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Resolution No. 2023-19 | Storm Drainage Master Plan | Adopted 04/03/2023
RESOLUTION NO. 2023-19
A RESOLUTION OF THE CITY COUNCIL OF THE CITY OF
GILROY ADOPTING THE STORM DRAINAGE SYSTEM
MASTER PLAN FOR THE CITY OF GILROY
WHEREAS, the 2022 Storm Drainage System Master (Plan) serves as a guide to
assess the current operations and functionality of the City’s existing s torm drain system;
and
WHEREAS, the City last developed a Storm Drainage System Master Plan in
2004, which identified capacity deficiencies in the existing storm drain system and
recommended improvements to alleviate existing deficiencies and serve future
developments in the Gilroy Planning Area; and
WHEREAS, Akel Engineering Group, Inc. was retained by the City Council in
August 2019 to prepare the Plan; and
WHEREAS, the objective of the Plan is to review and make recommendations on
how the current sewer system can be upgraded within the City to best suit the sewer
needs of the City in the future; and
WHEREAS, the projects identified in the Plan will be added to the City’s Capital
Improvement Program; and
NOW, THEREFORE, BE IT RESOLVED that the City Council of the City of Gilroy
hereby adopts the Storm Drain System Master Plan attached hereto and made a part
hereof.
PASSED AND ADOPTED by the City Council of the City of Gilroy at a regular
meeting duly held on the 3rd day of April 2023 by the following roll call vote:
AYES: COUNCIL MEMBERS: ARMENDARIZ, BRACCO, CLINE,
MARQUES, TOVAR, BLANKLEY
NOES: COUNCIL MEMBERS: NONE
ABSTAIN: COUNCIL MEMBERS: NONE
ABSENT: COUNCIL MEMBERS: HILTON
APPROVED:
Marie Blankley, Mayor
ATTEST:
_______________________
Thai Nam Pham, City Clerk
Marie Blankley (Apr 4, 2023 14:49 PDT)
Marie Blankley
2023 Storm DrainageSystem Master Plan
City of Gilroy
DRAF T
CITY OF GILROY
2023
STORM DRAINAGE
SYSTEM
MASTER PLAN
Final
March 2023
74 3 3 N. F I RS T S T RE E T , S UI T E 1 0 3 • F RE S NO , CA LI F O RN I A 9 37 2 0 • ( 5 59) 43 6 -06 0 0 • F A X ( 55 9) 4 3 6 -0622
www.akeleng.com
Smart Planning Our Water Resources
March 29, 2023
City of Gilroy
7351 Rosanna Street
Gilroy, CA 95020
Attention: Gary Heap, P.E.
City Engineer
Subject: 2023 Storm Drainage System Master Plan – Final Report
Dear Gary:
We are pleased to submit the draft report for the City of Gilroy Storm Drainage System Master
Plan. This master plan is a standalone document, though it was prepared as part of the
integrated infrastructure master plans for the water, sewer, and storm drainage master plans.
The master plan documents the following:
•Existing system facilities, acceptable hydrologic and hydraulic performance criteria, and
projected stormwater runoff consistent with the Urban Planning Area
•Development of the City’s GIS-based hydrologic and hydraulic stormwater models.
•Capacity evaluation of the existing system with improvements to mitigate existing
deficiencies and to accommodate future growth.
•Capital improvement program (CIP) with an opinion of probable construction costs and
suggestions for cost allocations to meet AB 1600.
We extend our thanks to you, Daryl Jordan, Director of Public Works and other City staff whose
courtesy and cooperation were valuable components in completing this study.
Sincerely,
AKEL ENGINEERING GROUP, INC.
Tony Akel, P.E., D. WRE
President
Enclosure: Report
Acknowledgements
City Council
Marie Blankley, Mayor
Dion Bracco , Mayor Pro Tempore
Rebeca Armendariz
Tom Cline
Zach Hilton
Carol Marques
Fred Tovar
Management Personnel
Jimmy Forbis, City Administrator
Daryl Jordan, P.E., Director of Public Works
Gary Heap, P.E., City Engineer
Jorge Duran, P.E., Senior City Engineer
Sharon Goei, Community Development Director
Matt Jones, Deputy Director of Public Works
Other City Engineering, Planning, and Operations Staff
City of Gilroy
Storm Drainage System Master Plan
TABLE OF CONTENTS PAGE NO.
March 2023 i City of Gilroy
Storm Drainage System Master Plan
0.0 EXECUTIVE SUMMARY ................................................................................................. ES-1
ES.1 STUDY OBJECTIVES ..................................................................................... ES-1
ES.2 INTEGRATED APPROACH TO MASTER PLANNING .................................... ES-1
ES.3 STUDY AREA DESCRIPTION......................................................................... ES-2
ES.4 SYSTEM PERFORMANCE AND DESIGN CRITERIA ..................................... ES-2
ES.5 HYDROLOGIC AND HYDRAULIC MODEL DEVELOPMENT.......................... ES-6
ES.6 CAPACITY EVALUATION ............................................................................... ES-6
ES.7 CAPITAL IMPROVEMENT PROGRAM ........................................................... ES-9
1.0 CHAPTER 1 - INTRODUCTION ...................................................................................... 1-1
1.1 BACKGROUND ............................................................................................... 1-1
1.2 SCOPE OF WORK .......................................................................................... 1-1
1.3 INTEGRATED APPROACH TO MASTER PLANNING .................................... 1-3
1.4 PREVIOUS MASTER PLANS .......................................................................... 1-3
1.5 RELEVANT REPORTS ................................................................................... 1-3
1.6 REPORT ORGANIZATION .............................................................................. 1-4
1.7 ACKNOWLEDGEMENTS ................................................................................ 1-4
1.8 UNIT CONVERSIONS AND ABBREVIATIONS ............................................... 1-5
1.9 GEOGRAPHIC INFORMATION SYSTEMS ..................................................... 1-5
2.0 CHAPTER 2 – PLANNING AREA CHARACTERISTICS ................................................ 2-1
2.1 STUDY AREA DESCRIPTION......................................................................... 2-1
2.2 WATERSHEDS AND DRAINAGE AREAS ...................................................... 2-1
2.2.1 Watersheds ....................................................................................... 2-3
2.2.2 Drainage Areas .................................................................................. 2-3
2.3 FLOODPLAINS ............................................................................................... 2-3
2.4 EXISTING SERVICE AREAS AND LAND USE ............................................... 2-6
2.5 HISTORICAL AND PROJECTED POPULATION............................................. 2-6
3.0 CHAPTER 3 – SYSTEM PERFORMANCE AND DESIGN CRITERIA ............................. 3-1
3.1 HYDROLOGIC CRITERIA ............................................................................... 3-1
3.1.1 Precipitation Characteristics .............................................................. 3-1
3.1.2 Design Storms ................................................................................... 3-1
3.1.3 Design Storm Events ......................................................................... 3-4
3.1.4 Soil Imperviousness........................................................................... 3-4
3.2 HYDRAULIC CRITERIA .................................................................................. 3-8
3.2.1 Gravity Conveyance Facilities ............................................................ 3-8
4.0 CHAPTER 4 – EXISTING FACILITIES AND MODEL DEVELOPMENT .......................... 4-1
4.1 HYDROLOGIC SYSTEM OVERVIEW ............................................................. 4-1
4.1.1 Watersheds ....................................................................................... 4-1
4.1.2 Drainage Areas .................................................................................. 4-1
4.1.3 Drainage Sub-catchment ................................................................... 4-3
4.2 HYDRAULIC SYSTEM OVERVIEW ................................................................ 4-4
4.2.1 Conveyance System .......................................................................... 4-4
4.2.2 Outfalls .............................................................................................. 4-4
4.3 MODEL DEVELOPMENT ................................................................................ 4-8
4.3.1 Modeling Software ............................................................................. 4-8
City of Gilroy
Storm Drainage System Master Plan
TABLE OF CONTENTS PAGE NO.
March 2023 ii City of Gilroy
Storm Drainage System Master Plan
4.3.2 Land Use and Routing Methodology .................................................. 4-8
4.3.3 Hydrological Soil Classification .......................................................... 4-9
4.3.4 Typical Channel Cross-Section .......................................................... 4-9
4.3.5 Physical Hydraulic Model Elements ................................................... 4-9
5.0 CHAPTER 5 – EVALUATION AND PROPOSED IMPROVEMENTS .............................. 5-1
5.1 OVERVIEW ..................................................................................................... 5-1
5.2 KNOWN AREAS OF CONCERN ..................................................................... 5-1
5.3 PIPELINE CONVEYANCE CAPACITY EVALUATION ..................................... 5-1
5.4 CAPACITY EVALUATION ANALYSIS ............................................................. 5-1
5.4.1 Alternative 1 - 8th Street 36-inch Pipeline Improvements .................... 5-4
5.4.2 Alternative 2 – Diverting Stormwater Runoff to Princevalle Channel .. 5-4
5.4.3 Ronan Channel Drainage Area .......................................................... 5-9
5.4.4 Miller Slough Drainage Area .............................................................. 5-9
5.4.5 North Uvas Drainage Area ................................................................. 5-10
5.4.6 South Uvas Drainage Area ................................................................ 5-10
5.4.7 Princevalle Drainage Area ................................................................. 5-11
5.4.8 Lower Miller Slough Drainage Area ................................................... 5-12
5.5 OUTFALLS ...................................................................................................... 5-12
6.0 CHAPTER 6 - CAPITAL IMPROVEMENT PROGRAM ................................................... 6-1
6.1 COST ESTIMATE ACCURACY ....................................................................... 6-1
6.2 COST ESTIMATE METHODOLOGY ............................................................... 6-2
6.2.1 Unit Costs .......................................................................................... 6-2
6.2.2 Construction Cost Index .................................................................... 6-2
6.2.3 Construction Contingency Allowance ................................................. 6-2
6.2.4 Project Related Costs ........................................................................ 6-2
6.3 CAPITAL IMPROVEMENT PROGRAM ........................................................... 6-4
6.3.1 Capital Improvement Costs ................................................................ 6-4
6.3.2 Pipelines ............................................................................................ 6-4
6.3.3 Recommended Cost Allocation Analysis ............................................ 6-9
City of Gilroy
Storm Drainage System Master Plan
TABLE OF CONTENTS PAGE NO.
March 2023 iii City of Gilroy
Storm Drainage System Master Plan
FIGURES
Figure ES.1 Regional Location Map ...................................................................... ES-3
Figure ES.2 Planning Area .................................................................................... ES-4
Figure ES.3 Existing Storm Drainage System ....................................................... ES-7
Figure ES.4 Capital Improvement Program ........................................................... ES-8
Figure 1.1 Regional Location Map .................................................................................... 1-2
Figure 2.1 Planning Area .................................................................................................. 2-2
Figure 2.2 Waterways and Watersheds ............................................................................ 2-4
Figure 2.3 FEMA 100 Year Flood Zones .......................................................................... 2-5
Figure 2.4 Existing Land Use ........................................................................................... 2-7
Figure 2.5 2040 General Plan Land Use .......................................................................... 2-8
Figure 3.1 10-Year/24-Hour Unit Hyetograph ................................................................... 3-5
Figure 4.1 Storm Drainage Areas ..................................................................................... 4-2
Figure 4.2 Existing Modeled Storm Drainage Systems ..................................................... 4-5
Figure 4.3 Hydrological Soil Classification ........................................................................ 4-10
Figure 4.4 Typical Channel Cross-Section ....................................................................... 4-11
Figure 5.1 Existing System Analysis 10-Year Design Storm ............................................. 5-2
Figure 5.2 Proposed Improvements Alternative 1 ............................................................. 5-5
Figure 5.3 Proposed Improvements Alternative 2 ............................................................. 5-6
Figure 6.1 Capital Improvement Program ......................................................................... 6-5
TABLES
Table ES.1 Hydrologic and Hydraulic Design Criteria ........................................................ ES-5
Table ES.2 Unit Costs ....................................................................................................... ES-10
Table ES.3 Capital Improvement Program ......................................................................... ES-11
Table 1.1 Unit Conversions ............................................................................................. 1-6
Table 1.2 Abbreviations and Acronyms ........................................................................... 1-7
Table 2.1 Existing and Future Land Use ......................................................................... 2-9
Table 2.2 Historical and Projected Population ................................................................. 2-10
Table 3.1 Precipitation Depth-Duration-Frequency Data ................................................. 3-2
Table 3.2 Precipitation Depth-Duration-Frequency Comparison ...................................... 3-3
Table 3.3 Relevant Design Storms .................................................................................. 3-6
Table 3.4 Soil Imperviousness and SCS Curve Numbers ............................................... 3-7
Table 3.5 Hydrologic and Hydraulic Design Criteria ........................................................ 3-9
Table 4.1 Existing GIS Storm Pipe Inventory .................................................................. 4-6
Table 4.2 Existing Modeled Storm Pipe Inventory ........................................................... 4-7
Table 5.1 Storm Drainage System Problem Areas .......................................................... 5-3
Table 5.2 Proposed Improvements ................................................................................. 5-7
Table 6.1 Unit Costs ....................................................................................................... 6-3
Table 6.2 Capital Improvement Program ......................................................................... 6-6
City of Gilroy
Storm Drainage System Master Plan
TABLE OF CONTENTS
March 2023 iv City of Gilroy
Storm Drainage System Master Plan
APPENDICES
Appendix A Hydrology Review
Appendix B Storm Drainage Area Hydrologic Routing
March 2023 ES-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
0.0 EXECUTIVE SUMMARY
This executive summary presents a brief background of the City of Gilroy’s (City) storm drainage
system, the planning area characteristics, the hydrology and hydraulic criteria, and the hydrology
and hydraulic model developments.
These hydrology and hydraulic models were used to evaluate the capacity adequacy of the
existing stormwater drainage system, for recommending improvements to mitigate existing
deficiencies and for servicing future growth. The prioritized capital improvement program accounts
for growth through the Gilroy Planning Area.
ES.1 STUDY OBJECTIVES
Recognizing the importance of planning, developing, and financing system facilities to provide
reliable stormwater system service to existing customers and for servicing anticipated growth
within the sphere of influence, the City initiated the 2022 Storm Drainage System Master Plan.
The City of Gilroy authorized Akel Engineering Group Inc. to complete the following tasks:
•Summarize the City’s existing stormwater system facilities.
•Document growth planning assumptions and known future developments.
•Update the stormwater system performance criteria and design storm event.
•Develop new hydrologic and hydraulic models.
•Evaluate the storm drainage facilities to address hydraulic capacity requirements from
existing and projected developments.
•Recommend a capital improvement program (CIP) with an opinion of probable
construction costs.
•Perform a cost allocation analysis for cost sharing purposes.
•Develop a 2023 Storm Drainage System Master Plan Report.
ES.2 INTEGRATED APPROACH TO MASTER PLANNING
The City implemented an integrated master planning approach and contracted the services of
Akel Engineering Group Inc. to prepare the following documents:
•2023 Water System Master Plan
•2023 Sewer System Master Plan
•2023 Storm Drainage System Master Plan
March 2023 ES-2 City of Gilroy
Storm Drainage System Master Plan
While each of these reports is published as a standalone document, it has been coordinated for
consistency with the City’s General Plan. Additionally, each document has been cross referenced
to reflect relevant analysis results with the other documents.
ES.3 STUDY AREA DESCRIPTION
The City of Gilroy is located in Santa Clara County near the west coast of California, south of City
of San Francisco. The City of Gilroy lies within the seismically active region of San Francisco Bay.
The City of Gilroy lies in the southern portion of the Santa Clara County and is the most southern
City located within the county. The City is located approximately 32 miles southeast of the City of
San Jose, 8 miles southeast of Morgan Hill, 25 miles east of City of Santa Cruz, and 16 miles
northwest of City of Hollister. The City limits currently encompass 16.5 square miles, with an
approximate population of 56,599 residents, according to Department of Finance as of January
2021. Figure ES.1 displays the City’s location.
The City’s service area is generally bound to the north by Fitzgerald Avenue, to the northeast by
San Ysidro Avenue, to the southeast by Camino Arroyo, to the west by Burchell Road and
Rancho Vista Drive, and to the south by Carnadero Avenue. U.S. Route 101 divides the City in a
southeast to northwest direction and the California State Route 152 (Hecker Pass Hwy) runs east-
west direction in the northern half of the City. The topography is generally flat in the middle of the
service area, with increasing slopes in the east and west side of the City due to the Santa Cruz
Mountains to the west and the Diablo Range to the east. Figure ES.2 displays the planning area
showing City limits, the Urban Growth Boundary (UGB) of the City and Planning Area / Sphere of
Influence (SOI).
The City operates and maintains a storm drainage system that covers the area within the City
Limits. Currently, stormwater runoff discharges to the Uvas Creek, Ronan Channel, Miller Slough
Channel, Princevalle Channel and Lower Miller Slough Channel.
