HomeMy WebLinkAboutNorth Marsh Summary
MEMORANDUM
Date:
June 8, 2018
To: Hal Hannula
Organization: Community Development
From: Robert Camacho
Title: Manager of Engineering Services
Project Name: Avila Ranch
Project Number: 0661-02-LP17
Topic: Chevron Offsite Remediation Summary
At the request of the City, this brief memo is meant to clarify the peak discharge rates used for analyzing Tank
Farm Creek as it enters the Avila Ranch Project and ultimately converges with the East Fork San Luis Creek, south
of the Buckley Bridge.
Tank Farm Creek currently flows through the Chevron “North Marsh” property prior to entering an existing
headwall located along the western property line. (see attachment 1)
The pre-remediation flowrates for Tank Farm Creek at the existing headwall are as follows (see
attachments)
Storm Event Peak Flow Rate (CFS)
2 yr 60.1
10 yr 81.8
25 yr 299.7
50 yr 377.0
100 yr 469.2
The Chevron “North Marsh Remediation and Restoration” work will include the excavation / removal of
approximately 85,7000 CY of soil, and the construction of a weir with removable boards to restrict flow. The
excavated area will then be restored to an elevation that will provide increased onsite detention volume. Based on
the revised Hydrology Study report prepared by August 13, 2014, the reduced flows are:
The post-remediation flowrates for Tank Farm Creek at the existing headwall are as follows (see table 2 in
attachment 1)
Storm Event Peak Flow Rate (CFS) Delta (Pre – Post) cfs
2 yr 18.4 -41.7
10 yr 48.5 -33.3
25 yr 62.6 -237.1
50 yr 70.8 -306.2
100 yr 78.8 -390.4
Since this work is already permitted and Chevron anticipates this work to be completed by the end of 2019, the
report for Avila Ranch uses the reduced flows to model to Tank Farm creek as it enters the project along the
northern boundary.
Attachment 1
(Portions of the revised Hydrology Study report prepared by August 13, 2014)
REVISED HYDROLOGY STUDY
Former San Luis Obispo Tank Farm
276 Tank Farm Road
San Luis Obispo, California
August 13, 2014
PREPARED FOR
Chevron Environmental Management Company
276 Tank Farm Road
San Luis Obispo, California 93401
PREPARED BY
Avocet Environmental, Inc.
1 Technology Drive, Suite C515
Irvine, California 92618-5302
Project No. 1212.010
REVISED HYDROLOGY STUDY
Former San Luis Obispo Tank Farm
276 Tank Farm Road
San Luis Obispo, California
August 13, 2014
August 13, 2014 Project No. 1212.010
Mr. Rik Williams
CHEVRON ENVIRONMENTAL MANAGEMENT COMPANY
276 Tank Farm Road
San Luis Obispo, California 93401
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road
San Luis Obispo, California
Dear Mr. Williams:
Enclosed is the Hydrology Study for the Former San Luis Obispo Tank Farm revised to include
recent developments between 2009 and 2014 within the project area and surrounding areas that
contribute to the overall hydrology of the site. If you have any questions or require additional
information, please do not hesitate to call.
Respectfully submitted,
AVOCET ENVIRONMENTAL, INC.
Robert Van Hyning, P.E.
Principal
RVH:sh
Enclosure
cc: Bill Almas – Chevron BRES (electronic only)
Eric Snelling – Padre Associates (electronic only)
Tim Walters – RRM Design Group (electronic only)
\\server4\projects\1212 Chevron_Padre_SLOTF\010_Finalize 2014 Hydrology Study\Revised Hydrology Study_2014_08_13.docx
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page i
San Luis Obispo, California August 13, 2014
TABLE OF CONTENTS
Page
LIST OF TABLES ......................................................................................................................... i
LIST OF FIGURES ...................................................................................................................... ii
LIST OF ABBREVIATIONS AND ACRONYMS ................................................................... iv
1.0 INTRODUCTION ................................................................................................................ 1
1.1 PURPOSE AND SCOPE ..................................................................................................... 1
1.2 BACKGROUND AND EXISTING SITE CONDITIONS ........................................................... 3
1.3 WATERSHED OVERVIEW ................................................................................................ 4
1.3.1 Regional Hydrologic Setting ............................................................................. 4
1.3.2 Tank Farm Creek Watershed ............................................................................ 5
1.3.3 Onsite Tributary to the East Fork of San Luis Obispo Creek ........................... 6
1.3.4 Onsite Isolated Catchments ............................................................................... 6
1.4 RELEVANT REGULATIONS AND GUIDANCE .................................................................... 6
1.5 ENHANCEMENT AND MITIGATION .................................................................................. 7
1.6 UNUSUAL OR SPECIAL CONDITIONS ............................................................................... 7
1.7 REPORT ORGANIZATION ................................................................................................ 8
2.0 ANALYSIS REQUIREMENTS AND METHODOLOGY .............................................. 9
2.1 HYDROLOGIC ANALYSES ............................................................................................. 10
2.2 HYDRAULIC ANALYSES ............................................................................................... 11
2.3 FLOODPLAIN ANALYSIS ............................................................................................... 12
2.4 LONG-TERM WATER BALANCE ................................................................................... 12
3.0 TANK FARM CREEK WATERSHED ........................................................................... 14
3.1 EXISTING CONDITION .................................................................................................. 14
3.1.1 Watershed Delineation .................................................................................... 14
3.1.2 Hydrology and Hydraulics .............................................................................. 16
