5.3. Existing Condition (2010) Results
5.3.1. Hydraulic Model Information
The Existing Condition sewer system is modeled using the Wastewater Capacity and Alignment Study El Camino Real & San Antonio Change Areas Project 14-48 (West Yost, 2017). Project sewer flow is assumed to discharge into the 8-inch diameter public sewer main in California Street
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5.3.2. Peak Wet Weather Flow (PWWF) Scenario – Pre and Post Project The sewer system does not have sufficient capacity downstream of the Project with the pre-Project or post- Project flows in the Existing Condition, as shown in Figure 10.
5.3.3. Deficiencies – Pre and Post Project
Existing Condition model results comparing pre- and post-Project d/D are presented in Table 5-4. In pre-Project conditions, there are deficient pipes downstream of the Project along Pachetti Way, Sondgroth Way, and San Antonio Circle. Several of the deficient pipes are surcharging (d/D ≥ 1). The Project adds flow to these deficient pipes, but does not increase the number of deficient pipes. The Project also increases the deficiency in existing deficient pipes along Pachetti Way, but does not result in an increase in the number of deficient pipes.
5.4. Future Cumulative Condition (2030) Results
5.4.1. Hydraulic Model Information
The Future Cumulative Condition model is created based on the West Yost (2017) model assuming the CIPs recommended in the West Yost (2017) report have been constructed per the Showers Drive Sanitary Sewer Improvement Project 35% Design Submittal (2017), which includes the relocation of the Alma Recorder further north of Alma Street. CIP#32 under the 2030 General Plan Update Utility Impact Study (2030 GPUUIS; IEC, October 2011) is assumed to be constructed for the Future Cumulative Condition. Previous UIS studies conducted by Schaaf & Wheeler within the San Antonio Precise Plan (SAPP) indicate that this CIP is still required to meet d/D performance Criteria for the FCC sewer flows. Project sewer flow is assumed to discharge into the 8-inch diameter public main in California Street.
5.4.2. Peak Wet Weather Flow (PWWF) Scenario – Pre and Post Project The system does meet d/D performance criteria downstream of the Project in the Future Cumulative Condition pre-Project with the CIPs recommended in the West Yost (2017) report and with CIP #32 from the GPUUIS.
The system does not meet d/D performance criteria under post-Project conditions for one pipe, the deficient pipe corresponding Pipe ID is 3737.
5.4.3. Deficiencies – Pre and Post Project
Table 5-4 presents the comparison of d/D criteria pre- and post-Project for pipes downstream of the Project development in the Future Cumulative Condition. The West Yost (2017) report identified several sewer mains along the flow path to be upsized, including pipes along San Antonio Circle and crossing Central Expressway.
An additional pipe (Pipe ID 3746) from CIP #32 of the GPUUIS has been recommended for upsizing to meet d/D requirements under pre-Project conditions. West Yost (2017) recommended pipe diameters are given in bold purple font in Table 5-5, the recommended pipe diameter for CIP #32 is given in bold green font.
Under post-Project conditions, one newly deficient pipe does not meet d/D performance criteria in the Future Cumulative Condition. Schaaf & Wheeler recommends upsizing Pipe ID 3737 from an 8-inch diameter to a 10- inch diameter pipe to accommodate additional sewer flows. This recommended CIP is identified as SW_SAPP01 in Table 5-6.
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5.4.4. Alma Recorder Contractual Limit
The Project is located in the Alma Recorder tributary area; flow from this area is contractually limited to two million gallons per day of PWWF. In the Existing Condition, flow to the Alma Recorder during PWWF is 14% and 13% below the contractual limit for pre-Project and post-Project respectively. In the Future Cumulative Condition, flow to the Alma Recorder is 8% below the contractual limit both pre- and post-Project.
