San Marcos City Council
July 23, 2024

Appendix E
Drainage Study

Preliminary Drainage Study
for
Pacific
Prepared: -DQXDU\

PREPARED FOR
William Lundstrom, R.C.E.

Lundstrom Engineering & Surveying
333 &DPLQR'HO5LR6RXWK, Suite 
San Diego, California 92108
(619) 814-1220

Drainage Study
Page ii
Declaration of Responsible Charge
I hereby declare that I am the engineer of wo rk for this project. That I have exercised responsible charge over the design of the project as defined in Section 6703 of the business and professions code, and that the design is consistent with current standards.

I understand that the check of project draw ings and specifications by the City of San Marcos is confined to a review only and does not relieve me, as engineer of work, of my responsibilities for project design.

William Lundstrom
Registered Civil Engineer 61630
Exp. Date: 06/30/23
Date
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Page iii

Table of Contents
Declaration of Responsible Charge ................................................................................. ii Table of Exhibits .............................................................................................................. iii List of Tables .................................................................................................................... iv Table of Appendices ......................................................................................................... iv Introduction ........................................................................................................................1 Purpose and Scope .........................................................................................................1 Section 1. Property Information ................................................................................2

1.1. Property Description ..........................................................................................2

1.2. Hydrologic Setting .............................................................................................2

1.3. Proposed Runoff Management Facilities ...........................................................3 Section 2. Design Criteria and Methodology ............................................................4

2.1. Hydrologic Design Methodology ......................................................................4 Section 3. Characterization of Project Runoff ..........................................................8

3.1. Hydrologic Effects of Project ............................................................................8 Section 4. Summary and Conclusions .......................................................................9 Table of Exhibits
Exhibit A Vicinity Map
Exhibit B FEMA FIRM Panel.

Exhibit C Existing Condition Hydrology Map
Exhibit D Proposed Condition Hydrology Map

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Page iv

List of Tables
Table 2-1 Rational Method Runoff Coefficients. .........................................................5 Table 3-1 Summary of Hydrology Analysis . ...............................................................9

Table of Appendices
Appendix A Hydrologic Information
Appendix B Hydrologic Calculations

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Introduction
Purpose and Scope
The City’s application process requires a hydrology/ drainage study on all properties at the time of application. This study provides the needed information to ensure that any drainage facilities proposed in the future ar e sized and located appropriately and will accommodate any future development.

The study reviews storm runoff under existing conditions (100 year event) and identifies existing drainage problems that may be caused, or aggravated, by future development.

The study is further used to determine impacts that might be caused downstream (erosion) and to identify proposed mitigation measures.

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Section 1. Property Information

1.1.Property Description

1.1.1 Property Location
The subject property is located at the northwestern corner of Linda Vista Drive and Las Posas Road in the city of San Marcos, CA. The property is bounded to the southwest by Linda Vista Rd, to the southeast by Las Posas Rd., to the northeast by La Mirada Dr., and to the northwest by Pacific St. Exhibit A provides a location map for the site.

1.1.2 Property Activities Description
The project is the development of Pacific, located at the northwest corner of Las Posas Road and Linda Vista Drive, comprised of APN's 219-222-01, 219-222-02, 219-222-03, and 219-222-04. the project proposed the development of approximately 449 residential units, including a mix of five story podium apartments, three story rowhomes, three story villas, and four story affordable flats on approximately 17.9 acres within the 33.2 acre project site. The project proposes a general plan amendment, rezone, specific plan, tentative map, and multi-family site development plan.

1.2.Hydrologic Setting
This section summarizes the project’s size and location in the context of the larger watershed perspective, t opography, soil and vegetation c onditions, percent impervious area, natural and infrastructure drainage features, and other relevant hydrologic and environmental factors to be protected specific to the project area’s watershed.

1.2.1 Topography
The topography slopes southeasterly toward Las Posas Rd and Linda Vista Dr. Elevations on site range from 557.1 Mean Sea Level to 524.7 Mean Sea Level.