3.1.3 Floodplain Analysis ........................................................................................ 19
3.1.4 Water Balance ................................................................................................. 19
3.2 CHANGES DUE TO REMEDIATION AND RESTORATION .................................................. 20
3.2.1 Watershed Delineation .................................................................................... 21
3.2.2 Hydrology and Hydraulics .............................................................................. 23
3.2.2.1 Downstream Peak Runoff Timing Analysis ................................... 25
3.2.3 Floodplain Analysis ........................................................................................ 26
3.3 CHANGES DUE TO ONSITE DEVELOPMENT ................................................................... 26
3.3.1 Watershed Delineation .................................................................................... 26
3.3.2 Hydrology and Hydraulics .............................................................................. 27
3.3.3 Floodplain Analysis ........................................................................................ 27
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page ii
San Luis Obispo, California August 13, 2014
Page
4.0 ONSITE CATCHMENTS TRIBUTARY TO THE EAST FORK OF SAN
LUIS OBISPO CREEK ......................................................................................................28
4.1 EXISTING CONDITIONS ................................................................................................. 28
4.2 CHANGES DUE TO REMEDIATION OR RESTORATION .................................................... 29
5.0 ONSITE ISOLATED CATCHMENTS ........................................................................... 30
5.1 EXISTING CONDITION .................................................................................................. 30
5.2 CHANGES DUE TO REMEDIATION AND RESTORATION .................................................. 30
5.2.1 Watershed Delineation .................................................................................... 30
5.2.2 Hydrology and Hydraulics .............................................................................. 31
5.2.3 Floodplain Analysis ........................................................................................ 31
5.3 CHANGES DUE TO DEVELOPMENT ............................................................................... 32
5.3.1 Watershed Delineation .................................................................................... 32
5.3.2 Hydrology and Hydraulics .............................................................................. 32
5.3.3 Floodplain Analysis ........................................................................................ 33
6.0 MINOR WATERSHEDS .................................................................................................. 34
7.0 CLOSING REMARKS ...................................................................................................... 35
REFERENCES ............................................................................................................................ 36
TABLES
FIGURES
APPENDIX A: REPORT CD
APPENDIX B: CLIMATE DATA
APPENDIX C: WATERSHED DEFINITIONS
APPENDIX D: HYDROLOGY ANALYSES
APPENDIX E: HYDRAULICS ANALYSES
APPENDIX F: FLOODPLAIN ANALYSES
APPENDIX G: WATER BALANCE ANALYSES
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page i
San Luis Obispo, California August 13, 2014
LIST OF TABLES
Table No. Title
1 Summary of Current and Post-Remediation Catchment
Areas
2 Comparison of Tank Farm Creek Watershed Offsite
Discharge Peak Flows and Volumes, and Onsite Storage
for the Existing and Post-Remediation Conditions
3 Summary of Peak Flows
4 Summary of 24-Hour Peak Storage Volumes at Selected
Ponds
5 Summary of Peak Flows and Velocities in Selected Tank
Farm Creek Watershed Channel Reaches
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page ii
San Luis Obispo, California August 13, 2014
LIST OF FIGURES
Figure No. Title
1 Site Location Map
2 Proposed Land Use Activities
3 Existing Site Conditions
4 Regional Hydrologic Basins
5 Watersheds Overview, Existing Conditions
6 Watersheds Overview, Post-Remediation and Post-
Development Conditions
7 Tank Farm Creek Watershed, Existing Conditions
8 Tank Farm Creek (DP-6) Hydrographs
9 Headwall Storage and Outflow Relationship
10 Round Culverts (DP-4) Hydrographs
11 Box Culverts (DP-5) Hydrographs
12 North Marsh Storage Hydrographs
13 North Marsh Storage and Outflow Relationship
14 100-Year Floodplain, Existing Conditions
15 Estimated Monthly Water Gains and Losses in the North
Marsh
16 Estimated Monthly Storage Volumes in the North Marsh
17 Water Levels and Precipitation Depths at the North Marsh
18 Tank Farm Creek Watershed, Post-Remediation Conditions
19 Typical Post-Remediation Tank Farm Creek Channel
20 100-Year Floodplain, Post-Remediation
21 Development-Related Stormwater Infrastructure
22 Onsite Watershed Tributary to the East Fork of San Luis
Obispo Creek, Existing Conditions
23 Grading Plan to Connect the Oxbow and Auxiliary Channel
to the East Fork of San Luis Obispo Creek
24 Onsite Isolated Catchments, Existing Conditions
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page iii
San Luis Obispo, California August 13, 2014
25 Onsite Isolated Catchments, Post-Remediation Conditions
26 Outflow Hydrograph for Watershed D Channel (DP-8),
Post-Remediation Conditions
27 Onsite Isolated Catchments, Post-Development Conditions
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page iv
San Luis Obispo, California August 13, 2014
LIST OF ABBREVIATIONS AND ACRONYMS
amsl above mean sea level
AST aboveground storage tank
BMP Best Management Practices
CEMC Chevron Environmental Management Company
CEQA California Environmental Quality Act
CLOMR Conditional Letter of Map Revision
cfs cubic feet per second
DDM Waterway Management Plan Volume III Drainage Design Manual
DWR State of California, Department of Water Resources
EGI England Geosystem, Inc.