ES.4 SYSTEM PERFORMANCE AND DESIGN CRITERIA
This report documents the City’s performance and design criteria that were used for evaluating
hydrologic and hydraulic systems within the City’s drainage watershed (Table ES.1). Hydrologic
criteria are developed to characterize the flood routing of rainfall runoff in a defined drainage
system. Akel Engineering Group Inc. obtained the services of Kasraie Consulting to review the
hydrologic modeling approach, applied rainfall depths and distributions, sub-catchment
delineations, model runoff, and outflow hydrographs of this project. The hydraulic criteria for the
storm drainage system were used to evaluate the capacity requirements of conveyance facilities .
Two design storms were used for the evaluation of the City’s existing storm drainage system, and
for the future drainage conveyance facilities. The 10-year 24-hour design storm was used for
evaluating the pipeline conveyance facilities as well as to size future pipeline while the 100-year
24-hour design storm was used to determine if street flooding exceeds one foot in depth.
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Figure ES.1
Regional Location Map
Storm Drainage System Master Plan
City of Gilroy
5Updated: September 21, 2020
GIS
0120.5 Miles
Legend
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Railroads
Highway
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Urban Growth Boundary
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Flie Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig1-1RLMap_092120.mxd
City of Gilroy
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Figure ES.2
Planning Area
Storm Drainage System Master Plan
City of Gilroy
5Updated: April 15, 2022
GIS
0120.5 Miles
Legend
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City Limits Area
Specific Plan Areas
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Urban Growth Boundary
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File Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig2-1PlanningArea_041522.mxd
Table ES.1 Hydrologic and Hydraulic Design Criteria
Hydrology and Hydraulics Design Criteria
Conveyance Facilities
Pipelines
10-Year 24-Hour design storm to determine if storm water
have a minimum of one foot of freeboard between the top
of curb and the Hydraulic Grade Line.
Streets
100-Year 24-Hour design storm to determine if flooding
exceeds one foot in depth and can flood buildings or create
safety hazards.
Receiving Waters
(Streams, Creeks, Channels)
FEMA 100-year water surface elevations used for
downstream control for facilities where 100-year flood maps
are available.
9/16/2020
Storm Drainage System Master Plan
City of Gilroy
March 2023 ES-6 City of Gilroy
Storm Drainage System Master Plan
ES.5 HYDROLOGIC AND HYDRAULIC MODEL DEVELOPMENT
Factors critical to the hydrologic model development include watersheds, drainage areas within
each watershed, overland flow routing within drainage sub-catchments, and conveyance that
makes full use of pipes as well as streets for routing 100-year design storm events. The hydraulic
model was developed to evaluate the capacity adequacy of the City’s storm drainage system. The
model was used to identify capacity deficiencies and to recommend improvements to mitigate
those deficiencies.
The hydrologic and hydraulic modeling software used for evaluating the capacity adequacy of the
City’s storm drainage system, InfoSWMM by Innovyze Inc. The software uses the fully dynamic
St. Venant’s Equation, which has a more accurate engine for simulating backwater and surcharge
conditions. The software also incorporates the use of the Manning Equation in other calculations
including upstream pipe flow conditions.
The physical hydraulic model was thus developed, as shown on Figure ES.3. The model was
populated to include rim elevations at manholes, invert elevations of pipelines, pipe sizes, pipe
slopes, pipe lengths, and outfall elevations. Tributary catchments contributing runoff to the City’s
drainage system were delineated based on 1 feet contours. The hydrologic model developed in
InfoSWMM was used to generate runoff from these tributary catchments. Inflow hydrographs from
the hydrologic model were then imported to the hydraulic model to simulate runoff entering the
piped system.
ES.6 CAPACITY EVALUATION
The City’s hydrology and hydraulic models were used to evaluate the capacity adequacy of the
existing system and for sizing future improvements.
Using the criteria described in the System Performance and Design Criteria chapter, the
hydrology and hydraulic models were used to recommend capacity improvements. The
recommended pipe improvements to mitigate existing system deficiencies and to serve future
growth are shown on Figure ES.4.
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NU-890Figure ES.4Capital ImprovementProgramStorm Drainage System Master Plan City of GilroyLegendProposed Improvements!COutfallPipesExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsNon-Modeled SystemPipesChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 15, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig6-1_CIP_Scen2_041522.mxd
March 2023 ES-9 City of Gilroy
Storm Drainage System Master Plan
ES.7 CAPITAL IMPROVEMENT PROGRAM
The Capital Improvement Program costs for the projects identified in this master plan for
mitigating existing system deficiencies and for serving anticipated future growth throughout the
City are summarized on Table ES.3, and are graphically represented on Figure ES.4. Each
improvement was assigned a uniquely coded identifier corresponding to its respective hydrologic
drainage area. The baseline costs for pipelines are shown in Table ES.2.
The estimated construction costs include the baseline costs plus 30 percent contingency
allowance to account for unforeseen events and unknown field conditions. Capital improvement
costs include the estimated construction costs plus 30 percent project-related costs (engineering
design, project administration, construction management and inspection, and legal costs).
The costs in this Storm Drainage System Master Plan were benchmarked using a 20-City national
average ENR CCI of 12,647, reflecting a date of November 2021. In total, the CIP project cost
totals approximately $34.8 million dollars.
Table ES.2 Unit Costs
Storm Drainage System Master Plan
City of Gilroy
Pipelines
(in)($/Lineal Foot)
24 245
27 283
30 302
36 358
42 414
48 489
54 545
60 564
66 622
72 678
84 866
90 928
Pipeline Casings
$22 per inch diameter per liner foot
Note :
3/29/2023
1. Unit costs are based on an ENR CCI Index Value
of 13,176 (March 2023).
Table ES.3 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Ronan Channel Drainage Area
RC-1 Sunrise Dr From Dream Catcher Way to Golden Sky Way 18 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
RC-2 Mantelli Dr From Muir Drive to Delta Dr 18 Replacement 24 1,250 245 306,506 307,000 400,000 520,000 100%0%520,000 0
RC-3 Zamzow Ct From approx. 1,250' n/o Cohansey Ave to
Cohansey Ave -New 48 1,200 489 586,990 587,000 764,000 994,000 8%92%74,621 919,379
RC-4 Cohansey Ave From approx. 350' e/o Monterey Rd to Murray
Ave -New 48 1,450 489 709,280 710,000 923,000 1,200,000 0%100%0 1,200,000
RC-5 Murray Ave From Cohansey Ave to Tomkins Ct -New 48 2,800 489 1,369,643 1,370,000 1,781,000 2,316,000 9%91%219,340 2,096,660
RC-6 Murray Ave From Tomkins Ct to Ronan Channel 24 Replacement 48 600 489 293,495 294,000 383,000 498,000 13%87%67,178 430,822
RC-7 No Name Uno/Las Animas
Ave
From approx. 1,500' s/o Buena Vista Ave to San
Ysidro Ave -New 36 3,400 358 1,216,516 1,217,000 1,583,000 2,058,000 91%9%1,875,920 182,080
RC-8 San Ysidro Ave From Las Animas Ave to approx. 500' n/o
Leavesley Rd -New 48 3,425 489 1,675,367 1,676,000 2,179,000 2,833,000 79%21%2,227,585 605,415
RC-9 Leavesley Rd From approx. 220' e/o Forest St to approx. 360'
e/o Forest St 18 Replacement 42 150 414 62,114 63,000 82,000 107,000 87%13%92,659 14,341
Subtotal - Ronan Channel Drainage Area 6,345,000 8,253,000 10,732,000 5,283,303 5,448,697
Miller Slough Drainage Area
MS-1 Wren Ave From approx. 290' n/o First St to Lawrence Dr 24 Replacement 30 2,325 302 700,991 701,000 912,000 1,186,000 100%0%1,186,000 0
MS-2 First St From approx. 180' e/o Princevalle St to Sargent St 18 Replacement 24 525 245 128,732 129,000 168,000 219,000 100%0%219,000 0
MS-3 Sargent St From First St to Broadway 18 Replacement 24 425 245 104,212 105,000 137,000 179,000 100%0%179,000 0
MS-4 Broadway From Sargent St to Church St 18/21 Replacement 30 1,075 302 324,114 325,000 423,000 550,000 100%0%550,000 0
MS-5 Church St From Broadway to approx. 100' s/o Howson St 21 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
MS-6 Monterey Rd From Second St to First St 18/24 Replacement 27 675 283 190,847 191,000 249,000 324,000 100%0%324,000 0
Subtotal - Miller Slough Drainage Area 1,572,000 2,047,000 2,664,000 2,664,000 0
North Uvas Drainage Area
NU-1 Hecker Pass Hwy From approx. 3,080' w/o Santa Teresa Blvd to
approx. 360' w/o Santa Teresa Blvd -New 30 2,825 302 851,742 852,000 1,108,000 1,441,000 0%100%0 1,441,000
NU-2 Hecker Pass Hwy From approx. 360' w/o Santa Teresa Blvd to
Santa Teresa Blvd 24 Replacement 30 350 302 105,526 106,000 138,000 180,000 0%100%0 180,000
NU-3 Santa Teresa Blvd From approx. 70' s/o Hecker Pass Hwy to Uvas
Creek 24 Replacement 30 2,575 302 776,367 777,000 1,011,000 1,315,000 0%100%0 1,315,000
NU-4 Westwood Dr From approx. 150' s/o Third St to Kentwood Ct 18 Replacement 24 425 245 104,212 105,000 137,000 179,000 100%0%179,000 0
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
Table ES.3 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
NU-5 Monterey Rd From Princevalle Channel to Victoria Dr -New 90 775 928 718,859 719,000 935,000 1,216,000 100%0%1,216,000 0
NU-6 Monterey Rd From Victoria Dr to approx. 950' n/o Farman Ln 42 Replacement 90 2,175 928 2,017,443 2,018,000 2,624,000 3,412,000 100%0%3,412,000 0
NU-7 ROW From Monterey Rd to approx. 970' n/o Uvas
Creek -New 90 975 928 904,371 905,000 1,177,000 1,531,000 100%0%1,531,000 0
NU-8 ROW From approx. 970' n/o Uvas Creek to Uvas Creek -New 90 1,000 928 927,560 928,000 1,207,000 1,570,000 100%0%1,570,000 0
Subtotal - North Uvas Drainage Area 6,410,000 8,337,000 10,844,000 7,908,000 2,936,000
South Uvas Drainage Area
SU-1 St Andrews Cir From Carnoustie Ct to approx. 380' nw/o
Carnoustie Ct 18 Replacement 36 375 358 134,175 135,000 176,000 229,000 100%0%229,000 0
Subtotal - South Uvas Drainage Area 135,000 176,000 229,000 229,000 0
Princevalle Drainage Area
PR-1 Third St From Santa Paula Dr to Wren Ave 27 Replacement 30 825 302 248,739 249,000 324,000 422,000 100%0%422,000 0
PR-2 Third St From Wren Ave to approx. 170' e/o Santa
Theresa Dr 27 Replacement 42 675 414 279,515 280,000 364,000 474,000 100%0%474,000 0
PR-3 Third St From approx. 170' e/o Santa Theresa Dr to Santa
Theresa Dr 27 Replacement 48 200 489 97,832 98,000 128,000 167,000 100%0%167,000 0
PR-4 Santa Theresa Dr From Third St to approx. 300' s/o Fourth St 27/30 Replacement 48 950 489 464,700 465,000 605,000 787,000 100%0%787,000 0
PR-5 ROW From Santa Theresa Dr to Miller Ave 30 Replacement 48 675 489 330,182 331,000 431,000 561,000 100%0%561,000 0
PR-6 Miller Ave From approx. 300' s/o Fourth St to Fifth St 30 Replacement 48 400 489 195,663 196,000 255,000 332,000 100%0%332,000 0
PR-7 Fifth St From Miller Ave to Princevalle St 30 Replacement 48 1,250 489 611,448 612,000 796,000 1,035,000 100%0%1,035,000 0
PR-8 Princevalle St From Third St to Fifth St 18 Replacement 30 1,250 302 376,877 377,000 491,000 639,000 100%0%639,000 0
PR-9 Princevalle St From Fifth St to Seventh St 42 Replacement 54 1,225 545 668,183 669,000 870,000 1,131,000 100%0%1,131,000 0
PR-10 Alexander St From Tenth St to Banes Ln 18 Replacement 24 825 245 202,294 203,000 264,000 344,000 84%16%288,575 55,425
PR-11 Banes Ln From Alexander St to Automall Pkwy 18 Replacement 24 625 245 153,253 154,000 201,000 262,000 89%11%234,103 27,897
PR-12 Monterey Rd From approx. 380' n/o 8th St to Angra Way -New 48 2,475 489 1,210,667 1,211,000 1,575,000 2,048,000 100%0%2,048,000 0
PR-13 Monterey Rd From Angra Way to Princevalle Channel 42 Replacement 48 775 489 379,098 380,000 494,000 643,000 100%0%643,000 0
Subtotal - Princevalle Drainage Area 5,225,000 6,798,000 8,845,000 8,761,678 83,322
Table ES.3 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
Lower Miller Slough Drainage Area
LM-1 Fourth St From Eigleberry St to Monterey Rd 18 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
LM-2 Monterey Rd From Fourth St to Sixth St 27 Replacement 36 1,250 358 447,248 448,000 583,000 758,000 100%0%758,000 0
LM-3 Monterey Rd From Sixth St to Hornlein Ct 27 Replacement 42 425 414 175,991 176,000 229,000 298,000 100%0%298,000 0
LM-4 Monterey Rd From Hornlein Ct to approx. 240' n/o Eighth St 33 Replacement 42 775 414 320,924 321,000 418,000 544,000 100%0%544,000 0
LM-5 ROW From Rosanna St to Monterey Rd 18 Replacement 24 1,125 245 275,855 276,000 359,000 467,000 100%0%467,000 0
LM-6 Alexander St From Sixth St to Eighth St 24 Replacement 36 1,300 358 465,138 466,000 606,000 788,000 100%0%788,000 0
Subtotal - Lower Miller Slough Drainage Area 1,808,000 2,353,000 3,061,000 3,061,000 0
Total Costs
Subtotal - Ronan Channel Drainage Area 6,345,000 8,253,000 10,732,000 5,283,303 5,448,697
Subtotal - Miller Slough Drainage Area 1,572,000 2,047,000 2,664,000 2,664,000 0
Subtotal - North Uvas Drainage Area 6,410,000 8,337,000 10,844,000 7,908,000 2,936,000
Subtotal - South Uvas Drainage Area 135,000 176,000 229,000 229,000 0
Subtotal - Princevalle Drainage Area 5,225,000 6,798,000 8,845,000 8,761,678 83,322
Subtotal - Lower Miller Slough Drainage Area 1,808,000 2,353,000 3,061,000 3,061,000 0
Total Improvement Costs 21,495,000 27,964,000 36,375,000 27,906,981 8,468,019
Notes:
3/29/2023
1. Cost estimates are based on the Engineering News Record (ENR) construction cost index (CCI) of 13,176 for March 2023
2. Baseline construction costs plus 30% to account for unforeseen events and unknown conditions.
3. Estimated construction cost plus 30% to cover other costs including: engineering design, project administration (developer and City staff), construction management and inspection, and legal costs.
March 2023 1-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
1.0 CHAPTER 1 - INTRODUCTION
This chapter provides a brief background of the City of Gilroy’s (City) Storm Drainage system, the
need for this master plan, and the objectives of the study. Abbreviations, and definitions are also
provided in this chapter.
1.1 BACKGROUND
The City of Gilroy is located approximately 32 miles southeast of the City of San Jose, 8 miles
southeast of Morgan Hill, 25 miles east of the City of Santa Cruz, and 16 miles northwest of City
of Hollister (Figure 1.1). The City provides storm drainage services to approximately 17,000
residential, commercial, industrial, and institutional establishments. The City currently owns and
operates more than 132 miles of storm drainage pipelines, with up to 84-inch pipe sizes.
In 2004, the City of Gilroy developed a Storm Drainage System Master Plan that identified
capacity deficiencies in the storm drainage system and recommended improvements to alleviate
deficiencies and serve future developments in the Gilroy Planning Area.
Recognizing the importance of planning, developing, and financing system facilities to provide
enhanced stormwater drainage system service to existing customers and for servicing anticipated
growth within the Gilroy Planning Area, the City initiated updating elements of the 2004 Storm
Drainage System Master Plan, to reflect current land use conditions and General Plan updates.
1.2 SCOPE OF WORK
City Council approved Akel Engineering Group Inc. to prepare this 2022 Storm Drainage System
Master Plan (SDSMP) and a concurrent Water System Master Plan and Sewer System Master
Plan. The 2022 SDSMP evaluates the City’s storm drainage system and recommends capacity
improvements necessary to service the needs of existing users and for servicing the future growth
of the City. This 2022 SDSMP is intended to serve as a tool for planning and phasing the
construction of future storm drainage infrastructure for the projected buildout of the City’s service
area. The planning area and horizon for this master plan is based on the City’s 2040 General
Plan. Should planning conditions change, and depending on their magnitude, adjustments to the
master plan recommendations might be necessary.
The master plan included the following tasks:
•Summarize the City’s existing stormwater system facilities.
•Document growth planning assumptions and known future developments.
