 1.8 9
Total 40
(Weighted average) 20.4 100%
BI 5452, Line G
Commercial 95 1.5 8
Education 82 0.8 4
Multi-Family Residential 80 12.2 68
Vacant 10 3.5 20
Total 68
(Weighted average) 18.0 100%
BI 5452, Line I
Commercial 95 1.6 7
Education 82 0.4 2
Multi-Family Residential 80 9.9 45
Single-Family Residential 42 9.2 41
Vacant 10 1.2 5
Total 62
(Weighted average) 22.3 100%
City Drain North
Commercial 95 6.0 12
Multi-Family Residential 80 40.7 84
Vacant 10 2.1 4
Total 79
(Weighted average) 48.8 100%
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Land Use Category % Imperviousness Area (acres) Percent of Area Polliwog Park
Commercial 95 8.8 2
Education 82 31.3 6
Industrial 91 6.4 1
Multi-Family Residential 80 19.0 4
Single-Family Residential 42 389.9 80
Vacant 10 32.2 7
Total 46
(Weighted average) 487.6 100%
The drainage areas are mostly comprised of multi-family and single-family residential land uses, as shown in Figure 3-2.
Figure 3-2 Land Use Map by Drainage Area
3.3 Results
Table 3-4 summarizes the results of the preliminary hydrologic analysis performed based on the methodology detailed above. The peak flow rate is not identified in the table, as it is recommended that more detailed modeling is performed during the design process to quantify the flow rate more accurately, as recommended above. Results are broken down by drainage area and the drainage areas included are
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based on the EWMP-identified area tributary to the beach infiltration project. Hydrographs were developed, which were then used to size infiltration systems using the proposed infiltration footprint and measured infiltration rates summarized in Section 2.3. The hydrographs determined the subsurface infiltration layouts which are included in Section 5.
Table 3-4 Results of Hydrologic Analysis by Drainage Area Drainage Area
RAA Event
(1-inch)
Volume (acre-feet)
85th Percentile
(0.85-inch)
Volume (acre-feet)
Difference in Volume
(acre-feet)
26th Street Parking Facility
(BI 0286) 62.9 53.5 9.5
BI 5452, Line E 0.7 0.6 0.1
BI 5452, Line G 1.0 0.8 0.2
BI 5452, Line I 1.1 0.9 0.2
City Drain North 3.0 2.0 1.0
Polliwog Park 18.8 16.0 2.8
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4. Sea Level Rise Analysis
The City is in the process of finalizing a Sea-Level Rise Risk, Hazards, and Vulnerability Assessment, which highlights existing conditions and future vulnerability of the City to projected sea-level rise, coastal flooding, and erosion. The assessment indicates that impacts have been observed in relation to sea-level rise, coastal flooding, and erosion and future impacts are expected to “increase in intensity and frequency due to sea-level rise and climate change” (City of Manhattan Beach, 2020c). Sea-level rise projections and available references were reviewed to assess potential impacts on the Project and to identify design constraints, especially for the alternatives proposed on/near the beach (as detailed throughout Section 5). This section summarizes the approach used to assess sea-level rise impacts on the Project and results/findings of the assessment.
4.1 Approach
The approach to perform a Project-specific sea-level rise assessment is illustrated in Figure 4-1. The goal was to leverage existing information to determine if projected sea-level rise could impact Project implementation and what constraints could be incorporated into the planning process to help minimize those impacts.
Figure 4-1 Approach to Sea-Level Rise Assessment
The City’s Sea-Level Rise Risk, Hazards, and Vulnerability Assessment summarizes various sources of information and includes sea-level rise projections from the California Ocean Protection Council’s (OCP’s) updated State of California Sea-Level Rise Guidance (OPC, 2018). OPC’s guidance document includes sea-level rise predictions along the coast through 2150 based on two global greenhouse gas emissions scenarios: high and low. A range of probabilistic projections of sea-level rise are also provided in the guidance document, which are intended to help guide decision makers (shown below with projected sea- level rise). A probabilistic projection denoted as H++ represents an extreme scenario that assumes rapid ice sheet loss on Antarctica which would significantly increase the rates of sea-level rise. Three risk aversion projections are also defined, which correspond to different levels of risk tolerance. Table 4-1 summarizes the definitions of the emissions scenarios and risk aversion projections.
Review
available
information
Identify
sea-level rise
projections
Identify
groundwater-
level rise
projections
Define design
constraints
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Table 4-1 Summary of OPC’s Emissions Scenarios and Risk Aversion Projections Terminology Summarized Definition
Emission’s Scenarios
High
Assumes a future where there are no significant global efforts to limit/reduce emissions, while there are modest rates of technological change and energy intensity improvements that lead to long-term high energy demand and emissions.
Low
Assumes more aggressive emissions reductions aligned with the goals of the 2015 Paris Agreement, which calls for limiting mean global warming to two degrees Celsius and achieving net-zero greenhouse gas emissions in the second half of the century.
This would require updated climate policies, concerted action by all countries, and a shift to a lower emissions service and information economy.
Risk Aversion Projections
Low Appropriate for adaptive, lower consequence projects (e.g., unpaved trail).
Medium-High
Appropriate as a precautionary projection that can be used for less adaptive, more vulnerable projects or populations that will experience medium to high consequences because of underestimating seal-level rise (e.g., coastal housing development).
Extreme
Appropriate for high consequence projects with little to no adaptive capacity and which could have considerable public health, public safety, or environmental impacts (e.g., coastal power plant, wastewater treatment plant, etc.).
Table 4-2 summarizes the sea-level rise projections based on the year, emission scenario, probabilistic projections, and risk aversion projection in Santa Monica, which is the closest OPC-assessed location to the Project (OPC, 2018).
Table 4-2 Sea-Level Rise Projections in Santa Monica (OPC, 2018) Emission Scenario and
Projected Year
Probabilistic Projections (feet)
Median Likely Range 1-in-20
Chance
1-in-200
Chance
H++
Scenario
50%
probability sea-
level rise
meets or
exceeds
66% probability sea-
level rise is
between…
5%
probability
sea-level rise
meets or
exceeds…
0.5%
probability sea-
level rise
meets or
exceeds…
Low
Risk
Aversion
Medium –
High Risk
Aversion
Extreme Risk
Aversion
High emissions
2020 0.4 0.3 - 0.5 0.6 0.8 1.0
2040 0.6 0.4 - 0.8 0.9 1.2 1.7
2050 0.8 0.6 - 1.1 1.3 1.9 2.6
Low emissions 2060 0.9 0.6 - 1.2 1.5 2.3 3.8 High emissions 2060 1.1 0.8 - 1.4 1.8 2.6 Low emissions 2070 1.0 0.7 - 1.4 1.9 3.0 5.1 High emissions 2070 1.3 1.0 - 1.8 2.3 3.4 Low emissions 2080 1.2 0.8 - 1.7 2.3 3.8 6.5 High emissions 2080 1.7 1.1 - 2.3 2.9 4.4 Low emissions 2090 1.3 0.8 - 2.0 2.7 4.6 8.1 High emissions 2090 2.0 1.3 - 2.8 3.5 5.5 City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Emission Scenario and
Projected Year
Probabilistic Projections (feet)
Median Likely Range 1-in-20
Chance
1-in-200
Chance
H++
Scenario
50%
probability sea-
level rise
meets or
exceeds
66% probability sea-
level rise is
between…
5%
probability
sea-level rise
meets or
exceeds…
0.5%
probability sea-
level rise
meets or
exceeds…
Low
Risk
Aversion
Medium –
High Risk
Aversion
Extreme Risk
Aversion
Low emissions 2100 1.5 0.9 - 2.3 3.1 5.5 10.0 High emissions 2100 2.3 1.5 - 3.3 4.3 6.8 Low emissions 2110* 1.6 1.0 - 2.4 3.3 6.1 11.7 High emissions 2110* 2.5 1.8 - 3.5 4.5 7.2 Low emissions 2120 1.7 1.0 - 2.7 3.8 7.3 14.0 High emissions 2120 2.9 2.0 - 4.0 5.2 8.5 Low emissions 2130 1.9 1.1 - 3.0 4.2 8.3 16.3 High emissions 2130 3.2 2.2 - 4.5 5.9 9.8 Low emissions 2140 2.0 1.1 - 3.2 4.7 9.4 18.9 High emissions 2140 3.5 2.4 - 5.1 6.7 11.3 Low emissions 2150 2.2 1.1 - 3.6 5.3 10.8 21.7 High emissions 2150 3.9 2.6 - 5.7 7.6 12.9 * Most of the available climate model experiments do not extend beyond 2100. The resulting reduction in model availability causes a small dip in projections between 2100 to 2110, along with a shift in uncertainty estimates.
Use of 2110 projections should be done with caution and with acknowledgement of increased uncertainty around these projections.
The Project is expected to have a useful life of about 30 to 50 years, depending on the type of infrastructure used and the preferred alternative. Manufactured subsurface infrastructure (drywells and/or storage systems/structures) typically can have a longer useful life (50 years) than more natural systems (rock-filled trench) (20-30 years). Assuming a useful life of around 50 years, the Project would be expected to last until a little after 2070. Sea-level rise projections for 2070 range from 1 to 5.1 feet based on the table above. Assuming a medium-high risk aversion, the expected sea-level rise is approximately 3.4 feet.
The City’s Sea-Level Rise Risk, Hazards, and Vulnerability Assessment also summarizes the Coastal Storm Modeling System (CoSMoS) developed by the United State Geological Survey (USGS) for use in sea-level rise planning. Coastal hazards were mapped for the City in 2016. A total of 40 scenarios were run combining sea-level rise and storm type: ten sea-level rise amounts (0 to 2 meters at 0.25 increments and 5 meters) were modeled with four coastal storm conditions (no storm, 1-year, 20-year, and 100-year). The main difference with this data as compared to the OPC’s guidance is that this data illustrates the impacts of sea-level rise at set intervals, while it does not specify an expected date in which those impacts would be realized.
The approach to identifying Project constraints related to sea-level rise involves using both sources of information. The expected sea-level rise over the Project’s useful life was identified using OPC’s guidance City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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(3.4 feet or just over 1 meter) and the impacts of that depth of sea-level rise can be visualized using the associated CoSMoS dataset.
4.2 Results
The 1-meter (just over 3 feet) CoSMoS dataset was mapped onto the Project area to visualize the impacts of sea-level rise over the life of the Project (50 years), as this depth matches the OPC guidance projections, as further explained in the subsection above. As the sea-level water rises, the groundwater level is expected to rise at a similar rate. Figure 4-2 illustrates the existing topography, existing sea-level, and sea-level with 1-meter of rise based on average conditions (no storm) and during the 1- and 100-year storm events (CoSMoS datasets shown in figure). The sea-level during the 1- and 100-year storm events are shown for reference, to demonstrate that sea-level rise could vary depending on different environmental conditions.
Figure 4-2 Sea-Level Rise Projections (1-Meter)
The Project improvements on the beach could be inundated with 1-meter of sea-level rise has occurred, specifically during extreme storm events (100-year), while those are not expected to occur often. If the Project was submerged infrequently, it is expected that it could be restored and used successfully once the water level subsides to average conditions. Under average conditions (no storm event) and small storm events, it is expected that the groundwater level would rise at the same rate as the sea-level.
Industry guidance documents suggest providing 10 feet of separation between the invert of an infiltration system and the anticipated groundwater level. There are instances that this rule is not followed, such as
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for similar beach/coastal infiltration projects. At a minimum, it is recommended that at least 5 feet of separation be provided.
Figure 4-3 illustrates the current and projected groundwater conditions at the Project site on the beach.
The design constraint identified based on the sea-level rise analysis and recommended separation between infiltration systems and groundwater is the allowable depth to the infiltration invert from the surface. The figure demonstrates the beach infiltration system invert must be no more than 4.1 feet beneath the surface. It is recommended that the system have a minimum cover of 2 feet, resulting in a system depth of about 2 feet. Aligning with the specified constraints will provide 5 feet of separation between the infiltration invert and projected future groundwater level due to sea-level rise. This constraint has been considered and incorporated into the alternatives presented in Section 5 for improvements on the beach. Sea-level rise is more likely to impact infiltration on the beach as compared to infiltration systems located inland or at higher elevations.
Figure 4-3 Design Constraints Considering Sea-Level Rise Projections
The City is currently in the design phase of a dune restoration project along the coastline, which will help protect the community from the impacts of sea-level rise. The project will enhance 0.6-miles of dunes along the Marvin Braude Bike Trail from 28
th Street to 36th Street. The dunes will be located close to where the infiltration systems is proposed to be located on the beach, as illustrated in Figure 4-4.
Infiltration systems included as part of the Project could be located behind the dunes. The dune restoration may be phased to allow for the Project to be constructed first, which would prevent the dunes from being demolished as part of the Project’s construction. Coordination is ongoing between the Project and dune restoration project.
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Figure 4-4 Dune Restoration Project Site Map
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5. Alternatives
The EWMP-identified project included a beach infiltration system that would capture runoff from each of the drainage areas analyzed, as summarized in Section 3, except for Polliwog Park, as that is part of the BI 0286 drainage area, as illustrated in Figure 1-1. As part of the feasibility assessment, alternatives have been developed and evaluated. Alternatives have been identified for each of the drainage areas, which can ultimately be pieced together to address the full capture area identified in the EWMP, which represents 66% (or two thirds) of the City’s area. Approximately half of the City’s area not captured by the Project is tributary to Dominguez Channel and the other half is tributary to the Santa Monica Bay outside of the Project drainage areas. The first subsection below summarizes infiltration approaches used in the alternatives identified in the subsequent subsections. Advantages, disadvantages, and preliminary layouts are included for each alternative. Schematics have also been developed and are included in Appendix B. Opportunities to enhance and refine the concepts will be evaluated as part of the design process when additional site-specific information becomes available.
5.1 Infiltration Approaches
Infiltration is a cost-effective approach for improving water quality, especially when measured infiltration rates are as high as those identified in the geotechnical investigation, as summarized in Section 2.3.
Two approaches to infiltrate runoff have been identified as alternatives based on the soil conditions and site configurations. This subsection summarizes those approaches, which are used to identify alternatives for each of the Project’s drainage areas in the following subsections.
5.1.1 Subsurface Infiltration Systems
Subsurface infiltration systems are underground systems that provide a surface area for runoff to infiltrate into the native ground, while providing additional storage volume. These systems may include precast concrete vaults, plastic chambers/storage cells, or engineered sections with gravel and pipes for storage (also referred to as an infiltration trench). An infiltration trench was identified in the EWMP as part of the Project and has been used to perform the alternatives analysis further detailed below. A preliminary cross section of the beach infiltration trench considered is included in Appendix B.
Table 5-1 tabulates some of the advantages and disadvantages of subsurface infiltration systems in general and alternative designs/materials used in subsurface infiltration systems.
Table 5-1 Advantages and Disadvantages of Subsurface Infiltration Systems Alternative Advantages Disadvantages
Subsurface infiltration
(in general)
Reduces discharges downstream
Reduces pollutant load
downstream
Provides groundwater recharge
Potential for significant storage
capacity
Lower costs compared to
treatment
Requires open space for
placement
May require significant excavation
Ineffective for high sediment
loads
Not cost-effective for steep
slopes
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Alternative Advantages Disadvantages
Precast concrete
High bearing capacity (concrete)
for shallow profile
Longer expected design life
Open interior space (easier to
maintain)
Bigger footprint
Heavy equipment necessary for
construction
More expensive
Plastic chambers/
storage cells
Lightweight High-Density
Polyethylene (HDPE) material
(easier to construct)
Shallow profile
Pretreatment row of chamber
Open space (easier to maintain)
Less expensive than concrete
HDPE creep (gradual change in
shape)
Shorter anticipated design life
Engineered gravel
systems (infiltration
trench)
Cost-effective
Materials readily available
Versatile layout configurations
Easy to construct (specialty
equipment not needed)
Difficult to maintain
May need to remove/replace
media layers over time
Higher potential for clogging
System interior is mostly
inaccessible
5.1.2 Drywells
A drywell is a bored, drilled, or driven shaft or hole where the depth is greater than its width. They may be considered another subsurface infiltration system, although the difference highlighted in this report is that the subsurface infiltration systems do not have depths greater than their widths. A drywell may either be an excavated pit filled with aggregate or a prefabricated storage chamber with a pipe segment draining into a gravel filled pit. Many drywell configurations exist and may be used, while the design considered as part the alternatives analysis is based on a precast concrete shaft near the surface with a deeper shaft filled with gravel, as illustrated in Appendix B. Drywells are designed to temporarily store runoff while infiltration can be accomplished, like the subsurface infiltration systems. One key difference is that drywells can infiltration on all sides as opposed to just the bottom footprint. Table 5-2 tabulates some of the advantages and disadvantages of drywells, especially in comparison to subsurface infiltration systems.
Table 5-2 Advantages and Disadvantages of Drywells
Alternative Advantages Disadvantages
Drywells
Reduces discharges downstream
Reduces pollutant load
downstream
Provides groundwater recharge
Large infiltration surface area
(vertical) with small footprint
Low installation costs
Fast installation timeframe
Can work in sloped areas
Does not require large excavation
Groundwater must be at least
30+ feet deep
Ineffective for high sediment
loads
Complete reconstruction required
for failed drywells
May experience reduced design
life depending on influent quality
Maintenance may require rock
removal (infrequently)
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5.2 BI 0286 Drainage Area (28th Street)
The BI 0286 drainage area (28th Street) is the largest drainage area included in the Project, ultimately generating the largest volume of runoff. Polliwog Park, the 26th Street Parking Facility, and portions of the beach are within the drainage area and can be used to capture runoff. The elevation, size, and layout of the 26th Street Parking Facility makes it ideal for drywells, as several can be placed, which maximizes infiltration vertically without requiring a significant footprint. Opportunities for subsurface infiltration systems were evaluated and could only provide 36% of the capture the drywell configuration could provide. Implementation of subsurface infiltration systems would also require the full parking facility to be demolished and reconstructed, while drywells can be implemented without removing and reconstructing the existing retaining walls. Drywells at the 26 th Street Parking Facility have been
previously discussed with the City and were used to prepare and submit a funding application for Proposition 1 Stormwater Grant Program funds administered through the State Water Resources Control Board (SWRCB). Drywells at the 26
th Street Parking Facility have been identified in all the alternatives identified for addressing runoff from this drainage area. The following alternatives are further described below and exhibits are included in Appendix B:
1. Drywells at the 26th Street Parking Facility and infiltration system at Polliwog Park
2. Drywells at the 26th Street Parking Facility and beach infiltration
3. Drywells at the 26th Street Parking Facility and infiltration systems at Polliwog Park and the beach
5.2.1 26th Street Parking Facility and Polliwog Park
Figure 5-1 illustrates the preliminary drywell layout at the 26 th Street Parking Facility. Runoff will be
diverted from the BI 0286 storm drain on 28th Street through a diversion pipe aligned southeast on Ocean Drive. Diverted runoff will be pretreated through a hydrodynamic separation unit or other trash and sediment removal device and pumped to a matrix of drywells located in and around the parking facility. BI 0286 is a reinforced concrete tunnel and was installed using tunneling methods. The system has a very mild slope and is approximately 40 feet deep near Manhattan Avenue. The diversion is located on Ocean Drive so that the pretreatment unit and pump wet well will not need to be excessively deep. The goal was to maximize capture at this location due to the high infiltration rates and the proximity to the beach outfall. Opportunities to capture runoff through a gravity diversion system will be evaluated during the design process. A total of 48 drywells are shown based on spacing requirements.
Table 5-3 summarizes the concept characteristics and capture volume. The Factor of Safety (FOS) is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-3 Concept Characteristics for Drywells at the 26th Street Parking Facility Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 0286 62.9 53.5 52.1 0.4 52.5
Configuration: 48 drywells, 25+ feet deep
Infiltration: 200 inches per hour with a FOS of 1.5
A low flow diversion exists on BI 0286 on The Strand, which includes a diversion and pump station. The diversion and pump station were designed to capture dry-weather runoff, which are much smaller than City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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the anticipated wet-weather runoff to be captured by the Project. It is recommended that opportunities to utilize and/or upgrade existing infrastructure be explored during the design phase, as this could help save money on overall implementation. The exact location of each drywell will be determined during final design and will be placed to avoid existing utilities, retaining walls, and other onsite features. The parking lot will be repaved as part of the Project. Opportunities to utilize pervious pavement will be evaluated, which will increase onsite runoff capture. The pump station capacity will need to be refined during the final design based on the more detailed hydrologic analysis recommended in Section 3.1.
Figure 5-1 26th Street Parking Facility Drywell Concept
Polliwog Park is in the BI 0286 drainage area and can be used to capture the volume differential between the goal for the drainage area and what is captured at the parking facility. The RAA event served as the volume capture goal for the concepts developed. Capturing 10.4 acre-feet at Polliwog Park (in addition to the drywells at the 26
th Street Parking Facility, as mentioned above) would result in the capture of the full BI 0286 drainage area.
The existing drainage system at Polliwog Park consists of three storm drains collecting runoff from a 488-acre drainage area in the east half of the City. The drains discharge into Polliwog Park which acts as a detention basin located near the center of the park, as described in Section 2.1 and Section 2.4.
LACFCD operates the pump station that delivers water from the lake into BI 0552 located in Manhattan Beach Boulevard. This drain discharges into BI 0286 and ultimately into Santa Monica Bay.
One approach to capturing runoff at this location would be to construct a diversion at each of the storm drains delivering runoff into the lake. This approach was evaluated, and it was found that a pump (or
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multiple pumps) would be required and piping would be needed around most of the lake. Alternatively, a diversion could be placed within the lake, allowing for a single diversion and pump system. The lake diversion was found to be the preferred approach. Runoff diverted from the lake will be pretreated and then pumped to a subsurface infiltration system located within the multi-purpose fields in the northwest corner of the park, near 18th Street and Peck Avenue, as illustrated in Figure 5-2. The infiltration rates in this location are not very high, which is why a subsurface infiltration system was found to be more beneficial, as it will provide additional storage capacity to hold runoff as it infiltrates.
Figure 5-2 Polliwog Park Subsurface Infiltration System Concept
Table 5-4 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time. It is recommended that opportunities to use the existing pump system and intake structure be considered during the final design process, as this may present an opportunity to reduce capital and long-term costs.