FEMA Federal Emergency Management Agency
ft/sec feet per second
HSA San Luis Obispo Creek Hydrologic Subarea
LID Low Impact Development
LOMR Letter of Map Revision
RWQCB California Regional Water Quality Control Board
SCS U.S. Soil Conservation Service
SWPPP Storm Water Pollution Prevention Plan
TOC time of concentration
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 1
San Luis Obispo, California August 13, 2014
1.0 INTRODUCTION
Union Oil Company of California (Union Oil) owns a 332-acre former tank farm property south
of the city of San Luis Obispo (Figure 1) that was constructed in 1910 to serve as the tidewater
accumulation point for a petroleum pipeline from San Joaquin Valley. This property, commonly
referred to as the San Luis Obispo Tank Farm, SLO Tank Farm, or Tank Farm, was slowly
withdrawn from operation during the latter decades of the twentieth century, and by the late
1990s, was formally decommissioned. Chevron Environmental Management Company (CEMC)
currently manages the property for Union Oil and they desire to convert the property to
alternative uses that are consistent with local planning, community needs, good environmental
stewardship, regulatory requirements, and efficient asset management.
CEMC has prepared a multi-use plan for the property that is illustrated in Figure 2. It includes
remediation of environmental concerns, restoration of riparian corridors, bike paths, 252.5 acres
of community-accessible open space, 64.9 acres of commercial development, and 15 acres
dedicated for public facilities. On June 16, 2008, CEMC submitted applications for vesting
tentative tract maps, specific and general plan amendments, and began negotiations with the City
of San Luis Obispo on a development agreement. The application included a Remedial Action
Plan (Avocet, December 18, 2007, revision in process) addressing environmental issues, a
Project Execution Plan (Padre, 2007) that provided extensive supporting documentation for
environmental review under the California Environmental Quality Act (CEQA), and
development maps (RRM, 2008). Although each plan contained or referenced some preliminary
hydrologic evaluations, it was recognized that further detailed analysis was required to support
the application. This report presents that detailed analysis.
This report was originally issued in January 2009. Since that time, changes due to development
of the surrounding properties and modifications to the proposed SLOTF development plans have
been significant enough to affect the hydrologic analysis. This report update has been prepared
to reflect those changes.
1.1 PURPOSE AND SCOPE
CEMC proposes to develop approximately 20 percent of the SLO Tank Farm in a manner that
implements the regional objectives of the City of San Luis Obispo’s Airport Area Specific Plan,
particularly in regard to basic infrastructure (e.g., water, sewer, flood control). The balance of
the property will be transformed into high-quality open space supporting a diverse and complex
habitat area. An important element of the project that overlaps these two goals is the site
hydrology and the approach for stormwater management and water quality.
To the degree practical, the City of San Luis Obispo, San Luis Obispo County, and the California
Regional Water Quality Control Board (RWQCB) have an institutional preference for natural or
natural-like waterways as a means of managing stormwater and preserving important ecological
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 2
San Luis Obispo, California August 13, 2014
resources. Remediation of the SLO Tank Farm provides an opportunity to locally satisfy this
preference while reducing area-wide flooding during heavy precipitation events.
A key element of the remediation project is restoration of the riparian condition of the drainages,
particularly Tank Farm Creek; improving ecological function; and mitigating the unavoidable
disturbances to important habitat that are a collateral effect of the site’s transformation. Where
possible, the restoration plan attempts to recreate the riparian conditions that existed on the Tank
Farm property prior to development. However, in the 100 years following original construction
of the Tank Farm, the surrounding area has been irreversibly developed. This includes essential
public transportation corridors such as Tank Farm Road. Tank Farm Creek is an important
component of the local flood management system and the restoration must, therefore, also ensure
public safety and preserve property.
A detailed 75 percent draft Restoration Plan, including exhaustive supporting inventories of
natural resources, was prepared by Padre Associates, Inc. (Padre) and WSP Environmental
Strategies (WSP) in 2008. That report presented a comprehensive analysis of the hydrologic
conditions associated with the current, post-remediation/restoration, and post-development
conditions. An update to the Restoration Plan, which will address changes and additions to the
restoration design between 2008 and the present, is currently being prepared and finalized by
Padre. Although stand-alone documents, these reports are complementary studies that are best
used together to fully understand restoration and stormwater management at the SLO Tank Farm.
This study is, however, an engineering report intended primarily for an engineering audience.