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Table 5-4: Existing Condition Model Results – Pre and Post Project Sewer
Main
ID
Upstream
MH ID
Downstream
MH ID
Existing
Diameter
(in)
Length
(ft)
Slope
(%)
ADWF PWWF
Pre-Project Post-Project Pre-Project Post-Project
Max
Flow
(MGD)
d/D
Max
Flow
(MGD)
d/D
Max
Flow
(MGD)
d/D Max Flow
(MGD) d/D
Pipe
Capacity
Remaining
(% of
Allowed
d/D)
821 F1-056 F1-101 8 298 0.351 0.069 0.2587 0.087 0.2889 0.106 0.3209 0.132 0.3598 28 868 F1-101 F1-017 8 309 0.461 0.073 0.3498 0.090 0.3723 0.111 0.4218 0.137 0.4512 10 867 F1-017 F2-058 8 263 0.694 0.247 0.3787 0.264 0.3925 0.339 0.4531 0.364 0.4718 6 795 F2-058 F2-104 8 50 2.423 0.252 0.3142 0.269 0.3251 0.351 0.3768 0.376 0.3918 22 785 F2-104 F2-272 8 19 2.183 0.255 0.3826 0.272 0.3961 0.355 0.4622 0.380 0.4812 4 3737 F2-272 F2-284 8 108 0.694 0.257 0.3980 0.274 0.4120 0.360 0.4788 0.385 0.4976 0 754 F2-284 F2-282 8 96 2.011 0.258 0.2839 0.275 0.2934 0.361 0.3382 0.386 0.3505 30 3744 F2-282 F2-276 8 25 10.164 0.258 0.2734 0.275 0.2879 0.362 0.6288 0.387 0.6333 0 725 F2-276 F1-034 10 269 0.298 0.709 0.6813 0.726 0.6935 0.981 1.0000 1.003 1.0000 0 686 F1-034 F1-028 10 259 0.367 0.718 0.6813 0.735 0.6926 0.995 1.0000 1.017 1.0000 0 659 F1-110 F1-108 10 23 0.516 0.736 0.6447 0.753 0.6595 1.024 1.0000 1.046 1.0000 0 3717 F1-028 F1-110 10 17 0.006 0.736 0.6386 0.753 0.6462 1.023 1.0000 1.045 1.0000 0 3746 F1-108 F1-018 10 261 0.307 0.737 0.7867 0.754 0.8041 1.025 1.0000 1.047 1.0000 0 618 F1-018 F1-012 10 312 0.337 0.767 0.7284 0.784 0.7400 1.083 0.8993 1.105 0.9024 0 580 F1-012 E1-033 15 108 0.554 1.238 0.4832 1.238 0.4873 1.724 0.5775 1.746 0.5919 21 570 E1-033 E1-037 15 13 0.171 1.487 0.3671 1.508 0.3701 2.493 0.4405 2.556 0.4497 40 567 E1-037 E1-035 15 23 10.448 1.223 0.2036 1.241 0.2051 1.846 0.2534 1.868 0.2547 66 564 E1-035 SPS 30 38 20.814 1.224 0.0711 1.241 0.0716 1.864 0.0871 1.881 0.0874 88
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Table 5-5: Future Cumulative Condition Model Results – Pre and Post Project Sewer
Main
ID
Upstrea
m MH
ID
Downstream
MH ID
Model
Diameter
(in)
Length
(ft)
Slope
(%)
ADWF PWWF
Pre-Project Post-Project Pre-Project Post-Project
Max Flow
(MGD) d/D
Max
Flow
(MGD)
d/D
Max
Flow
(MGD)
d/D Max Flow
(MGD) d/D
Pipe
Capacity
Remaining
(% of
Allowed
d/D)
821 F1-056 F1-101 8 298 0.351 0.069 0.2587 0.087 0.2887 0.106 0.3216 0.132 0.3602 28 868 F1-101 F1-017 8 309 0.461 0.073 0.3505 0.090 0.3729 0.112 0.4240 0.138 0.4532 9 867 F1-017 F2-058 8 263 0.694 0.249 0.3814 0.265 0.3951 0.344 0.4577 0.368 0.4762 5 754 F2-284 F2-282 8 96 2.011 0.265 0.2877 0.281 0.2970 0.372 0.3437 0.397 0.3558 29 3737 F2-272 F2-284 8/10 108 0.694 0.264 0.4034 0.281 0.4171 0.370 0.4869 0.395 0.5055/
0.3645 0/ 14
785 F2-104 F2-272 8 19 2.183 0.261 0.3877 0.278 0.4010 0.366 0.4703 0.391 0.4892 2 795 F2-058 F2-104 8 50 2.423 0.258 0.3182 0.275 0.3289 0.362 0.3831 0.387 0.3979 20 3744 F2-282 F2-276 8 25 10.164 0.265 0.2204 0.282 0.2273 0.373 0.2617 0.398 0.2705 46 725 F2-276 F1-034 10 269 0.372 0.288 0.4029 0.297 0.4095 0.388 0.4774 0.400 0.4858 3 686 F1-034 F1-028 10 259 0.367 0.300 0.4106 0.309 0.4169 0.407 0.4857 0.419 0.4936 1 659 F1-110 F1-108 12 23 0.516 0.319 0.3572 0.328 0.3616 0.436 0.4126 0.449 0.4179 16 3717 F1-028 F1-110 10 17 0.006 0.319 0.4204 0.327 0.4259 0.435 