Table 5-4 Concept Characteristics for Subsurface Infiltration at Polliwog Park Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 0286 18.81 16.0 2.5 7.9 10.4
Configuration: 76,000 square foot system, 5 feet deep
Infiltration: 1 inch per hour with a FOS of 1.5
1 Goal is 10.4 acre-feet based on the capture proposed at the 26th Street Parking Facility
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5.2.1.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for the two components of this alternative (Drywells at the 26th Street Parking Facility and subsurface infiltration at Polliwog Park). The cost opinions are preliminary and were developed using various sources of information as well as the engineer’s best judgement. Costs for planning, design, environmental documentation, permitting, and construction management are not included, as the opinions are for construction only. A contingency of 30% was included due to the preliminary nature of the alternative. The cost opinion of the 26th Street Parking Lot is included in Table 5-5, while the estimate for Polliwog Park is summarized in Table 5-6. The pump costs will be refined following a more detailed hydrologic analysis, which may result in significant changes to pump capacity and costs. Additionally, opportunities to incorporate a gravity diversion to maximize capture and minimize the demand for a pump will be evaluated as part of the design process.
Table 5-5 Preliminary Cost Opinion for Drywells at 26
th Street Parking Facility
Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $1,048,000 $1,048,000
Diversion Structure EA 1 $40,000 $40,000
Diversion Piping (Gravity) LF 60 $450 $27,000
Pretreatment System EA 1 $477,000 $477,000
Pump System and Appurtenances LS 1 $4,500,000 $4,500,000 Distribution Piping (Downstream of Pump) LF 1,580 $450 $711,000 Drywell EA 48 $72,000 $3,456,000
Parking Facility Improvements SF 24,000 $40 $960,000
Electrical LS 1 $300,000 $300,000
Subtotal: $11,519,000
Contingency (30%): $3,455,700
Total: $14,974,700
Table 5-6 Preliminary Cost Opinion for Subsurface Infiltration at Polliwog Park Description Unit Quantity Unit Cost Item Total
Mobilization (10%) 1 LS $982,000 $982,000
Diversion Structure EA 1 $40,000 $40,000
Pretreatment System EA 1 $171,000 $171,000
Pump System and Appurtenances LS 1 $1,800,000 $1,800,000 Distribution Piping (Force Main) LF 170 $450 $76,500
Subsurface Infiltration System GAL 2,575,000 $3 $7,725,000 Subtotal: $10,794,500
Contingency (30%): $3,238,400
Total: $14,032,900
The total cost for implementation of this alternative is $29,007,600, inclusive of improvements at both the 26
th Street Parking Facility and Polliwog Park. As mentioned above, the detailed hydrologic analysis recommended during design may result in increased pump costs.
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5.2.2 26th Street Parking Facility and Beach Infiltration
Another alternative for addressing runoff generated within the BI 0286 drainage area is to implement drywells at the 26th Street Parking Facility, as described in Section 5.2.1, and implement infiltration on the beach instead of capturing runoff at Polliwog Park. The concept for the parking facility is illustrated in Figure 5-1 and the Project characteristics would match those summarized in Table 5-3.
Infiltration on the beach includes diverting runoff from BI 0286 and routing it through pretreatment and to an engineered subsurface infiltration system (infiltration trench) located on the beach west of the existing bike path (in line/adjacent to the proposed dune restoration project mentioned in Section 4.2.
The infiltration system consists of gravel wrapped in filter fabric with 18-inch diameter perforated pipes to provide additional storage, as illustrated in Appendix B. Figure 5-3 illustrates the general layout of the beach infiltration approach.
Figure 5-3 Beach Infiltration Concept for Partial Diversion from BI 0286
Table 5-7 summarizes the concept characteristics and capture volume of the beach infiltration concept.
It is recommended that opportunities to combine the diversion, pretreatment, and pump system proposed with those proposed for the drywells at the parking facility be considered during the final design process, as this may present an opportunity to reduce capital and long-term costs. As mentioned above, opportunities to use/retrofit the existing low flow diversion infrastructure on BI 0286 near The Strand should also be evaluated. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
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Table 5-7 Concept Characteristics for Partial Beach Infiltration on BI 0286 Drainage Area Capture Goal (acre-feet) Concept Capture (acre-feet) RAA Event 85th Percentile Infiltration Storage Total
BI 0286 62.91 53.5 10.5 0.3 10.8
Configuration: 9,760 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 155.7 inches per hour with a FOS of 1.5
1 Goal is 10.4 acre-feet based on the capture proposed at the 26th Street Parking Facility
5.2.2.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for the two components of this alternative (drywells at the 26th Street Parking Facility and infiltration on the beach). The cost opinions are preliminary and were developed as described in Section 5.2.1.1. The cost opinion of the 26th Street Parking Facility is detailed in Table 5-5, while the estimate for the beach infiltration is summarized in Table 5-8.
Table 5-8 Preliminary Cost Opinion for Partial Beach Infiltration on BI 0286 Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $262,000 $262,000
Diversion Structure EA 1 $40,000 $40,000
Diversion Piping (Gravity) LF 60 $290 $17,400
Pretreatment System EA 1 $171,000 $171,000
Pump System and Appurtenances LS 1 $1,800,000 $1,800,000 Infiltration System SF 9,760 $60 $585,600
Subtotal: $2,876,000
Contingency (30%): $862,800
Total: $3,738,800
The total cost for implementation of this alternative is $18,713,500 inclusive of improvements at both the 26th Street Parking Facility and on the beach. As mentioned above, the detailed hydrologic analysis recommended during design may result in increased pump costs.
5.2.3 26th Street Parking Facility, Polliwog Park, and Beach Infiltration
The subsections above include recommendations for a more detailed hydrologic analysis to be performed during the final design process to better understand the anticipated flow rates within each of the drainage areas. The preliminary hydrologic analysis focused on runoff volume, while detailed hydrologic models inclusive of pipe flow will be necessary to understand the flow rates tributary to each Project site.
Ideally, one of the abovementioned alternatives for capturing runoff within the BI 0286 drainage area will provide sufficient capture to meet the volume capture goals. If the detailed hydrologic analysis results in high flow rates that cannot be captured fully in the alternatives described above, another alternative could be used in which drywells at the 26
th Street Parking Facility, subsurface infiltration at Polliwog Park, and subsurface infiltration at the beach be implemented. The footprints and layout would be dependent on the results of future analyses.
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The cost for this alternative would be based on a combination of the costs described above. It is likely that the configuration would not be identical to the concepts presented and that modifications would be made based on the results of the hydrologic analysis.
5.2.4 Comparison
Table 5-9 summarizes the advantages and disadvantages of the alternatives summarized above for addressing runoff within the BI 0286 drainage area.
Table 5-9 Advantages and Disadvantages of BI 0286 Alternatives Alternative Advantages Disadvantages
26th Street Parking
Facility and Polliwog
Park
No impact to the beach
Coordination with LACBH would
be minimized/eliminated
School site may offer education
benefits to students
Pollutant loads will be reduced
Two pump stations required
Infiltration rates at Polliwog Park
are much lower than the beach
Coordination required with the
school district
Higher cost
26th Street Parking
Facility and Beach
Infiltration
Lower cost
Faster construction timeline
More centralized construction
location (reduced mobilization)
Centralized maintenance
Pump station(s) required
Potential beach impact
Equipment access on the beach
may be limited/difficult
Sandy soils may cause
construction challenges
Beach infiltration may not remove
pollutant loads
26th Street Parking
Facility, Polliwog Park,
and Beach Infiltration
Achieve EWMP-identified capture
(if required based on analysis)
Most disadvantages above apply
Distributed implementation,
operation, and maintenance
Higher costs
5.3 BI 5452 Line E (21st Street)
Two alternatives were identified for the BI 5452 Line E drainage area, which are similar for each of the smaller drainage areas evaluated for inclusion in the Project (subsections below). This drainage area generates 0.7 acre-feet of runoff during the RAA design storm, as identified in Section 3.3. The following alternatives have been evaluated for addressing runoff in the BI 5452 Line E drainage area and exhibits are included in Appendix B:
1. Drywells upstream of the beach
2. Subsurface infiltration on the beach
5.3.1 Drywells Upstream of Beach
One alternative for addressing runoff in BI 5452 Line E on 21 st Street will be to install four drywells in
Ocean Drive. The system would include a diversion structure on Line E, pretreatment unit, and distribution piping to the drywells. Figure 5-4 illustrates the general concept and layout of the drywells.
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The drywells must be placed upstream of the beach, as adequate separation is needed between the infiltration invert (bottom of the drywell) and the groundwater level (additional discussion is included in Section 4.2). Additionally, the drywells must achieve a minimum depth to function as intended.
Figure 5-4 Drywell Concept on BI 5452 Line E
Table 5-10 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-10 Concept Characteristics for Drywells on BI 5452 Line E Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line E 0.7 0.6 0.7 <0.1 0.7
Configuration: 4 drywells, 25 feet deep
Infiltration: 200 inches per hour with a FOS of 1.5
5.3.1.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for drywells upstream of the beach and is summarized in Table 5-11. The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
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Table 5-11 Preliminary Cost Opinion for Drywells on BI 5452 Line E Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $38,000 $38,000
Diversion Structure EA 1 $20,000 $20,000
Diversion/Distribution Piping (Gravity) LF 150 $100 $15,000 Pretreatment System EA 1 $56,000 $56,000
Drywell EA 4 $72,000 $288,000
Subtotal: $417,000
Contingency (30%): $125,100
Total: $542,100
5.3.2 Beach Infiltration
Another alternative for BI 5452 on 21st Street is to install a subsurface infiltration system (infiltration trench) on the beach west of the existing bike path. The system would include a diversion structure on Line E pretreatment unit, distribution piping, and the infiltration system. Figure 5-5 illustrates the concept and layout of the beach infiltration alternative.
Figure 5-5 Beach Infiltration Concept on BI 5452 Line E
Table 5-12 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
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Table 5-12 Concept Characteristics for Beach Infiltration at BI 5452 Line E Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line E 0.7 0.6 0.7 <0.1 0.7
Configuration: 480 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 200 inches per hour with a FOS of 1.5
5.3.2.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for infiltration on the beach and is summarized in Table 5-13.
The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
Table 5-13 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line E Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $13,000 $13,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 190 $100 $19,000
Pretreatment System EA 1 $56,000 $56,000
Infiltration System SF 480 $60 $28,800
Subtotal: $136,800
Contingency (30%): $41,100
Total: $177,900
5.3.3 Comparison
Table 5-14 summarizes the advantages and disadvantages of the two alternatives summarized above for addressing runoff from BI 5452 Line E. These advantages and disadvantages are the same for each of the small drainage areas in the following subsections.
Table 5-14 Advantages and Disadvantages of BI 5452 Line E Alternatives Alternative Advantages Disadvantages
Drywells upstream of
beach
No impact to the beach
Easier construction
Compact installation (smaller
footprint)
Easier to access for maintenance
Pollutant loads will be reduced
Higher costs
Construction closer to residents
Beach infiltration Lower costs
Construction away from residents
Potential beach impact
Equipment access on the beach
may be limited/difficult
Sandy soils may cause
construction challenges
Beach infiltration may not remove
pollutant loads
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5.4 BI 5452 Line G (27th Street)
Two alternatives were identified for the BI 5452 Line G drainage area, which are similar for each of the smaller drainage areas evaluated for inclusion in the Project. This drainage area generates 1 acre-foot of runoff during the RAA design storm, as identified in Section 3.3. The following alternatives have been evaluated for addressing runoff in the BI 5452 Line G drainage area and exhibits are included in Appendix B:
1. Drywells upstream of the beach
2. Subsurface infiltration on the beach
5.4.1 Drywells Upstream of Beach
One alternative for BI 5452 Line G on 27
th Street is to install five drywells on Ocean Drive. The system would include a diversion structure on Line G, pretreatment unit, and distribution piping to the drywells.
Figure 5-6 illustrates the general concept and layout of the drywells alternative. Drywell placements upstream of the beach and depth requirements are described in Section 5.3.1.
Figure 5-6 Drywell Concept on BI 5452 Line G
Table 5-15 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
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Table 5-15 Concept Characteristics for Drywells on BI 5452 Line G Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line G 1.0 0.8 1.0 <0.1 1.0
Configuration: 5 drywells, 25 feet deep
Infiltration: 200 inches per hour with a FOS of 1.5
5.4.1.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for drywells upstream of the beach and is summarized in Table 5-16. The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
Table 5-16 Preliminary Cost Opinion for Drywells on BI 5452 Line G Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $47,000 $47,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 210 $100 $21,000
Pretreatment System EA 1 $63,000 $63,000
Drywell EA 5 $72,000 $360,000
Subtotal: $511,000
Contingency (30%): $153,300
Total: $664,300
5.4.2 Beach Infiltration
Another alternative for BI 5452 Line G on 27th Street is to install a subsurface infiltration system (infiltration trench) on the beach west of the existing bike path. The system would include a diversion structure on Line G, pretreatment unit, distribution piping, and infiltration system. Figure 5-7 illustrates the concept and layout of the beach infiltration alternative.
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Figure 5-7 Beach Infiltration Concept on BI 5452 Line G
Table 5-17 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-17 Concept Characteristics for Beach Infiltration at BI 5452 Line G Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line G 1.0 0.8 1.0 <0.1 1.0
Configuration: 640 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 200 inches per hour with a FOS of 1.5
5.4.2.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for infiltration on the beach and is summarized in Table 5-18.
The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
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Table 5-18 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line G Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $15,000 $15,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 200 $100 $20,000
Pretreatment System EA 1 $63,000 $63,000
Infiltration System SF 640 $60 $38,400
Subtotal: $156,400
Contingency (30%): $47,000
Total: $203,400
5.4.3 Comparison
The comparison between drywells upstream of the beach and infiltration on the beach are the same for each of the smaller drainage areas tributary directly to the beach. The advantages and disadvantages for this location are as summarized in Table 5-14 in Section 5.3.3.
5.5 BI 5452 Line I (24th Street)
Two alternatives were identified for the BI 5452 Line I drainage area, which are similar for each of the smaller drainage areas evaluated for inclusion in the Project. This drainage area generates 1.1 acre-feet of runoff during the RAA design storm, as identified in Section 3.3. The following alternatives have been evaluated for addressing runoff in the BI 5452 Line I drainage area and exhibits are included in Appendix B:
1. Drywells upstream of the beach
2. Subsurface infiltration on the beach
5.5.1 Drywells Upstream of Beach
One alternative for BI 5452 Line I on 24th Street is to install six drywells in the Ocean Drive. The system would include a diversion structure on Line I, pretreatment unit, and distribution piping to the drywells.
Figure 5-8 illustrates the general concept and layout of the drywells alternative. Drywell placements upstream of the beach and depth requirements are described in Section 5.3.1.
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Figure 5-8 Drywell Concept on BI 5452 Line I
Table 5-19 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-19 Concept Characteristics for Drywells on BI 5452 Line I Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line I 1.1 0.9 1.1 <0.1 1.1
Configuration: 6 drywells, 25 feet deep
Infiltration: 200 inches per hour with a FOS of 1.5
5.5.1.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for drywells upstream of the beach and is summarized in Table 5-20. The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
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Table 5-20 Preliminary Cost Opinion for Drywells on BI 5452 Line I Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $55,000 $55,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 220 $100 $22,000
Pretreatment System EA 1 $69,000 $69,000
Drywell EA 6 $72,000 $432,000
Subtotal: $598,000
Contingency (30%): $179,400
Total: $777,400
5.5.2 Beach Infiltration
Another alternative for BI 5452 Line I on 24th Street is to install a subsurface infiltration system (infiltration trench) on the beach west of the existing bike path. The system would include a diversion structure on Line I, pretreatment unit, distribution piping, and infiltration system. Figure 5-9 illustrates the concept and layout of the beach infiltration alternative.
Figure 5-9 Beach Infiltration Concept on BI 5452 Line I
Table 5-21 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
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Table 5-21 Concept Characteristics for Beach Infiltration at BI 5452 Line I Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
BI 5452 Line I 1.1 1.0 1.1 <0.1 1.1
Configuration: 720 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 200 inches per hour with a FOS of 1.5
5.5.2.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for infiltration on the beach and is summarized in Table 5-22.
The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
Table 5-22 Preliminary Cost Opinion for Beach Infiltration at BI 5452 Line I Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $15,000 $15,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 200 $100 $20,000
Pretreatment System EA 1 $63,000 $63,000
Infiltration System SF 720 $60 $43,200
Subtotal: $161,200
Contingency (30%): $48,400
Total: $209,600
5.5.3 Comparison
The comparison between drywells upstream of the beach and infiltration on the beach are the same for each of the smaller drainage areas tributary directly to the beach. The advantages and disadvantages for this location are as summarized in Table 5-14 in Section 5.3.3.
5.6 City Drain North (32nd Street)
Two alternatives were identified for the City Drain North drainage area, which are similar for each of the smaller drainage areas evaluated for inclusion in the Project. This drainage area generates 3 acre-feet of runoff during the RAA design storm, as identified in Section 3.3. The following alternatives have been evaluated for addressing runoff in the City Drain North drainage area and exhibits are included in Appendix B:
1. Drywells upstream of the beach
2. Subsurface infiltration on the beach
5.6.1 Drywells Upstream of Beach
One alternative for the City Drain North in 32
nd Street is to install eight drywells in the Ocean Drive and eight drywells in the Manhattan Avenue. The system would include a diversion structure on the City Storm Drain at each intersection with 32nd Street, pretreatment units, and distribution pipe to the City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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drywells. Figure 5-10 illustrates the general concept and layout of the drywells alternative. Two rows of drywells are required at this site due to the number needed to capture the design runoff volume and spacing requirements. Drywell placements upstream of the beach and depth requirements are described in Section 5.3.1.
Figure 5-10 Drywell Concept on City Drain North
Table 5-23 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-23 Concept Characteristics for Drywells on City Drain North Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
City Storm Drain 3.0 2.0 3.0 <0.1 3.0
Configuration: 16 drywells, 25 feet deep
Infiltration: 200 inches per hour with a FOS of 1.5
5.6.1.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for drywells upstream of the beach and is summarized in Table 5-24. The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
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Table 5-24 Preliminary Cost Opinion for Drywells on City Drain North Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $140,000 $140,000
Diversion Structure EA 2 $20,000 $40,000
Diversion Piping (Gravity) LF 570 $100 $57,000
Pretreatment System EA 2 $72,000 $144,000
Drywell EA 16 $72,000 $1,152,000
Subtotal: $1,533,000
Contingency (30%): $459,900
Total: $1,992,900
5.6.2 Beach Infiltration
Another alternative for the City Drain North on 32nd Street is to install a subsurface infiltration system (infiltration trench) on the beach west of the existing bike path. The system would include a diversion structure on the City Storm Drain, pretreatment unit, distribution piping, and infiltration system.
Figure 5-11 illustrates the concept and layout of the beach infiltration alternative.
Figure 5-11 Beach Infiltration Concept on City Drain North
Table 5-25 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
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Table 5-25 Concept Characteristics for Beach Infiltration at City Drain North Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
City Storm Drain 3.0 2.0 2.9 0.1 3.0
Configuration: 2,000 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 200 inches per hour with a FOS of 1.5
5.6.2.1 Preliminary Cost Opinion
A preliminary cost opinion was developed for infiltration on the beach and is summarized in Table 5-26.
The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
Table 5-26 Preliminary Cost Opinion for Beach Infiltration at City Drain North Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $28,000 $28,000
Diversion Structure EA 1 $20,000 $20,000
Diversion Piping (Gravity) LF 100 $100 $10,000
Pretreatment System EA 1 $126,000 $126,000
Infiltration System SF 2,000 $60 $120,000
Subtotal: $304,000
Contingency (30%): $91,200
Total: $395,200
5.6.3 Comparison
The comparison between drywells upstream of the beach and infiltration on the beach are the same for each of the smaller drainage areas tributary directly to the beach. The advantages and disadvantages for this location are as summarized in Table 5-14 in Section 5.3.3.
5.7 Full Capture Area (EWMP Project)
The Beach Cities EWMP identified a single project that captured each of the Project drainage areas in a beach infiltration system. One alternative for Project implementation includes the EWMP approach, as summarized in this subsection. This alternative serves as a comparison for the alternatives identified in the subsections above by drainage area. This approach ultimately is a combination of the beach infiltration alternatives by drainage area, resulting in five diversions and pretreatment systems, along with a 25,410 square foot infiltration system. A preliminary schematic is included in Figure 5-12 and an exhibit is included in Appendix B.
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Figure 5-12 Full Beach Infiltration Concept
Table 5-27 summarizes the concept characteristics and capture volume. The FOS is based on the soil type, site conditions, and accounts for degradation to infiltration capacity over time.
Table 5-27 Concept Characteristics for Full Beach Infiltration Drainage Area
Capture Goal (acre-feet) Concept Capture (acre-feet)
RAA Event 85th Percentile Infiltration Storage Total
Full Area 68.7 57.8 67.9 0.8 68.7
Configuration: 25,410 square foot infiltration trench system, 16 feet wide and 2 feet deep Infiltration: 155.7 inches per hour with a FOS of 1.5
A preliminary cost opinion was developed for infiltration on the beach and is summarized in Table 5-28.
The cost opinions are preliminary and were developed as described in Section 5.2.1.1.
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Table 5-28 Preliminary Cost Opinion for Full Beach Infiltration Description Unit Quantity Unit Cost Item Total
Mobilization (10%) LS 1 $884,000 $844,000
Diversion Structure (Five Total) LS 1 $120,000 $120,000 Diversion Piping (Gravity) LF 750 $130 $97,500
Pretreatment System (Five Total) LS 1 $791,000 $791,000 Pump System and Appurtenances LS 1 $6,300,000 $6,300,000 Infiltration System SF 25,410 $60 $1,524,600
Subtotal: $9,717,100
Contingency (30%): $2,915,200
Total: $12,632,300
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6. Proposed Alternative
The 26th Street Parking Facility and Beach Infiltration alternative, described in Section 5.2.2, was identified as the preferred implementation approach due to its centralized location and cost effectiveness.
This alternative is now referred to as the 28th Street Storm Drain Infiltration Project. A detailed Project Description was developed and is included in Appendix C. The proposed implementation approach involves implementing drywells at the 26th Street Parking Facility as Phase 1 and beach infiltration as Phase 2. The size and configuration of Phase 2 will depend on Phase 1 monitoring. The volume captured in Phase 1 will be maximized to reduce or eliminate Phase 2 implementation. Figure 6-1 illustrates the proposed improvements and phasing. The layout and configuration will be refined during the design process. Opportunities to implement a gravity diversion will be evaluated as a cost saving option. The sections below discuss the proposed alternative’s benefits related to water quality, water supply, and community investment; Disadvantaged Community (DAC) benefits; opportunities for nature- based solutions; vector concerns; and legal requirements.