1.2.2 FEMA Flood Insurance Rate Map
The site is located in Zone X of th e Flood Insurance Rate Map (FIRM) Panel 06073C0789H. Zone X is designated to be areas determined to be outside the 500-year floodplain. Exhibit B illustrates the project site within Flood Zone X.

1.2.3 Current and Adjacent Land Use
The 33.2 acre property is currently undeveloped. Adjacent land use is varied with shopping centers, light industrial, and recreation.

1.2.4 Soil and Vegetation Conditions
The majority of the property contains type D soils per USDA soils site. See Appendix A Drainage Study

1.2.5 Existing Drainage Patterns and Facilities (Narrative) The majority of the site flows southeasterly toward the northwest corner of Linda Vista Drive and Las Posas Road. This drainage is collected in a CMP (corrugated meatal pipe) riser which drains to an 11’x7’ RCB (reinforced concrete box) in Las Posas Rd. The remainder of the site surface drains to the surrounding streets. All surrounding streets drain via gutter flow to the same corner (Las Posas and Linda Vista) where runoff is collected by a pair of curb inlets which drain into the same 11’x7’ RCB in Las Posas Rd.

It is noted that some on-site run-off occurs from the property onto adjacent streets. The run-off is carried via the streets to the same RCB in Las Posas Road. There is no offsite run-on to the property.

1.2.6 Downstream Conditions
All site runoff leads to the RCB in Las Posas Rd.

1.3.Proposed Runoff Management Facilities
Any future development facilities for managing runoff from the site will include one or more of the following (examples listed below or equal):
ƒ Biofiltration basins.
ƒ Underground Storage
Per the preliminary geotechnical evaluation by GeoTek, Inc. No. 3649-SD dated January 2022, infiltration rates are 0.07 inches per ho ur so infiltration is not recommended.

Treated runoff will follow the same drainage pattern as currently existing.

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Section 2. Design Criteria and Methodology
This section summarizes the design criteri a and methodology applied during drainage analysis of the property. The design cr iteria and methodology follow the County of San Diego County Hydrology Manual (June 2003), San Diego County Hydraulic Drainage Design Manual (September 2014) , and Storm Water Standard s as appropriate for the property location.

2.1.Hydrologic Design Methodology

2.1.1 Rational Method: Peak Flow
Runoff calculations for this study were ac complished using the Rational Method. The Rational Method is a physically-based numeric al method where runoff is assumed to be directly proportional to rainfa ll and area, less losses for infiltration and depression storage. Flows were computed based on the Rational formula:

QC i A
where ... Q = Peak discharge (cfs);
C = runoff coefficient, based on land use and
soil type;
i = rainfall intensity (in/hr);
A = watershed area (acre)

The runoff coefficient represents the ratio of rainfall that runs off the watershed versus the portion that infiltrates to the soil or is held in depression storage. The runoff coefficient is dependent on the land use coverage and soil type.

For a typical drainage study, rainfall intens ity varies with the watershed time of concentration. The watershed time of concentr ation at any given point is defined as the time it would theoretically take runoff to travel from the most upstream point in the watershed to a concentration point, as calcu lated by equations in the San Diego County Hydrology Manual.

Drainage Study

Table 2-1 Rational Method Runoff Coefficients.

RUNOFF COEFFICIENT
(%) Hydrologic Soil Type
LAND USE (County Elements) Imperv. A B C D
Permanent Open Space 0.20 0.25 0.30 0.35
Residential, 1.0 DU/A or less 10 0.27 0.32 0.36 0.41
Residential, 2.0 DU/A or less 20 0.34 0.38 0.42 0.46
Residential, 2.9 DU/A or less 25 0.38 0.41 0.45 0.49
Residential, 4.3 DU/A or less 30 0.41 0.45 0.48 0.52
Residential, 7.3 DU/A or less 40 0.48 0.51 0.54 0.57
Residential, 10.9 DU/A or less 45 0.52 0.54 0.57 0.60
Residential, 14.5 DU/A or less 50 0.55 0.58 0.60 0.63
Residential, 24.0 DU/A or less 65 0.66 0.67 0.69 0.71
Residential, 43.0 DU/A or less 80 0.76 0.77 0.78 0.79
Neighborhood Commercial 80 0.76 0.77 0.78 0.79
General Commercial 85 0.80 0.80 0.81 0.82
Office Professional/Commercial 90 0.83 0.84 0.84 0.85
Limited Industrial 90 0.83 0.84 0.84 0.85
General Industrial 95 0.87 0.87 0.87 0.87