0.4897 0.447 0.4966 1 3746 F1-108 F1-018 15 261 0.384 0.319 0.3153 0.328 0.3193 0.438 0.3788 0.450 0.3838 49 618 F1-018 F1-012 15 312 0.337 0.792 0.4394 0.809 0.4436 1.126 0.5430 1.151 0.5489 27 580 F1-012 E1-033 15 108 0.433 1.297 0.5159 1.314 0.5200 1.846 0.6532 1.859 0.6567 12 WY-01 E1-033 E1-037 15 13 0.333 1.298 0.5518 1.315 0.5562 1.848 0.7001 1.861 0.7039 6 WY-02 E1-037 E1-035 15 23 0.301 1.299 0.4795 1.315 0.4832 1.849 0.5981 1.862 0.6010 20 WY-03 JCT-12 E1-035 15 16 0.500 1.299 0.4538 1.315 0.4569 1.849 0.5477 1.862 0.5498 27 WY-04 JCT-14 JCT-12 15 214 0.836 1.299 0.4090 1.315 0.4120 1.849 0.5015 1.862 0.5036 33 WY-05 E1-035 SPS 30 38 17.323 1.299 0.0764 1.316 0.0768 13.849 0.2397 13.863 0.2399 68 Notes: Pipe diameter in purple denotes West Yost CIP, pipe diameter in green denotes CIP #32 from the GPUUIS, pipe diameter in blue denotes new Project specific Schaaf & Wheeler CIP
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Table 5-6 Pipes Recommended for Upsizing and Percentage of Contributed Flow CIP
Sewer
Main
ID
Upstream
MH ID
Downstream
MH ID
Existing
Diameter
(in)
Proposed
Diameter
(in)
Total
Future
Cumulative
ADWF
Flow With
Project
(MGD)
Project Incremental
Contribution
City of Mountain View
Contribution
ADWF Flow (MGD)
Percentage
of Total
Flow (%)
ADWF Flow
(MGD)
Percentage
of Total
Flow (%)
SW_SAPP01 3737 F2-272 F2-284 8 10 0.281 0.017 6 0.264 94 CIP #32 3746 F1-108 F1-018 10 15 0.328 0.009 3 0.319 97 West Yost CIP
659 F1-110 F1-108 10 12 0.328 0.009 3 0.319 97
618 F1-018 F1-012 10 15 0.809 0.017 2 0.792 98
580 F1-012 E1-033 15 15 1.314 0.017 1 1.297 99
WY-01 E1-033 E1-037 #N/A 15 1.315 0.017 1 1.298 99
WY-02 E1-037 E1-035 #N/A 15 1.315 0.017 1 1.299 99
WY-03 JCT-12 E1-035 #N/A 15 1.315 0.017 1 1.299 99
WY-04 JCT-14 JCT-12 #N/A 15 1.315 0.017 1 1.299 99
WY-05 E1-035 SPS #N/A 30 1.316 0.017 1 1.299 99
Notes: Pipe diameter in purple denotes West Yost CIP, pipe diameter in green denotes CIP #32 from the GPUUIS, pipe diameter in blue denotes new Project specific Schaaf & Wheeler CIP
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Chapter 6. Storm System Impact
The storm drain system analysis for Project impact is based on the MIKE URBAN (MU) model developed as part of the City of Mountain View Storm Drain Master Plan (SDMP; Schaaf & Wheeler, 2019), which analyzed existing 12-inch and larger diameter storm drainage pipe. The Project site is within the Adobe Creek watershed, discharging to Adobe Creek near Alma Street. The Project is assumed to maintain the existing drainage direction by connecting to the existing 30-inch storm drain pipe along California Street. The Project’s impact is evaluated for the 10-year design storm.
6.1. Impervious Area Change and Hydrologic/Hydraulic Change The existing site impervious percentage is 79%, the Project development will increase the impervious percentage to 95% based on the Project Description provided March 11, 2021. This increase in impervious area results in an increase in peak runoff; the estimated incremental increase in flow to the existing storm drain network due to Project development is 0.09 cfs, or approximately 3% of the existing runoff peak flow. Model results show the hydraulic grade line at the connection point to the City’s storm drain system increases by 0.01 ft with Project development.