Figure 6-1 Proposed Alternative with Phasing
6.1 Quantified Benefits
The Project’s benefits related to water qualify, water supply, and community investment were quantified to support the Project’s request for funding under the Los Angeles County Measure W Safe, Clean Water Program (SCWP) for Infrastructure Projects. Quantified benefits are typically requested in funding
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applications and are also valuable during community outreach/engagement, as they provide measurable and tangible benefits. The sections below summarize the benefits of the proposed alternative.
6.1.1 Water Quality
The Project provides water quality benefits by reducing the pollutant loads reaching downstream receiving waters through the removal of runoff from the drainage system. Wet- and dry-weather runoff will be captured and infiltrated through a subsurface storage system (drywells and/or infiltration trench on the beach), which removes them, and their associated pollutant load, from the receiving water system. This benefit can be categorized as a volume and/or pollutant load removed from the system. The benefits and metrics are summarized in Table 6-1 below.
Table 6-1 Summary of Water Quality Benefits and Metrics Benefit Metric Description
Water Quality
Acre-feet (AF)
Acre-feet per year (AFY)
Volume of polluted runoff captured/diverted
by Project
Pounds (lbs)
Pounds per year (lbs/year)
Pollutant concentration multiplied by volume
capture = load removed
The SCWP module, which is used to submit an Infrastructure Program funding application, performs calculations to quantify pollutant load reductions and average annual runoff capture. The pollutant load reductions are quantified based on a 10-year hourly simulation with the Watershed Management Modeling System (WMMS). Table 6-2 summarizes the average annual runoff capture and pollutant load reductions anticipated following implementation of both Phase 1 and 2.
Table 6-2 Summary of Quantified Water Quality Benefits Metric Runoff from
Capture Area
Project
Inflow
Project
Outflow
Reduction
by Project
Percent
Reduction
Runoff volume (acre-feet) 611 588 110 478 81%
Total zinc (pounds) 327 310 69 241 78%
Total copper (pounds) 116 109 29 80 73%
Total lead (pounds) 62 58 16 42 73%
Total nitrogen (pounds) 6,844 6,618 1,037 5,581 84%
Total Phosphorous
(pounds) 949 913 164 749 82%
E. coli (#) 1.12E+15 1.06E+15 2.47E+14 8.14E+14 77%
6.1.2 Water Supply
The Project is located within the Coastal Plain of Los Angeles – West Coast groundwater basin, which produces approximately 52,000 acre-feet annually for the local communities. The groundwater basin is adjacent to the coastline and the volume extracted is limited due to water rights and to prevent seawater intrusion. A series of injection wells have been installed along the outer limits of the groundwater basin to create a seawater barrier, which protects the drinking water aquifer from seawater intrusion. Several injection wells are located within the City. The West Coast Seawater Barrier depends on imported water and high-quality recycled water to inject into the barrier.
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It is common for infiltrated runoff to provide water supply benefits through aquifer/groundwater recharge. Water recharged into the ground typically becomes available for use in the future. The Project is near the coast. Seawater intrusion occurs near the coast when saline water mixes with freshwater supplies beneath the ground surface. Water is not withdrawn in these areas for use due to the high saline content and extensive treatment that would be needed to deliver the water through the potable water system.
Other ways to achieve water supply benefits include using the captured runoff for irrigation or other non-potable uses or diverting runoff into a sanitary sewer system where it will be treated and used as recycled water. The Project does not meet either of these criteria for providing water supply benefits.
6.1.3 Community Investment
Community investment benefits are Project features that help improve the surrounding community.
SCWP identifies seven specific community benefits that project sponsors are encouraged to provide as part of their projects. Table 6-3 summarizes the SCWP-specified community investment benefits and whether they are provided by the Project.
Table 6-3 Summary of SCWP-Identified Community Investment Benefits Community Investment Provided? Description
Improve flood management, flood
conveyance, or flood risk mitigation No -
Create, enhance, or restore park space,
habitat, or wetland space No -
Improve public access to waterways No -
Create or enhance new recreational
opportunities No -
Create or enhance green spaces at school No -
Reduce local heat island effect and
increase shade Yes
Three new trees will be included in the
26th Street Parking Facility, which reduce
local heat island effect and increase shade
Increase shade or the number of trees or
other vegetation onsite Yes
Trees will be planted as mentioned above
and other vegetation/land cover is also
being assessed for inclusion
Other funding programs encourage community benefits, while not specifically identifying what they are.
The Project provides the following community benefits not specifically recognized by the SCWP:
Enhance accessibility through the construction of sidewalk ramps that meet the Americans with Disabilities Act (ADA) requirements at the 26th Street Parking Facility Incorporate educational signage to raise community awareness of water quality Public outreach events will be held to inform and educate the community
Public and stakeholder meetings will be held throughout Project development to seek community input, which may be used to shape some of the Project’s features. Several letters of support were obtained by City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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local non-profit organizations and community-based organizations, which are included in Appendix D for reference. Letters of support available at the time this document was prepared include the following:
Heal the Bay: letter indicates they support the Project, as it aligns with their mission Manhattan Beach Botanical Garden: letter indicates they support the Project for being one of many efforts needed to battle climate change
7th Generation Advisors: letter indicates they support the Project’s goal of improving water quality in the region and Santa Monica Bay
6.2 Disadvantaged Community Benefits
Several funding programs encourage and prioritize projects that provide benefits to DACs. The definition of a DAC may vary based on the funding program and/or source of information. The Department of Water Resources defines severe DACs as areas where the median household income is less than $42,737 and DACs as areas where the median household income is greater than $42,737 and less than $56,982 (based on 2018 datasets). Based on the 2016 dataset (illustrated in Figure 6-2), severe DACs were defined where the median household income was less than $38,270 and DACs were above $38,270 and under $51,026.
Figure 6-2 DAC Areas in Project Vicinity
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There are different ways to demonstrate that projects provide benefits to DACs. There are more clearly DAC benefits when a project is located within a DAC area and the project includes amenities and other features that will be used by the community. DAC benefits are not claimed formally by the Project, while some may be provided.
The Project may provide construction jobs for DAC residents within the South Bay area, including those within DACs in the Cities of Hawthorne, Inglewood, and Lawndale. The 26th Street Parking Facility will be enhanced and made more accessible by including ADA improvements, which will benefit beach goers, which may include DAC residents from outside the City. The parking facility provides parking spaces and beach access to visitors who cannot afford to live in the community. The parking facility will also have educational signage to teach visitors about the Project benefits, goals, and components (pervious pavement and infiltration drywells).
6.3 Effectiveness of Similar Projects
Infiltration is effective at reducing pollutant loads from the runoff diverted from the drainage system, as the runoff is removed and do not reach downstream water bodies, whereby the pollutants carried in the water also do not reach downstream water bodies. Infiltration is the preferred approach to improve water quality and is required when feasible for new/re-development projects. Several infiltration projects, including subsurface infiltration, drywells, and infiltration trenches, have been implemented throughout Los Angeles County due to their effectiveness in decreasing peak runoff rates, improving water quality, and decreasing the volume of stormwater runoff reaching downstream water bodies. They are effective in the built environment, as they do not require available space on the surface and can be buried, preserving surface uses. These systems are commonly placed within parking lots, as there is a large open space. The pretreatment systems proposed on the Project (hydrodynamic separators) are also commonly used throughout the region and is a California certified full capture device.
Drywells were chosen due to their cost effectiveness, small footprint, large infiltration surface area, and proven success for infiltrating large volumes of runoff. The drywells will infiltrate dry- and wet-weather runoff while reducing pollutant loads discharged into the Santa Monica Bay. Some examples of local projects that used drywells include Laurel Canyon Green Street in the City of Los Angeles, Foothill Boulevard Master Plan Implementation in the City of Claremont, and several projects led by LACPW.
The Hermosa Strand Infiltration Trench Project, which was completed in 2010, captures urban runoff before it reaches the Santa Monica Bay and diverts it to a buried infiltration trench approximately 1,000-feet long. Captured runoff is processed through a pretreatment system to remove debris, sediment, and hydrocarbons before discharging into a buried infiltration trench. The buried infiltration system includes structural units that provide void space for storage and allow for water to infiltrate. The runoff is then filtered by the existing sand/media layers to remove bacteria before discharging into the Santa Monica Bay. The Project is still in place and being used to meet the Santa Monica Bay Bacteria TMDL. The Project (Phase 2) would have a similar configuration along the coast.
6.4 Nature-Based Solutions
Nature-based solutions are emphasized under the SCWP. SCWP documentation describes nature-based solutions as “sustainable management and use of nature for undertaking socio-environmental challenges, including climate change, water security, water pollution, food security, human health, and disaster risk City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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management.” Nature-based solutions are to use natural processes that slow, detain, and infiltrate runoff. The SCWP emphasizes the following criteria related to nature-based solutions:
1. Project mimics natural processes to slow, detain, capture, and absorb/infiltrate water in a manner that protects, enhances, or restores habitat, green space, or usable open space.
2. Project uses natural materials, such as soils and vegetation, with a preference for native vegetation.
3. Impermeable surfaces are removed and replaced by pervious surfaces.
The Project will incorporate nature-based solutions based on the criteria specified above. The Project includes infiltration systems that will mimic natural processes by infiltrating runoff. Prior to urbanization and development, surfaces were pervious and allowed for infiltration to occur naturally. Implementing infiltration systems allows runoff to infiltrate as they did prior to urbanization. As mentioned in Section 5.2.1, the Project may include resurfacing the 26th Street Parking Facility with pervious pavement. Pervious pavement will mimic natural processes by allowing infiltration at the site in which rain falls, while it also aligns with the third goal defined above. Pervious pavement in the parking facility would reduce the impermeable area by approximately 20,000 square feet. Natural materials (native plants/soil) will be used in the parking facility to enhance existing vegetated areas and at the locations where new trees are proposed (as defined in Section 6.1.3).
6.5 Vector Concerns
Management of mosquitoes and other vectors in stormwater management structures, such as infiltration systems, is critical for protecting public health. A Vector Minimization Plan has been prepared for the Project based on the California Department of Public Health’s Checklist for Minimizing Vector Production in Stormwater Management Structures (2010). The Vector Minimization Plan includes a checklist of action items intended to lessen the short and long-term potential for vector production in stormwater management structures. The plan is included in Appendix E and it identifies Project components and divides them into wet or dry systems.
The dry systems cover the subsurface infiltration systems (drywells and infiltration trench), as water will not be standing for more than four days. The infiltration rate is high enough and the groundwater is deep enough to prevent long-term ponding, which discourages vector breeding. An operation and maintenance manual (further discussed in Section 8) will be used to promote routine maintenance, which will help prevent sediment accumulation that can cause standing water.
Wet systems include the pretreatment system, pump station, and vaults, as they will hold water permanently or longer than four days, typically within the sump areas. The wet systems will include covers, screens, and other barriers to deny access to mosquitos/vectors. Structures will have accessibility for inspection and maintenance and will be inspected for vectors on a periodic basis.
6.6 Legal Requirements
No legal requirements or obligations are anticipated as a result of constructing the Project.
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7. Cost and Schedule
This section summarizes the anticipated implementation cost and schedule of the proposed alternative.
The schedule is based on securing funding in 2021 and may be modified based on when funds become available. Phase 2 implementation will not be scheduled until Phase 1 has been implemented and monitored, as the need and size of Phase 2 will depend on whether Phase 1 achieves compliance.
7.1 Preliminary Cost Opinion
Preliminary cost opinions were developed for the recommended implementation approach. Table 7-1 summarizes the cost opinions for Phase 1 and Phase 2, which are based on Table 5-5 and Table 5-8.
The cost opinions were developed using various sources of information as well as the engineer’s best judgement. A contingency of 30 percent is included due to the preliminary nature of the opinion and will be decreased in design level opinions. The cost opinion is based on present value (2020-21 dollars) and does not account for inflation or changes in pricing that may occur by the time the Project is constructed.
Costs will be refined in the design phase and opportunities to reduce costs will be evaluated.
Table 7-1 Opinion of Probable Cost for Phase 1 and Phase 2 Item No. Description Unit Quantity Unit Price Total
Phase 1
1 Mobilization (10%) LS 1 $1,048,000 $1,048,000
2 Diversion Structure EA 1 $40,000 $40,000
3 Diversion Piping (Gravity) LF 60 $450 $27,000
4 Pretreatment System EA 1 $477,000 $477,000
5 Pump System and Appurtenances LS 1 $4,500,000 $4,500,000 6 Distribution Piping (Downstream
of Pump) LF 1,580 $450 $711,000
7 Drywell EA 48 $72,000 $3,456,000
8 Parking Facility Improvements SF 24,000 $40 $960,000
9 Electrical LS 1 $300,000 $300,000
Subtotal: $11,519,000
Contingency (30%): $3,455,700
Phase 1 Total: $14,974,700
Phase 2
10 Mobilization (10%) LS 1 $262,000 $262,000
11 Diversion Structure EA 1 $40,000 $40,000
12 Diversion Piping (Gravity) LF 60 $290 $17,400
13 Pretreatment System EA 1 $171,000 $171,000
14 Pump System and Appurtenances LS 1 $1,800,000 $1,800,000 15 Infiltration System SF 9,760 $60 $585,600
Subtotal: $2,876,000
Contingency (30%): $862,800
Phase 2 Total: $3,738,800
GRAND TOTAL: $18,713,500
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Table 7-2 summarizes overall implementation costs for both Phase 1 and Phase 2. Administrative costs are not currently accounted for separately. Construction costs are based on the cost opinion above and other implementation line items are based on the percentage indicated (percent of construction cost).
Table 7-2 Summary of Implementation Costs
Item No. Description Unit Quantity Total
Phase 1
A1 Planning (Actual Costs, Split) LS 1 $99,143
B1 Design % 5 $748,740
C1 Environmental/Permitting % 3 $449,250
D1 Construction Administration % 7 $1,048,230
E1 Construction (Opinion Above) LS 1 $14,974,700
Phase 1 Total: $17,320,063
Phase 2
A2 Planning (Actual Costs, Split) LS 1 $99,143
B2 Design % 5 $186,940
C2 Environmental/Permitting % 3 $112,170
D2 Construction Administration % 7 $261,720
E2 Construction (Opinion Above) LS 1 $3,738,800
Phase 2 Total: $4,398,773
Combined Phases (Total)
A Planning (Actual Costs) LS 1 $198,286
B Design % 5 $935,680
C Environmental/Permitting % 3 $561,420
D Construction Administration % 7 $1,309,950
E Construction (Opinion Above) LS 1 $18,713,500
GRAND TOTAL: $21,718,836
7.2 Anticipated Schedule
The schedule illustrated in Figure 7-1 is based on funding being secured for Project implementation in the first quarter of Year 1. The schedule is focused on Phase 1 implementation. Phase 1 will be implemented, monitored, and assessed to determine the need and sizing of Phase 2 implementation.
Phase 2 implementation will be scheduled in the future.
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Figure 7-1 Project Schedule
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7.3 Leveraged Funding
Many funding programs prefer that funding is leveraged for Project implementation, meaning that more than one source is used. This can be taken as a form of commitment to the Project, especially when the implementing agency is using their internal budget to support aspects of the Project. In some instances, funds from two different funding programs can be used to leverage (or match) each other and each funding program has its own requirements and priorities.
The City submitted a funding application under the Proposition 1 Stormwater Grant Program (Implementation, Round 2) administered by the SWRCB. This grant program requires a minimum match of 50 percent of the total project cost. Matching funds can be from any non-state funding source, which could include City’s General Fund, SCWP Infrastructure Program, and/or SCWP Municipal Program. The application submitted to SWRCB was for Phase 1 of the Project only, so that the match was only based on Phase 1 cost (50% of total project cost). A total of $8,535,000 was requested with a promise to match the funding (another $8,535,000), resulting in a total cost of $17,070,000. The matching funds in this application were planned to be from SCWP Municipal and/or Infrastructure Programs (application for Infrastructure Program funding was submitted after the Proposition 1 application). The grant application was developed before the cost opinions were refined to what is included above (total cost for Phase 1 varies slightly between Proposition 1 application and this document).
The SCWP Infrastructure Program also prioritizes and awards points to projects that leverage other funding sources. When the SCWP application was submitted, Proposition 1 funding had not been decided; therefore, the City did not show leveraged funds. In the SCWP application, leveraged funds were claimed for planning costs, as the City used internal budgets to pay for planning, along with life- cycle costs for operation, maintenance, and monitoring, as the City anticipates paying for those once the Project is constructed, unless additional funding is secured.
The Project was ultimately awarded SCWP Infrastructure Program funds, while the City continues to evaluate additional funding opportunities. The Project was waitlisted for Proposition 1 Stormwater Grant Program funding and may be awarded funds if other projects do not move forward. Additionally, the City is in coordination with the California National Resource Agency for Urban Flood Protection Grant funds.
The City continues to evaluate funding opportunities that can be leveraged for Project implementation.
7.4 Life-Cycle Cost
The SCWP module quantifies life-cycle costs based on planning, design, implementation (construction), operation, maintenance, and monitoring costs. The module applies an annual discount rate as specified below. The Project life cycle, specifically for Phase 1, is expected to be approximately 50 years.
Table 7-3 summarizes the 50-year life-cycle cost of the proposed alternative submitted to SCWP, which includes design of both phases and implementation of Phase 1 only. The life-cycle and annualized costs included in the table are based on the SCWP module calculations.
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Table 7-3 Life-Cycle Costs
Description Cost
Planning (Phase 1 and 2) $198,286
Design, Permitting, and Environmental (Phase 1 and 2) $1,497,0801 Construction/Construction Management (Phase 1) $16,022,930 Annual Maintenance Cost $101,0002
Annual Operation Cost $50,0003
Annual Monitoring Cost $50,0003
Life-Cycle Cost for Project* $22,541,070
Annualized Cost for Project* $939,450
1 Includes Phase 1 and 2 (off by $20 compared to Table 7-2 due to rounding). 2 Additional details included in Section 8.1.
3 Approximate costs based on similar projects in the region (will be confirmed following implementation). * Applies an annual discount rate as a static rate equal to 3.375%. The only costs not included in total life-cycle cost are the dismantling and replacement costs at the end of life.
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8. Operation, Maintenance, and Monitoring
Regular inspection and maintenance are necessary to preserve the Project’s effectiveness and longevity.
Monitoring will also determine the Project’s effectiveness and is typically required when grant funding is received, as it can be used to demonstrate to taxpayers that the money spent resulted in measurable outcomes. Additionally, monitoring will be used to determine the scope, timeline, and size of Phase 2 implementation. The following sections detail the necessary tasks required to operate, inspect, and maintain the Project’s components along with monitoring considerations.
8.1 Operations and Maintenance
Operation and Maintenance (O&M) is critical in project success and will prolong the Project’s lifespan and effectiveness. A preliminary O&M Manual has been prepared and is included in Appendix F. The O&M Manual will be finalized following construction of the Project, or once applicable submittals are approved, which will outline operation, inspection, and maintenance requirements based on the specific Project components utilized. The selected construction contractor will be required to complete the O&M Manual, or at a minimum, provide component cut sheets and O&M guidance documents, which will be incorporated into the manual. The final O&M Manual will include information relevant to the diversion system, conveyances, pump(s), pretreatment, drywells/infiltration system, proprietary systems, monitoring equipment, and electrical components.
Table 8-1 summarizes the anticipated inspection and maintenance requirements by component, including frequency. It is recommended that the Project be inspected more in the beginning to determine proper frequency. The City is committed to operating and maintaining the components for the useful life of the Project in accordance with the O&M Manual and the commitment letter submitted as part of the SCWP application is included in Appendix G.
Table 8-1 Summary of Anticipated O&M by Project Component Component Operation/Maintenance Inspection
Frequency
Diversion
System (inlet
and pipe)
Inspect for accumulated sediment and debris over
grate and within concrete basin
Remove accumulated sediment and debris (litter and
leaves) from the grate and inside the inlet
Inspect conveyance pipe for clogging
Remove accumulated materials from the pipe system
Before and after the
storm season (twice
per year)
Pretreatment
Unit
Inspect unit separation chamber and sump for
accumulated trash, debris, and sediment and remove,
as necessary
Inspect screen for clogging and remove accumulated
material as necessary
Before and after the
storm season (twice
per year)
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Component Operation/Maintenance Inspection
Frequency
Pump
Inspect pump well for sediment and debris and
remove as necessary
Check valves for operation and clogging
Clear material and replace valves as necessary
Inspect bearings and impeller for wear
Lubricate bearings as needed
Check pump for operation
Verify pump levels have been maintained
Consult manufacturer if pump has not been operated
in more than 12 months or if more extensive
maintenance is required
During operation, check pump for excessive noise,
vibration, or other abnormal conditions
Twice per year
Flow Meter
Inspect flow data to identify anomalies
Troubleshoot with manufacturer if data anomalies are
observed
Mostly maintenance free, but should be calibrated
annually
Once per year
Gate Valve (to
be determined
during design)
Inspect for excessive wear, corrosion, and/or leakage and replace affected part if necessary
Inspect for blockages or accumulated sediment and
remove as necessary
Cycle valve to make sure it functions properly
Once per year
Subsurface
Infiltration
System
(drywells)
Inspect storage for sediment and debris and remove
as necessary
Inspect pipes for clogging and remove debris as
necessary
Use vacuum truck to remove accumulated materials
Observe infiltration rates over time to confirm
infiltration areas are not clogged
Restore infiltration if rates impacted significantly
(maintenance requirements may vary based on issue)
Once per year
Subsurface
Infiltration
System (trench)
Inspect storage for sediment and debris and remove
as necessary
Inspect inlet pipe for clogging and remove debris as
necessary
Use vacuum truck to remove accumulated materials
Observe infiltration rates over time to confirm
infiltration areas are not clogged
Restore infiltration if rates impacted significantly
(maintenance requirements may vary based on issue)
Once per year
Pervious
Pavement
Visually inspect pavement for accumulated
sediment/debris and remove as necessary
Inspect pavement surface for deterioration or spalling and replace/restore as necessary
Perform sweeping and/or vacuuming (depending on
pervious material used)
Observe infiltration rates over time to confirm
infiltration areas are not clogged
Restore infiltration if rates are impacted significantly (maintenance requirements vary based on issue)
Once per year
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Component Operation/Maintenance Inspection
Frequency
Mosquito/Vector
Screens/Barriers
Inspect screen/barrier for rips or holes
Replace screen/barrier if damage is observed Quarterly
There are costs associated with maintenance activities as shown in Table 8-2. The cost is based on the anticipated number of crews/staff required, staff level expertise, and hours of maintenance per year. The hourly rate is based on general industry averages and may vary depending on whether the City will maintain or hire a maintenance team. It is expected that at least two laborers are onsite performing maintenance. The hourly projections may also vary depending on the final design and elements constructed. These projections will be updated following construction. In addition to the maintenance costs projected below, it is anticipated that approximately $50,000 of operational costs will exist per year in association with utility (electrical) fees. Costs will likely increase about three percent (3%) annually based on inflation, which is not shown in the table below.