•Update the stormwater system performance criteria and design storm event.
•Develop new hydrologic and hydraulic models.
!(
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Morgan Hill
Watsonville
Gilroy
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£¤101
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UV152
UV152
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UV152
UV129
UV129 UV129
UV1
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Calero
Reservoir
Anderson
Reservoir
Coyote
Reservoir
Uvas
Reservoir
Figure 1.1
Regional Location Map
Storm Drainage System Master Plan
City of Gilroy
5Updated: September 21, 2020
GIS
0120.5 Miles
Legend
!(Cities
Railroads
Highway
City Limits
Urban Growth Boundary
Study Area
Elevation (ft)
51 - 100
101 - 250
251 - 500
501 - 1,000
1,001 - 2,000
2,001 - 3,0009 - 50
3,001 - 3,792
Waterbodies
Flie Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig1-1RLMap_092120.mxd
March 2023 1-3 City of Gilroy
Storm Drainage System Master Plan
• Evaluate the storm drainage facilities to address hydraulic capacity requirements from
existing and projected developments.
• Recommend a capital improvement program (CIP) with an opinion of probable
construction costs.
• Perform a cost allocation analysis for cost sharing purposes.
• Develop a 2023 Storm Drainage System Master Plan Report.
1.3 INTEGRATED APPROACH TO MASTER PLANNING
This City implemented an integrated master planning approach and contracted the services of
Akel Engineering Group Inc. to prepare the following documents:
• 2023 Water System Master Plan
• 2023 Sewer System Master Plan
• 2023 Storm Drainage System Master Plan
While each of these reports is published as a standalone document, it has been coordinated for
consistency with the City’s 2040 General Plan. Additionally, each document has been cross
referenced to reflect relevant analysis results with the other documents.
1.4 PREVIOUS MASTER PLANS
The City’s most recent storm drainage master plan was completed in 2004. This master plan
included evaluation of servicing growth to the planning area, evaluating the existing system, and
identifying storm drainage improvements required to serve future growth. Additionally, the 2004
master plan included the development of the hydrologic and hydraulic models, which were used
for evaluating the storm drainage system. Improvements were recommended for servicing existing
and future growth areas, and a corresponding Capital Improvement Program was developed to
quantify the costs associated with each improvement.
1.5 RELEVANT REPORTS
The City has completed several special studies intended to evaluate localized growth. These
reports were referenced and used during the capacity analysis. The following lists relevant reports
that were used in the completion of this master plan, as well as a brief description of each
document:
• City of Gilroy 2040 General Plan, November 2020 (2040 GP). The City’s 2040 General
Plan provides future land use planning, and growth assumptions for the Planning Area.
Additionally, this report establishes the planning horizon for improvements in this master
plan.
March 2023 1-4 City of Gilroy
Storm Drainage System Master Plan
•City of Gilroy Storm Drainage System Master Plan, May 2004 (2004 SDSMP). This
report documents the planning and performance criteria, evaluates the storm drainage
system, recommends improvements, and provides an estimate of costs.
1.6 REPORT ORGANIZATION
The Storm Drainage System Master Plan report contains the following chapters:
Chapter 1 – Introduction. This chapter provides a brief background of the City’s Storm Drainage
system, the need for this master plan, and the objectives of the study. Unit conversions,
abbreviations, and definitions are also provided in this chapter.
Chapter 2 – Planning Area Characteristics. This chapter presents a discussion of the planning
area characteristics affecting the hydrologic and hydraulic analysis of this master plan. These
characteristics include soil, topography, floodplains, and land use.
Chapter 3 – System Performance and Design Criteria. This chapter presents the City’s
planning and design criteria that were used for evaluating hydrologic and hydraulic systems within
the City’s drainage watershed.
Chapter 4 –Existing Facilities Model Development. This chapter defines the hydrologic
delineation of storm drainage areas, routing to their respective receiving facilities, and includes the
hydrologic model development. Additionally, this chapter includes an overview of the storm
drainage system, and the hydraulic model development.
Chapter 5 – Evaluation and Proposed Improvements. This chapter presents a summary of the
storm drainage system evaluation and identifies improvements needed to mitigate existing
deficiencies, as well as improvements needed to expand the system and service future growth.
Chapter 6 – Capital Improvement Program. This chapter provides a summary of the
recommended storm drainage system improvements intended to mitigate existing capacity
deficiencies and for accommodating anticipated future growth. This chapter also presents the cost
criteria and methodologies for developing the Capital Improvement Program (CIP). Finally, a cost
allocation analysis, usually used for cost sharing purposes, is also included.
1.7 ACKNOWLEDGEMENTS
Obtaining the necessary information to successfully complete the analysis presented in this
report, and developing the long-term strategy for mitigating the existing system deficiencies and
for accommodating future growth, was accomplished with the strong commitment and very active
input from dedicated team members including:
•Daryl Jordan, P.E.; Director of Public Works
•Gary Heap, P.E.; City Engineer
•Jorge Duran, P.E.; Senior City Engineer
March 2023 1-5 City of Gilroy
Storm Drainage System Master Plan
•Matt Jones, Deputy Public Works Director
1.8 UNIT CONVERSIONS AND ABBREVIATIONS
Engineering units were used in reporting flow rates and volumes pertaining to the design and
operation of various components of the storm drainage system. Where it was necessary to report
values in smaller or large quantities, different sets of units were used to describe the same
parameter. Values reported in one set of units can be converted to another set of units by
applying a multiplication factor. A list of multiplication factors for units used in this report are
shown on Table 1.1.
Various abbreviations and acronyms were also used in this report to represent relevant storm
drainage system terminologies and engineering units. A list of abbreviations and acronyms is
included in Table 1.2.
1.9 GEOGRAPHIC INFORMATION SYSTEMS
This master planning effort made extensive use of Geographic Information Systems (GIS)
technology, for completing the following tasks:
•Developing the physical characteristics of the hydraulic model.
•Delineating stormwater tributary areas, and outlining watershed areas.
•Extracting ground elevations along the storm drainage system from available contour
maps and digital elevation models.
•Generating maps and exhibits used in this master plan.
Table 1.1 Unit Conversions
Storm Drainage System Master Plan
City of Gilroy
Volume Unit Calculations
To Convert From:To:Multiply by:
acre feet gallons 325,857
acre feet cubic feet 43,560
acre feet million gallons 0.3259
cubic feet gallons 7.481
cubic feet acre feet 2.296 x 10-5
cubic feet million gallons 7.481 x 10-6
gallons cubic feet 0.1337
gallons acre feet 3.069 x 10-6
gallons million gallons 1 x 10-6
million gallons gallons 1,000,000
million gallons cubic feet 133,672
million gallons acre feet 3.069
Flow Rate Calculations
To Convert From:To:Multiply By:
ac-ft/yr mgd 8.93 x 10-4
ac-ft/yr cfs 1.381 x 10-3
ac-ft/yr gpm 0.621
ac-ft/yr gpd 892.7
cfs mgd 0.646
cfs gpm 448.8
cfs ac-ft/yr 724
cfs gpd 646300
gpd mgd 1 x 10-6
gpd cfs 1.547 x 10-6
gpd gpm 6.944 x 10-4
gpd ac-ft/yr 1.12 x 10-3
gpm mgd 1.44 x 10-3
gpm cfs 2.228 x 10-3
gpm ac-ft/yr 1.61
gpm gpd 1,440
mgd cfs 1.547
mgd gpm 694.4
mgd ac-ft/yr 1,120
mgd gpd 1,000,000
9/16/2020
Table 1.2 Abbreviations and Acronyms
Storm Drainage System Master Plan
City of Gilroy
Abbreviation Expansion Abbreviation Expansion
2016 SDSMP 2016 Storm Drainage System
Master Plan ft Feet
10Yr-24Hr 10-Year 24-Hour fps Feet per Second
100Yr-24Hr 100-Year 24-Hour GIS Geographic Information Systems
AC Acre HDPE High Density Polyethylene Pipe
ACP Asbestos Cement Pipe in Inch
Akel Akel Engineering Group, Inc.in/hr Inch per Hour
Caltrans California Department of
Transportation LF Linear Feet
City City of Gilroy mi Miles
CCI Construction Cost Index NRCS National Resource Conservation
Service
cfs Cubic Feet per Second NOAA National Oceanic and Atmospheric
Administration
CI Cast Iron Pipe PVC Polyvinyl Chloride Pipe
CIP Capital Improvement Program RCP Reinforced Concrete Pipe
County County of Santa Clara ROW Right of Way
CMP Corrugated Metal Pipe SCS Soil Conservation Service
DIP Ductile Iron Pipe SOI Sphere of Influence
DDF Depth Duration Frequency SSURGO Soil Survey Geographic database
ENR Engineering News Record SWMM Stormwater Management Model
EPA Environmental Protection Agency UGB Urban Growth Boundary
DOF Department of Finance UWMP Urban Water Management Plan
FEMA Federal Emergency Management
Agency VCP Verified Clay Pipe
9/16/2020
March 2023 2-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
2.0 CHAPTER 2 – PLANNING AREA CHARACTERISTICS
This chapter presents a discussion of the planning area characteristics affecting the hydrologic
and hydraulic analysis of this master plan. These characteristics include soil, topography,
floodplains, and land use.
2.1 STUDY AREA DESCRIPTION
The City of Gilroy is located in Santa Clara County near the west coast of California, south of City
of San Francisco. The City of Gilroy lies within the seismically active region of San Francisco Bay.
The City of Gilroy lies in the southern portion of the Santa Clara County and is the most southern
City located within the county. The City is located approximately 32 miles southeast of the City of
San Jose, 8 miles southeast of Morgan Hill, 25 miles east of City of Santa Cruz, and 16 miles
northwest of City of Hollister. The City limits currently encompass 16.5 square miles, with an
approximate population of 57,000 residents, according to Department of Finance as of January
2021. Figure 1.1 displays the City’s location.
The City’s service area is generally bound to the north by Fitzgerald Avenue, to the northeast by
San Ysidro Avenue, to the southeast by Camino Arroyo, to the west by Burchell Road and
Rancho Vista Drive, and to the south by Carnadero Avenue. U.S. Route 101 divides the City in a
southeast to northwest direction and the California State Route 152 (Hecker Pass Hwy) runs east-
west direction in the northern half of the City. The topography is generally flat in the middle of the
service area, with increasing slopes in the east and west side of the City due to the Santa Cruz
Mountains to the west and the Diablo Range to the east. Figure 2.1 displays the planning area
showing City limits, the Urban Growth Boundary (UGB) of the City and Planning Area / Sphere of
Influence (SOI).
The City operates and maintains a storm drainage system that covers the area within the City
Limits. Currently, stormwater runoff discharges to the Uvas Creek, Ronan Channel, Miller Slough
Channel, Princevalle Channel and Lower Miller Slough Channel, as describe in the following
section.
2.2 WATERSHEDS AND DRAINAGE AREAS
The City of Gilroy maintains a rich diversity of land use types, which contribute to a varying degree
of stormwater runoff containment needs. Several creeks, a vast array of agricultural lands, small
pockets of development, and the City itself comprises the wide array of runoff generation and
conveyance within the planning area.
City of Gilroy
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Figure 2.1
Planning Area
Storm Drainage System Master Plan
City of Gilroy
5Updated: April 15, 2022
GIS
0120.5 Miles
Legend
City Limits
City Limits Area
Specific Plan Areas
Urban Service Area
Urban Growth Boundary
Sphere of Influence Boundary
General Plan Area
Roads
Highways
Railroads
Rivers & Creeks
Waterbodies
File Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig2-1PlanningArea_041522.mxd
March 2023 2-3 City of Gilroy
Storm Drainage System Master Plan
2.2.1 Watersheds
The City has multiple major watersheds within the Urban Growth Boundary that collect and
convey stormwater runoff, and are as follows: Uvas Creek, Llagas Creek, and Pajaro River
(Figure 2.2). The following are brief descriptions of the three watersheds:
• Uvas Creek Watershed – The portion of the Uvas Creek Watershed within the City limits is
generally defined as the area bounded by Rancho Vista Drive to the west and Santa
Teresa Boulevard to the east, with the northern and southern boundaries approximately
represented by Uvas Creek and Santa Cruz Mountain respectively.
• Llagas Creek Watershed – The portion of the Lower Coyote Creek Watershed within the
City limits is generally bounded by Olea Court to the west and Llagas Creek to the east,
with the northern and southern boundaries approximately represented by Fitzgerald
Avenue and Luchessa Avenue respectively.
• Pajaro River Watershed – The Pajaro River Watershed within the City limits is generally
defined as the area bounded by Uvas Creek to the west and Llagas Creek to the east, with
the northern and southern boundaries approximately represented by Luchessa Avenue
and Southside Drive respectively.
2.2.2 Drainage Areas
Watersheds are typically comprised of smaller drainage areas defined by topographical features.
Additionally, these drainage areas may comprise multiple soil types as defined by Natural
Resources Conservation Service (NRCS). The City’s storm drainage system consists of a series
of conveyance networks. These networks have different means of disposing of stormwater runoff,
which include outfall points to Uvas Creek or Llagas Creek, or being conveyed to the canals and
sloughs throughout the City.
2.3 FLOODPLAINS
Floodplains are important for delineating the extent of water-level rise during major floods.
Typically, floodplains are estimated for the historic 100-Year and 500-Year flood. Most of the
floodplains for the City are located southeast boundary of the City limits, as shown on Figure 2.3.
The Federal Emergency Management Agency (FEMA) produces Flood Insurance Rate Maps that
show areas subject to flooding during major storm events. The flood risk information shown on
these maps is based on historical data, including meteorological, hydrological, and hydraulic data
for the specified area. The map creation is a result of the 1968 National Flood Insurance Program,
aimed at reducing or preventing property owner losses due to flooding by allowing premiums to be
paid for those in need of protection.
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,,,,,,,,,,,,,,,,
,
,,,,
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,,,,,,,,,,,,,,,DAY RDPUEBLO STUvas CreekWatershedLlagas CreekWatershedPajaro RiverWatershedFigure 2.2Waterwaysand WatershedsStorm Drainage System Master PlanCity of GilroyLegendExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsNon-Modeled SystemPipesChannelsWatershed BoundariesLlagas CreekPajaro RiverUvas CreekRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 14, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig2-2_Watersheds_041522.mxd
!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!CV
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GOLDEN GATE AVBUENA VISTA AVM A R C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A N T A T E R E S A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAGLE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekLlagas CreekUvas Creek303042182442483024481860335442722118184242361818183024364824242430242718243048302418181818241824302121183 6
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,,,,,,,,,,,,,,,DAY RDPUEBLO STFigure 2.3FEMA 100 YearFlood ZonesStorm Drainage System Master PlanCity of GilroyLegendFEMA 100 Year Flood Zones0.2% Annual Chance Flood Hazard1% Annual Chance Flood HazardRegulatory FloodwayArea of Minimal Flood HazardArea with Reduced Risk Due to LeveeArea of undetermined Flood HazardExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsNon-Modeled SystemPipesChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 14, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig2-3_FEMA_Flood_041522.mxd
March 2023 2-6 City of Gilroy
Storm Drainage System Master Plan
2.4 EXISTING SERVICE AREAS AND LAND USE
The City’s storm drainage system services residential and non-residential lands within the City
limits, as summarized on Table 2.1, and shown graphically on Figure 2.4. Areas within the City
limit include 4,197 acres of developed lands and 4,905 acres of undeveloped lands.
At ultimate development of the General Plan, the City’s storm drainage system is anticipated to
service approximately 3,045 acres of residential land use, 4,750 acres of non-residential land use,
and 2,477 acres of open space land use, for a total of 10,272 acres inside the City’s Urban
Growth Boundary (Table 2.1). The land use designations utilized in this master plan are
consistent with the Land Use Element of the City’s 2040 General Plan received from the City’s
planning division, and shown on Figure 2.5.
2.5 HISTORICAL AND PROJECTED POPULATION
According to California Department of Finance (DOF) population estimates, the 2021 City
population is approximately 57,000 people. From 2015 to present, the City’s service area has
observed an average annual growth rate of approximately 0.7%. This 2022 SDSMP is consistent
with the City’s 2020 Urban Water Management Plans (UWMP’s) annual growth rate factor of
1.5%. The current and projected service area population is summarized in Table 2.2.