Consequently, its focus is on evaluations of peak flow and storage, flood stage, long-term water
balance, and water quality. The purposes of these evaluations are to:
• Identify hydrologic changes resulting from necessary site remediation and the
proposed development;
• Provide hydraulic data needed to design stormwater management infrastructure;
• Provide an analysis of the existing and post-development floodplain boundaries
onsite in support of Conditional Letter of Map Revision (CLOMR)/Letter of Map
Revision (LOMR) applications;
• Prepare preliminary engineering drawings for the stormwater management
infrastructure;
• Provide hydrologic and water balance information needed to engineer habitat
restoration and to improve ecologic function; and
• Ensure that hydrologic impacts due to development are properly mitigated.
This analysis addresses the hydrologic setting within the property boundary under current
conditions and for the reasonably foreseeable future. This includes two distinct future situations;
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 3
San Luis Obispo, California August 13, 2014
post-remediation, which includes restoration, and post-development. After remediation, much of
the mass grading for development will have been completed, including the rough surface for the
extension of Santa Fe Road. Construction in the development areas will immediately follow
remediation, but will be spread over 25 years or more. Over this period, the proportion of the
development area that is impervious will increase, resulting in changes to quantity and timing of
runoff. Development-related stormwater management infrastructure must be sized to address
these changes. This study identifies the changes that occur after remediation of the site and after
development. However, for purposes of analysis under CEQA, we believe that the relevant
differences are between the existing (baseline) condition and post-development, which represents
conclusion of the project. Post-remediation is only an interim step to the end of the project.
Another important consideration for the future scenarios is the effect of planned upstream
development. To the extent that information is available, changes due to known or expected
development have been included in the evaluation. The analysis does not analyze hydrologic
conditions downstream of the property in any detail, but does provide a brief analysis of
estimated peak stormwater runoff times downstream of the property to show the effect of
stormwater management onsite in relation to other components of the overall watershed of the
East Fork San Luis Obispo Creek, as well as provide suggestions on possible regional
approaches to managing stormwater.
The report provides general recommendations on maintaining and improving surface water
quality from the site but does not include fate and transport analyses such as total maximum
daily load estimates for potential pollutants.
Erosion control measures are generally discussed in this report, although detailed information on
restoration of the riparian corridors will be provided in the upcoming Restoration Plan (Padre,
November 12, 2008, revision in process) and will be consistent with the conclusions of this
Hydrology Study. The Remedial Action Plan (Avocet, December 18, 2007, revision in process)
describes the proposed erosion control measures that will be used during remediation, and also
includes specific information on the drainage of particular portions of the remediation such as
cap perimeter ditches.
1.2 BACKGROUND AND EXISTING SITE CONDITIONS
The following provides a brief history of site operations and an overview of the existing site
conditions. More detailed information can be found in the Remedial Action Plan (Avocet,
December 18, 2007), Feasibility Study (Avocet, March 15, 2007), or the various assessment
reports listed in the references.
Union Oil’s 332-acre SLO Tank Farm is located at 276 Tank Farm Road in western San Luis
Obispo County, south of the city of San Luis Obispo and northwest of the San Luis Obispo
regional airport. As shown in Figure 3, the site is divided into north and south sections by Tank
Farm Road and is bordered by light commercial and industrial development, the San Luis Obispo
regional airport, agricultural and pastoral lands, with scattered residences and a trailer park. The
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 4
San Luis Obispo, California August 13, 2014
existing buildings in the northwest part of the site are mostly unoccupied and will be removed
from the site during the first phases of remedial construction activities. Aside from the existing
buildings (with approximately 2 acres of impervious pavement) and scattered remnants from
historical oil storage operations, the site is otherwise undeveloped land that contains a variety of
flora and fauna, including natural and man-made wetlands, the latter resulting from water
accumulating in artificial operations-related depressions.
From 1910 until the early 1980s, the Tank Farm was used for storing crude oil transported via
pipeline from the San Joaquin Valley. Although the site no longer handles petroleum, the
remnants of that use are still found across the site. The most prominent features are the vestiges
of the former storage facilities. There were six large earthen reservoirs (numbered 2 through 7)
ranging in capacity between 775,000 and 1,350,000 barrels. The reservoirs were constructed by
lining circular and elliptical depressions with wire-mesh-reinforced concrete and constructing
reinforced concrete walls either vertically or integrated with the side slopes of the depressions.
Initially, 21 steel aboveground storage tanks (ASTs) capable of holding 55,000 barrels each were
located north of Tank Farm Road. Their historical locations are also shown in Figure 3.
On April 7, 1926, a series of lightning strikes resulted in a massive fire at the Tank Farm. By
April 11, 1926, all but a few thousand barrels in inventory had been burned to coke or spread
across the site. Following the 1926 fire, four large reservoirs were repaired and continued in
service for several more decades. Ten of the ASTs were rebuilt and used through the 1980s.
The last AST, which was used most recently to store fire water, was removed from the site in
August 2000. The reconstruction effort included expanding fire breaks and enlarging the
containment areas surrounding the reservoirs and ASTs. This created numerous hydraulically
closed depressions across the site, many of which became wetlands and habitat for
environmentally protected species.