Table 8-2 Anticipated Annual Maintenance Cost
Component Frequency
Hours by Position per Year
Equipment
Rental
Total
Cost Superintendent Foreman Laborer
$150 $120 $75
Diversion System
(inlet and pipe)
Twice per
year 12 36 72
Equipment
used for
many of the
components
$11,520
Pretreatment Unit Twice per
year 12 36 72 $11,520
Pump Twice per
year 24 72 144 $23,040
Flow Meter Once per
year 4 12 24 $3,840
Gate Valve Once per
year 4 12 24 $3,840
Subsurface
Infiltration
System (drywells)
Once per
year 14 42 84 $13,440
Subsurface
Infiltration
System (trench)
Once per
year 14 42 84 $13,440
Pervious
Pavement
Once per
year 8 24 48 $7,680
Mosquito/Vector
Screens/Barriers Quarterly 4 12 24 $3,840
Total (Maintenance): 96 288 576 $8,840 $101,000
8.2 Post-Construction Monitoring
Monitoring is used to measure the Project’s effectiveness and can be used as part of a cost-benefit analysis. Post-construction monitoring is also typically required when public funds are received, as it quantifies the benefits provided through tax-payer dollars. A preliminary monitoring plan was developed and submitted as part of the SCWP application process. The plan will need to be updated with Project- specific information, constituents to be monitored, and monitoring techniques following design. The preliminary plan is included in Appendix H.
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The Project includes a flow meter downstream of the proposed diversion from the 28th Street Storm Drain (BI 0286). The flow meter will collect volumetric data that will become an integral part of quantifying Project benefits. The readings captured on the flow meter will be downloaded periodically to identify how much volume was captured and infiltrated. These readings will provide the water quality benefits in acre-feet (AF) or acre-feet per year (AFY), which match the metrics used to quantify benefits, as described in Section 6.1.
To quantify the pollutant load reduction provided by the Project, grab samples could be taken within the diversion structures or pretreatment system to measure pollutant load of the system inflow. An existing TMDL monitoring site is located on the BI 0286 storm drain and opportunities to leverage data collected at that site will be evaluated, as additional grab samples may not be required. A detailed monitoring plan will need to be developed to identify grab sample procedures, exact location(s), and frequency in accordance with SCWP Infrastructure Program or other funding program requirements. It is anticipated that bacteria loads will be quantified in association with the Santa Monica Bay Bacteria TMDL.
Results from water quality monitoring (concentrations) could be used with the volume capture readings from the flow meter to approximate the total pollutant load removed by the Project. If the existing monitoring site was used, then data could be evaluated for compliance with TMDL to determine if the Project achieves the required load reductions. The data collected with the flow meter and grab samples would be used to quantify the water quality benefit as pounds of pollutant removed or pounds of pollutant removed per year. If bacteria are being analyzed for, then the metrics will not be in pounds, but in Most Probable Number (MPN).
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9. Environmental and Permitting
The Project must meet environmental requirements related to the California Environmental Quality Act (CEQA) and will require coordination with various agencies throughout the design phase to obtain proper permits and approval. It is recommended that applications be submitted with the 50-60 percent design plans to expedite the permitting process. The selected contractor will also need to obtain construction permits from the City. The subsections below detail environmental and permitting requirements.
9.1 California Environmental Quality Act
CEQA is California's broadest environmental law and requires projects to prepare documentation and go through a public review process prior to implementation, especially if the project may result in environmental impacts. It requires local
government agencies to inform decision makers and the public about potential environmental impacts of proposed projects, and to reduce those environmental impacts to the extent feasible. The Project will fall under CEQA requirements and an Initial Study and Mitigated Negative Declaration (IS/MND) are anticipated as being required, with the City as lead agency. The MND will incorporate mitigation measures to reduce potential Project impacts. Draft CEQA documents (IS/MND) are typically posted for public review with the 50-60 percent design plans. LACBH owns the parcel where the parking facility is located and will be a responsible agency under CEQA. LACFCD has requested to review the draft CEQA documents as a responsible agency based on the connection to their storm drain. The CEQA documents will then need to be publicized and made available for a 30-day public review. Following public review and revisions, the City Council will approve the IS/MND and a Notice of Determination (NOD) will be filed with the County Clerk.
Prior to the preparation of CEQA documents (IS/MND), various environmental studies and processes will be required, which are summarized in Table 9-1. The information collected through the studies and processes described in the table will be incorporated into the IS/MND and any other CEQA documentation.
Table 9-1 Required Environmental Studies/Process
Study/Process Purpose/Requirement
Records Search
Involves looking up historical/cultural records that have been identified in the area (required for Cultural Assessment and Assembly Bill 52 (AB 52) consultation.
Cultural Assessment
Documents findings of records search and includes information regarding records (if found) within the study area, specifically related to cultural significance.
AB 52 Consultation
Tribal consultations are required prior to implementation, especially when excavation/subsurface work is included, as there is the potential to encounter tribal artifacts. The City will need to provide Project information and notify tribes who have requested consultations.
Noise Analysis
An analysis of baseline noise in the study area will be performed and models will be used to simulate construction noise to determine potential noise-related impacts, especially when residential areas surround study area.
Vibration Analysis A model will be used to simulate potential construction-related vibrations to determine if there are potential impacts to nearby residential properties.
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9.2 Permitting Requirements
This subsection summarizes the permitting requirements applicable to the proposed alternative.
Coordination with LACFCD will be required for the connection to their storm drain (BI 0286 – 28th Street Storm Drain), which runoff is being diverted from. A Coastal Permit will also be required for work being conducted in the coastal zone, which the proposed alternative is within. Coordination will also be required with LACBH and LACPW Transportation Planning and Programs Division, as the beach area and parking facility lands are owned by LACBH, and LACPW operates The Strand and Marvin Braude Bike Trail. The selected contractor will need to obtain construction permits from the City and other agencies, while those are not detailed in this subsection.
9.2.1 LACFCD Connection Permit
The Project will divert runoff from BI 0286 (28th Street Storm Drain), which is owned and operated by LACFCD. Modifications to LACFCD-owned/operated facilities require a Connection Permit prior to construction that will allow the City to modify existing LACFCD facilities. The following items must be submitted to LACPW’s online platform (EPIC LA) prior to receiving the Connection Permit:
Permit application
Plan checking and inspection fees
Construction/engineering plans
As-built plans with the sketch of the proposed connection location Hydraulic model input and output
Allowable discharge rate and Hydraulic Grade Line (HGL) Contractor’s license information and insurance documents including the endorsement that names the County as additional insured
Additional information for non-stormwater discharges (if applicable)
LACFCD Connection Permit guidelines are included in Appendix I. A hydraulic analysis conforming to LACFCD requirements will be required for the diversion on their storm drain and the results will be submitted as part of the permitting process.
Concurrent to the engineering review, a Use and Maintenance Agreement will be prepared by LACFCD, which will need to be approved by both the City and LACFCD. This agreement will identify the maintenance responsibility of the City versus LACFCD. Typically, the City will be required to maintain the diversion structure and 50 feet of the storm drain upstream and downstream from the diversion structure. An exhibit will be prepared and submitted to LACFCD illustrating these responsibilities. It is recommended that the Use and Maintenance Agreement be developed concurrent to the engineering review, as both elements will need to be approved prior to issuance of the permit. Once the engineering review and UMA are completed, the selected contractor will be able to pull the permit with their bond and insurance information. The permit will need to be obtained prior to diversion structure construction.
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9.2.2 Coastal Permit
The CCC was established by a voter initiative in 1972 (Proposition 20) and was later made permanent by the Legislature through adoption of the California Coastal Act of 1976. The CCC partners with local coastal cities and counties to plan and regulate the use of land and water in the coastal zone.
Development activities require a Coastal Permit from either the CCC or the local government acting on behalf of the CCC. Development within the coastal zone may not commence until a coastal development permit has been issued by either the CCC or a local government that has a Commission-certified Local Coastal Program (LCP). After certification of LCP, which include Land Use Plans (LUPs), coastal development permit authority is delegated to the appropriate local government, but the CCC retains original permit jurisdiction over certain specified lands (such as tidelands and public trust lands).
The proposed alternative is within the coastal zone and will require a Coastal Permit. The City has an LCP, which will be responsible for approving the Project and issuing the Coastal Permit. The review process is in place to confirm projects will:
Preserve the coastal zone as a distinct and valuable natural resource Protect the natural and scenic resources of the coastal zone Promote public safety, health, and welfare
Protect public and private property, wildlife, marine fisheries, and other ocean resources and the natural environment
Preserve the ecological balance of the coastal zone and prevent its deterioration and destruction Preserve the economic and social well-being of the community, especially those working within the coastal zone
The applications typically request general project information that can help demonstrate there will not be significant impacts, especially as they relate to the goals identified above. Another focus of the application process is related to parking and the project’s impact to both short- and long-term parking.
The proposed alternative will have some temporary parking impacts while construction takes place at the 26 th Street Parking Facility. Strategies to preserve parking and beach access during the construction process may be required as part of the review process, while the Project will ultimately result in more accessible parking in the long-term based on ADA enhancements planned.
Early coordination has been ongoing with CCC, who agreed the Project falls within the LCP. CCC anticipates further coordination with the LCP and City will take place through the Coastal Permit review and issuance process (City of Manhattan Beach, 2020b). Initial comments provided are summarized as follows for both phases of the proposed alternative:
Phase 1:
No specific requirements for public access/parking
Demonstrate good faith effort in providing access (could be signage identifying alternative parking areas)
Sequence construction to minimize impacts during peak months City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Phase 2:
Concerned about environmental impacts
Demonstrate efforts to evaluate alternative locations Show sea-level rise was accounted for
Biological survey will be required
Habitat restoration may require 4:1 restoration to impact Suggest installing infiltration trench under the Marvin Braude Bike Trail
9.2.3 Los Angeles County Beaches and Harbors
LACBH operates and manages 20 beaches along the County’s coast, including Manhattan Beach. LACBH owns the parcels along the beach where Phase 2 would be implemented. They also own the 26th Street Parking Facility parcels, which are then leased out to the City for use. A Right of Entry Permit will be required when construction work, including but not limited to staging, repair, or installation activities require access through, or use of, the County’s property, which includes County-operated beaches and parking lots. The permit application is included in Appendix J. The following items must be submitted to LACBH prior to receiving the permit:
Certificate of insurance meeting specific requirements (this will be from the contractor) Work plan with a description of the planned work, site map, staging plan, and applicable specifications
Permit application
Processing fees
Regulatory permits and approvals
Early coordination has been ongoing with LACBH and they recommended continuing coordination efforts through design, as that would likely lead to an easier and faster permitting process once the contractor is selected. Initial concerns were not voiced by LACBH, while they did emphasize review of the ultimate site map and staging area delineation.
9.2.4 Los Angeles County Roads Permit
Projects that will temporarily or permanently impact Los Angeles County-owned trails, including The Strand and Marvin Braude Bike Trail, will require approval prior to implementation. A Roads Permit will be required from LACPW when the existing trails will be impacted. This approval will be required for Phase 2 implementation and may be required for Phase 1, depending on where the diversion is located.
The permit application and supporting documentation will be submitted using the EPIC LA online permitting platform, the same one used for a Connection Permit through LACFCD. The submittal must include:
Completed application (copy included in Appendix K)
Applicable fees
Design plans
As-built plans of existing road/right-of-way
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Early coordination has been ongoing with LACPW in relation to the Project. During coordination efforts, it was emphasized that temporary (or permanent) detours would need to be provided if trails would be closed during construction. Detour plans would need to be approved as part of the permitting process.
Traffic control plans may also be required. LACPW staff indicated that trail detours have been accomplished in the region by placing temporary surfaces on the sand, as opposed to directing users to the City streets.
9.2.5 Additional Permits and Coordination
In addition to the permits summarized above; the selected contractor will be required to obtain applicable City permits. These may include Electrical, Encroachment, Excavation, Mechanical, and/or Right-of-Way Construction Permits. Coordination for these permits is not necessary during the design process.
Coordination with stakeholders will be necessary prior to construction. Coordination has been ongoing with the Los Angeles County Lifeguards, as they have a facility adjacent to the 26 th Street Parking Facility.
Phase 1 construction in the parking facility could restrict/impact access to the Lifeguard facility. Classes are held weekly at the facility and sometimes include up to 30 people in addition to employees, many of which use the 26th Street Parking Facility. Coordination will be necessary to determine how access will be maintained and how parking/classes could be accommodated during construction, if possible.
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10. Outreach/Engagement Approach
The goal of outreach/engagement is to gain support for the Project and identify challenges that may hold up the Project’s progress, such that they can be mitigated early in the process. Outreach and engagement have been implemented early in the planning process and will continue during design and into implementation. Permanent outreach strategies will also be used following implementation, which will allow the community to understand how the Project works and how it is beneficial. Outreach methods include the following:
Online media
Email blasts
Social media
Website publications
Local media
Newsletters
Newspaper advertisements (local and/or regional)
Local radio/television advertisements
Grassroots
Door-to-door canvasing
Phone banking
Surveys/focus groups
Distribution of flyers/printed materials.
In addition to outreach, engagement events will be used to meet with the public to solicit, address, and seek input from the community for the Project. Engagement events may occur as part of any public meeting, such as City Council meetings, Commission/Committee meetings, Project-specific public meetings, festivals, fairs, and/or open houses. This section describes outreach/engagement that has occurred prior to development of this study and what is planned during the design/implementation process. Some funding programs will have specific requirements related to outreach/engagement. The goals and strategies summarized below will be modified based on additional requirements that become applicable during the design and implementation phases.
10.1 Goals
A combination of outreach and engagement will be used to throughout the Project implementation process. Stakeholder coordination will also be used and is not the focus of this section, while stakeholders will be invited to participate as part of the public outreach/engagement. The goals related to outreach/engagement include the following:
Broaden public perspective
Promote public awareness and support
Educate stakeholders and the local community
Solicit feedback/input
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Provide a platform to engage with neighbors who live near the Project site
Outreach/engagement events will highlight Project goals and benefits while creating and maintaining a positive relationship between the community and City. The City will build and maintain this relationship through communication, effective dialogue, and providing a public platform to receive questions and concerns.
10.2 Outreach
Prior to the development of this study, the City performed outreach in the form of website development, social media posts, and flyer/handout distribution (mostly through the website). These forms of outreach will continue into the design and implementation phases. The Project-specific webpage is www.citymb.info/InfiltrationProject, which includes information on upcoming meetings (as applicable), Project introduction, description, frequently asked questions, glossary of terms, letters of support, and contact information. The City plans to keep the webpage up to date as the Project progresses. The webpage includes the ability to download applicable flyers, handouts, and/or reference documents.
The City posted on their Facebook (@citymb) and Twitter (@citymb) pages about the Project prior to the development of this study. The posts included a picture associated with the Project, simplified overview, webpage address, and information on an engagement event. The public was able to like, share, and comment on the posts. The City will continue to use their social media platforms to share Project information and advertise engagement events.
The City has also collected community contact information for future email correspondences. Community members interested in receiving Project updates have provided their email addresses, which the City intends to use during the design and implementation phases to help inform them of Project updates and additional engagement events.
10.3 Engagement Events
A virtual public engagement meeting was conducted on September 17, 2020 via Zoom. The public and nearby residents were notified about the meeting though social media, postcards, and a posting on the City’s website (as mentioned above). A presentation was made to the community that included the Project background/purpose, proposed improvements, and schedule. The presentation included figures to illustrate the layout of Project components and photos of similar construction activities. Feedback was received regarding the use and closure of the 26
th Street Parking Facility. Additional engagement events will be held during the design and construction phases to address concerns, answer questions, and give updates. The postcard mailed to nearby residents is illustrated in Figure 10-1. The City has also had the Project on several City Council agendas and that will continue to be the case during the design and implementation phases. City Council meetings also provide a platform for the public to make comments and provide feedback on the Project.
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Figure 10-1 Informational Postcard
The SCWP Infrastructure Program asked that the Project address any anti-displacement policies. The Project will be located within public right of way and will not lead to the displacement of nearby residents. The Project will comply with any County-wide displacement policies and anti-displacement requirements associated with various funding sources.
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11. References
Barnard, P.L., Erikson, L.H., Foxgrover, A.C., Limber, P.W., O'Neill, A.C., and Vitousek, S. 2018. Coastal Storm Modeling System (CoSMoS) for Southern California, v3.0, Phase 2 (ver. 1g, May 2018): United States Geological Survey data release. https://doi.org/10.5066/F7T151Q4.
Beach Cities EWMP Group. March 2018. Enhanced Watershed Management Program (EWMP) for the Beach Cities Watershed Management Area (Santa Monica Bay and Dominguez Channel Watersheds).
Includes the Cities of Hermosa Beach, Manhattan Beach, Redondo Beach, and Torrance, and the Los Angeles County Flood Control District.
California Department of Public Health. September 2010. Checklist for Minimizing Vector Production in Stormwater Management Structures.
https://www.cdph.ca.gov/Programs/CID/DCDC/CDPH%20Document%20Library/ChecklistforVectorPreven tioninBMPs.pdf.
California Department of Water Resources. DAC Mapping Tool. https://gis.water.ca.gov/app/dacs/ .
City of Manhattan Beach. May 2020 (2020a). GIS data provided by email (includes catch basins, contours, sewer manholes, sewer pipes, storm drain manholes, storm drain pipes, streets, and water pipes).
City of Manhattan Beach. October 2020 (2020b). Follow-Up: Meeting with the CCC (email). Email from Mamerto Estepa Jr. (City of Manhattan Beach) to Katie Harrel (CWE) and Vik Bapna (CWE) on October 13, 2020.
City of Manhattan Beach. December 2020 (2020c). Draft Sea-Level Rise Risk, Hazards, and Vulnerability Assessment, City of Manhattan Beach. Prepared by ESA and WaClim LLC.
CWE. September 2020. 28
th Street Storm Drain Infiltration Project Stakeholder Meeting (Video/Conference Call): Meeting Summary. Meeting with City of Manhattan Beach, CWE, and Los Angeles Beaches and Harbors, Flood Control District, Public Works, and Transportation Planning and Programs Departments.
Los Angeles County Flood Control District (LACFCD). August 1955 (Latest Revision). Project No. 286 Manhattan Beach Storm Drain As-Builts. February 1954. PD027536.
Los Angeles County Flood Control District (LACFCD). October 1967 (Latest Revision). Project No. 552 Manhattan Beach As-Builts. July 1961. PD031562.
Los Angeles County Flood Control District (LACFCD). November 1968 (Latest Revision). Project No. 5401 Unit I Manhattan Beach As-Builts. October 1966. PD038461.
Los Angeles County Flood Control District (LACFCD). June 1973 (Latest Revision). Project No. 5452 – Unit 2 Manhattan Beach As-Builts. January 1974. PD042270.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
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Los Angeles County Flood Control District (LACFCD). September 2019. Safe, Clean Water Program Feasibility Study Guidelines. https://safecleanwaterla.org/wp-content/uploads/2019/09/Feasibility-Study- Guidelines-20190917-FINAL-1.pdf.
Los Angeles County Flood Control District (LACFCD). March 2020. Draft Template: Transfer Agreement between the Los Angeles County Flood Control District and [Insert Project Developer]. Safe, Clean Water Program – Regional Program.
https://safecleanwaterla.org/wp-content/uploads/2020/03/SCWP-Transfer- Agreement-Regional-Template-20200306-CLEAN.pdf.
Los Angeles County GIS Data Portal. 2018. https://egis3.lacounty.gov/dataportal/ .
Los Angeles County Public Works (LACPW). January 2006. Hydrology Manual.
Los Angeles County Public Works (LACPW). April 2019. Manhattan Beach Pump Station Hydrology & Hydraulic Analysis. Prepared by Michael Baker International.
Los Angeles Regional Water Quality Control Board (LARWQCB). 2018. Order No. R4-2018-0125, General NPDES Permit No. CAG994004, Waste Discharge Requirements for Discharges of Groundwater from Construction and Project Dewatering to Surface Waters in Coastal Watersheds of Los Angeles and Ventura Counties. Effective Date: November 13, 2018.
State of California. 2020. Public Resources Code Division 20 California Coastal Act.
Terracon Consultants, Inc. July 6, 2020. Geotechnical Engineering Report for Manhattan Beach Subsurface Infiltration System Feasibility Study. Terracon Project No. 60205008. Tustin, California.
United States Geological Survey (USGS). January 2017. CoSMos v3.0 Phase 2 – Los Angeles County.
Website referenced citation included under Barnard… (above). https://www.sciencebase.gov/catalog/item/5845d431e4b04fc80e52356c .
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix A
Geotechnical Report
REPORT COVER PAGE
Geotechnical Engineering Report
__________________________________________________________________________ Manhattan Beach Subsurface Infiltration System Feasibility Study Manhattan Beach, Los Angeles County, California
July 6, 2020
Terracon Project No. 60205008
Prepared for:
CWE
Fullerton, California
Prepared by:
Terracon Consultants, Inc.
Tustin, California
Terracon Consultants, Inc. 1421 Edinger Ave Suite C Tustin, California 92780 P [949 ] 261 0051 F [949 ] 261 6110 terracon.com
REPORT COVER LET TER T O SIGN
July 6, 2020
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California 92831
Attn: Ms. Katie Harrel
E: [email protected]
Re: Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study Manhattan Beach, Los Angeles County, California
Terracon Project No. 60205008
Dear Ms. Harrel:
We have completed the Geotechnical Engineeringservices for the above referenced project.This study was perform ed in general accordance with Terracon Proposal No. P602 05008 dated January 20, 2020 . This report presents the findings of the subsurface exploration and provides geotechnical recommendations concerningwhich will assist in the design and construction of the planned underground infiltration systemsfor the proposedproject.
We appreciate the opportunity to be of s ervice to you on this project. If you have any questions concerning this report or if we may be of further service, please contact us.
Sincerely,
Terracon Consultants, Inc.
Abigail K. McCranie, E.I.T. Sivasubramaniam (Raj) Pirathiviraj, P.E., G.E.