£¤101£¤101UV152UV152UV25L la g a s C r e e k
Miller SloughUvas CreekBuena Vista AveFitzgerald AveM o n te r e y R d
Day RdSunrise DrMantelli DrS a n ta T e r e s a Bl Wren AveLeavesley RdC a m in o A rro y o
Tenth StSouthside DrUvasPark D r Castro Valley RdF ra zie r L a k e R d Bloomfield AveSheldon AveDavidson AveLas Animas AveCohansey AveGilman RdHecker Pass RdFirst StRucker AvPajaro RiverShore RdLegendExisting Land UseLow Density ResidentiaMedium Density ResidentialHigh Density ResidentialNeighborhood DistrictPublic/Quasi-Public FacilityEducational FacilityProfessional OfficeGeneral Services CommercialVisitor-Serving CommercialIndustrialOpen SpacePark and Recreation FacilityVacantCity LimitsRoadsRailroadsRivers & CreeksWaterbodiesFigure 2.4Existing Land UseStorm Drainage System Master PlanCity of Gilroy5Updated: April 15, 2022GIS0120.5MilesFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig2-4_ExistLandUse_041522.mxd
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Table 2.1 Existing and Future Land Use
Storm Drainage System Master Plan
City of Gilroy
Existing Existing Lands -
Redeveloping
New Lands -
Redevelopment
New
Development
(acre)(acre)(acre)(acre)(acre)(acre)(acre)(acre)
Residential
Rural Residential -0 0 0 0 0 0 0 0
Hillside Residential Hillside Residential 444 -1 442 112 344 8 464 907
Low Density Residential Low Density Residential 1,704 -121 1,583 122 45 44 211 1,794
Medium Density Residential Medium Density Residential 100 -42 58 177 4 2 183 240
High Density Residential High Density Residential 249 -182 67 18 15 3 36 104
Subtotal - Residential 2,496 -346 2,151 429 408 58 895 3,045
Non-Residential
Visitor Serving Commercial Visitor Serving Commercial 99 -67 32 176 0 0 176 208
General Services Commercial General Services Commercial 524 -91 432 106 55 44 205 637
Professional Office 1 0 1 0 0 0 0 1
General Industrial General Industrial 320 -71 249 213 255 389 856 1,105
Campus Industrial 56 -12 44 0 0 0 0 44
Educational Facility Educational Facility 12 -5 7 1 0 0 1 8
Public/Quasi-Public Facility Public/Quasi-Public Facility 596 -44 552 152 102 28 281 833
Neighborhood District Neighborhood District 94 -94 0 38 22 336 395 395
City Gateway District -0 0 0 20 7 0 27 27
Downtown Specific Plan Area -0 0 0 57 32 0 89 89
Mixed Use Corridor High -0 0 0 25 7 0 33 33
Hecker Pass Special Use District -0 0 0 328 88 0 416 416
Glen Loma Ranch -0 0 0 103 237 0 341 341
Industrial Park -0 0 0 43 40 0 83 83
Employment Center -0 0 0 279 30 222 530 530
Subtotal - Non-Residential 1,701 -384 1,317 1,541 876 1,017 3,434 4,750
Other
Agriculture -0 0 0 0 0 0 0 0
Park and Recreation Facility Park and Recreation Facility 1,232 -238 995 0 66 88 154 1,149
Vacant Vacant 2,391 -2,391 0 0 0 0 0 0
Open Space Open Space 1,282 -1,030 253 0 1,041 4 1,045 1,298
Rural County -0 0 0 28 0 3 30 30
Subtotal - Other 4,905 -3,658 1,247 28 1,107 95 1,230 2,477
Total
Total Developed Area 9,102 -4,388 4,714 1,997 2,391 1,170 5,558 10,272
3/10/2022
Notes:
1. Source: City of Gilroy General Plan Alternatives Report, Public Review Draft July 2019.
2. Specific Plan land use types (Downtown, Hecker Pass, Glen Loma) are comprised of multiple land use types. As such, existing land use acreages reflect existing Specific Plan development under current existing land use designations.
Total
DevelopmentGeneral Plan Land Use
Classification
Existing Land Use
Classification
Future Development
Urban Growth
Boundary
Within City Limits
Existing Development
Subtotal
Existing
Development -
Unchanged
Within City Limits Subtotal
Future
Development
Table 2.2 Historical and Projected Population
Storm Drainage System Master Plan
City of Gilroy
Annual Growth
(%)
Historical
2000 41,464 -
2001 42,436 2.3%
2002 43,144 1.7%
2003 43,866 1.7%
2004 45,026 2.6%
2005 45,782 1.7%
2006 46,446 1.5%
2007 47,047 1.3%
2008 48,353 2.8%
2009 48,627 0.6%
2010 48,821 0.4%
2011 49,622 1.6%
2012 50,716 2.2%
2013 52,475 3.5%
2014 53,325 1.6%
2015 54,233 1.7%
2016 54,849 1.1%
2017 55,811 1.8%
2018 56,030 0.4%
2019 56,635 1.1%
2020 56,704 0.1%
Projected
2021 57,555 1.5%
2022 58,418 1.5%
2023 59,294 1.5%
2024 60,184 1.5%
2025 61,086 1.5%
2026 62,003 1.5%
2027 62,933 1.5%
2028 63,877 1.5%
2029 64,835 1.5%
2030 65,807 1.5%
2031 66,794 1.5%
2032 67,796 1.5%
2033 68,813 1.5%
2034 69,845 1.5%
2035 70,893 1.5%
2036 71,957 1.5%
2037 73,036 1.5%
2038 74,131 1.5%
2039 75,243 1.5%
2040 76,372 1.5%
8/31/2021
Notes:
1. Historical populations extracted from California Department of Finance, Population Estimates E-4.
2. Projected population assuming medium annual growth rate of 1.5% per the City of Gilroy 2020 Urban Water Management Plan.
Year Population1,2
March 2023 3-1 City of Gilroy
Storm Drainage System Master Plan
2022 City of Gilroy
3.0 CHAPTER 3 – SYSTEM PERFORMANCE AND DESIGN CRITERIA
This chapter presents the City’s planning and design criteria that were used for evaluating
hydrologic and hydraulic systems within the City’s drainage watershed.
3.1 HYDROLOGIC CRITERIA
Hydrologic criteria are developed to characterize the flood routing of rainfall runoff in a defined
drainage system. Akel Engineering Group obtained the services of Kasraie Consulting to review
the hydrology modeling approach, applied rainfall depths and distributions, sub-catchment
delineations, model runoff, and outflow hydrographs of this project. The hydrology sub-catchments
review is shown graphically in Appendix A. This section discusses the precipitation
characteristics, the design storms used in this master plan, and soil imperviousness.
3.1.1 Precipitation Characteristics
A dominating factor in the generation of rainfall in California is the oscillation of the semi-
permanent high pressure area of the north Pacific Ocean. This high pressure center moves north
in the summer and south in the winter, adjusting the flow of moisture into California. In the winter,
when the high pressure center moves further south, moisture can move south and bring
widespread rainfall.
In certain instances when circulation patterns allow for subtropical moisture to enter California
from a southwesterly direction, rainfall amounts can be quite heavy, and can result in widespread
flooding. The City receives an average 20.5 inches of total precipitation per year. The City’s wet
weather season typically starts in November and ends in April.
3.1.2 Design Storms
Design storms are typically defined by three important features: depth, duration and frequency.
• Depth. The depth of the storm identifies the amount of precipitation occurring during a
specific time interval.
This master plan uses the precipitation depth-duration-frequency equations included in the Santa
Clara County Drainage Manual, and converted to reflect City of Gilroy specific precipitation
documented on Table 3.1. The depth-duration-frequency values reflect rainfall events specific to
the City and lists precipitations, in inches and inches per hour, for return intervals up to 100 years.
For comparison purposes, Table 3.2 lists the difference in precipitation between the 2004 Master
Plan, County Drainage Manual and NOAA Atlas 14 Volume 6 Version 2. While 2004 Master Plan
and Santa Clara Drainage Manual values are relatively close, NOAA precipitation values have
Table 3.1 Precipitation Depth-Duration-Frequency
Storm Drainage System Master Plan
City of Gilroy
2-Year 5-Year 10-Year 25-Year 100-Year
(in)(in/hr)(in)(in/hr)(in)(in/hr)(in)(in/hr)(in)(in/hr)
5-min 0.15 1.78 0.21 2.50 0.24 2.93 0.29 3.43 0.34 4.12
10-min 0.21 1.24 0.29 1.71 0.33 1.99 0.39 2.33 0.47 2.79
15-min 0.24 0.97 0.33 1.33 0.39 1.57 0.46 1.85 0.56 2.26
30-min 0.33 0.67 0.45 0.91 0.53 1.06 0.62 1.25 0.76 1.51
1-hr 0.48 0.48 0.63 0.63 0.73 0.73 0.85 0.85 1.02 1.02
2-hr 0.73 0.36 0.95 0.47 1.09 0.54 1.25 0.62 1.48 0.74
3-hr 0.94 0.31 1.22 0.41 1.39 0.46 1.60 0.53 1.88 0.63
6-hr 1.36 0.23 1.80 0.30 2.08 0.35 2.40 0.40 2.85 0.48
12-hr 1.83 0.15 2.51 0.21 2.94 0.24 3.44 0.29 4.15 0.35
24-hr 2.29 0.10 3.27 0.14 3.91 0.16 4.69 0.20 5.81 0.24
Note:
10/20/2021
1. Source: Santa Clara County Drainage Manual.
Duration
Table 3.2 Precipitation Depth-Duration-Frequency Comparison
Storm Drainage System Master Plan
City of Gilroy
2004 Master Plan Santa Clara County Drainage Manual NOAA Atlas 141
10-Year 100-Year 10-Year 100-Year 10-Year 100-Year
(in)(in/hr)(in)(in/hr)(in)(in/hr)(in)(in/hr)(in)(in/hr)(in)(in/hr)
5-min 0.18 2.16 0.27 3.24 0.24 2.93 0.34 4.12 0.26 3.10 0.41 4.97
10-min 0.26 1.56 0.39 2.34 0.33 1.99 0.47 2.79 0.37 2.22 0.59 3.56
15-min 0.32 1.28 0.49 1.96 0.39 1.57 0.56 2.26 0.45 1.79 0.72 2.87
30-min 0.47 0.94 0.71 1.42 0.53 1.06 0.76 1.51 0.62 1.24 0.99 1.98
1-hr 0.69 0.69 1.04 1.04 0.73 0.73 1.02 1.02 0.87 0.87 1.40 1.40
2-hr 1.01 0.51 1.52 0.76 1.09 0.54 1.48 0.74 1.33 0.67 2.14 1.07
3-hr 1.26 0.42 1.89 0.63 1.39 0.46 1.88 0.63 1.68 0.56 2.71 0.90
6-hr 1.84 0.31 2.77 0.46 2.08 0.35 2.85 0.48 2.39 0.40 3.86 0.64
12-hr 2.69 0.22 4.05 0.34 2.94 0.24 4.15 0.35 3.40 0.28 5.46 0.46
24-hr 3.93 0.16 5.92 0.25 3.91 0.16 5.81 0.24 4.69 0.20 7.51 0.31
Note:
10/20/2021
1. Source: NOAA Atlas 14 Volume 6 Version 2 for Gilroy.
Duration
March 2023 3-4 City of Gilroy
Storm Drainage System Master Plan
been consistently increasing over the years. City staff selected to maintain consistency with Santa
Clara Drainage Manual.
• Duration. The duration of the storm can be little as 5-minutes and as high as 24-hours as
documented on Table 3.1. This storm drainage master plan uses 24-hours as the
duration, and the distribution is based on NOAA Atlas 14 Balanced Centered Hyetograph.
The balance centered hyetograph provides a design storm with the peak intensity
occurring during the middle of the day (Figure 3.1).
• Frequency. The frequency of the storm is the recurrence interval at which the storm may
occur at a given area.
3.1.3 Design Storm Events
The design storms used in this evaluation, and which are specific to the City, are listed on Table
3.3. The values used in evaluating the storm drainage system are as follows:
• 10-year 24-hour. This design storm is used to evaluate pipeline conveyance facilities as
well as to size future ones. This storm was quantified at 3.91 inches.
• 100-year 24-hour. This design storm is used in evaluating City street performance for
conveying stormwater flows, while allowing up to one foot of street flooding; this storm was
quantified at 5.81 inches.
3.1.4 Soil Imperviousness
In determining the quantity of rainfall runoff generated from a given land use type, three factors
dictate the volume of water that enters the storm drainage system: effective imperviousness,
ineffective imperviousness, and effective pervious area.
• Effective Impervious. An effective impervious area is the percentage of impervious area
that generates stormwater runoff entering the storm drainage system. The effective
impervious percentages are based on land uses identified in the 2040 General Plan, and
are included in Table 3.4.
• Ineffective Impervious. An ineffective impervious area is land that has no flow path, or
the flow path results in delayed timing of the runoff, to the storm drainage system. These
areas are typically noted as residential backyards, pools, or dense shrub landscaping. The
values for these areas are shown on Table 3.4.
• Effective Pervious. Effective pervious areas contribute to runoff based on the National
Resource Conservation Service (NRCS) Soil Classification Group, and the subsequent
SCS Curve Number. The Curve Number is listed by soil group in Table 3.4.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 2 4 6 8 10 12 14 16 18 20 22Unit Intensity (Unitless)Time (hours)
Rainfall Unit Intensity (Design Storm: 3.91 inches)
January 15, 2021
Figure 3.1
10-Year/24-Hour
Unit Hyetograph
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Table 3.3 Relevant Design Storms
Storm Drainage System Master Plan
City of Gilroy
Design Criteria Design Storm
10-year 24-hour
Used to evaluate if Hydraulic Grade Line have a minimum
of one foot of freeboard between top of curb. Quantified
at 3.91 inches.
100-year 24-hour
Used in evaluating street conveyance capacity and
determining if street flooding exceeds one foot in depth.
Quantified at 5.81 inches.
10/20/2021
Table 3.4 Soil Imperviousness and SCS Curve Numbers
Storm Drainage System Master Plan
City of Gilroy
AMC-II Pervious
Soil B
AMC-II Pervious
Soil C
AMC-II Pervious
Soil D
(%)(%)(CN)(CN)(CN)
Rural Residential 5 0 63 73 82
Hillside 15 5 65 75 83
Low Density 35 15 67 77 83
Medium Density 50 5 69 79 84
High Density 60 0 71 81 86
Neighborhood District 2 0 78 85 89
Commercial 95 0 80 87 90
Industrial 70 0 76 84 88
Hecker Pass Special Use 1 0 61 73 80
Other Uses
Water Surface, Ponds, Streams, Canals 100 0 99 99 99
Natural Grassland 1 0 65 77 82
Floodplains 1 0 55 70 76
Parks, Athletic Fields, Cemetaries 0 74 83 87 0
Government, Schools 80 0 79 83 89
Note:
9/16/2020
1. Source: City of Gilroy 2004 Storm Drainage System Master Plan.
Land Use Category
Effective Percent
Impervious
Ineffective Percent
Impervious
Effective Pervious
March 2023 3-8 City of Gilroy
Storm Drainage System Master Plan
3.2 HYDRAULIC CRITERIA
The hydraulic criteria for the storm drainage system were used to evaluate the capacity
requirements of conveyance facilities.
3.2.1 Gravity Conveyance Facilities
Gravity pipeline capacities depend on several factors including: material and roughness of the
pipe, the limiting velocity and slope, and the maximum allowable depth of flow. The hydraulic
modeling software used for evaluating the capacity adequacy of the City storm drainage system is
InfoSWMM by Innovyze Inc. This software uses the fully dynamic St. Venant’s Equation, and has
a more accurate engine for simulating backwater and surcharge, in addition to manifolded force
mains. The software also incorporates the use of the Manning Equation in other calculations
including upstream pipe flow conditions.
Storm Hydraulic Design
The 10-year 24-hour design storm was used to evaluate the City’s existing stormwater system as
well as to size pipelines to serve areas of future growth. (Table 3.5). For the the 100-year 24-hour
design storm, City streets were allowed to flood in some cases to provide reasonable conveyance
and storage capacity, thus reducing additional costly improvements.
Manning’s Equation for Pipe Capacity
The Continuity Equation and the Manning’s Equation for steady-state flow are used for calculating
pipe capacities in open channel flow. Open channel flow can consist of either open conduits or, in
the case of gravity pipelines, partially full closed conduits. Gravity full flow occurs when the
conduit is flowing full but has not reached a pressure condition.
• Continuity Equation: Q = V A
Where:
Q = peak flow, in cubic feet per second (cfs)
V = velocity, in feet per second (fps)
A = cross-sectional area of pipe, in square feet (sq. ft.)
• Manning Equation: V = (1.486 R2/3 S1/2)/n
Where:
V = velocity, in feet per second (fps)
n = Manning’s roughness coefficient
R = hydraulic radius, area divided by wetted perimeter (ft)
S = slope of pipe, in feet per foot (ft/ft)
Table 3.5 Hydrologic and Hydraulic Design Criteria
Hydrology and Hydraulics Design Criteria
Conveyance Facilities
Pipelines
10-Year 24-Hour design storm to determine if storm water
have a minimum of one foot of freeboard between the top
of curb and the Hydraulic Grade Line.
Streets
100-Year 24-Hour design storm to determine if flooding
exceeds one foot in depth and can flood buildings or create
safety hazards.
Receiving Waters
(Streams, Creeks, Channels)
FEMA 100-year water surface elevations used for
downstream control for facilities where 100-year flood maps
are available.