1.3 WATERSHED OVERVIEW
This section provides the reader with a brief orientation to the important hydrologic elements of
the SLO Tank Farm. Detailed discussions of the watersheds, including changes resulting from
remediation, restoration, and development, are provided in later sections of this report.
Watersheds are defined by the design points selected for analysis. Watershed design points were
selected based on key existing and proposed hydraulic structures and points of interest such as
runon locations entering the site and discharge locations for runoff exiting the site.
1.3.1 Regional Hydrologic Setting
Figure 4 shows the location of the site relative to the hydrologic boundaries of the central coast.
The State of California, Department of Water Resources (DWR) initially described these
boundaries in a 1958 study of San Luis Obispo and Santa Barbara Counties (DWR, 1958). The
RWQCB defines slightly different hydrologic units in the Water Quality Control Plan, Central
Coast Region (RWQCB, 1994). The table below presents the names of the various hydrologic
units, their sources, and how they correspond.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 5
San Luis Obispo, California August 13, 2014
Regional Hydrologic Units and Areas
DWR Name Area RWQCB Name Area
Central Coast Hydrologic Area 6,600 miles2 Central Coast Hydrologic Planning Area 11,274 miles2
No corresponding unit Estero Bay Hydrologic Unit 490,205 ac
San Luis Obispo Hydrologic 168,680 ac Point Buchon Hydrologic Area 145,448 ac
San Luis Obispo Hydrologic
Subunit 54,292 ac San Luis Obispo Creek Hydrologic Study
Area 54,292 ac
This study will use the RWQCB names in describing the various hydrologic units. The Tank
Farm is located within the San Luis Obispo Creek Hydrologic Subarea (HSA). This unit
comprises approximately 37 percent of the Point Buchon Hydrologic Area but is only 0.7 percent
of the total Central Coast Hydrologic Planning Area.
The eponymously named San Luis Obispo Creek HSA is drained by San Luis Obispo Creek.
The creek starts in the foothills of the Lopez Mountains and discharges to the Pacific Ocean at
Avila Beach. Its course is approximately 1,500 feet from the Tank Farm site at its closest point.
The Tank Farm itself lies across or adjacent to tributaries to San Luis Obispo Creek. This is
relevant since the site lies within the special management zone that the City of San Luis Obispo
and San Luis Obispo County have established for managing San Luis Obispo Creek.
The SLO Tank Farm, in its current condition, has three primary watersheds or catchments, as
shown in Figure 5. The first is Tank Farm Creek, the second is the small portion of the site
tributary to the East Fork of San Luis Obispo Creek, and the third is the collection of isolated
depressions and basins created during the historical petroleum storage operations. A small
portion of the site along the east property line (Watershed D) drains via sheet flow onto the
adjoining property. Other minor catchments formed by the berm along the southeast property
line account for the remainder of the site area. Remediation, restoration and development will
alter the catchments in both configuration and hydrologic characteristics. Table 1 summarizes
the current catchment areas and the areas estimated for the post-remediation site condition.
1.3.2 Tank Farm Creek Watershed
Tank Farm Creek is a tributary to the East Fork of San Luis Obispo Creek. Approximately
4,000 feet of the creek (31 percent of its total length) is on the SLO Tank Farm. This includes a
branching of the creek that occurs just north of Tank Farm Road. The watershed area
(Watershed A) tributary to the headwall where Tank Farm Creek leaves the site is approximately
636 acres. It is estimated that 160 acres of this watershed is located within the property
boundary of the SLO Tank Farm.
Most of the site development will occur on the eastern portion of the property (Figure 2), and
most of the grading that supports it will occur during remediation. To analyze the post-
remediation and development conditions, it was, therefore, necessary to subdivide Watershed C,
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 6
San Luis Obispo, California August 13, 2014
as shown in Figure 6. The analysis also considers the eventual pavement of the identified
development areas.
1.3.3 Onsite Tributary to the East Fork of San Luis Obispo Creek
Approximately 19 acres south of Tank Farm Road discharge to the East Fork of San Luis Obispo
Creek. This is a strip of land surrounding the southeast and southwest quadrants of Reservoir 3.
It includes an old meander of the East Fork of San Luis Obispo Creek and is connected to the
active channel by a12-inch-diameter gate valve that appears to have rusted open.
Restoration activities propose to engage the old oxbow during higher-than-average storm events.
This will be accomplished by opening the berm at an elevation approximately 4 feet above the
adjacent channel bottom.
1.3.4 Onsite Isolated Catchments
As described previously, the historical oil storage operations left numerous depressions that have
no hydrologic connection to either Tank Farm Creek or the East Fork of San Luis Obispo Creek.
In effect, changes in storage occur only from direct precipitation, evapotranspiration, or
infiltration. Except under extraordinary circumstances there is no runon or runoff.