Field Engineer Senior Engineer
APR review provided by F. Fred Buhamdan, P.E.
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REPORT TOPICS
INTRODUCTION............................................................................................................. 1 SITE CONDITIONS AND PROJECT DESCRIPTION..................................................... 2 GEOTECHNICAL CHARACTERIZATION...................................................................... 4 SEISMIC CONSIDERATIONS .............................. ERROR! BOOKMARK NOT DEFINED.
LIQUEFACTION ............................................................................................................. 5 CORROSIVITY................................................................................................................ 6 STORMWATER MANAGEMENT ................................................................................... 6 GEOTECHNICAL OVERVIEW ............................. ERROR! BOOKMARK NOT DEFINED.
EARTHWORK....................................................... ERROR! BOOKMARK NOT DEFINED.
GENERAL COMMENTS................................................................................................. 8 Note: This report was originally delivered in a web -based format. Orange Bold text in the report indicates a referenced section heading. The PDF version also includes hyperlinks which direct the reader to that section and clicking on theGeoReport logo will bring you back to this page. For more interactive features, please view your project online at client.terracon.com.
ATTACHMENTS
EXPLORATION AND TESTING PROCEDURES
SITE LOCATION PLAN, EXPLORATION PLANS, AND EXPLORATION RESULTS SUPPORTING INFORMATION (General Notes and Unified Soil Classification System) Responsive ■ Resourceful ■ Reliable 1
INTRODUC TION
Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study Project Address
Manhattan Beach, Los Angeles County, California
Terracon Project No. 60205008
July 6, 2020
INTRODUCTION
This report presents the results of our subsurface exploration and geotechnical engineer ing services performed for the proposed infiltration system planned through the following areas in Manhattan Beach, Los Angeles County, California:
■ Site 1: Public Beach/Beach Path ■ Site 3: Bruce’s Beach ■ Site 2: Polliwog Park ■ Site 4: 26th Street (Parking Lot) The purpose of these services is to provid e information and geotechnical engineering recommendations relative to:
■ Subsurface soil conditions ■ Infiltration Design and Considerations ■ Groundwater conditions ■ Site preparation and earthwork The geotechnical engineering Scope of Services for this project included the advancement of eighteen (18) test borings as follows:
Project Site Boring Type of Boring Boring Depth (feet)
Site 1: Public
Beach
PI-1 Percolation Boring 7
PI-2 Percolation Boring 5
PI-3 Percolation Boring 7
PI-4 Percolation Boring 5
BI-1 Hollow-Stem Auger Boring 16.5
BI-2 Hollow-Stem Auger Boring 11.5
BI-3 Hollow-Stem Auger Boring 16.5
BI-4 Hollow-Stem Auger Boring 11.5
Site 1: Beach
Path
PI-5 Percolation Boring 10
BI-5 Hollow-Stem Auger Boring 21.5
Site 2: Polliwog
Park PP-1 Percolation Boring 16.5
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Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
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Project Site Boring Type of Boring Boring Depth (feet)
Site 2: Polliwog
Park
(continued)
PP-2 Percolation Boring 16.5
BP-1 Hollow-Stem Auger Boring 26.5
BP-2 Hollow-Stem Auger Boring 26.5
Site 3: Bruce’s
Beach PB-1 Percolation Boring 16.5
Site 4: 26th Street
Parking Lot
PL-1 Percolation Boring 31.5
PL-2 Percolation Boring 16.5
BL-1 Hollow-Stem Auger Boring 51.5
Maps showing the site and boring locations are shown in the Site Location and Exploration Plan sections, respectively. The results of the laboratory testing performed on soil samples obtained from the site during the field exploration are included on the boring logsand as separate graphs in the Exploration Results section.
SITE CONDITIONS AND PROJECT DESCRIPTION
All sites are included within the City of Manhattan Beach, California. The following description of site conditions is derived from our site visit in association with the field exploration and our review of publicly available geologic and topographic maps.
■ Site 1: Public Beach /Beach Path: This project site is located along an approximately ¾ mile strip of Manhattan Beach and beach walking path running southeast from 36th Street to 20th street.
■ Site 2: Polliwog Park: This project site is located at 1601 Manhattan Beach Boulevardand includes the park area bounded by North Redondo Avenue, Manhattan Beach Middle School, Manhattan Beach Boulevard, North Peck Avenue, and 18th Street. ■ Site 3: Bruce’sBeach: This project site is located within Bruce’s Beach Park located at 2600 Highland Avenue and includes the grass areas bounded by Highland Avenue, 26 th Street, Manhattan Avenue, and 27th Street.
■ Site 4: 26th Street (Parking Lot): This project is located within the parking area located at 587 26th Street, and is bounded by 26 th Street, Ocean Drive, 27th Street, and Manhattan Avenue.
Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
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Item Description
Parcel
Information
Approximate coordinates for the center of the project sites are:
■ Site 1: Public Beach/Beach Path: 33.89235°N, 118.41699°W ■ Site 2: Polliwog Park: 33.88866°N, 118.38620°W
■ Site 3: Bruce’s Beach: 33.89434°N, 118.41549°W
■ Site 4: 26th Street (Parking Lot): 33.89408°N, 118.41627°W Proposed
Systems
The Manhattan Beach Infiltration System is proposed along a section of public beach adjacent to The Strand walkway. The final size of the proposed infiltrationsystem may be reduced/eliminated based on any combination of the following:
■ Site 2: Polliwog Park: Use of the park to detain runo ff from its tributary area (estimated 470 acres)
■ Site 3: Bruce’s Beach: Construction of underground infiltration galleries or drywells
■ Site 4: 26th Street (Parking Lot): Construction of underground infiltration galleries or drywells
Current
Ground
Cover
■ Site 1: Public Beach: Exposed soils/sand
■ Site 2: Polliwog Park – Grassed areas with light vegetation and concrete paved walkways
■ Site 3: Bruce’s Beach: Grassed areas with light vegetation and concrete paved walkways and stairways
■ Site 4: 26th Street (Parking Lot): Asphalt paved parking and drive lanes with associated concrete hardscapes and landscape areas
Existing
Topography
■ Site 1: Public Beach: The beach slopes up from the ocean edge towards the Bike Trail to an approximate elevation of 12 to 14 feet above mean sea level. ■ Site 2: Polliwog Park: The site has an elevation ranging between 67 and 125 feet above mean sea level. Polliwog lake is located in the middle of the park and in general, the existing ground elevation is descending towards the lake. ■ Site 3: Bruce’s Beach: The site is located along the existing 1:6 slope and has an approximate elevation ranging between 59 and 102 feet above mean sea level.
In general, the existing ground surface is sloping towards the beach. ■ Site 4: 26th Street (Parking Lot): The site is relatively flat and contains two levels along the existing slope. The first level has an approximate elevation of 44 feet above mean sea level and the second level has an approximate elevation of 52 feet above mean sea level.
Geology
The site is situated within the eastern Transverse Range Geomorphic Province in Southern California. Geologic structures within the Transverse Ranges Province trend mostly east west, in contrast to the prevailing northwest trend elsewhere in the state.
The Transverse Range Province contains the highest peaks composed of pre - Phanerozoic rocks south of the Sierra Nevada, four of the eight islands off the southern California coast, and is both bounded and transected by several major fault zones.1, 2 1 Harden, D. R., “California Geology, Second Edition ,” Pearson Prentice Hall, 2004. 2 Norris, R. M. and Webb, R. W., “Geology of California, Second Edition,”John Wiley & Sons, Inc., 1990.
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Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
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GEOTECHNICAL CHARACTERIZATION
Based on the results of the borings, the subsurface conditions encountered at each project site are presented below:
n Site 1: Public Beach: Predominantly loose to dense poorly graded sand to the maximum depth explored of 16.5 feet bgs.
n Site 1:Beach Path: Predominantly medium dense to dense poorly graded sand with variable amounts of silt to the maximum depth explored of 21.5 feet bgs. n Site 2: Polliwog Park: Predominantlyloose to dense sand with variable amounts of silt and clay to the maximum depth explored of 26.5 feet bgs.A 2-foot layer of medium stiff sandy lean clay was encountered in BP-2 at approximately 5 feet bgs. n Site 3: Bruce’s Beach:Predominantly loose to medium dense poorly graded sand with variable amounts of silt to themaximum depth explored of 16.5 feet bgs. n Site 4:26th Street Parking Lot: Predominantlylooseto very dense sand with variable amounts of silt to the maximum depth explored of 51.5 feet bgs.
Conditions encountered at each boring location are indicated on the individual boring logs shown in the Exploration Results section and are attached to this report. Stratification boundaries on the boring logs represen t the approximate location of changes in native soil types; in situ, the transition between materials may be gradual.
Lab Results
Laboratory tests were conducted on selected soil samples and the test results are presented in the Exploration Results section and on the boring logs.
Groundwater Conditions
Groundwater observations for each site are reflected below . These observations represent groundwater conditions at the time of the field exploration and may not be indicative of other times, or at other locations.
n Site 1: Public Beach: Groundwater was observed in borings BI -1 and BI -3 at a depth of approximately 12.5 feet bgs.
n Site 1:Beach Path:Groundwater wasnot encountered in either of the borings at this project site.
n Site 2:Polliwog Park:Groundwater wasnot encountered in any of the borings at this project site.
n Site 3: Bruce’s Beach:Groundwater wasnot encountered in the boring at this project site n Site 4: 26th Street Parking Lot: Groundwater was observed in boring BL -1 at a depth of approximately 47 feet bgs.
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Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
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Groundwater level fluctuations occur dueto seasonal variations in the amount of rainfall, runo ff and other factors not evident at the time the borings were performed. Therefore, groundwater levels during construction or at other times in the life of the structure may be higher or lower than the levels indicated on the boring logs. The possibility of groundwater level fluctuations should be considered when developing the design and construction plans for the project.
The historic high groundwater elevatio ns for each site according to data collected from the Los Angeles County Public Works Water DataLibrary for the State of Californiaare presented below:
n Site 1:Public Beach and Beach Path, Site 3:Bruce’s Beach, Sites 4:26th Street Parking Lot:
According to groundwater data collected from nearby borings performed by Gregg Drilling at the intersec tion of Sepulveda and Manhattan Beach Boulevard, the highest groundwater elevation level was recorded at a depth greater than 100 feet bgs.3 n Site 2: Polliwog Park: According to data collected from the Los Angeles County Public Works Water Data Library for the State of California from well number 701G, located approximately 0.29 miles southwest of Polliwog Park, the highest groundwater level, between June 13, 1950 and October 28, 2008, was recorded at a depth greater than 100 feet. 4
LIQUEFACTION
Liquefaction is a mode of ground failure that results from the generation of high pore water pressures during earthquake ground shaking, causing loss of shear strength. Liquefaction is typically a hazard where loose sandy soils exist below groundwater. The California Geological Survey (CGS) has designated certain areas as potential liquefaction hazard zones. These are areas considered at a risk of liquefaction -related ground failure during a seismic event, based upon mapped surficial deposits and the presence of a relatively shallow water table.
The project site s are not located within a liquefaction hazard zone as designated by the CGS.
Based on CGS maps, liquefaction hazard potential at the site is considered low. Other geologic hazards related to liquefaction, such as lateral spreading, are therefore also considered low. 3 Data collected from Gregg Drilling at the intersection of Sepulveda and Manhattan Beach Boulevard, Manhattan Beach, CA (http://www.greggdrilling.com/resources/#guides) 4 Groundwater elevation was obtained from a monitoring well (well id: 701G) located at a distance of approximately 0.29 miles southwest of the project site (www.dpw.lacounty.gov/general/ wells#).
Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
Responsive ■ Resourceful ■ Reliable 6
CORROSIVITY
The table below lists the results of laboratory soluble sulfate, soluble chloride, electrical resistivity, and pH testing. The values may be used to estimate potential corrosive characteristics of the on- site soils with respect to contact with the various underground materials which will be used for project construction.
Corrosivity Test Results Summary
Site Boring
Sample
Depth
(ft)
Soil Description
Soluble
Sulfate
(%)
Chlorides
(ppm)
Electrical
Resistivity
(Ω-cm)
pH
Site 1: Public Beach
BI-2 0.0-5.0 Poorly Graded Sand 0.04 94 19,000 8.3
BI-3 15.0-16.5 Poorly Graded Sand 0.03 98 8,200 8.2
Site 1: Beach Path BI-5 0.0-5.0 Poorly Graded Sand
with silt 0.04 17 14,000 8.4
Site 2: Polliwog Park
BP-2 0.0-5.0 Silty Clayey Sand 0.05 143 4,100 7.7
PP-1 0.0-5.0 Silty Sand 0.02 150 3,000 7.8
Site 4: 26th Street
Parking Lot BL-1 0.0-5.0 Poorly Graded Sand 0.02 94 7,100 7.5 Results of soluble sulfate testing indicate samples of the on -site soils tested possess negligible sulfate concentrations when classified in accordance with Table 19.3.1.1 of the ACI Design Manual. Concrete should be designed in accordance with the exposure class S0 provisions of the ACI Design Manual, Section 318, Chapter 19.
STORMWATER MANAGEMENT
Ten (10) in-situ percolation tests were performed to approximate depths of 5 and 31 feet bgs. A 2-inch thick layer of gravel was placed in the bottom of each boring after the borings were drilled to investigate the soil profile. A 3-inch diameter perforated pipe was installed on top of the gravel layer in each boring. Gravel was used to backfill between the perforated pipes and the boring sidewall. The borings were then filled with water for a pre-soak period of 24 hours. Testing began after a pre-soak period. At the beginning of the test, the pipes were refilled with water and readings were taken at standardized time intervals. Percolation rates are provided in the following table:
Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
Responsive ■ Resourceful ■ Reliable 7
TEST RESULTS
Project Site Test Location
(depth, feet bgs)
Slowest Measured
Percolation Rate
(in/hr.)
Correlated
Infiltration Rate1
(in/hr.)
Water
Head (in)
Site 1: Public
Beach
PI-1 (2 to 7 ft) >500 >200 84
PI-2 (0 to 5 ft) >500 155.7 38
PI-3 (2 to 7 ft) >500 >200 74
PI-4 (0 to 5 ft) >500 >200 48
Site 1: Beach Path PI-5 (5 to 10 ft) 126.0 9.2 61
Site 2: Polliwog
Park
PP-1 (10 to 16 ft) 21.6 1.0 80
PP-2 (10 to 16 ft) 49.7 4.0 50
Site 3: Bruce
Beach Street PB-1 (10 to 16 ft) 97.2 9.6 42
Site 4: 26th Street
Parking Lot
PL-1 (25 to 31 ft) >500 >200 12
PL-2 (10 to 16 ft) 224 17.1 67
1If proposed infiltration system will mainly rely on vertical downward seepage, the correlated infiltration rates should be used based on County of LA reduction factor
The field test results are not intended to be design rates. They represent the result of our tests, at the depths and locations indicated, as described above. The design rate should be determined by the designer by applying an appropriate factor of safety. Based on the County of Los Angeles Department of Public Works GS200.2 document, the following reduction factors are recommended:
LA County Reduction Factor Value
RFt1 2
RFv2 1
RFs3 14
RF, Total Reduction Factor
RF=RFtxRFvxRFs
25 1. Utilized for Boring Percolation and Dry Well Percolation Tests
2. Used to account for site variability, number of tests, and thoroughness of investigation.
3. Used to account for Long-term siltation, plugging, and maintenance
4. This factor considers that stormwater will be pretreated. If stormwater will not be pretreated, the factor should be increased to 2.
5. This factor considers that stormwater will be pretreated. If stormwater will not be pretreated, the factor should be increased to 4.
Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
Responsive ■ Resourceful ■ Reliable 8
With time, the bottoms of infiltration systems tend to plug with organics, sediments, and other debris. Long term maintenance will likely be required to remove these deleterious materials to help reduce decreases in actual percolation rates.
The percolation tests were performed with clear water, whereas the storm water will likely not be clear, but may contain organics, fines, and grease/oil. The presence of these deleterious materials will tend to decrease the rate that water percolates from the infiltration systems. Design of the storm water infiltration systems should account for the presence of these materials and should incorporate structures/devices to remove these deleterious materials.
Based on the soils encountered in our borings, we expect the percolation rates of the soils could be different than measured in the field due to variations in fines and gravel content. The design elevation and size of the proposed infiltration system should account for this expected variability in infiltration rates.
Infiltration testing should be performed after construction of the infiltration system to verify the design infiltration rates. It should be noted that siltation and vegetation growth along with other factors may affect the infiltration rates of theinfiltration areas. The actual infiltration rate may vary from the values reported here. Infiltration systems should be located a minimum of 10 feet from any existing foundation system.For proposed drywell system areas, a minimum spacing between drywells equal to the depth of the drywells should be applied.
GENERAL COMMENTS
Our analysis and opinions are based upon our understanding of the project, the geotechnical conditions in the area, and the data obtained from oursite exploration. Natural variations will occur between exploration point locations or due to the modifying effec ts of construction or weather.
The nature and extent of such variations may not become evident until during or after construction.
Terracon should be retained as the Geotechnical Engineer, where noted in this report, to provide observation and testing services during pertinent construction phases. If variations appear, we can provide further evaluation and supplemental recommendations. If variations are noted in the absence of our observation and testing services on -site, we should be immediately notified so that we can provide evaluation and supplemental recommendations.
Our Scope of Services does not include either specifically or by implication any environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or prevention of pollutants, hazardous materials or conditions. If the owner is concerned about the potential for such contamination or pollution, other studies should be undertaken.
Our services and any correspondence or collaboration through this system are intended for the sole benefit and exclusive use of our client for specific application to the project discussed and are accomplished in accordance with generally acceptedgeotechnical engineering practices with Geotechnical Engineering Report
Manhattan Beach Subsurface Infiltration System Feasibility Study■ Manhattan Beach, Los Angeles County, California
July 6, 2020 ■ Terracon Project No. 60205008
Responsive ■ Resourceful ■ Reliable 9
no third-party beneficiaries intended. Any third-party access to services or correspondence is solely for information purposes to support the services provided by Terracon to our client.
Reliance upon the services and any wo rk product is limited to our client and is not intended for third parties. Any use or reliance of the provided information by third parties is done solely at their own risk. No warranties, either express or implied, are intended or made.
Site characteristics as provided are for design purposes and not to estimate excavation cost. Any use of our report in that regard is done at the sole risk of the excavating cost estimator as there may be variations on the site that are not apparent in the data that could significantly impact excavation cost. Any parties charged with estimating excavation costs should seek their own site characterization for specific purposes to obtain the specific level of detail necessary for costing.
Site safety, and cost estimating incl uding, excavation support, and dewatering requirements/design are the responsibility of others. If changes in the nature, design, or location of the project are planned, our conclusions and recommendations shall not be considered valid unless we review the changes and either verify or modify our conclusions in writing.
Responsive ■ Resourceful ■ Reliable
ATTACHMENTS
Responsive ■ Resourceful ■ Reliable
SITE LOCATION PLAN, EXPLORATION PLANS, AND EXPLORATION
RESULTS
SITE LOCATIONS
Manhattan Beach Subsurface Infiltration System Feasibility Study ■ Manhattan Beach, CA June 30, 2020 ■ Terracon Project No. 60205008
DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS NOT
INTENDED FOR CONSTRUCTION PURPOSES
TOPOGRAPHIC MAP IMAGE COURTESY OF THE U.S. GEOLOGICAL SURVEY QUADRANGLES INCLUDE: VENICE, CA (1/1/1981) and INGLEWOOD, CA (1/1/1981).
EXPLORATION PLAN – SITE 1: PUBLIC BEACH AND BEACH PATH
Manhattan Beach Subsurface Infiltration System Feasibility Study ■ Manhattan Beach, CA June 29, 2020 ■ Terracon Project No. 60205008
DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS
NOT INTENDED FOR CONSTRUCTION PURPOSES
AERIAL PHOTOGRAPHY PROVIDED
BY MICROSOFT BING MAPS
LEGEND
BI-1: APPROXIMATE BORING LOCATION
PI-1: APPROXIMATE BORING AND
PERCOLATION TEST LOCATION
6-12-12N=24
6-7-8N=15
6-15-18N=33
3.0 2
POORLY GRADED SAND (SP), tan
medium dense
denseBoring Terminated at 16.5 Feet16.5
Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8973° Longitude: -118.4188°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. BI-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
At completion of drillingWATER LEVEL OBSERVATIONS
SAMPLE TYPE
7-14-16N=30
3-5-6N=11
32.8 POORLY GRADED SAND (SP), tan
dense
medium denseBoring Terminated at 11.5 Feet11.5
Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8948° Longitude: -118.4178°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. BI-2CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
6-7-6N=13
2-3-3N=6
24-29-31N=60
23.3 POORLY GRADED SAND (SP), tan
medium dense
loose
trace gravel, coarse grained, very denseBoring Terminated at 16.5 Feet16.5 Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8918° Longitude: -118.4162°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. BI-3CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-19-2020
1421 Edinger Ave, Ste CTustin, CA
At completion of drillingWATER LEVEL OBSERVATIONS
SAMPLE TYPE
3-4-6N=10
5-5-4N=9
02.5 POORLY GRADED SAND (SP), tan
medium dense
looseBoring Terminated at 11.5 Feet11.5
Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8906° Longitude: -118.4157°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. BI-4CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
3-5-8N=136-7-8N=15
8-14-17N=31
13-22-25N=47
259
CONCRETE, 6'' ThicknessPOORLY GRADED SAND WITH SILT (SP-SM), light brown SILTY SAND (SM), brown, medium dense
POORLY GRADED SAND (SP), golden brown, dense
Boring Terminated at 21.5 Feet
0.5 5.0
15.0 21.5
Beach Path Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 20
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8972° Longitude: -118.4183°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. BI-5CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
11-22-24N=468-11-13N=24 225.7
POORLY GRADED SAND (SP), tan
dense
medium dense
Boring Terminated at 7 Feet7.0
Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8973° Longitude: -118.4189°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. PI-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
3 POORLY GRADED SAND (SP), tan, encountered wellrounded gravel and concrete material at about 0-2' Boring Terminated at 5 Feet5.0
Public Beach Location
Stratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8949° Longitude: -118.4178°GRAPHIC LOGDEPTH
Advancement Method:Hand Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. PI-2CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
13-15-16N=315-8-8N=16 1
POORLY GRADED SAND (SP), tan
Boring Terminated at 7 Feet7.0
Public Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8919° Longitude: -118.4163°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. PI-3CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
0.3 POORLY GRADED SAND (SP), tan
Boring Terminated at 5 Feet5.0
Public Beach Location
Stratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8907° Longitude: -118.4157°GRAPHIC LOGDEPTH
Advancement Method:Hand Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. PI-4CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
CONCRETE, 6'' ThicknessPOORLY GRADED SAND WITH SILT (SP-SM), light brown SILTY SAND (SM), brown
Boring Terminated at 10 Feet
0.5 5.0
10.0 Beach Path Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8972° Longitude: -118.4183°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with cement-bentonite grout uponcompletion.