Rational Method calculations were accomp lished using the Advanced Engineering Software Rational Method Analysis (Sout hern California County Methods) (AES- RATSCx) computer software packages. Peak discharges were computed for 100-year and 50-year storm return frequencies.

2.1.2 Time of Concentration
The Time of Concentration (T
c) is the time required for runoff to flow from the most remote part of the drainage area to the point of interest. The T c is composed of two components: initial time of concentration (Ti) and the travel time (T t). The Ti is the time required for runoff to travel across the surface of the most remote subarea in the study, or “initial subarea”. Guidelines for designation the initial subarea are provided within the discussion of computation of T
i. The T i is the time required for the runoff to flow in a watercourse (e.g., swale, channel, gutter, pipe) or series of watercourses from the initial subarea to the point of interest . For the Rational Method, the T c at any point within the drainage area is given by:

Tc = Ti + Tt
Methods of calculation differ for natural watersheds (nonurbanized) and for urban drainage systems. When analyzing storm dr ain systems, the designer must consider the possibility that an existing natural watershed may become urbanized during the useful life of the storm drain system. Future land uses must be used for T c and runoff calculations,
and can be determined from the local Community General Plan.

Drainage Study

2.1.3 Initial Time of Concentration
The initial time of concentration is typically based on sheet flow at the upstream end of a drainage basin. The Overland Time of Flow is approximated by an equation developed by the Federal Aviation Agency (FAA) for analyzing flow on runways (FAA, 1970). The usual runway configuration consists of a crown, like most freeways, with sloping pavement that directs flow to either side of the runway. This type of flow is uniform in the direction perpendicular to the velocity and is very shallow. Since these depths are ¼ of an inch in magnitude, the relative roughne ss is high. Some higher relative roughness values for overland flow are presented in the HEC-1 Flood Hydrograph Package User’s Manual (USACE, 1990).

The sheet flow that is predicted by the F AA equation is limited to conditions that are similar to runway topography. Some considerations that limit the extent to which the FAA equation applies are identified below:
™ Urban Areas – This “runway type” runoff includes:
o Flat roofs, sloping at 1% +/-
o Parking lots at the extreme upstr eam drainage basin boundary (at the “ridge” of a catchment area.) Even a parking lot is limited in the amounts of sheet flow. Parked or moving ve hicles would “break-up” the sheet flow, concentrating runoff into streams that are not characteristic of sheet flow.
o Driveways are constructed at the upstream end of catchment areas in some developments. However, if flow from a roof is directed to a driveway through a downspout or other conveya nce mechanism, flow would be concentrated.
o Flat slopes are prone to meandering fl ow that tends to be disrupted by minor irregularities and obstructions. Maximum Overland Flow lengths are shorter for the flatter slopes.
™ Rural or Natural Areas –The FAA equation is applicable to these conditions since (0.5% to 10%) slopes that are uniform in width of flow have slow velocities consistent with the equation. Irregul arities in terrain limit the length of application.
o Most hills and ridge lines have a re latively flat area near the drainage divide. However, with flat slopes of 0.5% +/-, minor irregularities would cause flow to concentrate into streams.
o Parks, lawns and other vegetated areas would have slow velocities that are consistent with the FAA Equation.

Drainage Study

The Initial Time of Concentration is reflective of the general land-use at the upstream end of a drainage basin.