As shown in Figure 5 the isolated catchments are estimated to measure approximately 140 acres,
or nearly 40 percent of the entire site. Most of the isolated catchments will continue to function
in the same manner after remediation and restoration, as they did prior to any disturbance.
Development, particularly the extension of Santa Fe Road envisioned by the Airport Area
Specific Plan, connects 3.0 acres of isolated catchment (Watershed E) with the East Fork of San
Luis Obispo Creek (Figure 6). The proposed development also requires a separate evaluation of
22.9 acres located on the east portion of the site south of Tank Farm Road, identified as
Catchment C2 in Figure 6. Runoff from this area will be directed to a detention basin, and
retained onsite. Any overflow from this detention basin would be directed back into
Catchment C. If it were deemed ecologically advantageous, runoff from this area could be
discharged to the East Fork of San Luis Obispo Creek or one of the adjacent oxbows.
1.4 RELEVANT REGULATIONS AND GUIDANCE
Numerous documents were consulted for guidance in this analysis (a complete listing is provided
in the reference section). A few of these, however, either defined the structural framework for
the study, or provided essential information.
Key among these was the Waterway Management Plan Volume III Drainage Design Manual
(DDM) developed cooperatively between the City of San Luis Obispo Public Works Department
and the County of San Luis Obispo Flood Control District (SLOPWD/SLOCoFCD, February
2003). The site is currently in San Luis Obispo County, but it is anticipated that it will ultimately
be annexed into the city of San Luis Obispo. As noted above, the site is within the San Luis
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 7
San Luis Obispo, California August 13, 2014
Obispo Creek Watershed. Accordingly, the DDM requirements are the underlying basis for
many of the analyses.
Another important consideration in preparation of this report has been the Low Impact
Development (LID) standards compiled by the RWQCB. These are being enacted through the
City of San Luis Obispo (the City) by a draft ordinance, and have been implemented by the
County of San Luis Obispo (the County) through their Storm Water Management Program
(SLOCoDPW, June 2006).
Other important resources have been three hydrologic analyses performed by adjacent
landowners. One was performed by TEC Civil Engineering Consultants in support of the
proposed King Ventures and Serra Meadows (formerly Sierra Gardens) development north of the
western portion of the SLO Tank Farm (TEC, 2008). A subsequent hydrologic analysis was
performed by RRM Design Group to address further development in the area (RRM, October 26,
2011). The third was by Mead & Hunt, Inc. (Mead & Hunt, 2006) in support of the San Luis
Obispo County Regional Airport runway extension.
One of the objectives of this Hydrology Study is to provide floodplain analyses to support
CLOMR and LOMR applications for the project. In doing so, Title 44 CFR Part 65
(Identification and Mapping of Special Hazard Areas) was used to determine data and formatting
requirements, as well as requirements for floodway revisions.
1.5 ENHANCEMENT AND MITIGATION
This project includes substantial enhancement and mitigation. Most of this effort is concentrated
in the Riparian corridor associated with Tank Farm Creek. More focused activities are proposed
adjacent to the East Fork of San Luis Obispo Creek. A thorough description of the proposed
enhancement and mitigation actions is found in Landscape Restoration for the San Luis Obispo
Tank Farm (Restoration Plan) prepared by WSP (2008). A revised Restoration Plan is currently
being drafted, which will further discuss enhancement and mitigation actions.
An important consideration for hydrology studies within the San Luis Obispo Creek Watershed
are bank stability and erosion issues. In general, the terrain is so flat on the SLO Tank Farm that
banks are limited in size. However, the issue is specifically addressed in the Restoration Plan.
1.6 UNUSUAL OR SPECIAL CONDITIONS
Remediation and restoration of the SLO Tank Farm is a significant local project. It is believed
that the project will be of substantial benefit to the community, but its implementation is
complex and requires permitting through multiple agencies. Beyond grading and development
approvals, the project requires permits for various disturbances, working in rivers and streams,
authorization to disturb federally protected habitat, and operating conditions that will be imposed
by regulatory agencies such as the RWQCB.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 8
San Luis Obispo, California August 13, 2014
To the degree practical, the flood management components of the site will be configured in as
natural a condition as possible. To achieve this goal, the extent of stormwater discharge from the
site will be lengthened in order to reduce peak flows leaving the site and allow downstream
stormwater infrastructure to function as intended. Artificially created open water wetlands will
be modified for remedial purposes to allow for proper drainage to improve airport safety and, to
the extent practical, re-create the pre-Tank Farm riparian corridor conditions. Nevertheless,
certain aspects of the site will require long-term maintenance, especially where water is
temporarily impounded or routed through artificial conduits. Those components directly related
to development (e.g., storm drains, catch basins) will be operated and maintained by the
developer or the successor. Operation and maintenance of regional infrastructure will be
addressed in an open space management plan that will be prepared to support dedication of the
open space.