6.2. Project Contribution to Existing Deficiencies
The SDMP identifies the California Street and San Antonio storm drain system as having sufficient capacity to convey site runoff. However, there are existing drainage deficiencies downstream of the Project. The existi ng
6.5-foot diameter trunk line connecting the San Antonio Road storm drain trunk line to Adobe Creek does not have the capacity to convey the 10 -year runoff when creek levels are high. The existing line currently operates in a surcharged condition in the hydraulic model at various points along its length. Additional conveyance capacity is required to convey existing runoff and maintain the 10-year service level. The SDMP recommends a Capital Improvement Project (CIP) that includes upsizing the 6.5-foot diameter pipe to an 8-foot diameter pipe or adding a parallel line which provides the equivalent capacity.
The Project is not anticipated to have a significant impact on the existing storm drain network or result in additional deficiencies downstream of the Project. With Project development, the peak runoff is anticipated to increase by approximately 0.03 cfs, or approximately 3% of the existing runoff peak flow. Additionally, model results indicate that the hydraulic grade line at the connection point to the City’s storm drain system increases by 0.01 ft with Project development. At existing surcharge locations downstream of the Project, surcharge levels remain approximately equal to the pre-Project levels (i.e., less than 0.01 ft different).
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APPENDIX A:
Additional Considered Projects
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Table A-1: Additional Considered Projects
ID Address Project Description Status
29 2645 - 2655 Fayette Drive Mixed-use; 44-unit condominium building and a 6,900 square foot commercial building Approved
35 369 North Whisman Road Two office buildings totally 180,773 square feet Approved 36 1411 – 1495 West El Camino Real 53-unit condominium building Approved 37 701 West Evelyn Avenue commercial building; 6,500 square feet retail and 28,090 square feet of office Approved
38 851 - 853 Sierra Vista Avenue 9 unit rowhouse development Approved 39 1958 Latham Street 6 unit rowhouse development Approved 40 231 - 235 Hope Street 8 unit condo Approved
44 1555 West Middlefield Road 115-unit rowhouse development Approved 45 410 - 414 Sierra Vista Avenue 14-unit rowhouse development Approved 46 2019 Leghorn Street 12,050 square-foot office building Approved 47 855 - 1023 West El Camino Real mixed-use 60-bed senior memory care facility with 5,883 square feet of ground floor retail and restaurant uses Approved 49 2300 West El Camino Real 75,824 square foot, 153-room hotel Approved 50 1313 and 1347 West El Camino Real mixed-use project with 24 apartment units above ground floor commercial space Approved
52 676 West Dana Street
20,166 square foot mixed-use project with three floors of residential condominiums, 4,791 square feet of ground-
floor commercial
Approved
53 828 and 836 Sierra Vista Avenue 20 unit rowhouse development Approved 54 2580 and 2590 California Street / 201 San
Antonio Circle
632 residential units and 20,000 square feet of commercial space Under Construction
56 257 - 279 Calderon Avenue 16-unit rowhouse development Under Construction 58 2044 and 2054 Montecito Avenue 52-unit rowhouse development Under Construction Source: ICF
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Table A-1: Additional Considered Projects (Continued)
ID Address Project Description Status
59 2700 West El Camino Real 211 apartment units and 2,000 square feet of ground floor commercial Under Construction
60 858 Sierra Vista Avenue 4 small-lot, single-family homes Under Construction 64 1045 - 1085 La Avenida
Renovate existing office buildings and construct an
additional office building. The net new square footage is 128,000 square feet added to the existing 515,000 square feet of office space
Under Construction
67 315 and 319 Sierra Vista Avenue 15-unit rowhouse development Under Construction 70 1950 Montecito Avenue 33-unit rowhouse development Under Construction 71 950 West El Camino Real 71-unit affordable studio apartment development Under Construction 72 1720 Villa Street 226-unit apartment complex Under Construction 73 2005 Rock Street 8 rowhouse units and 7 townhome units Under Construction 74 2310 Rock Street 55-unit rowhouse development Under Construction 75 982 Bonita Avenue 8 condominium units Under Construction 76 250 San Antonio Circle 3,350 square foot building for the Community School of Music and Arts Under Construction
Source: ICF
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APPENDIX B:
Figures
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