9/16/2020
Storm Drainage System Master Plan
City of Gilroy
March 2023 3-10 City of Gilroy
Storm Drainage System Master Plan
St. Venant’s Equation for Pipe Capacity
A dynamic type of modeling facilitates the analysis of unsteady and non-uniform flows (dynamic
flows) within a gravity conveyance system. Some hydraulic modeling programs have the ability to
analyze these types of flows using the St. Venant Equation, which take into account unsteady and
non-uniform conditions that occur over changes in time and cross-section within system pipes.
The St. Venant Equations are a set of two equations, a Continuity Equation and a Dynamic
Equation, used to analyze dynamic flows within a system. The first equation, the Continuity
Equation, relates the continuity of flow mass within the system pipes in terms of: (A) the change in
the cross-sectional area of flow at a point over time, and (B) The change of flow over the distance
of piping in the system. The continuity equation is shown as follows:
• Continuity Equation: 𝜕𝐴
𝜕𝑡+𝜕𝑄
𝜕𝑥=0
(A) (B) __
Where:
t = time
x = distance along the longitudinal direction of the channel
Q = discharge flow
A = flow cross-sectional area perpendicular to the x directional axis
The second equation, the Dynamic Equation, relates changes in flow to fluid momentum in the
system using: (A) Changes in acceleration at a point over time, (B) Changes in convective flow
acceleration, (C) Changes in momentum due to fluid pressure at a given point, (D) Changes in
momentum from the friction slope of the pipe, and (E) Fluid momentum provided by gravitational
forces. The Dynamic Equation is provided as follows:
• Dynamic Equation: 𝜕𝑄
𝜕𝑡+𝜕
𝜕𝑡(𝛽𝑄2
𝐴)+𝑔𝐴𝜕𝑥
𝜕𝑥+𝑔𝐴𝑆𝑓−𝑔𝐴𝑆𝑜=0
• (A) (B) (C) (D) (E) __
Where:
t = time
x = distance along the longitudinal direction of the channel
Q = discharge flow
A = flow cross-sectional area perpendicular to the x directional axis
y = flow depth measured from the channel bottom and normal to the x
directional axis
Sf = friction slope
So = channel slope
β = momentum
g = gravitational acceleration
Use of this method of analysis provides a more accurate and precise analysis of flow conditions
within the system compared to steady state flow analysis methods. It must be noted that two
March 2023 3-11 City of Gilroy
Storm Drainage System Master Plan
assumptions are made for use of St. Venant Equations in the modeling software. First, flow is one
dimensional. This means it is only necessary to consider velocities in the downstream direction
and not in the transverse or vertical directions. Second, the flow is gradually varied. This means
the vertical pressure distribution increases linearly with depth within the pipe.
Manning’s Roughness Coefficient (n)
The Manning Roughness Coefficient ‘n’ is a friction coefficient that is used in the Manning formula
for flow calculation in open channel flow. In conveyance systems, the coefficient can vary between
0.011 and 0.017 depending on pipe material, size of pipe, depth of flow, root intrusion,
smoothness of joints, and other factors.
For the purpose of this evaluation, an “n” value of 0.013 was used for both existing and proposed
gravity pipes unless directed otherwise by City staff based on pipe structural condition. This “n”
value is an acceptable practice in planning studies.
March 2023 4-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
4.0 CHAPTER 4 – EXISTING FACILITIES AND MODEL DEVELOPMENT
This chapter defines the hydrologic delineation of storm drainage areas, routing to their respective
receiving facilities, and includes the hydrologic model development. Additionally, this chapter
includes an overview of the storm drainage system, and the model development.
4.1 HYDROLOGIC SYSTEM OVERVIEW
Factors critical to the hydrologic model development include watersheds, drainage areas within
each watershed, overland flow routing within drainage sub-catchments, and conveyance that
makes full use of pipes as well as streets for routing 100-year design storm events. This section
discusses the existing watersheds, and the delineation of drainage areas and sub-catchments.
4.1.1 Watersheds
Watersheds within the City Planned Area Boundary and outlying areas were defined based on the
receiving tributary creek or river system, with topographical or physical barriers dictating the limits
of the watershed. The City has multiple natural watersheds that collect and convey stormwater
runoff within Urban Growth Boundary: Uvas Creek Watershed, Llagas Creek Watershed, and
Pajaro River Watershed (Figure 2.2).
4.1.2 Drainage Areas
Drainage areas are typically defined by existing or natural conveyance systems within each
watershed. The City’s drainage area boundaries are generally dictated by the open channel
conveyance facilities throughout the City (Figure 4.1). Drainage areas in the City discharge
stormwater runoff into creeks, channels or sloughs that flow through the City.
Each drainage area was assigned a unique identifier intended for cross-referencing purposes.
The identifier reflects the name of the receiving water body or storm drainage infrastructure that
receives the stormwater runoff. The project divided the City into seven drainage areas, which are
briefly summarized as follows:
• Llagas Drainage Area: The Llagas drainage area is generally defined as the area east of
Highway 101 and west of Llagas Creek, with the northern and southern boundaries
approximately represented by Miller Slough Channel and Southside Drive respectively.
• Miller Slough Drainage Area. The Miller Slough drainage area is generally defined as the
area east of Santa Teresa Boulevard and west of Highway 101, with the northern and
southern boundaries approximately represented by La Primavera Way and First Street
respectively. This drainage area generally flows in a northerly direction towards the Miller
Slough Channel.
£¤101UV152SUNRISE DRSANTA TERESA BLKERN AVWREN AVTHIRD STSECOND STP RI N C E V A L L E S TSIXTH STTENTH STUVAS PARK DRM U R R AY A V RENZ LNLEWIS STM O N T E R E Y S T
GOLDEN GATE AVBUENA VISTA AVV
M AR C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A NT A T E RES A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAG LE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekLlagas CreekMiller SloughR o n a n Ch a n n e lSouthUvasNorthUvasNorthUvasPrincevalleLlagasLowerMiller SloughMillerSloughRonanChannelLowerMiller SloughRonanChannelUvas CreekLegendHydrologic Drainage AreasLlagasMiller SloughLower Miller SloughNorth UvasPrincevalleRonan ChannelSouth UvasChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodiesPrincevalleM ille rChannelL o w e r S lo u g h Figure 4.1Storm DrainageAreasStorm Drainage System Master Plan City of Gilroy5Updated: April 15, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig4-2_DrainageBasins_41522.mxd
March 2023 4-3 City of Gilroy
Storm Drainage System Master Plan
• Lower Miller Slough Drainage Area. The Lower Miller Slough drainage area is generally
defined as the area east of Carmel Street and west of Highway 101, with the northern and
southern boundaries approximately represented by Second Street and Tenth Street
respectively. This drainage area is generally flows in easterly direction towards the Lower
Miller Slough Channel.
• North Uvas Drainage Area. The North Uvas drainage area is generally defined as the
area east of Burchell Road and Santa Barbara Drive, with northern and southern
boundaries approximately represented by Olea Court and Uvas Creek respectively. This
drainage area is generally flows in southerly direction towards the Uvas Creek.
• Princevalle Drainage Area. The Princevalle drainage area is generally defined as the
area east of Santa Barbara Drive and west of Cameron Boulevard, with the northern and
southern boundaries approximately represented by First Street and Luchessa Avenue
respectively. This drainage area generally flows in a southeasterly direction, though it
currently is drained, via storm drainage system, in a southerly direction, towards the
Princevalle Channel.
• Ronan Channel Drainage Area. The Ronan Channel drainage area is generally defined
as the area east of Jean Ellen Court and west of Highway 101, with northern and southern
boundaries approximately represented by Fitzgerald Avenue and First Street respectively.
• South Uvas Drainage Area. The South Uvas drainage area is generally defined as the
area east of the Rancho Vista Drive and west of Highway 101, with the southeastern
boundaries approximately represented by Uvas Creek. This drainage area generally flows
in a northeasterly direction towards the Uvas Creek.
4.1.3 Drainage Sub-catchment
Each drainage area shown on Figure 4.1 was further divided into smaller sub-catchments for the
purpose of routing tributary hydrologic stormwater flows to appropriate stormwater manhole. The
connectivity within each sub-catchment is very critical for accurate analysis and was thus
graphically documented on exhibits included in Appendix B. Each sub-catchment included
hydrologic modeling information to mimic Overland Flow Routing and Combined Pipe Street
Conveyance.
• Overland Flow Routing. This element consists of routing rainfall runoff from sub-
catchments to stormwater manhole. Overland flow routing is dependent on many factors
such as land use, subsurface soil infiltration characteristics and physical barriers blocking
the flow paths within the drainage sub-catchment. The nonlinear reservoir method is
typically used to calculate overland flow and was built into the hydrologic component of the
model.
March 2023 4-4 City of Gilroy
Storm Drainage System Master Plan
• Combined Pipe Street Conveyance. During typical storm events, and up to 10-year 24-
hour design storms, streets are used to convey rainfall runoff from house gutters to
nearest catchments where it enters the conveyance system.
To mitigate very costly conveyance facility improvements during more intense storm
events and up to the 100-year 24-hour design storm, this analysis assumed that existing
streets may also provide additional capacity for routing rainfall runoff at a flood depth not to
exceed one foot.
4.2 HYDRAULIC SYSTEM OVERVIEW
The City’s storm drainage system consists of approximately 132 miles of up to 84-inch diameter
gravity pipelines that convey stormwater runoff as shown on Figure 4.2. A system-wide pipeline
inventory, listing the total length by pipe diameter and material, is shown on Table 4.1. This table
is based on information extracted from the City’s GIS database and was updated to reflect the
review of construction drawings provided by City staff. The 15-inch and 18-inch diameter pipeline
accounts for 50 percent of the total storm drainage pipeline lengths. The following section
discusses the components of the storm drainage system.
4.2.1 Conveyance System
The modeled storm drainage system includes approximately 55 miles of stormwater conveyance
to local water bodies, or channels (Table 4.2). Pipe sizes range from 18-inches to 84-inches in
diameter, and are shown on Figure 4.2. The modeled storm conveyance system is predominantly
composed of 18-, 24-, and 36-inch diameter pipelines.
The City-owned open channel conveyance systems includes the following: Ronan Channel, Miller
Slough and Princevalle Channel. These channels play an important role in relieving stormwater
runoff during intense rainfall events.
4.2.2 Outfalls
The modeled system consists of more than 60 outfall points that discharge runoff to natural
creeks, sloughs, and channels. These outfalls were located along the Uvas Creek, Princevalle
Channel, Miller Slough Channel, Lower Miller Slough Channel, Ronan Channel, and Llagas Creek
as shown in Figure 4.2.
!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!CV
£¤101UV152SUNRISE DRSANTA TERESA BLKERN AVWREN AVTHIRD STSECOND STP RI N C E V A L L E S TSIXTH STTENTH STUVAS PARK DRM U R R A Y A V RENZ LNLEWIS STMO N T E R E Y S T
GOLDEN GATE AVBUENA VISTA AVM AR C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A NT A T E RES A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAGLE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekLlagas CreekUvas Creek303042182442483024481860335442722118184242361818183024364824242430242718243048302418181818241824302121183 6
2 448 363 0 424 2 42182142243624242418242418241818181818182 4
1827 242 4
2 4
2 4
184 8 607 218182 4
2 7
183 3
2 4 3636183636363 0 364224
6 05442
362418
1 8
27 3 6 2 1
72
241 8 24848448303024181824181 8
241 8
6 0
331821301821242730/42182724302418242427272418365418
1836361836181 8
18271 8
42181821181824244 242
1 8 1 8
184 2
1 8
1 8
181818423636Miller SloughR o n a n Ch a n n e l214 23648
3021PrincevalleM ille rChannelL o we r S lo u g h
242424363642246 0
6 0,,,,,,,,,,,,
,,,,,,,,,,,,,,,,
,
,,,,
,
,,,
,,,,,,,,,,,,,,,DAY RDPUEBLO STFigure 4.2Existing Modeled StormDrainage SystemStorm Drainage System Master Plan City of GilroyLegendExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsNon-Modeled SystemPipesChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 14, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig4-1_ExistingSys_041422.mxd
Table 4.1 Existing GIS Storm Pipe Inventory
Storm Drainage System Master Plan
City of Gilroy
Length In Material
RCP CMP CIP Concrete HDPE PVC VCP ACP DIP Unknown
(in)(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft)(ft)
Unknown Diameter 32,806 88 0 83 0 12 0 0 0 33,525 66,514
4 19 0 0 0 71 3,165 0 0 0 0 3,255
6 2,733 0 0 0 16 5,802 603 115 0 944 10,214
8 5,816 0 177 0 0 500 0 351 0 817 7,661
10 2,200 27 226 0 0 362 0 354 0 0 3,169
12 22,978 491 283 1,031 2,205 3,996 39 197 0 377 31,597
15 110,396 576 157 608 1,614 1,397 1,097 41 0 1,024 116,910
18 204,862 609 19 1,666 4,399 8,219 0 0 394 551 220,720
21 12,428 257 0 362 0 0 0 0 0 0 13,047
24 74,554 810 0 0 2,705 446 0 0 0 648 79,163
27 14,574 147 0 904 0 0 0 0 0 0 15,625
30 27,537 112 0 192 0 22 0 0 0 422 28,285
33 3,829 0 0 0 0 413 0 0 0 0 4,242
36 31,597 180 1,144 0 333 0 0 0 0 0 33,254
42 18,409 0 0 0 0 0 335 0 0 0 18,743
48 22,485 808 0 0 0 0 0 0 0 0 23,293
54 3,028 0 0 0 0 0 0 0 0 0 3,028
60 7,495 162 0 0 0 0 0 0 0 0 7,656
72 4,208 0 174 0 0 2,281 0 0 0 0 6,664
84 1,137 0 0 0 0 0 0 0 0 0 1,137
Total (ft)603,090 4,268 2,179 4,846 11,343 26,617 2,073 1,058 394 38,309 694,176
Total (miles)114.2 0.8 0.4 0.9 2.1 5.0 0.4 0.2 0.1 7.3 131.5
Note:
9/28/2020
1. Source: GIS information provided by City Staff on September 04, 2020.
Pipe Size Total Length
Table 4.2 Existing Modeled Storm Pipe Inventory
Storm Drainage System Master Plan
City of Gilroy
Pipe Size Length Percent of Total
System
(in)(ft)(miles)(%)
18 101,964 19.3 35.14%
21 12,908 2.4 4.45%
24 58,024 11.0 19.99%
27 12,974 2.5 4.47%
30 23,095 4.4 7.96%
33 3,609 0.7 1.24%
36 28,224 5.3 9.73%
42 18,721 3.5 6.45%
48 16,129 3.1 5.56%
54 2,624 0.5 0.90%
60 6,830 1.3 2.35%
72 3,976 0.8 1.37%
84 1,117 0.2 0.38%
Total 290,194 55.0 100%
10/5/2021
March 2023 4-8 City of Gilroy
Storm Drainage System Master Plan
4.3 MODEL DEVELOPMENT
A GIS-based model of the City’s storm drainage system was developed to identify capacity
deficiencies and to recommend improvements to mitigate those deficiencies. This section
discusses the modeling software, land use and routing methodology.
4.3.1 Modeling Software
The hydrologic component of the drainage model generates runoff through synthetic design storm
events and sub-catchment parameters that mimic actual surface response to rainfall. The
hydraulic component of the drainage model routes stormwater runoff from source (loading
manholes) to the discharge points (outfalls) by using physical characteristics of the storm drainage
system (pipelines). The hydraulic model then performs calculations and solves series of equations
to simulate flows in pipes, including backwater calculations for surcharged conditions.
There are several network analysis software products released by different manufacturers that
can equally perform the hydrologic and hydraulic analysis satisfactorily. The selection of a
particular software depends on user preferences, the storm drainage system’s unique
requirements, and the costs for purchasing and maintaining the software.
The modeling software used for evaluating the capacity adequacy of the Gilroy storm drainage
system, InfoSWMM by Innovyze Inc. This software uses the fully dynamic St. Venant’s Equation,
and has a more accurate engine for simulating backwater and surcharge. The software also
incorporates the use of the Manning Equation in other calculations including upstream pipe flow
conditions. The St Venant’s and Manning’s Equations were discussed in the System Performance
and Design Criteria chapter.
4.3.2 Land Use and Routing Methodology
The sub-catchment parameters in the model were populated to account for land use types, flood
routing, and routing methodology.
• Land Use. Land use was used to define impervious area and the SCS Curve Number for
pervious runoff. Residential areas included soil classifications to define non-effective
runoff mostly confined in backyards.
• Flood Routing. Flood routing consists of determining the flow path and connectivity to the
storm drainage collection system.
• Routing Methodology. Nonlinear reservoir method is used for transforming precipitation
into runoff for overland flow routing, street flow, and pipe conveyance. This is typically
defined by the time of concentration, which represents the time it takes for overland flow to
reach from the furthest point in a sub-catchment to the outfall. It is calculated as the sum of
the inlet time required to reach a manhole and the travel time in the conveyance system
from the manhole to the outfall.