Another unusual condition is associated with the outlet to Tank Farm Creek. At present, the
creek downstream of the SLO Tank Farm outlet is routed through an existing industrial
development, across land slated for future development, and finally across agricultural lands
until joining the East Fork of San Luis Obispo Creek. It is believed that superior hydraulic
performance and better flood management could be achieved by routing Tank Farm Creek
through the southern property line (Figure 6) and establishing a riparian corridor that continues
the SLO Tank Farm restoration project to the downstream confluence. However, developing and
implementing that proposal requires a regional working group that would include land owners
tributary to Tank Farm Creek, the City of San Luis Obispo, and the Flood Management
Department of San Luis Obispo County, as well as various regulatory agencies. Chevron is
willing to actively participate and contribute to such a group, but the required analysis and
coordination is beyond the scope of this study, and outside the immediate requirements of the
proposed project. Consequently, rerouting of Tank Farm Creek was taken into consideration
during placement of proposed wetland mitigation areas and is discussed in the Restoration Plan.
1.7 REPORT ORGANIZATION
Including the introduction, this report is organized into seven sections. The next section,
Section 2.0, discusses the analysis requirements and general methodology used in this study.
Sections 3.0, 4.0, 5.0, and 6.0 address Tank Farm Creek, the onsite catchments tributary to the
East Fork of San Luis Obispo Creek, the isolated onsite catchments, and other minor catchments,
respectively. As necessary, these sections include detailed discussions of the existing, post-
remediation, and post-development conditions. Section 7.0 presents concluding remarks for the
report, and the signature and stamp of the principal author.
Figures and tables are attached at the end of this report. Appendix A is a CD that contains a
bookmarked PDF version of this report. The other supporting appendices are included with the
PDF. They are not reproduced in hard copies of this report.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 9
San Luis Obispo, California August 13, 2014
2.0 ANALYSIS REQUIREMENTS AND METHODOLOGY
This section describes the analytical requirements stipulated in the regulatory documents
(primarily the DDM) that are applicable to this project, and presents the general approach
employed in the hydrologic, flood stage, and long-term wetland water balance analyses that were
used to satisfy those requirements.
As described previously, the site lies within the greater San Luis Obispo Creek Watershed. It is
also within a Special Floodplain Management Zone defined in the DDM, which makes the
objectives of that document particularly important. The DDM identifies the following as its core
objectives:
• Maintaining discharge at natural locations.
• Proper engineering of onsite conveyances.
• Developing stormwater infrastructure that ensures no adverse downstream
impacts, including maintaining peak flows within 5 percent of current levels.
• Channel impact assessment, design, and mitigation for channel modifications
within a Federal Emergency Management Agency (FEMA)-designated 100-year
floodplain.
• Compliance with floodplain management requirements, including In-Fill and
Managed Fill floodplain regulations for the Special Floodplain Management
Zones. This includes limiting increases in floodwater elevations to less than
2.5 inches or limiting increases in discharge velocities to 0.3 foot per second
(ft/sec), and ensuring that the net precipitation storage volume within the
floodplain does not decrease for any flood stage up to the 100-year event.
• Ensuring stabilized stream banks.
• Controlling erosion and managing stormwater quality.
• Supporting future channel maintenance and management.
The SLO Tank Farm has features beyond what is typical for a development-only project. Its
historical use to store petroleum necessitates substantial disturbance in the form of grading in
order to mitigate various environmental impacts. A further objective of the project is to at least
restore, and if possible improve upon, the original riparian and ecological function of the creek
systems found onsite. Consequently, the study considers three conditions (where applicable): the
current environment, post-remediation (including restoration), and post-development.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 10
San Luis Obispo, California August 13, 2014
The study also includes four distinct analyses: hydrology, hydraulics, flood-stage, and water
balance. The first three are essential to meeting the core objectives of the DDM. The latter
analysis is used to support plant selection and maintenance planning for the restoration effort.
Although the water balance evaluation is limited to the North Marsh, it does provide information
regarding groundwater levels that are useful for engineering floodplain management
infrastructure.
The following sections describe the general approach used for each analysis. This includes
software, the basis for selecting certain parameters, and other considerations relevant to
evaluating hydrologic conditions at the site. The results of each analysis for the specific
watersheds are discussed in subsequent chapters.
2.1 HYDROLOGIC ANALYSES
Hydrologic analysis is the process of apportioning precipitation within a given watershed into
runoff, infiltration, and storage. In defining catchments and channels within the watershed, it is
also possible to calculate peak flow, velocity, and changes in storage. The hydrologic analysis
provides the fundamental data for developing most of the information required by the DDM.
The first task in the hydrologic analysis is to identify the design points. A watershed is, by
definition, the area contributing runoff to a given design point. Once the design points were
known, it was a simple task to delineate the watersheds. As described in Section 1.3, the design
points were either obvious existing natural discharge points or were more subtle discharge points
that were identified in the field during a March 2008 precipitation event. The only added
discharge point for the site is necessitated by the northern extension of Santa Fe Road, which
diverts runoff from approximately 2 acres of the SLO Tank Farm into Acacia Creek.
In February 2007, a topographic map of the SLO Tank Farm was created to support preparation
of the Remedial Action Plan (Avocet, December 18, 2007) based on the National Geodetic
Vertical Datum of 1929 (NGVD29). In March 2013, a new topographic map was prepared using
the North American Vertical Datum of 1988 (NAVD88). The 2013 topographic map has been
used to prepare this Hydrology Study. Caution should be used in extrapolating contours beyond
the site.