Notes:
Project No.: 60205008Drill Rig: CME-75 LAR
BORING LOG NO. PI-5CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 06-18-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 06-18-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
EXPLORATION PLAN - SITE 2: POLLIWOG PARK
Manhattan Beach Subsurface Infiltration System Feasibility Study ■ Manhattan Beach, CA June 29, 2020 ■ Terracon Project No. 60205008
DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS
NOT INTENDED FOR CONSTRUCTION PURPOSES
AERIAL PHOTOGRAPHY PROVIDED
BY MICROSOFT BING MAPS
LEGEND
BP-1: APPROXIMATE BORING LOCATION
PP-1: APPROXIMATE BORING AND
PERCOLATION TEST LOCATION
18th Street
ORGANIC CLAY/ORGANIC SILT (OL), with sand, darkbrownSILTY SAND (SM), dark brown and black dark brown
brown
SILTY CLAYEY SAND (SC-SM), brown
SILTY SAND (SM), orangish brown
POORLY GRADED SAND WITH SILT (SP-SM), orangeBoring Terminated at 26.5 Feet
0.5 15.0
20.0 25.026.5
Polliwog Park Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 20
25 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8905° Longitude: -118.3867°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with sod Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. BP-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-20-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-20-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
2-6-8N=14
2-7-8N=15
10-14-21N=35
7-18-23N=41
11-18-25N=43
55 6
11.1 ORGANIC CLAY/ORGANIC SILT (OL), with sand, darkbrownSILTY CLAYEY SAND (SC-SM), brown SANDY LEAN CLAY (CL), dark brownish gray, medium stiff
SILTY SAND (SM), tan, medium dense
POORLY GRADED SAND WITH SILT (SP-SM), tan, dense
orangish tan
tanBoring Terminated at 26.5 Feet
0.5 5.0
7.0 15.0
26.5 Polliwog Park Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 20
25 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8902° Longitude: -118.386°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with sod Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. BP-2CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-20-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-20-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
2-5-7N=12
3-3-5N=8
6-8-11N=19 27
9.2 ORGANIC CLAY/ORGANIC SILT (OL), with sand, darkbrownSILTY SAND (SM), dark brown brown, medium dense
loose
SILTY CLAYEY SAND (SC-SM), brown, medium denseBoring Terminated at 16.5 Feet
0.5 15.016.5
Polliwog Park Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8906° Longitude: -118.3867°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with sod Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. PP-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-20-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-20-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
2-6-10N=16
5-10-12N=22
10-17-24N=41
15 9.8
10.9 ORGANIC CLAY/ORGANIC SILT (OL), with sand, darkbrownSILTY CLAYEY SAND (SC-SM), brown SILTY SAND (SM), tan, medium dense
denseBoring Terminated at 16.5 Feet
0.5 5.0
16.5 Polliwog Park Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8902° Longitude: -118.386°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with sod Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. PP-2CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-20-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-20-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
SITE LOCATION - SITE 3: BRUCE'S BEACH AND SITE 4: 26TH ST PARKING LOT Manhattan Beach Subsurface Infiltration System Feasibility Study ■ Manhattan Beach, CA June 29, 2020 ■ Terracon Project No. 60205008
DIAGRAM IS FOR GENERAL LOCATION ONLY, AND IS
NOT INTENDED FOR CONSTRUCTION PURPOSES
AERIAL PHOTOGRAPHY PROVIDED
BY MICROSOFT BING MAPS
LEGEND
BL-1: APPROXIMATE BORING LOCATION
PL-1/PB-1: APPROXIMATE BORING AND
PERCOLATION TEST LOCATION
3-4-6N=10
2-6-9N=15
3-5-10N=15
11-14-15N=29
12-7-15N=22
10-19-28N=47
8 4
4.8 4.2
ASPHALT, 4" ThicknessAGGREGATE BASE COURSE, 4" ThicknessPOORLY GRADED SAND WITH SILT (SP-SM), dark tan tan, medium dense
dark tan to orange
orange
POORLY GRADED SAND (SP), trace silttan, medium dense
dense
0.30.7 25.0
26th Street Parking Lot Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 20
25 30
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8941° Longitude: -118.4162°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with asphalt Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. BL-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-19-2020
1421 Edinger Ave, Ste CTustin, CA
While drillingWATER LEVEL OBSERVATIONS
SAMPLE TYPE
9-16-22N=38
6-11-19N=30
22-35-33N=68
11-20-24N=44
5.4 21
POORLY GRADED SAND (SP), trace silt (continued)
POORLY GRADED SAND WITH SILT (SP-SM), tracegravel, brown with black, dense, observed thin gravel layers tan, very dense
SILTY SAND (SM), trace gravel, brown, denseBoring Terminated at 51.5 Feet
40.0 50.051.5
26th Street Parking Lot Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
35 40
45 50
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8941° Longitude: -118.4162°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with asphalt Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. BL-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-19-2020
1421 Edinger Ave, Ste CTustin, CA
While drillingWATER LEVEL OBSERVATIONS
SAMPLE TYPE
4-7-10N=17
5-12-17N=29
2-4-5N=9
3 4.4
12.2 ORGANIC CLAY/ORGANIC SILT (OL), with sand, darkbrownPOORLY GRADED SAND (SP), trace silt, dark tan medium dense
SILTY SAND (SM), reddish brown, looseBoring Terminated at 16.5 Feet
0.5 15.016.5
Bruce's Beach Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8943° Longitude: -118.416°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with sod Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. PB-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
4-7-11N=18
3-4-6N=10
4-5-5N=10
8-13-15N=28
8-15-20N=35
8-13-15N=28
7 6.1
3.7 ASPHALT, 3.5" ThicknessAGGREGATE BASE COURSE, 4" ThicknessPOORLY GRADED SAND (SP), trace silt, tan brownish tan, medium dense
SILTY SAND (SM), reddish brown, medium dense
reddish brown and dark tan
POORLY GRADED SAND WITH SILT (SP-SM), dark tan,medium dense orange, dense
POORLY GRADED SAND (SP), trace silt & gravel, lightorange to light tan, medium denseBoring Terminated at 31.5 Feet
0.30.6 10.0
20.0 30.031.5
26th Street Parking Lot Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 20
25 30
STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8942° Longitude: -118.4164°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with asphalt Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. PL-1CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
3-6-7N=13
3-4-5N=9
3-5-7N=12 17
7.8 6
ASPHALT, 4" ThicknessAGGREGATE BASE COURSE, 5" ThicknessPOORLY GRADED SAND (SP), trace silt, tan medium dense
SILTY SAND (SM), reddish brownloose
medium denseBoring Terminated at 16.5 Feet
0.30.8 9.5
16.5 26th Street Parking Lot Location
Hammer Type: AutomaticStratification lines are approximate. In-situ, the transition may be gradual.
THIS BORING LOG IS NOT VALID IF SEPARATED FROM ORIGINAL REPORT. GEO SMART LOG-NO WELL 60205008 MANHATTAN BEACH S.GPJ TERRACON_DATATEMPLATE.GDT 6/26/20
WATER LEVELOBSERVATIONS
DEPTH (Ft.)
5 10
15 STRENGTH TEST
FIELD TESTRESULTS
COMPRESSIVESTRENGTH(tsf)
TEST TYPE
STRAIN (%)
PERCENT FINES
WATERCONTENT (%)DRY UNITWEIGHT (pcf)
ATTERBERGLIMITS
LL-PL-PI
LOCATIONSee Exploration PlanLatitude: 33.8939° Longitude: -118.4163°GRAPHIC LOGDEPTH
Advancement Method:Hollow Stem Auger
Abandonment Method:Boring backfilled with grout upon completion and surfacecapped with asphalt Notes:
Project No.: 60205008Drill Rig: CME-75
BORING LOG NO. PL-2CWE CorpCLIENT:Fullerton, CA
Driller: 2R DrillingBoring Completed: 05-19-2020
PROJECT: Manhattan Beach Subsurface Infiltration
System Feasibility Study
See Exploration and Testing Procedures for adescription of field and laboratory procedures usedand additional data (If any).See Supporting Information for explanation ofsymbols and abbreviations.
Manhattan Beach, CASITE:
Boring Started: 05-19-2020
1421 Edinger Ave, Ste CTustin, CA
WATER LEVEL OBSERVATIONSNot encountered
SAMPLE TYPE
ANAHEIM TEST LAB, INC
196 Technology Drive, Unit D
Irvine, CA 92618
Phone (949)336-6544
DATE: 05/29/2020
Terracon Consultants, Inc.
1421 Edinger Ave. P.O. NO.: Chain of Custody Tustin, CA 92780
LAB NO.: C-3831, 1-3
SPECIFICATION: CTM-643/417/422
MATERIAL: Soil
Project No.: 60205008
Project: Beach Park Grass Parking Lot
Manhattan Beach, CA
ANALYTICAL REPORT
CORROSION SERIES
SUMMARY OF DATA
pH MIN. RESISTIVITY SOLUBLE SULFATES SOLUBLE CHLORIDES per CT. 643 per CT. 417 per CT. 422 ohm-cm (% by weight) ppm
1) BP-2 @ 0’-5’ 7.7 4,100 0.0489% 143
2) BL-1 @ 0’-5’ 7.5 7,100 0.0176% 94
3) PP-1 @ 0’-5’ 7.8 3,000 0.0238% 150
RESPECTFULLY SUBMITTED
________________________________
WES BRIDGER LAB MANAGER
ANAHEIM TEST LAB, INC
196 Technology Drive, Unit D
Irvine, CA 92618
Phone (949)336-6544
DATE: 06/26/2020
Terracon Consultants, Inc.
1421 Edinger Ave. P.O. NO.: Chain of Custody Tustin, CA 92780
LAB NO.: C-3891, 1-3
SPECIFICATION: CTM-643/417/422
MATERIAL: Soil
Project No.: 60205008
Project: Manhattan Beach Inflitration
ANALYTICAL REPORT
CORROSION SERIES
SUMMARY OF DATA
pH MIN. RESISTIVITY SOLUBLE SULFATES SOLUBLE CHLORIDES per CT. 643 per CT. 417 per CT. 422 ohm-cm (% by weight) ppm
1) BI-2 @ 0’-5’ 8.3 19,000 0.0444% 94
2) BI-3 @ 15’-16.5’ 8.2 8,200 0.0255% 98
3) BI-5 @ 0’-5’ 8.4 14,000 0.0399% 17
RESPECTFULLY SUBMITTED
________________________________
WES BRIDGER LAB MANAGER
Responsive ■ Resourceful ■ Reliable
SUPPORTING INFORMATION
UNIFIED SOIL CLASSIFICATION SYSTEM
Responsive ■ Resourceful ■ Reliable
UNIFIED SOI L CLASSI FICATI ON SYSTEM
Criteria for Assigning Group Symbols and Group Names Using Laboratory Tests A Soil Classification
Group
Symbol Group Name B
Coarse-Grained Soils:
More than 50% retained
on No. 200 sieve
Gravels:
More than 50% of
coarse fraction
retained on No. 4 sieve
Clean Gravels:
Less than 5% fines C
Cu 4 and 1 Cc 3 E GW Well-graded gravel F
Cu 4 and/or [Cc<1 or Cc>3.0] E GP Poorly graded gravel F Gravels with Fines:
More than 12% fines C
Fines classify as ML or MH GM Silty gravel F, G, H
Fines classify as CL or CH GC Clayey gravel F, G, H
Sands:
50% or more of coarse
fraction passes No. 4
sieve
Clean Sands:
Less than 5% fines D
Cu 6 and 1 Cc 3 E SW Well-graded sand I
Cu 6 and/or [Cc<1 or Cc>3.0] E SP Poorly graded sand I Sands with Fines:
More than 12% fines D
Fines classify as ML or MH SM Silty sand G, H, I
Fines classify as CL or CH SC Clayey sand G, H, I
Fine-Grained Soils:
50% or more passes the
No. 200 sieve
Silts and Clays:
Liquid limit less than 50
Inorganic:
PI 7 and plots on or above “A”
line J
CL Lean clay K, L, M
PI 4 or plots below “A” line J ML Silt K, L, M
Organic: Liquid limit - oven dried
0.75 OL Organic clay K, L, M, N
Liquid limit - not dried Organic silt K, L, M, O
Silts and Clays:
Liquid limit 50 or more
Inorganic: PI plots on or above “A” line CH Fat clay K, L, M PI plots below “A” line MH Elastic Silt K, L, M
Organic: Liquid limit - oven dried
0.75 OH Organic clay K, L, M, P
Liquid limit - not dried Organic silt K, L, M, Q
Highly organic soils: Primarily organic matter, dark in color, and organic odor PT Peat A Based on the material passing the 3 -inch (75-mm) sieve.
B If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name.
C Gravels with 5 to 12% fines require dual symbols: GW -GM well-graded gravel with silt, GW-GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay.
D Sands with 5 to 12% fines require dual symbols: SW -SM well-graded sand with silt, SW-SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP-SC poorly graded sand with clay .
E Cu = D60/D10 Cc =
6010
2 30
DxD
)(D
F If soil contains 15% sand, add “with sand” to group name.
G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
H If fines are organic, add “with organic fines” to group name.
I If soil contains 15% gravel, add “with gravel” to gr oup name.
J If Atterberg limits plot in shaded area, soil is a CL -ML, silty clay.
K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel,” whichever is predominant.
L If soil contains 30% plus No. 200 predominantly sand, add “sandy” to group name.
M If soil contains 30% plus No. 200, predominantly gravel, add “gravelly” to group name.
N PI 4 and plots on or above “A” line.
O PI 4 or plots below “A” line.
P PI plots on or above “A” line.
Q PI plots below “A” line.
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix B
Preliminary Design Concepts
25TH ST
24TH ST
23RD ST
MARINE AVE
BIKE TRAILTHE STRANDTHE STRANDBIKE TRAILINFILTRATION2.420'x10.5' LINE "I"
26TH ST
27TH ST
28TH ST
29TH ST
BIKE TRAIL
BIKE TRAIL
THE STRANDBI 0286LINE "G"INFILTRATION GALLERY2,420'x10.5'
THE STRAND
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT1-1 CITY OF MANHATTAN BEACHFULL - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDY
MATCHLINE - SEE TOP RIGHT
MATCHLINE - SEE BOTTOM LEFTMATCHLINE - SEE SHEET 1-2
LEGEND
32ND ST
BIKE TRAILTHE STRAND
30TH ST
31ST ST
THE STRANDBIKE TRAILINFILTRATION GALLERY2,420'x10.5'"CITY DRAIN"
THE STRAND
PREPARED BY:PROJECT NO.20365CITY OF MANHATTAN BEACHPROJECT TITLE SHEET TITLEFULL - BEACH INFILTRATIONEXHIBIT1-2 BEACH INFILTRATION FEASIBILTY STUDY MATCHLINE - SEE SHEET 1-1
LEGEND
26TH ST
27TH ST
THE STRANDOCEAN DRMANHATTAN AVE
THE STRANDOCEAN DRMANHATTAN AVE
BI 0286
INFILTRATIONGALLERY610'x16'
28TH ST
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT2 CITY OF MANHATTAN BEACHPARTIAL - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDYLEGEND 21ST ST
LINE "E"
THE STRAND
OCEAN DR
INFILTRATIONGALLERY30'x16'
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT3
LEGENDCITY OF MANHATTAN BEACHLINE E - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDY
THE STRAND
OCEAN DR
MANHATTAN AVE
INFILTRATIONGALLERY40'x16'
27TH STLINE "G"
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT4
LEGENDCITY OF MANHATTAN BEACHLINE G - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDY
THE STRAND
MANHATTAN AVE
LINE "I"24TH ST INFILTRTIONGALLERY45'x16'
OCEAN DR
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT5
LEGENDCITY OF MANHATTAN BEACHLINE I - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDY 32ND ST "CITY DRAIN"THE STRAND
OCEAN DR
INFILTRATIONGALLERY125'x16'
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT6 CITY OF MANHATTAN BEACHCITY DRAIN - BEACH INFILTRATIONBEACH INFILTRATION FEASIBILTY STUDYLEGEND 21ST ST
LINE "E"
THE STRAND
OCEAN DR
LEGENDPREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT7 CITY OF MANHATTAN BEACHLINE E - DRYWELLSBEACH INFILTRATION FEASIBILTY STUDY
THE STRAND
OCEAN DR
MANHATTAN AVE
27TH STLINE "G"
LEGENDPREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT8 CITY OF MANHATTAN BEACHLINE G - DRYWELLSBEACH INFILTRATION FEASIBILTY STUDY
THE STRAND
MANHATTAN AVE
LINE "I"24TH ST
OCEAN DR
LEGENDPREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT9 CITY OF MANHATTAN BEACHLINE I - DRYWELLSBEACH INFILTRATION FEASIBILTY STUDY 32ND ST
THE STRAND
OCEAN DR
MANHATTAN AVE
"CITY DRAIN"ABANDONED 24"
BAYVIEW DR
THE STRANDLEGENDPREPARED BY:PROJECT NO.20365 PROJECT TITLE SHEET TITLEEXHIBIT10 CITY OF MANHATTAN BEACHCITY DRAIN - DRYWELLSBEACH INFILTRATION FEASIBILTY STUDY 26TH ST
27TH STTHE STRAND
OCEAN DR
MANHATTAN AVE
28TH ST
HIGHLAND AVE
BI 0286
LEGENDPREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEEXHIBIT11 CITY OF MANHATTAN BEACH26TH STREET PARKING FACILITY DRYWELLSBEACH INFILTRATION FEASIBILTY STUDY
N PECK RD
INFILTRATION GALLERY380' LENGTH x 200' WIDTH x 5' DEPTH PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEPOLLIWOG PARK - INFILTRATIONLEGENDCITY OF MANHATTAN BEACHBEACH INFILTRATION FEASIBILTY STUDYEXHIBIT12 PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLE
A
A
SECTION A-ADRYWELL - SECTIONCITY OF MANHATTAN BEACHBEACH INFILTRATION FEASIBILTY STUDYEXHIBIT13
SECTION C-CSECTION A-A
STORAGE PLAN - 16' WIDEA
A
CC
B CC
BSTORAGE PLAN - 10.5' WIDE
SECTION B-B
PREPARED BY:PROJECT NO.20365 PROJECT TITLE
SHEET TITLEBEACH INFILTRATIONCITY OF MANHATTAN BEACHBEACH INFILTRATION FEASIBILTY STUDYEXHIBIT14 City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix C
Project Description
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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1. Introduction
The City of Manhattan Beach (City) proposes to implement the 28th Street Storm Drain Infiltration Project (Project) to improve water quality at the beach and for the Santa Monica Bay. The primary goal of the Project is to improve water quality locally, on the beach, and in the Santa Monica Bay by:
➢ Reducing bacterial discharges from the storm drain system and ➢ Reducing trash/debris discharge from the storm drain system.
The Project will generate additional benefits, including:
➢ Enhancing beach conditions,
➢ Reducing the potential for beach closures,
➢ Providing an enhanced environment for marine life,
➢ Creating educational and outreach opportunities for the local community, and ➢ Improving the 26th Street Parking Facility.
Implementation of a beach infiltration system was identified as the highest priority capital project for the City in the Beach Cities Enhanced Watershed Management Program (EWMP). The EWMP was developed in a collaborative effort involving the Cities of Manhattan Beach, Hermosa Beach, Redondo Beach, Torrance, and the Los Angeles County Flood Control District (LACFCD). The EWMP identified capital improvement projects in the participating jurisdictions to help improve water quality and address applicable pollutants in the Santa Monica Bay, specifically related to bacteria. Figure 1-1 illustrates the Project area, surrounding drainage areas, and the Beach Cities EWMP Group.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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Figure 1-1 Surrounding Watersheds and Watershed Group
The Project’s drainage area covers over 1,500 acres. A drainage area is defined as the land area where rain falls and is then captured in a single drainage system (28th Street Storm Drain). Almost the whole area is within the City of Manhattan Beach, as illustrated in Figure 1-2. The Project captures 60% of the City’s area. The pollutants generated in the drainage area are mitigated by the Project, preventing them from reaching the beach and Santa Monica Bay.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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Figure 1-2 Project Drainage Area
1.1 Project Conceptual Approach
The City seeks to implement the Project to achieve the water quality goals identified in the Beach Cities EWMP. The Project addresses discharges that drain into the 28th Street Storm Drain and ultimately the Santa Monica Bay. Figure 1-3 illustrates the general concept of the Project. The Project will reduce pollutants sent to downstream water bodies by capturing runoff, such as rainwater and irrigation overspray. Captured runoff will be discharged into an underground system that facilitates infiltration.
Figure 1-3 General Project Concept
Divert urban
runoff from storm
drain
Pretreat runoff
Pump runoff to
an underground
infiltration system
Infiltrate runoff
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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1.2 Benefits
The Project provides multiple benefits, which are summarized below:
➢ Improve water quality locally, on the beach, and in the Santa Monica Bay ▪ Reduce bacteria discharges from drainage system
▪ Reduce discharge of trash/debris from drainage system ➢ Enhance beach conditions
▪ Reduced debris from the upstream drainage system
▪ Reduce the potential for odors related to drainage system ➢ Reduce the potential for beach closures
➢ Create opportunities for education and outreach in local communities ▪ Inform the community of water quality challenges and strategies to improve it ▪ Install permanent educational signage
▪ Engage in dialogue with stakeholders and the public through community meetings ➢ Provide an enhanced environment for marine life
➢ Improve the 26th Street Parking Facility
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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2. Proposed Improvements
The City proposes to use a phased approach to accomplish the goals of the Project. Phase 1 includes improvements at the 26th Street Parking Facility, while Phase 2 includes improvements on the beach.
Figure 2-1 illustrates the Project areas for each phase.
Phase 1 will maximize, to the greatest extent practicable, the volume of water diverted and infiltrated.
The City will assess improvements made to water quality through the implementation of Phase 1 to determine the need for Phase 2 implementation. In total, the two phases will ultimately capture runoff from the drainage area illustrated in Figure 1-2.