2.1.4 Travel Time
The T t is the time required for the runoff to flow in a watercourse or series of watercourses from the initial subarea to the point of interest. The T t is computed by dividing the length of the flow path by the co mputed flow velocity. Since the velocity normally changes as a result of each change in flow rate or slope, such as at an inlet or grade break, the total T t must be computed as the sum of the T t’s for each section of the flow path.

2.1.5 Rational Method: Runoff Volume
For designs that are dependent on the to tal storm volume, a hydrograph must be generated to account for the entire volume of runoff from the 6-hour storm event. The hydrograph for the entire 6-hour storm ev ent is generated by creating a rainfall distribution consisting of blocks of rain, creating an incremental hydrograph for each block of rain, and adding the hydrographs from each block of rain. This process creates a hydrograph that contains runoff from all the bl ocks of rain and accounts for the entire volume of runoff from the 6-hour storm even t. The total volume under the resulting hydrograph is equal to the following equation:

VOL = CP6A
Where: VOL = volume of runoff (acres-inches)
P 6 = 6-hour rainfall (inches)
C = runoff coefficient
A = area of the watershed (acres)
Drainage Study

Section 3. Characterization of Project Runoff

3.1.Hydrologic Effects of Project
Any future development will be designed such that it will not significantly alter drainage patterns on the site. Table 3-1 summarizes the hydrologic effects of the existing site.

Table 3-1 Summary of Hydrology Analysis.

EEXISTTINNG
NODE TC (MIN.) AREA
(ACRES) C I100
(in/hr)
Q100 RUN-OFF
(CFS)
DMA1 18.7 0.33 0.35 3.71 0.43
DMA2 18.7 0.70 0.35 3.71 0.91
DMA3 18.7 0.78 0.35 3.71 1.01
DMA4 18.7 4.32 0.35 3.71 5.61
DMA5 18.7 2.43 0.35 3.71 3.20
DMA6 18.7 24.05 0.35 3.71 31.36
DMA7 18.7 0.53 0.35 3.71 0.70
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PPROPOSED
NODE TC (MIN.) AREA
(ACRES) C I100
(in/hr) Q100 RUN-OFF (CFS)
DMA 1-3 10.7 13.3 0.79 5.74 60.3 (7.5 mitigated)
DMA 4 6.4 1.8 0.79 7.41 10.5 (0.9 mitigated)
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Section 4. Summary and Conclusions
This hydrology and hydraulic study has evaluated the potential e ffects of rainfall, runoff, and drainage on the property. In addition, the report has addressed the methodology used to analyze the pre-construction and to the parameters for post-c onstruction condition, which was based on the San Diego County H ydrology and Design Manual. This section provides a summary discussion that evaluates the potential effects of any future proposed project.
™ The proposed project will not substantially alter the existing drainage patterns on the site or area, including through the alteration of the existing drainage course, in which would not result in substantial erosion or siltation on- or off-site and not exceed the capacity of downstream storm drain.
™ The proposed project does not place housi ng or structures within 100-year flood area in which would impede or redirect flows.
™ The project will add new impervious area to the site, increasing unmitigated storm water runoff rates and volume from the existing condition. Proposed biofiltration basins and detention storage are sized to mitigate peak 100 year runoff rates. ™ In my professional opinion, the proposed wo rk and improvements, as they relate to this project, will not increase the flow rates or velocity of surface flows to the detriment of downstream landowners and/or facilities.

Drainage Study

THIS PAGE BUILDS TABLE OF APPENDICES AND EXHIBITS

DO NOT DELETE THIS PAGE

DO NOT INCLUDE THIS PAGE WITH PUBLISHED DOCUMENT

TABLE OF EXHIBITS

Exhibit A Vicinity Map
Exhibit B FEMA FIRM Panel.

Exhibit C Existing Condition Hydrology Map
Exhibit D Proposed Condition Hydrology Map

Drainage Study

EXHIBITS

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Exhibit B
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Exhibit C
Existing Condition Hydrology Map

Exhibit D

APPENDIX A
Hydrologic Information
This Section Contains:
• Precipitation Analysis