March 2023 4-9 City of Gilroy
Storm Drainage System Master Plan
4.3.3 Hydrological Soil Classification
Soils are classified into four hydrologic soil units. These groups are A, B, C, and D, where A soil
are characterized by having high infiltration rates and low potential runoff and D soil are
characterized by having very slow infiltration rates and high runoff potential. The City hydrological
soil classification extracted from Soil Survey Geographic database (SSURGO) is shown
graphically on Figure 4.3.
4.3.4 Typical Channel Cross-Section
The majority of the City’s storm drainage system are pipelines that drain into the major channels
discussed in the previous section. Typical channel cross-sections are shown graphically on
Figure 4.4 and were used to represent some of the channels in the models.
4.3.5 Physical Hydraulic Model Elements
The City’s GIS database was used as a primary source to define physical hydraulic model
elements. Data gaps related to inverts and ground elevations were resolved through review of as-
built drawings. The physical hydraulic model was populated to include rim elevations at manholes,
invert elevations of pipelines, pipe sizes, pipe slopes, pipe lengths, and outfall elevations.
£¤101UV152SUNRISE DRSANTA TERESA BLKERN AVWREN AVTHIRD STSECOND STP R I N C E V A L L E S TSIXTH STTENTH STUVAS PARK DRM U R R A Y A V RENZ LNLEWIS STM O N T E R E Y S T
GOLDEN GATE AVBUENA VISTA AVV
M A R C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A N T A T E R E S A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAG LE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekUvas CreekLlagas CreekMiller SloughR o n a n Ch a n n e lFigure 4.3Hydraulical SoilClassificationStorm Drainage System Master PlanCity of Gilroy5Updated: February 9, 2022GIS00.510.25MileLegendSubbasin Soil TypeType BType CType DChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodiesFile Path & Name: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig4-3_SoilClassification_020922.mxdPrincevalleM ille rChannelL o w e r S lo u g h
Miller Slough
X
2
16
28
42
Lower Miller Slough
X
2
28.4
42.4
68.8
February 26, 2021
0
1
2
3
4
5
6
7
8
9
0 10 20 30 40 50Depth, ftMiller Slough
Figure 4.4
Channel Cross-Section
Storm Drainage System Master
Plan
City of Gilroy
3
LEGEND
T = 40'
y = 8'
b = 12'
1
1.75
0
2
4
6
8
10
12
14
0 20 40 60 80 100 120Depth, ftRonan
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15 20 25 30 35Depth, ftPrincevalle
0
2
4
6
8
10
12
14
0 10 20 30 40 50 60 70 80Depth, ftLower Miller Slough
T = 67'
y = 12'1
2.2
T =102'
y = 13'1
2
T = 30'
y = 9'1
1.5
b = 14'
b = 50'
b = 3'
T: Top Width
b: Bottom Width
y:Depth
Note:
Channel cross-section estimated using SCVWD contours.
March 2023 5-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
5.0 CHAPTER 5 – EVALUATION AND PROPOSED IMPROVEMENTS
This chapter presents a summary of the storm drainage system evaluation and identifies
improvements needed to mitigate existing deficiencies as well as improvements needed to
expand the system to service future growth.
5.1 OVERVIEW
The City’s stormwater model was used to evaluate the current storm drainage system, identify
capacity deficiencies and to recommend improvements required to serve future growth based on
the relevant design storms. The criteria used for evaluating the capacity adequacy of the storm
drainage system and used for sizing recommended improvements were discussed and
summarized in the System Performance and Design Criteria chapter.
5.2 KNOWN AREAS OF CONCERN
The capacity evaluation phase also considered a detailed review of known areas of concern. City
staff have maintained an inventory of known areas of concern that may experience surface water
ponding during large storm events. The known areas of concern provided by City staff are
documented on Table 5.1. The capacity improvement recommended in this chapter are intended
to alleviate some of these problem areas.
5.3 PIPELINE CONVEYANCE CAPACITY EVALUATION
The 10-year 24-hour design storm was used for evaluating the capacity adequacy of pipeline
conveyance facilities. The existing capacity analysis indicates that the conveyance facilities are
generally adequate to accommodate the design storm, with certain areas experiencing
surcharging and flooding as shown on Figure 5.1.
As part of the Master Plan, an analysis of the recently updated NOAA Atlas 14 precipitation was
completed and corresponding recommendations were presented to City staff for review. City staff
reviewed and requested a similar analysis using the precipitation data from the Santa Clara
County Drainage Manual. City staff chose to maintain the precipitation data from the Santa Clara
County Drainage Manual for the purposes of developing the capital improvement program (CIP)
discussed in a later chapter.
5.4 CAPACITY EVALUATION ANALYSIS
This Master Plan evaluated the storm drainage system infrastructure and recommend
improvements to mitigate existing system deficiencies and future development at the buildout.
Two alternatives were considered and evaluated, discussed as follows:
!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!C!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(!(V
£¤101UV152SUNRISE DRSANTA TERESA BLKERN AVWREN AVTHIRD STSECOND STP R IN C E V A L L E S TSIXTH STTENTH STUVAS PARK DRM U R R A Y A V RENZ LNLEWIS STM O N T E R E Y S T
GOLDEN GATE AVBUENA VISTA AVM A R C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A N T A T E R E S A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAGLE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekLlagas CreekUvas Creek303042182442483024481860335442722118184242361818183024364824242430242718243048302418181818241824302121183 6
2 448 363 0 424 2 42182142243624242418242418241818181818182 4
1 8
27242 4
2 4
2 4
184 8 607 218182 4
2 7
183 3
2 4 3636183636363 0 364224
6 05442
36241 8
1 8
27 3 6 2 1
7 2
241 8 24848448303024181824181 8
241 8
6 0
331821301821242730/4218272430241824242727241836541 8
1836361836181 8
18271 8
42181821181824244 242
1 8 1 8
184 2
1 8
1 8
181818423636Miller SloughR o n a n Ch a n n e l214 23648
3021PrincevalleM ille rChannelL o w e r S lo u g h
242424363642246 0
6 0,,,,,,,,,,,,
,,,,,,,,,,,,,,,,
,
,,,,
,
,,,
,,,,,,,,,,,,,,,DAY RDPUEBLO STFigure 5.1Existing System Analysis10-Year Design StormStorm Drainage System Master PlanCity of GilroyLegend!(Flooding Manholes!(Surcharging ManholesExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 15, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig5-1_10YrStorm_041522.mxd
Table 5.1 Storm Drainage System Problem Areas
Storm Drainage System Master Plan
City of Gilroy
No.Type of Concern Location
1 Leaves on Catch Basin Monterey Road & First Street
2 Flooding First Street & Church Street
3 Flooding Third Street & Church Street
4 Leaves on Catch Basin Sixth Street & Church Street
5 Flooding Seventh Street & Church Street
6 Flooding Luchessa Avenue between Monterey Road &
Chestnut Street
7 Flooding Chestnut Street between Eighth Street & Ninth
Street
8 Leaves on Catch Basin Church Street & Howson Street
9 Flooding Welburn Avenue & Hanna Street
10 Leaves on Catch Basin Welburn Avenue & Westwood Drive
11 Flooding Wren Avenue & Ronan Avenue
12 Leaves on Catch Basin Driftwood Terrace & Westwood Drive
13 Flooding Hanna Street & Broadway
14 Flooding Hanna Street & Third Street
15 Flooding Rosanna Street & Third Street
16 Flooding Dowdy Street & Fifth Street
17 Leaves on Catch Basin Fourth Street & Filice Drive
18 Leaves on Catch Basin Fourth Street & Monte Vista Way
19 Leaves on Catch Basin Fourth Street & Miller Avenue
20 Leaves on Catch Basin Sixth Street & Princevalle Street
21 Flooding Forest Street & Martin Street
Note:
12/8/2021
1. Storm drainage problem locations provided by City staff on March 24, 2021.
March 2023 5-4 City of Gilroy
Storm Drainage System Master Plan
5.4.1 Alternative 1 - 8th Street 36-inch Pipeline Improvements
This alternative consists of a total 10.1 miles of pipeline improvements, including paralleling the
existing 36-inch diameter storm drainage pipeline along 8th Street under Highway 101. However,
this alternative requires coordination with the California Department of Transportation (Caltrans)
for the 36-inch diameter parallel pipeline. In summary, this alternative will require the following
improvements, which are shown graphically on Figure 5.2.
• 10.1 miles of new pipeline ranging in size from 24 to 90 inches in diameter.
As part of this master plan, preliminary costs were prepared as a means of comparing
infrastructure improvements alternatives. The total approximate cost estimate for this alternative is
at $40.7 million.
5.4.2 Alternative 2 – Diverting Stormwater Runoff to Princevalle Channel
Following the review of the first alternative improvements, city staff requested Akel to evaluate the
feasibility of diverting stormwater runoff south to Princevalle Channel, as a second alternative.
This alternative consists of a total 9.3 miles of pipeline improvements, none of the which
improvements are crossing Highway 101 and therefore do not require coordination with Caltrans.
In summary, this alternative will require the following improvements, which are shown graphically
on Figure 5.3.
• 9.3 miles of new pipeline ranging in size from 24 to 90 inches in diameter.
The total estimated cost for this alternative is estimated at
$36.4 million. This alternative was selected by City staff
due to the lower capital improvement costs and
construction feasibility. In comparison, alternative 1 costs
were estimated at $40.7 million and that alternative
require the construction of 36-inch pipeline under
Highway 101.
Since Alternative 2 is the recommended alternative, a
more detailed listing of improvements is summarized on Table 5.2, and which are shown
graphically and cross reference on Figure 5.3. Each pipeline has been assigned a uniquely coded
identifier corresponding to the drainage area in which it resided; this unique identifier is intended
to aid in defining the location of the improvement for mapping purposes. The Alternative 2
improvements are also briefly described in this section, by drainage area:
Alternative 1
Proposed Improvement Costs: $40.7
million dollars. Also requires construction
of 36-inch pipeline under Highway 101.
Alternative 2 (Recommended)
Recommended Improvement Costs:
$36.4 million dollars
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Table 5.2 Proposed Improvements
Storm Drainage System Master Plan
City of Gilroy
Diameter Length
(in)(in)(ft)
Ronan Channel Drainage Area
RC-1 Sunrise Dr From Dream Catcher Way to Golden Sky Way 18 Replacement 30 400
RC-2 Mantelli Dr From Muir Drive to Delta Dr 18 Replacement 24 1,250
RC-3 Zamzow Ct From approx. 1,250' n/o Cohansey Ave to
Cohansey Ave -New 48 1,200
RC-4 Cohansey Ave From approx. 350' e/o Monterey Rd to Murray
Ave -New 48 1,450
RC-5 Murray Ave From Cohansey Ave to Tomkins Ct -New 48 2,800
RC-6 Murray Ave From Tomkins Ct to Ronan Channel 24 Replacement 48 600
RC-7 No Name Uno/Las Animas
Ave
From approx. 1,500' s/o Buena Vista Ave to San
Ysidro Ave -New 36 3,400
RC-8 San Ysidro Ave From Las Animas Ave to approx. 500' n/o
Leavesley Rd -New 48 3,425
RC-9 Leavesley Rd From approx. 220' e/o Forest St to approx. 360'
e/o Forest St 18 Replacement 42 150
Miller Slough Drainage Area
MS-1 Wren Ave From approx. 290' n/o First St to Lawrence Dr 24 Replacement 30 2,325
MS-2 First St From approx. 180' e/o Princevalle St to Sargent St 18 Replacement 24 525
MS-3 Sargent St From First St to Broadway 18 Replacement 24 425
MS-4 Broadway From Sargent St to Church St 18/21 Replacement 30 1,075
MS-5 Church St From Broadway to approx. 100' s/o Howson St 21 Replacement 30 400
MS-6 Monterey Rd From Second St to First St 18/24 Replacement 27 675
North Uvas Drainage Area
NU-1 Hecker Pass Hwy From approx. 3,080' w/o Santa Teresa Blvd to
approx. 360' w/o Santa Teresa Blvd -New 30 2,825
NU-2 Hecker Pass Hwy From approx. 360' w/o Santa Teresa Blvd to Santa
Teresa Blvd 24 Replacement 30 350
NU-3 Santa Teresa Blvd From approx. 70' s/o Hecker Pass Hwy to Uvas
Creek 24 Replacement 30 2,575
NU-4 Westwood Dr From approx. 150' s/o Third St to Kentwood Ct 18 Replacement 24 425
NU-5 Monterey Rd From Princevalle Channel to Victoria Dr -New 90 775
NU-6 Monterey Rd From Victoria Dr to approx. 950' n/o Farman Ln 42 Replacement 90 2,175
NU-7 ROW From Monterey Rd to approx. 970' n/o Uvas
Creek -New 90 975
NU-8 ROW From approx. 970' n/o Uvas Creek to Uvas Creek -New 90 1,000
South Uvas Drainage Area
SU-1 St Andrews Cir From Carnoustie Ct to approx. 380' nw/o
Carnoustie Ct 18 Replacement 36 375
Improv.
No.Alignment Limits Existing
Diameter
Pipeline Improvements
New/Parallel/
Replace
Table 5.2 Proposed Improvements
Storm Drainage System Master Plan
City of Gilroy
Diameter Length
(in)(in)(ft)
Improv.
No.Alignment Limits Existing
Diameter
Pipeline Improvements
New/Parallel/
Replace
Princevalle Drainage Area
PR-1 Third St From Santa Paula Dr to Wren Ave 27 Replacement 30 825
PR-2 Third St From Wren Ave to approx. 170' e/o Santa Theresa
Dr 27 Replacement 42 675
PR-3 Third St From approx. 170' e/o Santa Theresa Dr to Santa
Theresa Dr 27 Replacement 48 200
PR-4 Santa Theresa Dr From Third St to approx. 300' s/o Fourth St 27/30 Replacement 48 950
PR-5 ROW From Santa Theresa Dr to Miller Ave 30 Replacement 48 675
PR-6 Miller Ave From approx. 300' s/o Fourth St to Fifth St 30 Replacement 48 400
PR-7 Fifth St From Miller Ave to Princevalle St 30 Replacement 48 1,250
PR-8 Princevalle St From Third St to Fifth St 18 Replacement 30 1,250
PR-9 Princevalle St From Fifth St to Seventh St 42 Replacement 54 1,225
PR-10 Alexander St From Tenth St to Banes Ln 18 Replacement 24 825
PR-11 Banes Ln From Alexander St to Automall Pkwy 18 Replacement 24 625
PR-12 Monterey Rd From approx. 380' n/o 8th St to Angra Way -New 48 2,475
PR-13 Monterey Rd From Angra Way to Princevalle Channel 42 Replacement 48 775
Lower Miller Slough Drainage Area
LM-1 Fourth St From Eigleberry St to Monterey Rd 18 Replacement 30 400
LM-2 Monterey Rd From Fourth St to Sixth St 27 Replacement 36 1,250
LM-3 Monterey Rd From Sixth St to Hornlein Ct 27 Replacement 42 425
LM-4 Monterey Rd From Hornlein Ct to approx. 240' n/o Eighth St 33 Replacement 42 775
LM-5 ROW From Rosanna St to Monterey Rd 18 Replacement 24 1,125
LM-6 Alexander St From Sixth St to Eighth St 24 Replacement 36 1,300
4/11/2022
March 2023 5-9 City of Gilroy
Storm Drainage System Master Plan
5.4.3 Ronan Channel Drainage Area
This section documents improvements within the Ronan Channel drainage service area.
• RC-1: Replace the existing 18-inch diameter pipeline on Sunrise Drive with a new 30-inch
diameter pipeline from Dream Catcher Way to Golden Sky Way.
• RC-2: Replace the existing 18-inch diameter pipeline on Mantelli Drive with a new 24-inch
diameter pipeline from Muir Drive to Delta Drive.
• RC-3: Construct a new 48-inch diameter pipeline on Zamzow Court from approximately
1,250 feet north of Cohansey Avenue to Cohansey Avenue.
• RC-4: Construct a new 48-inch diameter pipeline on Cohansey Avenue from
approximately 350 feet east of Monterey Road to Murray Avenue.
• RC-5: Construct a new 48-inch diameter pipeline on Murray Avenue from approximately
Murray Avenue to Tomkins Court.
• RC-6: Replace the existing 24-inch diameter pipeline on Murray Avenue with a new 48-
inch diameter pipeline from Tomkins Court to Ronan Channel.
• RC-7: Construct a new 36-inch diameter pipeline along No Name Uno and Las Animas
Avenue from approximately 1,500 feet south of Buena Vista Avenue to San Ysidro
Avenue.
• RC-8: Construct a new 48-inch diameter pipeline on San Ysidro Avenue from Las Ysidro
Avenue to approximately 500 feet north of Leavesley Road.
• RC-9: Replace the existing 18-inch diameter pipeline on Leavesley Road with a new 42-
inch diameter pipeline from approximately 220 feet east of Forest Street to approximately
360 feet east of Forest Street.
5.4.4 Miller Slough Drainage Area
This section documents improvements within the Miller Slough drainage service area.
• MS-1: Replace the existing 24-inch diameter pipeline on Wren Avenue with a new 30-inch
diameter pipeline from approximately 290 feet north of First Street to Lawrence Drive.