Figure 2-1 Concept Layout
2.1 Phase 1
Phase 1 improvements focus on the 26th Street Parking Facility, which is bordered by Manhattan Avenue, Ocean Drive, 27th Street, and 26th Street, as shown in the figure above. Runoff will be diverted from the storm drain on 28th Street through a pipe aligned southeast on Ocean Drive. Opportunities to include a gravity diversion upstream will be evaluated during the design process. The diverted runoff will be pretreated to remove trash and sediment and pumped to a matrix of drywells located in and around the parking facility.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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The storm drain is a reinforced concrete tunnel that is approximately 40 feet deep near Manhattan Avenue. The diversion along Ocean Drive will reduce the amount of excavation required for the pretreatment unit and pump station.
The intent of Phase I is to maximize capture at this location due to the high infiltration rates and the proximity to the beach outfall. A total of 48 drywells are anticipated to be installed. Captured runoff will fill the drywells, which are six-foot diameter shafts, partially filled with gravel. Water will be able to infiltrate through the soil, improving water quality. Figure 2-2 illustrates the preliminary drywell layout at the 26th Street Parking Facility.
Figure 2-2 Phase 1: 26th Street Parking Facility Drywell Concept
The exact location of each drywell will be determined during final design and will be placed to avoid existing utilities, retaining walls, and other onsite features. The parking lot will be repaved as part of the Project using pervious pavement, to maximize the infiltration on site. Improvements to the landscaping and existing walls of the facility may also be incorporated.
Approximately 55 acre-feet of runoff is expected to be captured during a single storm event following implementation of Phase 1. The parking facility will look the same as what is there now with some upgrades (new pavement, new landscape, signage, etc.). Manholes will be in the parking facility above each drywell, like what exists in the street for various utilities, and the rest of the improvements will be underground.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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2.2 Phase 2
Following implementation of Phase 1, the improvements to water quality will be monitored and assessed to determine the need and goals for Phase 2 implementation. Like Phase 1, a diversion, pretreatment system, and pump will be required to capture runoff from the existing storm drain on 28 th Street. It is expected that at least part of the system used for Phase 1 will also be used for Phase 2, which will reduce construction impacts and costs. Runoff will be pumped into a buried infiltration trench on the beach, as is illustrated in Figure 2-3. The infiltration trench is essentially a pit filled with gravel and perforated pipes wrapped in a filter fabric. The pipes and void space from the gravel allow for water to be stored while infiltration occurs into the sand. Other buried structures that meet the Project objective are also under consideration as alternatives. The system will be buried under the surface and potential future impacts associated with sea level rise have been considered.
Figure 2-3 Phase 2: Beach Infiltration Concept
Up to 10 acre-feet of runoff is expected to be captured during a single storm event under Phase 2. In total, the two phases of the Project aim to capture up to 63 acre-feet of runoff during a single storm event, or 1-inch of rainfall generated within the 1,500-acre drainage area.
Following Phase 2 implementation, the surface will be restored to match existing conditions and dune restoration may also be included as part of the Project. Manholes will be visible on the street at the diversion, pretreatment, and pump, while the rest of the infrastructure will be underground.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Project Description
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Figure 2-4 illustrates the improvements of both phases, which is the eventual goal. Specific sizes and locations may move slightly during the design process, while the overall concept will remain the same.
Figure 2-4 Full Project Implementation Concept
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix D
Letters of Support
September 14, 2020
City of Manhattan Beach
Mo Estepa Jr., P.E., Project Manager
3621 Bell Avenue
Manhattan Beach, California 90266
28th Street Storm Drain Infiltration Project
Letter of Support
Dear Mr. Estepa,
On behalf of 7th Generation Advisors, we submit our formal support of the 28th Street Storm Drain Infiltration Project in Manhattan Beach.
7th Generation Advisors works with government, non-profits and the private sector to create and implement best-practice projects that improve water quality and sustainability, and environmental resilience. The mission of 7th Generation Advisors is to empower individuals and organizations to create and adopt solutions that protect our environment and fight climate change, based on the ancient First Nations philosophy that the decisions we make today should result in a sustainable world generations into the future.
We support the 28th Street Storm Drain Infiltration Project because it will significantly improve the quality of a large amount of urban runoff and other storm drainage into the Pacific Ocean and onto the beaches.
We are aware that the phased project will capture runoff from the 28th Street storm drain system for infiltration, reducing the pollutant loading onto the beach and into the Santa Monica Bay. The technologies and methodologies presented ensure that if built accordingly the project will have enormous and lasting impact in the region. The Project takes steps towards meeting water quality goals while also enhancing community benefits through tree planting, parking facility improvements, and public outreach. 7th Generation Advisors is excited to support Manhattan Beach and to anticipate improved water quality in the region and Santa Monica Bay.
If we can be of assistance to you in this project, or otherwise, it would be our pleasure. Please do not hesitate to contact us.
With thanks and respect,
Portia P. Cohen, Esq., Advisor
7th Generation Advisors
Phone: 310.991.7366
Email: [email protected]
Website: www.7thgenerationadvisors.org
April 2, 2020
California Natural Resources Agency
Proposition 68 Urban Flood Protection Grant Program
1416 Ninth Street, Suite 1311
Sacramento, CA 95814
From: Heal the Bay
Re: Letter of Support for Funding of the Manhattan Beach Stormwater Infiltration Project
To the Prop 68 UFP Grant Program:
Heal the Bay is pleased to offer support for the Manhattan Beach Stormwater Infiltration Project. Heal the Bay is an environmental organization with over 30 years of experience and 15,000 members dedicated to making the coastal waters and watersheds of greater Los Angeles safe, healthy, and clean. The proposed project aligns well with Heal the Bay’s mission.
The project is proposing to achieve flood protection and water quality goals through the installation of a stormwater capture and infiltration system. This project will help protect essential stormwater infrastructure in the City of Manhattan Beach from the effects of coastal flooding and will also help to achieve water quality objectives for trash, indicator bacteria and other pollutants in Santa Monica Bay. As detailed in the Beach Cities Enhanced Watershed Management Program (EWMP), the Manhattan Beach Stormwater Infiltration Project is the highest priority project in the City for addressing water quality impairments in the Santa Monica Bay, specifically for meeting the dry and wet weather Bacteria Total Maximum Daily Loads (TMDLs) established to protect and restore year-round recreational beach water quality.
An additional component of the project will be the restoration of approximately 3.5 acres of coastal dune habitat, which will increase the extent and condition of a key ecosystem in this coastal area and help to create a more resilient shoreline.
It is for these reasons that Heal the Bay supports the Manhattan Beach Stormwater Infiltration Project and encourages the Natural Resources Agency to give it serious consideration for funding.
Sincerely,
Nancy Shrodes
Associate Director of Policy & Outreach
September 29, 2020
City of Manhattan Beach
Attention: Mo Estepa Jr.
3621 Bell Avenue
Manhattan Beach, California 90266
28th Street Storm Drain Infiltration Project
Letter of Support
Dear Mr. Mo Estepa Jr.,
We understand the City of Manhattan Beach (City) is moving forward with the 28th Street Storm Drain Infiltration Project to improve water quality and the environment. We are aware that the phased project will capture runoff from the 28th Street storm drain system for infiltration, reducing the pollutant loading onto the beach and into the Santa Monica Bay.
The Manhattan Beach Botanical Garden demonstrates the benefits of water conservation and care of native plants without fertilizers in a pesticide and herbicide free environment. We also encourage landscaping in ways to capture as much rain runoff as possible for irrigation and soil infiltration. This projects aligns with our mission of water conservation and will help infuse water into the ground to prevent salt water infiltration along the coast allowing plant life to continue to thrive in the immediate area. This is one of many efforts needed to battle the effects of climate change and prevent environmental degradation and will help the City meet federal clean requirements and regulations to clean up the stormwater runoff.
MBBG supports efforts to improve water quality in the region and Santa Monica Bay. This project takes steps toward meeting water quality goals while enhancing community benefits through tree planting, parking facility improvements, and public engagement.
If you have any questions or require additional information, please contact Charlotte Marshall at 310-374-1290 or mbbgarden.org.
Respectfully submitted,
The Manhattan Beach Botanical Garden Board of Directors Charlotte Marshall
Charlotte Marshall
MBBG, president
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix E
Vector Minimization Plan
City of Manhattan Beach
28th Street Storm Drain Infiltration Project
Vector Minimization Plan
Prepared for:
City of Manhattan Beach
1400 Highland Avenue
Manhattan Beach, CA 90266
TEL (310) 802-5000
Prepared by:
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California, 92831
TEL (714) 526-7500 | FAX (714) 526-7004 | www.cwecorp.com
September 2020
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Vector Minimization Plan
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1. Introduction
Management of mosquitoes and other vectors in stormwater management structures, such as flood control basins and Best Management Practices (BMPs), is critical for protecting public health. With careful planning, such structures can be designed, built, operated, and maintained in a manner that minimizes opportunities for the proliferation of vectors. This Vector Minimization Plan has been prepared based on the Checklist for Minimizing Vector Production in Stormwater Management Structures developed by the State of California Health and Human Services Agency (https://www.cdph.ca.gov/Programs/CID/ DCDC/CDPH%20Document%20Library/ChecklistforVectorPreventioninBMPs.pdf). The checklist provides action items intended to lessen the short and long-term potential for vector production in stormwater management structures while reducing dependence on pesticides to the maximum extent possible.
The checklist items are incorporated into this Plan. Narrative discussions are included as necessary to clarify applicability. The Plan is broken into sections for wet and dry systems, consistent with the referenced checklist. Each section identifies what project components fall into each category. The Project does not include any permanent water features or wetlands; therefore, associated portions of the checklist are not included in this Plan.
2. Project Overview
This Vector Minimization Plan is prepared in association with the Manhattan Beach 28th Street Storm Drain Infiltration Project (Project) being implemented by the City of Manhattan Beach (City) in response to the priority project identified in the Beach Cities Enhanced Watershed Management Program (EWMP).
The Project will improve water quality and provide additional benefits to the community. The goal of the Project is to minimize wet-weather discharges and eliminate dry-weather discharges into the Santa Monica Bay as measures to reduce pollutant loading. Runoff will be diverted from the Los Angeles County Flood Control District (LACFCD)-owned storm drain, Bond Issue (BI) 0286 (also referred to the 28th Street Storm Drain), and infiltrate the runoff back into the ground. The Project is being implemented in response to the Santa Monica Bay Beaches Bacteria Total Maximum Daily Load (TMDL).
3. Dry Systems
Dry systems are defined as any structure designed to drain completely following capture and/or treatment of runoff. Examples identified in the checklist include flood control basins, extended detention basins, infiltration basins and trenches, Austin sand filters, swales and strips, drain inlet inserts, and/or linear-radial gross solids removal devices. It is recommended that dry systems discharge all captured water in four (4) days or less to address vector concerns.
The following dry systems are included as part of the Project:
➢ Subsurface infiltration system (drywells and infiltration trench)
Water will not be standing for more than four (4) days. The total time it will take to infiltrate will depend on the storm duration. This allows the Project to address vector concerns related to the subsurface infiltration system.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Vector Minimization Plan
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3.1 Dry System Checklist
Dry systems will drain completely within four (4) days, avoiding vector concerns. The chec klist below is based on the Checklist for Minimizing Vector Production in Stormwater Management Structures for dry systems. The checklist is completed based on the Project and narrative is included, as necessary, to provide additional Project information and clarification.
Is the structure designed to discharge all captured water in four (4) days or less? Additional Clarification/Response (if necessary):
As described above, water will not be standing (still) for more than four (4) days.
Has every effort been made to trace and eliminate persistent non-stormwater flows (e.g. irrigation runoff) that may enter the system and jeopardize non-chemical vector control efforts? Additional Clarification/Response (if necessary):
The Project will capture dry-weather (non-stormwater) flows from the Project drainage area and convey them to the infiltration system. Surrounding irrigation runoff may be captured and diverted to the system.
Has groundwater depth been carefully evaluated to ensure that the structure will not be permanently or seasonally flooded (i.e. is the base of the basin higher than the local groundwater table)?
Additional Clarification/Response (if necessary):
Geotechnical explorations have encountered groundwater, which has been used to design the Project.
Geotechnical explorations at the 26th Street Parking Facility found groundwater 47 feet beneath the ground surface, while explorations at the beach found groundwater approximately 12 feet beneath the surface. The dry system components will not be placed deep enough to encounter groundwater .
Does the design provide an adequate slope between the inlets and outlets, with special attention given to ensure corners are above grade?
Additional Clarification/Response (if necessary):
True for gravity pipe conveyances.
Has soil been compacted adequately during grading to minimize subsidence, which can result in pools of standing water?
Additional Clarification/Response (if necessary):
Adequate compaction required in the Project specifications. Lower compaction will be specified beneath the subsurface infiltration system to preserve infiltration rates.
Does the design slope take into consideration the inevitable accumulation of sediment and debris between maintenance periods that can result in standing water, especially in and around the inlet? Additional Clarification/Response (if necessary):
Slope is sufficient. Pretreatment system is also being implemented to reduce sediment and debris accumulation.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Vector Minimization Plan
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Does the design minimize the use of features that increase the potential for standing water, such as loose riprap and concrete curbs?
Additional Clarification/Response (if necessary):
Not applicable (no new rip rap or surface drainage features).
Does the structure include a concrete or earthen low-flow channel to concentrate (i.e. minimize available surface area) and direct non-stormwater flows to the outlet? Additional Clarification/Response (if necessary):
Not applicable (no low-flow channels).
Is the distribution piping sloped adequately and smooth (not corrugated) on the inside to prevent standing water?
Additional Clarification/Response (if necessary):
Diversion pipe will be sloped adequately and smooth pipes will be used.
Are the inlet structures and energy dissipaters designed and sloped sufficiently to prevent scour depressions?
Additional Clarification/Response (if necessary):
Not applicable (no energy dissipater structures and scour is not anticipated, as improvements are mostly concrete).
Are the outlets designed with debris screens or other features that reduce the potential for clogging?
Additional Clarification/Response (if necessary):
Not applicable, no outlet system. Flows will infiltrate and when the infiltration system is full, a valve may close and prevent flows from entering the system.
Is the structure designed with safe and sufficient access for inspection, maintenance, and/or vector control activities when needed?
Additional Clarification/Response (if necessary):
Manholes (or other access points) are proposed along subsurface infiltration system. Manholes are proposed at each drywell in the 26th Street Parking Facility. The diversion, pretreatment, and pump system will also include manholes/vault access (wet system).
Does the operation and maintenance plan include a minimum of quarterly inspections to ensure that vegetation overgrowth, sediment accumulation, or other factors have not created areas of standing water?
Additional Clarification/Response (if necessary):
The Operation and Maintenance (O&M) Manual describes the maintenance procedures for the proposed components, including the subsurface infiltration system.
Does the operation and maintenance plan include a minimum annual maintenance to remove vegetation overgrowth, remove sediment and debris accumulation, and otherwise return the structure to “as-designed” conditions?
Additional Clarification/Response (if necessary):
The O&M Manual describes the maintenance procedures for Project components as described above.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Vector Minimization Plan
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Is signage provided and clearly visible with minimum information indicating the type of structure (e.g. extended detention basin), ownership, and contact information? Additional Clarification/Response (if necessary):
Not applicable, as the system is underground and not accessible to people.
4. Wet Systems
Wet systems are defined as any structure designed with features such as sumps, vaults, and/or basins that hold water permanently, or longer than four (4) days. Examples identified in the checklist include open catch basins, concrete retention basins, Delaware sand filters, and a variety of belowground proprietary devices. It is recommended that wet systems are designed to deny mosquito access to standing water by using covers, screens, and/or other barriers.
The following wet system is included as part of the Project:
➢ Pretreatment system (hydrodynamic separator)
➢ Pump station (standing/still water not anticipated, as flow is anticipated on a semi-continuous basis [will capture wet- and dry-weather runoff])
➢ Vaults containing flow meter/valve
The wet systems will include covers, screens, and other barriers to deny access to mosquitos/vectors.
This allows the Project to address vector concerns related to the identified component.
4.1 Wet System Checklist
Wet systems will include covers, screens, and other barriers to prevent mosquitos/vectors from entering the systems with standing water, allowing the Project to avoid vector concerns. The checklist below is based on the Checklist for Minimizing Vector Production in Stormwater Management Structures for wet systems. The checklist is completed based on the Project and narrative is included, as necessary, to provide additional Project information and clarification.
Have sumps, vaults, or basins that hold water permanently, or longer than 4 days, been completely or partially sealed against adult mosquito entry? Additional Clarification/Response (if necessary):
Systems will be sealed with mosquito screens/barriers implemented at the manhole access points for the pretreatment system, pump well, and flow meter vaults.
If used, are covers tight fitting, with gaps or holes of no greater than 1/16” (2 mm)? Additional Clarification/Response (if necessary):
See response above about the use of screens/barriers.
If used, are aluminum or nylon screens for sealing small openings secured with gaps or holes of no greater than 1/16” (2 mm)?
Additional Clarification/Response (if necessary):
Design will require the gap to be minimized.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Vector Minimization Plan
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If cast iron manhole covers are used, are pick holes sealed or is a mosquito-proof insert provided below?
Additional Clarification/Response (if necessary):
See response above about the use of screens/barriers.
Where feasible, are the inlet and/or outlet conveyance pipes submerged to prevent adult mosquito entry into the main water storage area?
Additional Clarification/Response (if necessary):
Not applicable (inlets/outlets cannot be submerged at some facilities).
Where feasible, are conveyance pipes fitted with flapper valves, collapsible fabric tubes, or other barriers to prevent adult mosquito entry into the main water storage area? Additional Clarification/Response (if necessary):
Not applicable, vectors will not be able to get to main water storage area (subsurface infiltration system) due to screens and barriers (see response above).
Is the structure designed with safe and sufficient access to permanent water areas for inspection, maintenance, and/or vector control activities when needed? Additional Clarification/Response (if necessary):
Structures will have accessibility for inspection and maintenance.
Does the operation and maintenance plan include a minimum of quarterly inspections to ensure that barriers to mosquito entry are intact and in place as designed? Additional Clarification/Response (if necessary):
Where possible, is signage provided with minimum information indicating type of structure (e.g.
CDSTM), ownership, and contact information?
Additional Clarification/Response (if necessary):
Pretreatment access, manholes, and vault covers will include some information pertaiing to equipment and/or the City.
5. Vector Minimization Summary
In summary, the following vector minimization strategies are included as part of the Project:
➢ Subsurface infiltration system, pretreatment, and valve vaults will be accessible and have mosquito/vector screens/barriers, which will be inspected/maintained quarterly
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix F
O&M Manual
City of Manhattan Beach
28th Street Storm Drain Infiltration Project
Operation and Maintenance Manual
Prepared for:
City of Manhattan Beach
1400 Highland Avenue
Manhattan Beach, CA 90266
TEL (310) 802-5000
Prepared by:
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California, 92831
TEL (714) 526-7500 | FAX (714) 526-7004 | www.cwecorp.com
September 2020
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Table of Contents
TABLE OF CONTENTS .................................................................................................................. i LIST OF FIGURES ....................................................................................................................... i LIST OF TABLES .......................................................................................................................... i ACRONYMS .................................................................................................................................ii
1. INTRODUCTION ............................................................................................................... 1
1.1 PURPOSE ................................................................................................................................. 1
1.2 PROPOSED IMPROVEMENTS .......................................................................................................... 1
1.3 LOCATION OF THE PROJECT ......................................................................................................... 3
1.4 REFERENCES ............................................................................................................................ 4
2. PROJECT DESCRIPTION .................................................................................................. 5
2.1 OPERATION ............................................................................................................................. 5
2.2 MAINTENANCE AND INSPECTIONS .................................................................................................. 5
2.3 REPAIR, REPLACEMENT, REHABILITATION, AND REMOVAL .................................................................... 7
2.4 REGULATORY REQUIREMENTS ....................................................................................................... 7
2.5 THIRD PARTY AGREEMENTS ......................................................................................................... 7
3. COST ................................................................................................................................ 8
Appendices
Appendix A As-Built Drawings
Appendix B Technical Specifications
Appendix C Catalog Cut Sheets and Pertinent Information Appendix D Inspection and Maintenance Logs
List of Figures
Figure 1-1 Phase 1 Proposed Conditions .............................................................................................. 2 Figure 1-2 Phase 2 Proposed Conditions .............................................................................................. 3 Figure 1-3 Project Site Map ................................................................................................................ 4
List of Tables
Table 2-1 Inspection and Maintenance Summary ................................................................................. 5 Table 3-1 Anticipated Maintenance Cost .............................................................................................. 8
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Acronyms
BI Bond Issue
EWMP Enhanced Watershed Management Program
LACFCD Los Angeles County Flood Control District
O&M Operation and Maintenance
TMDL Total Maximum Daily Load
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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1. Introduction
The City of Manhattan Beach (City) is implementing the 28th Street Storm Drain Infiltration Project (Project) to improve water quality. The City participated with adjacent agencies, including the Los Angeles County Flood Control District (LACFCD) and the Cities of Hermosa Beach, Redondo Beach, and Torrance, in developing the Beach Cities Enhanced Watershed Management Program (EWMP). The EWMP identified capital improvement projects to help improve water quality and address applicable impairments in the Santa Monica Bay, specifically related to the bacteria Total Maximum Daily Load (TMDL). The Beach Cities EWMP identified a beach infiltration system as a high priority capital project within the City.
A detailed Project Description includes additional information regarding the Project goals, benefits, and layout. In summary, a diversion structure is proposed on the Bond Issue (BI) 0286 storm drain (also referred to as the 28th Street Storm Drain), which is owned and operated by the LACFCD. Flows captured through the diversion will be pretreated before being pumped into a subsurface infiltration system. There are two phases included in the Project. Phase 1 includes a subsurface infiltration system consisting of drywells in the 26th Street Parking Facility, while Phase 2 includes a shallow subsurface infiltration system on the beach. The diversion system will include a flow meter and vault to measure the volume of runoff captured by the Project. It is expected that a single diversion, pretreatment system, and pump will be used for both Phase 1 and 2. Opportunities to utilize a gravity diversion upstream are under consideration, while the functionality of the Project would remain as is explained above.
This Operation and Maintenance (O&M) Manual has been prepared to provide guidance for maintenance crews on the inspection and maintenance of proposed Project components. The manual outlines operation, inspection, and maintenance requirements and will include specifications, as-built plans, and maintenance/inspection logs following Project implementation.
1.1 Purpose
The Project is being implemented by the City to meet water quality goals as identified in the Beach Cities EWMP. The Project will address discharges that drain into the 28th Street Storm Drain and ultimately the Santa Monica Bay. The Project will reduce pollutant loading to downstream water bodies by capturing wet- and dry-weather runoff. Captured runoff will be discharged into a subsurface system that will facilitate infiltration. The 26th Street Parking Facility will be improved as part of the Project and will provide community enhancements. The parking facility will be repaved with pervious pavement to facilitate additional capture and incorporate nature-based solutions.
1.2 Proposed Improvements
The following improvements are proposed as part of the Project:
➢ Diversion structure (modified manhole with grated drop inlet):
▪ Along the 28th Street Storm Drain (on BI 0286)
▪ Schematics identify diversion structure near the coast, while opportunities for a gravity diversion upstream are under consideration
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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➢ Pretreatment device:
▪ Downstream of diversion structure
➢ Pump:
▪ Downstream of pretreatment device
➢ Piping to connect various components
➢ Flow meter and gate valve (pending final design) downstream of pump system) ➢ Subsurface infiltration system:
▪ Phase 1: drywells in the 26th Street Parking Facility (48 expected) ▪ Phase 2: infiltration trench/system on beach
➢ Pervious pavement in 26th Street Parking Facility
Figure 1-1 illustrates the improvements that will be constructed as part of the Project during Phase 1 and Figure 1-2 illustrates the improvements expected as part of Phase 2. As mentioned above, opportunities to implement a single diversion, pretreatment, and pump station will be explored during the design process. Additionally, a gravity diversion may be implemented upstream to minimize the pump sizing near the beach. As-builts will be included in Appendix A following construction of the Project.
Figure 1-1 Phase 1 Proposed Conditions
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Figure 1-2 Phase 2 Proposed Conditions
1.3 Location of the Project
The Project is located between 25th Street, 28th Street, Manhattan Avenue, and the beach, as shown in the figures above. The Project was identified in the Beach Cities EWMP and will capture runoff generated within the Santa Monica Bay Frontal Watershed. Figure 1-3 illustrates the Project area within the City, surrounding drainage areas, and the Beach Cities EWMP Group. The Beach Cities EWMP Group encompasses land tributary to both the Santa Monica Bay and the Long Beach Harbor (through Dominguez Channel).
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Figure 1-3 Project Site Map
1.4 References
The Project components will be inspected, operated, and maintained as described in this manual.
References and technical specifications for the pretreatment unit, pump, and subsurface infiltration systems, particularly those supplied by vendors, are attached in Appendix B and Appendix C.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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2. Project Description
Regular maintenance and inspection schedules are necessary to preserve the Project’s effectiveness and longevity. The following sections detail the necessary tasks required to operate, inspect, and maintain the Project’s components.
2.1 Operation
The main system components includes a diversion system, pretreatment unit, pump station with appurtenances, flow meter/gate valve, and subsurface infiltration system. Once in place, the diversion system and pretreatment unit will not require any specific operations, as they will function on their own.
Inspections and maintenance related to the diversion system and pretreatment unit are further discussed below.
The pump station, including the flow meter/valves, will operate automatically based on controls put into place during construction. The system will have multiple pumps for redundancy in case one of the pumps breaks down or needs to be serviced. The pumps will alternate operations to reduce overuse and prolong their lifespan. The pump system does not require manual operation. Inspection and maintenance related to the pump system and flow meter/valves is further discussed below.
2.2 Maintenance and Inspections
Descriptions of the operation/maintenance and inspection frequency of the Project’s components are listed in Table 2-1.
The City will be responsible for inspection, operation, and maintenance. Contact information is as follows:
The City of Manhattan Beach
Department of Public Works
3621 Bell Avenue
Manhattan Beach, CA 90266
(310) 802-5300
Table 2-1 Inspection and Maintenance Summary
Component Operation/Maintenance Inspection Frequency
Diversion
System (inlet
and pipe)
➢ Inspect for accumulated sediment and debris
over grate and within concrete basin
➢ Remove accumulated sediment and debris
(litter and leaves) from the grate and inside
the inlet
➢ Inspect conveyance pipe for clogging
➢ Remove accumulated materials from the pipe
system
Before and after the storm
season (twice per year)
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Component Operation/Maintenance Inspection Frequency
Pretreatment
Unit
➢ Inspect unit separation chamber and sump for
accumulated trash, debris, and sediment and
remove as necessary.
➢ Inspect screen for clogging and remove
accumulated material as necessary
Before and after the storm
season (twice per year)
Pump
➢ Inspect pump well for sediment and debris
and remove as necessary
➢ Check valves for operation and clogging
➢ Clear material and replace valves as necessary
➢ Inspect bearings and impeller for wear
➢ Lubricate bearings as needed
➢ Check pump for operation
➢ Verify pump levels have been maintained
➢ Consult manufacturer if pump has not been
operated in more than 12 months or if more
extensive maintenance is required
➢ During operation, check pump for excessive
noise, vibration, or other abnormal conditions
Twice per year
Flow Meter
➢ Inspect flow data to identify anomalies
➢ Troubleshoot with manufacturer if data
anomalies are observed
➢ Mostly maintenance free, but should be
calibrated annually
Once per year
Gate Valve (to
be determined
during design)
➢ Inspect for excessive wear, corrosion, and/or
leakage and replace affected part if necessary
➢ Inspect for blockages or accumulated
sediment and remove as necessary
➢ Cycle valve to make sure it functions properly
Once per year
Subsurface
Infiltration
System
(drywells)
➢ Inspect storage for sediment and debris and
remove as necessary
➢ Inspect pipes for clogging and remove debris
as necessary
➢ Use vacuum truck to remove accumulated
materials
➢ Observe infiltration rates over time to confirm
infiltration areas are not clogged
➢ Restore infiltration if rates impacted
significantly (maintenance requirements may
vary based on issue)
Once per year
Subsurface
Infiltration
System (trench)
➢ Inspect storage for sediment and debris and
remove as necessary
➢ Inspect inlet pipe for clogging and remove
debris as necessary
➢ Use vacuum truck to remove accumulated
materials
➢ Observe infiltration rates over time to confirm
infiltration areas are not clogged
➢ Restore infiltration if rates impacted
significantly (maintenance requirements may
vary based on issue)
Once per year
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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Component Operation/Maintenance Inspection Frequency
Pervious
Pavement
➢ Visually inspect pavement for accumulated
sediment/debris and remove as necessary
➢ Inspect pavement surface for deterioration or
spalling and replace/restore as necessary
➢ Perform sweeping and/or vacuuming
(depending on pervious material used)
➢ Observe infiltration rates over time to confirm
infiltration areas are not clogged
➢ Restore infiltration if rates impacted
significantly (maintenance requirements may
vary based on issue)
Once per year
Mosquito/Vector
Screens/Barriers
➢ Inspect screen/barrier for rips or holes
➢ Replace screen/barrier if damage is observed Quarterly
2.3 Repair, Replacement, Rehabilitation, and Removal
The Project components shall be inspected and maintained on a regular basis, as detailed in Table 2-1.
It is recommended that the specialized components be maintained by qualified personnel. The City will either contract out some of the maintenance or be trained on how to maintain the components. Any damage observed shall be repaired, replaced, rehabilitated, and/or removed, depending on the damage and manufacturer (or specialist) recommendations. Detailed product information will be submitted to the City by the selected Contractor, including O&M protocols, which will be included in this manual following construction.
2.4 Regulatory Requirements
All the described inspection and maintenance activities are exempt from local or regulatory permits.
2.5 Third Party Agreements
There are no third party agreements in place for the inspection, operation, and/or maintenance of the Project components described in this O&M Manual at this time.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
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3. Cost
Table 3-1 summarizes the anticipated maintenance cost. The maintenance cost is based on the anticipated number of crews/staff required, staff level expertise, and hours of maintenance per year. The hourly rate is based on general industry averages and may vary depending on whether the City will maintain or hire a maintenance team. It is expected that at least two laborers are onsite performing maintenance. The hourly projections may also vary depending on the final design and elements constructed. These projections will be updated following construction. In addition to the maintenance costs projected below, it is anticipated that approximately $50,000 of operational costs will exist per year in association with inspections and utility (electrical and other) fees. Costs will likely increase about 3% annually based on inflation, which is not shown in the table below.
Table 3-1 Anticipated Maintenance Cost
Component Frequency
Hours by Position per Year
Equipment
Rental
Total
Cost Superintendent Foreman Laborer
$150 $120 $75
Diversion
System (inlet
and pipe)
Twice per
year 12 36 72
Equipment
used for
many of the
components
$11,520
Pretreatment
Unit
Twice per
year 12 36 72 $11,520
Pump Twice per
year 24 72 144 $23,040
Flow Meter Once per
year 4 12 24 $3,840
Gate Valve Once per
year 4 12 24 $3,840
Subsurface
Infiltration
System
(drywells)
Once per
year 14 42 84 $13,440
Subsurface
Infiltration
System (trench)
Once per
year 14 42 84 $13,440
Pervious
Pavement
Once per
year 8 24 48 $7,680
Mosquito/Vector
Screens/Barriers Quarterly 4 12 24 $3,840
TOTAL: 96 288 576 $8,840 $101,000
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Appendix A
As-Built Drawings
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
As-built drawings will be included upon Project completion City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Appendix B
Technical Specifications
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Technical specifications will be included upon Project completion City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Appendix C
Catalog Cut Sheets and Pertinent Information
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Catalog cut sheets and pertinent data will be included upon Project completion City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Appendix D
Inspection and Maintenance Logs
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Diversion Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Pretreatment Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Pump Station Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Flow Meter Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Gate Valve Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Subsurface Infiltration System (Drywells) Inspection and Maintenance Log Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Subsurface Infiltration System (Infiltration Trench) Inspection and Maintenance Log Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Pervious Pavement Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Operation and Maintenance Manual
Mosquito Screen/Barrier Inspection and Maintenance Log
Date Inspection/Maintenance
Personnel Inspection/Maintenance Performed Additional Notes
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix G
Commitment Letter
CITY OF MANHATTAN BEACH PUBLIC WORKS
3621 Bell Avenue, Manhattan Beach, CA 90266
ENGINEERING DIVISION
www.citymb.info • [email protected] • (310) 802-5000
October 1, 2020
Los Angeles County Flood Control District
Safe Clean Water Program
900 South Fremont Avenue
Alhambra, CA 91803
SUBJECT: 28 TH Street Storm Drain Infiltration Project Letter of Commitment
To Whom It May Concern:
The City of Manhattan Beach (City) is seeking fundi ng for the 28 th Street Storm Drain
Infiltration Project (Project) through the Regional Safe Clean Water Program (SCWP). The regional multi-benefit project provides water quali ty, environmental, and community enhancements. Runoff will be diverted from the 28 th Street Storm Drain (Bond Issue [BI] 0286), pretreated, and pumped into subsurface infil tration systems. The subsurface storage system will include drywells in the 28 th Street Parking Facility (Phase 1) and an infiltrat ion trench on the beach (Phase 2). Captured runoff will be infiltrated, which will result in the removal of pollutants from the beach and Santa Moni ca Bay in alignment with the Municipal Separate Storm Sewer (MS4) Permit and Santa Monica Bay Bacteria Total Maximum Daily Load (TMDL). Pollutant removal will contribute to a n improved marine environment and reduced potential for beach closures. The 26 th Street Parking Facility will be repaved with pervious concrete and new trees and landscaping wil l be planted.
The City commits to operate and maintain the Projec t in accordance with the Operation and Maintenance (O&M) Manual prepared and submitted as part of the Regional SCWP application process for the useful life of the Proj ect. The O&M Manual will be updated following construction based the parts and componen ts implemented. We understand the following major elements will require maintenance:
D
iversion structure and piping,
Pretreatment system,
Pump station,
Subsurface infiltration system (drywells and infiltration trench), and Flow meter/valves.
If you have any questions or require additional information, please contact Mamerto (Mo) Estepa, Senior Civil Engineer at (310) 802-5325 or [email protected] .
R
espectfully submitted,
Stephanie Katsouleas
Public Works Director
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix H
Preliminary Monitoring Plan
City of Manhattan Beach
28th Street Storm Drain Infiltration Project
Preliminary Monitoring Plan
Prepared for:
City of Manhattan Beach
1400 Highland Avenue
Manhattan Beach, CA 90266
TEL (310) 802-5000
Prepared by:
1561 E. Orangethorpe Avenue, Suite 240
Fullerton, California, 92831
TEL (714) 526-7500 | FAX (714) 526-7004 | www.cwecorp.com
September 2020
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
- 1 -
1. Background
The City of Manhattan Beach (City) is implementing the 28th Street Storm Drain Infiltration Project (Project) to improve water quality. The City participated with adjacent agencies, including the Los Angeles County Flood Control District (LACFCD) and the Cities of Hermosa Beach, Redondo Beach, and Torrance, in developing the Beach Cities Enhanced Watershed Management Program (EWMP). The EWMP identified capital improvement projects to help improve water quality and address applicable impairments in the Santa Monica Bay, specifically related to bacteria. The Beach Cities EWMP identified a beach infiltration system as a high priority capital project within the City. Figure 1-1 illustrates the Project area, City boundary, surrounding drainage areas, and the Beach Cities EWMP Group.
Figure 1-1 Surrounding Watersheds and Watershed Group
The Project’s drainage area is over 1,500 acres. Almost the entire area is within the City of Manhattan Beach, as illustrated in Figure 1-2. The Project captures 60% of the City’s area. The pollutants generated in the drainage area are mitigated by the Project, preventing them from reaching the beach and Santa Monica Bay.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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Figure 1-2 Project Drainage Area
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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2. Project Purpose and Goals
The Project is being implemented by the City to meet water quality goals as identified in the Beach Cities EWMP. The Project will address discharges that drain into the 28th Street Storm Drain and ultimately the Santa Monica Bay. Figure 2-1 illustrates the general concept of the Project. The Project will reduce pollutant loading to downstream water bodies by capturing wet- and dry-weather runoff. Captured runoff will be discharged into a subsurface system that will facilitate infiltration.
Figure 2-1 General Project Concept
The Project provides multiple benefits, which are summarized below:
➢ Improve water quality locally, on the beach, and in the Santa Monica Bay ▪ Reduce bacteria discharges from drainage system
▪ Reduce discharge of trash/debris from drainage system ➢ Enhance beach conditions
▪ Reduced debris from the upstream drainage system
▪ Reduce the potential for odors related to drainage system ➢ Reduce the potential for beach closures
➢ Create opportunities for education and outreach in local communities ▪ Inform the community of water quality challenges and strategies to improve it ▪ Install permanent educational signage
▪ Engage in dialogue with stakeholders and the public through community meetings ➢ Provide an enhanced environment for marine life
➢ Improve the 26th Street Parking Facility
Divert urban
runoff from storm
drain
Pretreat runoff
Pump runoff to
subsurface
infiltration system
Infiltrate runoff
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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3. Proposed Improvements
The City is using a phased approach to improve water quality as part of the Project. Phase 1 includes improvements at the 26th Street Parking Facility, while Phase 2 includes improvements on the beach.
Figure 3-1 illustrates the Project areas for each phase. The volume of water being captured under Phase 1 will be maximized to the extent practicable based on the capture goal, space constraints, and soil conditions. The City will assess improvements made to water quality through the implementation of Phase 1 to determine the specific area needed for Phase 2 implementation. In total, the two phases will ultimately capture flows from this drainage area illustrated above.
Figure 3-1 Concept Layout
3.1 Phase 1
Phase 1 improvements will be focused in the 26th Street Parking Facility, which is bound by Manhattan Avenue, Ocean Drive, 27th Street, and 26th Street, as shown in the figure above. Flows will be diverted from the storm drain on 28th Street (Bond Issue [BI] 0286) through a diversion pipe aligned southeast on Ocean Drive. Diverted runoff will be pretreated through a trash and sediment removal device and pumped to a matrix of drywells located in and around the parking facility. The storm drain is a reinforced concrete tunnel and was installed using tunneling methods. The system has a very mild slope and is approximately 40 feet deep near Manhattan Avenue. The diversion is located on Ocean Drive so that the
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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pretreatment unit and pump wet well will not require extensive excavation. The goal is to maxim ize capture at this location due to the high infiltration rates and the proximity to the beach outfall. A total of 48 drywells are anticipated based on spacing requirements. Captured flows will fill the drywells, which are six-foot diameter shafts, partially filled with gravel. Water will be able to infiltrate through the soils and into the groups, improving water quality. Figure 3-2 illustrates the preliminary drywell layout at the 26th Street Parking Facility.
Figure 3-2 Phase 1: 26th Street Parking Facility Drywell Concept
The exact location of each drywell will be determined during final design and will be placed to avoid existing utilities, retaining walls, and other onsite features. The parking lot will be repaved as part of the Project using pervious pavement, which will increase capture. Other parking lot upgrades may also be incorporated.
Up to 52.5 acre-feet of runoff is expected to be captured during a single storm event.
3.2 Phase 2
Following implementation of Phase 1, improvements in water quality will be monitored and assessed to identify the precise goals for Phase 2 implementation. Phase 2 may need to capture about 10.8 acre -feet of flow. Like Phase 1, a diversion, pretreatment system, and pump will be required to capture runoff from the existing storm drain on 28th Street (BI 0286). It is expected that at least part of the system
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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used for Phase 1 will also be used for Phase 2, which will reduce construction impacts and costs. Flows will be pumped into a buried infiltration trench on the beach, as is illustrated in Figure 3-3. The infiltration trench may be a pit filled with gravel and perforated pipes wrapped in a filter fabric or a prefabricated system. The system will provide void space to allow for water to be stored while infiltration occurs into the sand. The system will be buried under the surface and potential future impacts associated with sea level rise have been considered.
Figure 3-3 Phase 2: Beach Infiltration Concept
Up to 10.8 acre-feet of runoff is expected to be captured during a single storm event. In total, the two phases of the Project may result in capturing up to 63 acre-feet of runoff during a single storm event.
The capture of Phase 1 will be maximized, and improvements will be monitored following implementation to determine the need and sizing for Phase 2 improvements.
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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4. Project Benefits
The Project provides water quality benefits by reducing the pollutant loads reaching downstream receiving waters through the removal of flows from the system. Wet- and dry-weather runoff will be captured and infiltrated through a subsurface storage system (drywells and/or infiltration trench), which removes them, and their associated pollutant load, from the receiving water system. This benefit can be categorized as a volume and/or pollutant load removed from the system. While the Project is utilizing recharge, water supply benefits are not applicable, as the recharge is not within a drinking water aquifer due to the proximity to the coast. The benefits and metrics are summarized in Table 4-1 below.
Table 4-1 Summary of Project Benefits and Metrics
Benefit Metric Description
Water Quality
Acre-feet (AF)
Acre-feet per year (AFY)
Volume of polluted runoff captured/diverted
by Project.
Pounds (lbs)
Pounds per year (lbs/year)
Pollutant concentration multiplied by volume
capture = load removed.
The Project will also provide many community and environmental benefits, as summarized below:
➢ Reduce the potential for beach closures
➢ Engage in dialogue with stakeholders and the public through community meetings ➢ Install permanent educational signage to inform the community ➢ Provide an enhanced environment for marine life
➢ Improve the 26th Street Parking Facility
City of Manhattan Beach 28th Street Storm Drain Infiltration Project Preliminary Monitoring Plan
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5. Monitoring Approach
The City anticipates being awarded funding for the Project, which typically requires implementation of a monitoring program to determine if the benefits anticipated are provided by the Project, specifically in the initial time following implementation. The Project includes a flow meter downstream of the proposed diversion from the 28th Street Storm Drain. The flow meter will collect volumetric data that will become an integral part of quantifying Project benefits. The readings captured on the flow meter will be downloaded periodically to identify how much volume was captured and infiltrated. These readings will provide the water quality benefits in acre-feet (AF) or acre-feet per year (AFY).
If there is a need to quantify the pollutant load reduction provided by the Project, then the City will take grab samples within the diversion structures or pretreatment system to quantify the pollutant load of the system inflow. A monitoring plan will be developed to identify the grab sample procedures, exact location, and frequency. It is anticipated that bacteria loads will be quantified at a minimum in association with the Santa Monica Bay Bacteria Total Maximum Daily Load (TMDL). Results from water quality monitoring (concentrations) will be used with the volume capture readings from the flow meter to approximate the total pollutant load removed by the Project. The data collected with the flow meter and grab samples would be used to quantify the water quality benefit as pounds of pollutant removed or pounds of pollutant removed per year. If bacteria are being analyzed for, then the metrics will not be in pounds, but in Most Probable Number (MPN).
City of Manhattan Beach Subsurface Infiltration Trench and Alternatives Feasibility Study
Appendix I
LACFCD Connection Permit References
COUNTY OF LOS ANGELES
DEPARTMENT OF PUBLIC WORKS
"To Enrich Lives Through Effective and Caring Service"
GAIL FARBER, Director
900 SOUTH FREMONT AVENUE
ALHAMBRA, CALIFORNIA 91803-1331
Telephone: (626) 458-5100
http://dpw.lacounty.gov ADDRESS ALL CORRESPONDENCE TO:
P.O. BOX 1460
ALHAMBRA, CALIFORNIA 91802-1460
IN REPLY PLEASE
REFER TO FILE: SWP -~
October 14, 2020
Mamerto (Mo) Estepa Jr.
Senior Civil Engineer
City of Manhattan Beach
3621 Bell Avenue
Manhattan Beach, CA 90266
Dear Mr. Estepa:
NOTICE OF CONCEPTUAL APPROVAL FOR SAFE, CLEAN WATER PROGRAM
CONSIDERATION OF INFRASTRUCTURE PROJECTS FUNDING
Los Angeles County Flood Control District has been engaged to review the following project and is hereby providing conceptual approval:
28th Street Storm Drain Infiltration Project
City of Manhattan Beach
South Santa Monica Bay
We understand the proposed two-phase project will divert runoff from Project No. 286 (Manhattan Beach Storm Drain) for infiltration with the goal of reducing pollutant loading onto the beach and into the Santa Monica Bay. The District supports projects that are consistent with its goal of improving watershed health and promoting beneficial uses.
The Project is not currently inconsistent with any District plans, policies, or goals.
Conceptual approval for this Project does not indicate the District's consent to support or even permit the Project. If funding is ultimately allocated to the Project, it is required that the developer remain closely engaged with the District throughout each subsequent project phase and comply with any eventual applicable agreement and/or permit provisions. Please upload a copy of this letter in the Projects Module application when responding to the Regional Program Call for Projects.
Mr. Mamerto (Ma) Estepa Jr.
October 14, 202Q