• MS-2: Replace the existing 18-inch diameter pipeline on First Street with a new 24-inch
diameter pipeline from approximately 180 feet east of Princevalle Street to Sargent Sreet.
• MS-3: Replace the existing 18-inch diameter pipeline on Sargent Street with a new 24-inch
diameter pipeline from First Street to Broadway.
March 2023 5-10 City of Gilroy
Storm Drainage System Master Plan
• MS-4: Replace the existing 18 and 21-inch diameter pipeline on Broadway with a new 30-
inch diameter pipeline from Sargent Street to Church Street.
• MS-5: Replace the existing 21-inch diameter pipeline on Church Street with a new 30-inch
diameter pipeline from Broadway to approximately 100 feet south of Howson Street.
• MS-6: Replace the existing 18 and 24-inch diameter pipeline on Monterey Road with a
new 27-inch diameter pipeline from Second Street to First Street.
5.4.5 North Uvas Drainage Area
This section documents improvements within the North Uvas drainage service area.
• NU-1: Construct a new 30-inch diameter pipeline on Hecker Pass Highway from
approximately 3,080 feet west of Santa Teresa Boulevard to approximately 360 feet west
of Santa Teresa Boulevard.
• NU-2: Replace the existing 24-inch diameter pipeline on Hecker Pass Highway with a new
30-inch diameter pipeline from approximately 360 feet west of Santa Teresa Boulevard to
Santa Teresa Boulevard.
• NU-3: Replace the existing 24-inch diameter pipeline on Santa Teresa Boulevard with a
new 30-inch diameter pipeline from approximately 70 feet south of Hecker Pass Highway
to Uvas Creek.
• NU-4: Replace the existing 18-inch diameter pipeline on Westwood Drive with a new 24-
inch diameter pipeline from approximately 150 feet south of Third Street to Kentwood
Court.
• NU-5. Construct a new 90-inch diameter pipeline on Monterey Road from Princevalle
Channel to Victoria Drive.
• NU-6: Replace the existing 42-inch diameter pipeline on Monterey Road with a new 90-
inch diameter pipeline from Victoria Drive to approximately 950 feet north of Farman Lane.
• NU-7. Construct a new 90-inch diameter pipeline in right-of-way from Monterey Road to
approximately 970 feet north of Uvas Creek.
• NU-8. Construct a new 90-inch diameter pipeline in right-of-way from approximately 970
feet north of Uvas Creek to Uvas Creek.
5.4.6 South Uvas Drainage Area
This section documents improvements within the South Uvas drainage service area.
March 2023 5-11 City of Gilroy
Storm Drainage System Master Plan
• SU-1: Replace the existing 18-inch diameter pipeline on St Andrews Circle with a new 36-
inch diameter pipeline from Carnoustie Court to approximately 380 feet northwest of
Carnoustie Court.
5.4.7 Princevalle Drainage Area
This section documents improvements within the Princevalle drainage service area.
• PR-1: Replace the existing 27-inch diameter pipeline on Third Street with a new 30-inch
diameter pipeline from Santa Paula Drive to Wren Avenue.
• PR-2: Replace the existing 27-inch diameter pipeline on Third Street with a new 42-inch
diameter pipeline from Wren Avenue to approximately 170 feet east of Santa Theresa
Drive.
• PR-3: Replace the existing 27-inch diameter pipeline on Third Street with a new 48-inch
diameter pipeline from approximately 170 feet east of Santa Theresa Drive to Santa
Theresa Drive.
• PR-4: Replace the existing 27 and 30-inch diameter pipeline on Santa Theresa Drive with
a new 48-inch diameter pipeline from Third Street to approximately 300 feet south of
Fourth Street.
• PR-5: Replace the existing 30-inch diameter pipeline in right-of-way with a new 48-inch
diameter pipeline from Santa Theresa Drive to Miller Avenue.
• PR-6: Replace the existing 30-inch diameter pipeline on Miller Avenue with a new 48-inch
diameter pipeline from approximately 300 feet south of Fourth Street to Fifth Street.
• PR-7: Replace the existing 30-inch diameter pipeline on Fifth Street with a new 48-inch
diameter pipeline from Miller Avenue to Princevalle Street.
• PR-8: Replace the existing 18-inch diameter pipeline on Princevalle Street with a new 30-
inch diameter pipeline from Third Street to Fifth Street.
• PR-9: Replace the existing 42-inch diameter pipeline on Princevalle Street with a new 54-
inch diameter pipeline from Fifth Street to Seventh Street.
• PR-10: Replace the existing 18-inch diameter pipeline on Alexander Street with a new 24-
inch diameter pipeline from Tenth Street to Banes Lane.
• PR-11: Replace the existing 18-inch diameter pipeline on Banes Lane with a new 24-inch
diameter pipeline from Alexander Steet to Automall Parkway.
• PR-12: Construct a new 48-inch diameter pipeline on Monterey Road from approximately
380 feet north of Eighth Street to Angra Way.
March 2023 5-12 City of Gilroy
Storm Drainage System Master Plan
• PR-13: Replace the existing 42-inch diameter pipeline on Monterey Road with a new 48-
inch diameter pipeline from Angra Way to Princevalle Channel.
5.4.8 Lower Miller Slough Drainage Area
This section documents improvements within the Lower Miller Slough drainage service area.
• LM-1: Replace the existing 18-inch diameter pipeline on Fourth Street with a new 30-inch
diameter pipeline from Eigleberry Street to Monterey Road.
• LM-2: Replace the existing 27-inch diameter pipeline on Monterey Road with a new 36-
inch diameter pipeline from Fourth Street to Sixth Street.
• LM-3: Replace the existing 27-inch diameter pipeline on Monterey Road with a new 42-
inch diameter pipeline from Sixth Street to Hornlein Court.
• LM-4: Replace the existing 33-inch diameter pipeline on Monterey Road with a new 42-
inch diameter pipeline from Hornlein Court to approximately 240 feet north of Eighth
Street.
• LM-5: Replace the existing 18-inch diameter pipeline in right-of-way with a new 24-inch
diameter pipeline from Rosanna Street to Monterey Road.
• LM-6: Replace the existing 24-inch diameter pipeline on Alexander Street with a new 36-
inch diameter pipeline from Sixth Street to Eighth Street.
5.5 OUTFALLS
The City’s storm drainage system discharges stormwater runoff into several outfalls that discharge
into: Ronan Channel, Miller Slough Channel, Princevalle Channel, Uvas Creek, and Llagas Creek.
As part of this master plan, outfalls were assumed in adequate condition, and capable of
conveying stormwater flows from upstream facilities.
March 2023 6-1 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
6.0 CHAPTER 6 - CAPITAL IMPROVEMENT PROGRAM
This chapter provides a summary of the recommended storm drainage system improvements to
mitigate existing capacity deficiencies and for accommodating anticipated future growth. This
chapter also presents the cost criteria and methodologies for developing the Capital Improvement
Program (CIP). Finally, a cost allocation analysis, usually used for cost sharing purposes, is also
included.
6.1 COST ESTIMATE ACCURACY
Cost estimates presented in the Capital Improvement Program were prepared for general master
planning purposes and, where relevant, for further project evaluation. Final costs of a project will
depend on several factors including the final project scope, costs of labor and material, and
market conditions during construction.
The Association for the Advancement of Cost Engineering (AACE International), formerly known
as the American Association of Cost Engineers, has defined three classifications of assessing
project costs. These classifications are presented in order of increasing accuracy: Order of
Magnitude, Budget, and Definitive.
• Order of Magnitude Estimate. This classification is also known as an “original estimate”,
“study estimate”, or “preliminary estimate”, and is generally intended for master plans and
studies.
This estimate is not supported with detailed engineering data about the specific project,
and its accuracy is dependent on historical data and cost indices. It is generally expected
that this estimate would be accurate within -30 percent to +50 percent.
• Budget Estimate. This classification is also known as an “official estimate” and generally
intended for pre-design studies. This estimate is prepared to include flow sheets and
equipment layouts and details. It is generally expected that this estimate would be
accurate within -15 percent to +30 percent.
• Definitive Estimate. This classification is also known as a “final estimate” and prepared
during the time of contract bidding. The data includes complete plot plans and elevations,
equipment data sheets, and complete specifications. It is generally expected that this
estimate would be accurate within -5 percent to + 15 percent.
Costs developed in this study should be considered “Order of Magnitude” and have an expected
accuracy range of -30 percent and +50 percent.
March 2023 6-2 City of Gilroy
Storm Drainage System Master Plan
6.2 COST ESTIMATE METHODOLOGY
Cost estimates presented in this chapter are opinions of probable construction and other relevant
costs developed from several sources including cost curves, Akel experience on other master
planning projects, and input from City staff on the development of public and private cost sharing.
Where appropriate, costs were escalated to reflect the more current Engineering News Records
(ENR) Construction Cost Index (CCI).
This section documents the unit costs used in developing the opinion of probable construction
costs, the Construction Cost Index, the land acquisition costs, and markups to account for
construction contingency and other project related costs.
6.2.1 Unit Costs
The unit cost estimates used in developing the Capital Improvement Program are summarized on
Table 6.1. The unit costs are intended for developing the Order of Magnitude estimate, and do not
account for site specific conditions, labor or material costs during the time of construction, final
project scope, implementation schedule, detailed utility and topography surveys, investigation of
alternative routes for pipelines, and other various factors. These factors are assumed included in
the contingencies applied to the final capital improvement cost.
6.2.2 Construction Cost Index
Costs estimated in this study are adjusted utilizing the Engineering News Record (ENR)
Construction Cost Index (CCI), which is widely used in the engineering and construction
industries.
The costs in this Storm Drainage System Master Plan were benchmarked using a 20-City national
average ENR CCI of 12,176, reflecting a date of March 2023.
6.2.3 Construction Contingency Allowance
Knowledge about site-specific conditions for each proposed project is limited at the master
planning stage; therefore, construction contingencies were used. The estimated construction
costs in this master plan include a 30 percent contingency allowance to account for unforeseen
events and unknown field conditions.
6.2.4 Project Related Costs
The capital improvement costs also account for project-related costs, comprising of engineering
design, project administration (developer and City staff), construction management and
inspection, and legal costs. The project related costs in this master plan were estimated by
applying an additional 30 percent to the estimated construction costs.
Table 6.1 Unit Costs
Storm Drainage System Master Plan
City of Gilroy
Pipelines
(in)($/Lineal Foot)
24 245
27 283
30 302
36 358
42 414
48 489
54 545
60 564
66 622
72 678
84 866
90 928
Pipeline Casings
$22 per inch diameter per liner foot
Note :
3/29/2023
1. Unit costs are based on an ENR CCI Index Value
of 13,176 (March 2023).
March 2023 6-4 City of Gilroy
Storm Drainage System Master Plan
6.3 CAPITAL IMPROVEMENT PROGRAM
This section documents the capital improvement program, contingencies included in the costs,
and the allocation of costs to meet the requirements of AB1600.
6.3.1 Capital Improvement Costs
The Capital Improvement Program costs for the projects identified in this master plan for
mitigating existing deficiencies and for servicing future growth throughout the City are summarized
on Table 6.2.
Each improvement was assigned a unique code identifier associated with the improvement
pipelines drainage area, and is summarized graphically on Figure 6.1. The estimated construction
costs include the baseline costs plus 30 percent contingency allowance to account for unforeseen
events and unknown field conditions, as described in a previous section. Capital Improvement
Costs include the estimated construction costs plus 30 percent project-related costs (engineering
design, project administration, construction management and inspection, and legal costs).
6.3.2 Pipelines
The recommended pipeline improvements are grouped by drainage area and listed on Table 6.2.
Each improvement includes a general description of the street alignment and limits, as well as
existing pipe diameter and length. Improvements are shown on Figure 6.1.
The following pipeline improvements categories were identified:
• New Pipeline. A new pipeline is proposed where none exists.
• Replacement Pipeline. This improvement is intended as a replacement to an existing
pipeline, and along the same alignment. The existing pipeline should be abandoned when
the replacement pipeline has been constructed.
The opinion of probable construction costs, for the projects included in this master plan, are based
on the pipe unit costs summarized on Table 6.1.
It is assumed that any replacement pipes will be in the same alignment and at the same slope as
the existing pipe. However, this study recommends a field survey of the alignment during the pre-
design stage of each project.
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GOLDEN GATE AVBUENA VISTA AVM A R C E L L A A V LEAVESLEY RDS A N Y S ID R O A V
A R R O Y O C IR
M O N T E R E Y S T SOUTHSIDE DRS A N T A T E R E S A B L
S A N T A T E R E S A B L CASTRO VALLEY RDEAGLE RIDG E DRCLUB DRDAY RDMANTELLI DRUV152UV152£¤101L la g a s C r e e k
Uvas CreekLlagas CreekUvas Creek30304224424830244818603354427221181842423618181830243648242430242718304830241818181824182430213 6
2 448 363 0 424 2 42182142243618242418241818181818181 8
27242 4
2 4
184 8 607 218182 4
3 6
304 2 36363636363 0 364224
6 05442
362427 3 6 2 1
7 2
241 8 24848448303024181824181 8
241 8
6 0
331821301821242742/1827242418242427241836541 8
18361836181 8
18271 8
42181821181824241 8
1 8
1 8
181818423636Miller SloughR o n a n Ch a n n e l2136483021PrincevalleM ille rChannelL o w e r S lo u g h
242424363642246 0
6 0,,,,,,,,,,,,
,,,,,,,,,,,,,,,,
,
,,,,
,
,,,
,,,,,,,,,,,,,,,DAY RDPUEBLO ST2 4 24P R -1 0 PR-119 0
9 0
N U -6
N U -5 NU-79024LM-54 8 P R -1 2
4 8
P R -1 3
LM-1L M -2 L M -4
LM-33 6
L M -6
2 7
M S -6
3024MS-2MS-43 0
MS-54 8
3 6
4 8
4 8
4 8
42RC-9R C -3
R C -5RC-7
R C -8
48RC-43 6
4 8
,R C -7
R C -8
R C -6
30MS-13024RC-230RC-1303036NU-1NU-3SU-1NU-2303024NU-430424848PR-1PR-2PR-4PR-5PR-7PR-6PR-348483 0 5 4
PR -9
P R -8
NU -6
NU-890Figure 6.1Capital ImprovementProgramStorm Drainage System Master PlanCity of GilroyLegendProposed Improvements!COutfallPipesExisting Modeled System!COutfallsPipes by Diameter18" or Less21" - 36"42" or GreaterChannelsNon-Modeled SystemPipesChannelsRoadsRailroadsCity LimitsUrban Growth BoundaryRivers & CreeksWaterbodies5Updated: April 15, 2022GIS00.510.25MileFile Path: P:\xGIS\GIS_Projects\Gilroy\Storm\200626-MP\GL_Fig6-1_CIP_Scen2_041522.mxd
Table 6.2 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Ronan Channel Drainage Area
RC-1 Sunrise Dr From Dream Catcher Way to Golden Sky Way 18 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
RC-2 Mantelli Dr From Muir Drive to Delta Dr 18 Replacement 24 1,250 245 306,506 307,000 400,000 520,000 100%0%520,000 0
RC-3 Zamzow Ct From approx. 1,250' n/o Cohansey Ave to
Cohansey Ave -New 48 1,200 489 586,990 587,000 764,000 994,000 8%92%74,621 919,379
RC-4 Cohansey Ave From approx. 350' e/o Monterey Rd to Murray
Ave -New 48 1,450 489 709,280 710,000 923,000 1,200,000 0%100%0 1,200,000
RC-5 Murray Ave From Cohansey Ave to Tomkins Ct -New 48 2,800 489 1,369,643 1,370,000 1,781,000 2,316,000 9%91%219,340 2,096,660
RC-6 Murray Ave From Tomkins Ct to Ronan Channel 24 Replacement 48 600 489 293,495 294,000 383,000 498,000 13%87%67,178 430,822
RC-7 No Name Uno/Las Animas
Ave
From approx. 1,500' s/o Buena Vista Ave to San
Ysidro Ave -New 36 3,400 358 1,216,516 1,217,000 1,583,000 2,058,000 91%9%1,875,920 182,080
RC-8 San Ysidro Ave From Las Animas Ave to approx. 500' n/o
Leavesley Rd -New 48 3,425 489 1,675,367 1,676,000 2,179,000 2,833,000 79%21%2,227,585 605,415
RC-9 Leavesley Rd From approx. 220' e/o Forest St to approx. 360'
e/o Forest St 18 Replacement 42 150 414 62,114 63,000 82,000 107,000 87%13%92,659 14,341
Subtotal - Ronan Channel Drainage Area 6,345,000 8,253,000 10,732,000 5,283,303 5,448,697
Miller Slough Drainage Area
MS-1 Wren Ave From approx. 290' n/o First St to Lawrence Dr 24 Replacement 30 2,325 302 700,991 701,000 912,000 1,186,000 100%0%1,186,000 0
MS-2 First St From approx. 180' e/o Princevalle St to Sargent St 18 Replacement 24 525 245 128,732 129,000 168,000 219,000 100%0%219,000 0
MS-3 Sargent St From First St to Broadway 18 Replacement 24 425 245 104,212 105,000 137,000 179,000 100%0%179,000 0
MS-4 Broadway From Sargent St to Church St 18/21 Replacement 30 1,075 302 324,114 325,000 423,000 550,000 100%0%550,000 0
MS-5 Church St From Broadway to approx. 100' s/o Howson St 21 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
MS-6 Monterey Rd From Second St to First St 18/24 Replacement 27 675 283 190,847 191,000 249,000 324,000 100%0%324,000 0
Subtotal - Miller Slough Drainage Area 1,572,000 2,047,000 2,664,000 2,664,000 0
North Uvas Drainage Area
NU-1 Hecker Pass Hwy From approx. 3,080' w/o Santa Teresa Blvd to
approx. 360' w/o Santa Teresa Blvd -New 30 2,825 302 851,742 852,000 1,108,000 1,441,000 0%100%0 1,441,000
NU-2 Hecker Pass Hwy From approx. 360' w/o Santa Teresa Blvd to
Santa Teresa Blvd 24 Replacement 30 350 302 105,526 106,000 138,000 180,000 0%100%0 180,000
NU-3 Santa Teresa Blvd From approx. 70' s/o Hecker Pass Hwy to Uvas
Creek 24 Replacement 30 2,575 302 776,367 777,000 1,011,000 1,315,000 0%100%0 1,315,000
NU-4 Westwood Dr From approx. 150' s/o Third St to Kentwood Ct 18 Replacement 24 425 245 104,212 105,000 137,000 179,000 100%0%179,000 0
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
Table 6.2 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
NU-5 Monterey Rd From Princevalle Channel to Victoria Dr -New 90 775 928 718,859 719,000 935,000 1,216,000 100%0%1,216,000 0
NU-6 Monterey Rd From Victoria Dr to approx. 950' n/o Farman Ln 42 Replacement 90 2,175 928 2,017,443 2,018,000 2,624,000 3,412,000 100%0%3,412,000 0
NU-7 ROW From Monterey Rd to approx. 970' n/o Uvas
Creek -New 90 975 928 904,371 905,000 1,177,000 1,531,000 100%0%1,531,000 0
NU-8 ROW From approx. 970' n/o Uvas Creek to Uvas Creek -New 90 1,000 928 927,560 928,000 1,207,000 1,570,000 100%0%1,570,000 0
Subtotal - North Uvas Drainage Area 6,410,000 8,337,000 10,844,000 7,908,000 2,936,000
South Uvas Drainage Area
SU-1 St Andrews Cir From Carnoustie Ct to approx. 380' nw/o
Carnoustie Ct 18 Replacement 36 375 358 134,175 135,000 176,000 229,000 100%0%229,000 0
Subtotal - South Uvas Drainage Area 135,000 176,000 229,000 229,000 0
Princevalle Drainage Area
PR-1 Third St From Santa Paula Dr to Wren Ave 27 Replacement 30 825 302 248,739 249,000 324,000 422,000 100%0%422,000 0
PR-2 Third St From Wren Ave to approx. 170' e/o Santa
Theresa Dr 27 Replacement 42 675 414 279,515 280,000 364,000 474,000 100%0%474,000 0
PR-3 Third St From approx. 170' e/o Santa Theresa Dr to Santa
Theresa Dr 27 Replacement 48 200 489 97,832 98,000 128,000 167,000 100%0%167,000 0
PR-4 Santa Theresa Dr From Third St to approx. 300' s/o Fourth St 27/30 Replacement 48 950 489 464,700 465,000 605,000 787,000 100%0%787,000 0
PR-5 ROW From Santa Theresa Dr to Miller Ave 30 Replacement 48 675 489 330,182 331,000 431,000 561,000 100%0%561,000 0
PR-6 Miller Ave From approx. 300' s/o Fourth St to Fifth St 30 Replacement 48 400 489 195,663 196,000 255,000 332,000 100%0%332,000 0
PR-7 Fifth St From Miller Ave to Princevalle St 30 Replacement 48 1,250 489 611,448 612,000 796,000 1,035,000 100%0%1,035,000 0
PR-8 Princevalle St From Third St to Fifth St 18 Replacement 30 1,250 302 376,877 377,000 491,000 639,000 100%0%639,000 0
PR-9 Princevalle St From Fifth St to Seventh St 42 Replacement 54 1,225 545 668,183 669,000 870,000 1,131,000 100%0%1,131,000 0
PR-10 Alexander St From Tenth St to Banes Ln 18 Replacement 24 825 245 202,294 203,000 264,000 344,000 84%16%288,575 55,425
PR-11 Banes Ln From Alexander St to Automall Pkwy 18 Replacement 24 625 245 153,253 154,000 201,000 262,000 89%11%234,103 27,897
PR-12 Monterey Rd From approx. 380' n/o 8th St to Angra Way -New 48 2,475 489 1,210,667 1,211,000 1,575,000 2,048,000 100%0%2,048,000 0
PR-13 Monterey Rd From Angra Way to Princevalle Channel 42 Replacement 48 775 489 379,098 380,000 494,000 643,000 100%0%643,000 0
Subtotal - Princevalle Drainage Area 5,225,000 6,798,000 8,845,000 8,761,678 83,322
Table 6.2 Capital Improvement Program
Storm Drainage System Master Plan
City of Gilroy
Existing
Diameter
New/Parallel/
Replace Diameter Length Unit Cost Infr. Cost Existing
Users
Future
Users Existing Users Future users
(in)(in)(ft)($/lf)($)($)($)($)(%)(%)($)($)
Cost Sharing
Baseline
Constr. Costs
Estimated
Constr. Costs
Capital Improv.
Costs
Improv.
No.Alignment Limits
Pipeline Improvements Infrastructure Costs Suggested Cost
Allocation
Lower Miller Slough Drainage Area
LM-1 Fourth St From Eigleberry St to Monterey Rd 18 Replacement 30 400 302 120,601 121,000 158,000 206,000 100%0%206,000 0
LM-2 Monterey Rd From Fourth St to Sixth St 27 Replacement 36 1,250 358 447,248 448,000 583,000 758,000 100%0%758,000 0
LM-3 Monterey Rd From Sixth St to Hornlein Ct 27 Replacement 42 425 414 175,991 176,000 229,000 298,000 100%0%298,000 0
LM-4 Monterey Rd From Hornlein Ct to approx. 240' n/o Eighth St 33 Replacement 42 775 414 320,924 321,000 418,000 544,000 100%0%544,000 0
LM-5 ROW From Rosanna St to Monterey Rd 18 Replacement 24 1,125 245 275,855 276,000 359,000 467,000 100%0%467,000 0
LM-6 Alexander St From Sixth St to Eighth St 24 Replacement 36 1,300 358 465,138 466,000 606,000 788,000 100%0%788,000 0
Subtotal - Lower Miller Slough Drainage Area 1,808,000 2,353,000 3,061,000 3,061,000 0
Total Costs
Subtotal - Ronan Channel Drainage Area 6,345,000 8,253,000 10,732,000 5,283,303 5,448,697
Subtotal - Miller Slough Drainage Area 1,572,000 2,047,000 2,664,000 2,664,000 0
Subtotal - North Uvas Drainage Area 6,410,000 8,337,000 10,844,000 7,908,000 2,936,000
Subtotal - South Uvas Drainage Area 135,000 176,000 229,000 229,000 0
Subtotal - Princevalle Drainage Area 5,225,000 6,798,000 8,845,000 8,761,678 83,322
Subtotal - Lower Miller Slough Drainage Area 1,808,000 2,353,000 3,061,000 3,061,000 0
Total Improvement Costs 21,495,000 27,964,000 36,375,000 27,906,981 8,468,019
Notes:
3/29/2023
1. Cost estimates are based on the Engineering News Record (ENR) construction cost index (CCI) of 13,176 for March 2023
2. Baseline construction costs plus 30% to account for unforeseen events and unknown conditions.
3. Estimated construction cost plus 30% to cover other costs including: engineering design, project administration (developer and City staff), construction management and inspection, and legal costs.
March 2023 6-9 City of Gilroy
Storm Drainage System Master Plan
6.3.3 Recommended Cost Allocation Analysis
Cost allocation analysis is needed to identify improvement funding sources, and to establish a
nexus between development impact fees and improvements needed to service growth. In
compliance with the provisions of Assembly Bill AB 1600, the analysis differentiates between the
project needs of servicing existing users and for those required to service anticipated future
developments. Table 6.2 lists each improvement and separates the cost by responsibility between
existing and future users. The cost responsibility is based on model parameters for existing and
future land use, and may change depending on the nature of development.
March 2023 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
APPENDIX A
Hydrology Review
SU002
RC001
SU024
SU072
SU090
RC058
RC056
RC082
SU055
SU012
SU066
RC005A
RC062
RC003
RC060
SU068
SU088
SU098
SU071
LL051
NU002A
LL050
RC123
RC008
SU060
SU057
NU010
SU044
NU012
RC005D
LM042
MS094
RC054
NU040A
LL046
SU078
RC098
MS082
RC011
SU010
RC114
LM035
PR020
PR078
MS130
NU018
PR084
RC074
SU032
RC083
RC066
RC016
LM021
NU013
RC086
LM054
LM052
MS116
PR068
NU014
NU011
LM020
LM032
RC006
RC050A
RC110
RC099
SU059
PR076
PR051
NU045
PR040
NU015
RC002
NU020
PR002NU021
PR010
NU023
RC088
RC064A
LM024
MS110
NU005
PR064
MS122
SU080
PR022
RC041
RC100A
RC064
LL044
PR004
SU035
NU052
SU079 SU080A
SU067
PR012
NU028
MS106
RC092
MS102
PR014
PR098
NU034
RC038
LL042
RC052
RC044
LL047
RC029
RC084
SU008
RC122
NU042
MS107 MS120
SU100
RC055
RC005B
RC048
RC022
RC004
RC030
PR026
PR080
PR085
RC014
MS090
SU048
NU016
NU006
LM046
SU083A
PR053
RC007 RC067
RC085
RC009
PR032
LL048
MS096
PR071
MS086
SU042
MS115
NU017
NU040
PR034
RC043
PR070
MS108
RC024
PR008
RC005C
RC028 RC072
PR042
RC051 MS100
NU009
LM038
RC044A
MS124
MS098
RC068A
MS091
PR036
RC108
RC051A
SU042A
RC118
NU019
PR086
RC027
RC039
PR075
RC042
MS098A
RC090
RC106
PR066
LM022
LM050
NU024
SU069
NU048
RC100
LM033
NU043NU036
NU053
RC102
RC012
MS109
PR028
PR024
SU034
LM030
PR052
RC031
RC121
PR050
SU011
PR074
MS132
RC112A
PR006
SU021
LM044
PR061
SU074
RC013
PR030
RC051B
RC021
SU083
RC015
LM028
MS100A
NU004B
SU096
NU030
SU050
PR063
LM040
PR047
SU058
NU004
SU070
SU020
MS128
PR091
RC014A
PR005
SU036
RC026
RC014B
SU056
RC019
NU022
PR046
RC068
PR031
MS107A
SU042B
MS100C
NU026
LM037
NU004C
RC093
RC112
SU084B
PR090
SU046
NU050
RC036
RC052A
RC046
RC021A
MS104
PR040A
RC019A
MS092
RC023
NU004A
MS126
SU076
PR016
SU091
SU084
RC020
PR054
RC032
SU026
MS090A
MS126A
RC040
SU092
MS092B
MS100B
RC070
RC025
PR044
SU038
MS092A
SU033
SU018
SU030
MS128A
RC018
SU035A
SU036A
PR062
RC116RC120
RC050B
PR055
RC017
SU084A
RC050
SU028
SU030A
NU032
SU040SU014
NU038
PR054A
10-Year Yield (CFS/ACRE)
Hydrology Subcatchments
Legend
10 YEAR YIELD CFS/ACRE
≤0.26
≤0.43
≤0.59
≤0.69
≤0.86
0 2,0004,000
Feet1 inch equals 4,000 feet
March 2023 City of Gilroy
Storm Drainage System Master Plan
2023 City of Gilroy
APPENDIX B
Storm Drainage Area Hydrologic Routing
PR008
PR072 PR076
PR074
PR014
March 3,2022
Princevalle Drainage
Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Princevalle Channel
PR002
Flow Combination
Hydrologic Sub-
catchment Area
PR020
PR040
PR044
PR038
PR036PR042
PR032PR034PR030
PR028
PR024PR026
PR022
PR018 PR016
PR010
PR012
PR006
PR002 PR004
PR046 PR050 PR052 PR070 PR080 PR084 PR086
PR048
PR056
PR058
PR066
PR068
PR062
PR054
PR060
PR064
PR078
PR082
PR102
PR094 PR100
PR090
PR096
PR098
PR088
PR092
March 4,2022
North Uvas Drainage
Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
North Uvas Creek
NU002
Flow Combination
Hydrologic Sub-
catchment Area
NU002 NU004 NU006
NU048
NU050
NU042
NU044
NU056
NU058
NU060 NU062
NU064
NU066
NU068
NU072
NU074
NU076
NU052
NU054
NU046
NU028
NU008
NU010NU012
NU014 NU016
NU018 NU020
NU022
NU070
NU034
NU040
NU030
NU032
NU036
NU038
NU024
NU026
March 4,2022
Lower Miller Slough
Drainage Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Lower Miller Slough
LM002
Flow Combination
Hydrologic Sub-
catchment Area
LM006 LM002 LM008
LM026
LM024
LM004
LM010LM012
LM014
LM016
LM018
LM020 LM022
LM032 LM036
LM034
LM038 LM030
LM028
March 4,2022
Llagas Drainage Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Llagas Creek
LL002
Flow Combination
Hydrologic Sub-
catchment Area
LL004 LL008
LL002LL006
LL016
LL010LL014LL012
March 8,2022
Miller Slough
Drainage Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Miller Slough
MS002
Flow Combination
Hydrologic Sub-
catchment Area
MS008
MS050
MS052
MS044
MS048
MS058
MS060
MS056
MS054
MS040
MS038
MS012
MS004 MS006
MS030
MS028
MS032
MS034
MS002
MS016MS018
MS010
MS014
MS022
MS020
MS024
MS026
MS036
MS046
MS042
MS062
MS066
MS064
MS068
March 4,2022
Ronan Channel
Drainage Area -1
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Ronan Channel
RC002
Flow Combination
Hydrologic Sub-
catchment Area
RC002RC014
RC004RC008RC010
RC012 RC006
RC014RC016
RC126
RC128
RC124
RC018 -RC114
(See Ronan Channel Drainage Area
-2)
RC116
RC136
RC118
RC120
RC122
RC130
RC132
RC134
RC138
RC140
RC142
RC144
RC162
RC146
RC148
RC152
RC154
RC150
RC156
RC168
RC164
RC166
RC170
RC158
RC160
RC172
RC174
RC176
RC178
RC186
RC180
RC182
RC184
RC188
RC190
RC196
RC192
RC194
March 4,2022
Ronan Channel
Drainage Area -2
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
Ronan Channel
RC002
Flow Combination
Hydrologic Sub-
catchment Area
RC114
RC098
RC096
RC094
RC088
RC092
RC090
RC076
RC078
RC080
RC074
RC082
RC084
RC086
RC100
RC102
RC106
RC108
RC044
RC068
RC070
RC064
RC072
RC020
RC022
RC038
RC040
RC042
RC054
RC058
RC056
RC050
RC048
RC052
RC046
RC112
RC104
RC110
RC018RC024RC026RC028 RC062RC060
RC032RC030RC034
RC036
March 9,2022
South Uvas Drainage
Area
Storm Drainage System Master
Plan
City of Gilroy
LEGEND
South Uvas Creek
SU002
Flow Combination
Hydrologic Sub-
catchment Area
SU002
SU006
SU030SU004
SU008
SU014
SU018
SU010
SU016
SU026
SU022
SU020
SU024
SU012
SU066
SU068
SU062
SU064
SU060
SU052
SU058
SU054
SU056
SU040 SU044
SU042
SU034 SU032
SU038
SU036
SU048
SU050
SU046
SU078
SU102
SU122
SU076
SU094
SU096
SU098
SU112
SU088
SU084
SU090
SU108
SU028
SU092SU086
SU110
SU120
SU100
SU070 SU072
SU074
SU080
SU082
SU104
SU106
SU118
SU116
SU114
CERTIFICATE OF THE CLERK
I, THAI NAM PHAM, City Clerk of the City of Gilroy, do hereby certify that the
attached Resolution No. 2023-19 is an original resolution, or true and correct copy of a
city Resolution, duly adopted by the Council of the City of Gilroy at a Regular Meeting of
said held on Council held Monday, April 3, 2023, at which meeting a quorum was
present.
IN WITNESS WHEREOF, I have hereunto set my hand and affixed the Official
Seal of the City of Gilroy this Monday, April 3, 2023.
____________________________________
Thai Nam Pham, CMC, CPMC
City Clerk of the City of Gilroy
(Seal)