Avocet has used Bentley’s SewerGEMS v8i software package, which is built upon the
CivilStorm platform, for the hydrology and hydraulic calculations since many of the watersheds
exceed 100 acres in size. It is similar to HEC-HMS and calculates based on the U.S.
Environmental Protection Agency’s SWMM engine, but offers several advantages that were
considered essential to this analysis. First the program has an easy to use graphical interface,
which makes building the model much simpler. Analytically, the model has a variety of solution
options. Most importantly, the software allows the user to model looped networks, an unusual,
but prominent feature of the SLO Tank Farm. The model also features numerous graphical
profiling tools and comprehensive tabulation tools that speed validation and make report
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 11
San Luis Obispo, California August 13, 2014
preparation more efficient. This software was used to determine the peak flow rates and pond
storage volumes reported in this document.
Due to the size of the Tank Farm Creek Watershed, the Soil Conservation Service (SCS) unit
hydrograph method was used to estimate runoff. It was most appropriate, given the catchment
sizes and the degree of storage that occurs within the catchments. Since no stream gauge data
are available for this area, the SCS synthetic method was used.
Each hydrologic evaluation was considered as a 24-hour storm. Results were computed for the
2-, 10-, 25-, 50-, and 100-year recurrence events. Total precipitation quantities for the storms
were taken from the hydrology report prepared for the proposed adjacent upstream development
(TEC, 2008) and are tabulated in the following table:
Precipitation Event Cumulative Rainfall
(inches)
2-Year 24-Hour Storm 2.9
10-Year 24-Hour Storm 4.7
25-Year 24-Hour Storm 5.7
50-Year 24-Hour Storm 6.4
100-Year 24-Hour Storm 7.1
Beyond the watershed area, there are a few fundamental parameters that must be defined to
create the model. The first task is to determine what portions of a given watershed are served by
urban infrastructure (e.g., paved ditches, conduits), and what portions route stormwater overland.
The next task is to define the physical features of the relevant stormwater infrastructure, such as
ditches, ponds, conduits, and the like. Those parameters include channel geometry, length,
slope, inlet and outlet elevations, and storage capacity, and are either measured directly in the
field or taken from topographical surveys. Finally, values are selected that define the cover and
soil type in the model (the SCS curve number) for each catchment. Those factors affect the
degree of infiltration and the speed at which runoff travels across a given catchment. Catchment
definition and parameter selection for the model are discussed in greater detail in the sections for
each watershed. Tabulations of the watershed parameters are found in Appendix C.
The analysis produces hydrographs for each catchment (or design point) and for each feature
with storage capacity. Those hydrographs are discussed in the sections corresponding to each
watershed following this one, and the detailed output for the hydrologic analyses is provided in
Appendix D. Summary hydrographs for the design points are provided as figures. Peak flow,
total offsite discharge (in 24 hours), and changes in storage are tabulated. Tabulations for other
results are also provided for each catchment, including water surface elevations and velocity.
2.2 HYDRAULIC ANALYSES
The proposed project includes construction of new hydraulic structures or modification of
existing structures. Design of these structures is performed in conjunction with the various
hydrologic analyses, which define the level of service each hydraulic structure must provide.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 12
San Luis Obispo, California August 13, 2014
The analyses are frequently iterative as the designer attempts to optimize the size of various
structures balancing size and capacity against cost.
The purpose and function of the various structures, or proposed modifications, are described in
the sections for each watershed. Those sections also discuss the results of the hydraulic analyses.
Detailed calculations from the modeling software package are provided in Appendix D
Hydrology Analyses
Tabulations of the hydraulic structures show physical geometry (e.g., width, depth), slope (if
applicable), storage capacity, and hydrographs for each of the design storm events. Where
hydraulic structures are inadequate for a particular storm event, the analysis shows the peak flow
escaping the structure and the quantity of water lost.
2.3 FLOODPLAIN ANALYSIS
A 100-year floodplain analysis was performed for the Tank Farm Creek Watershed. The grade
of this area will be substantially modified to accommodate the remediation and restoration
efforts. Further modification, though of a lesser extent, will occur due to the development
activities. Floodplain analysis was not deemed necessary for the other watersheds. There is no
proposed change to the onsite tributary to the East Fork of San Luis Obispo Creek, other than
opening the berm to engage the oxbows and historical meanders. Since this will not significantly
increase peak flow contributions but will increase the potential flood storage volume, a detailed
floodplain analysis was not considered necessary. A small portion of the isolated catchments
(Watershed E in Figure 6) will contribute to the East Fork of San Luis Obispo Creek following
the extension of Santa Fe Road; rough grading for this occurs as part of remediation. The area is
approximately 3 acres and contributes less than 10 cubic feet per second (cfs) to the creek during
the 100-year storm (Table 3). Precipitation onto the other isolated catchments continues to be
retained onsite.
Floodplain calculations were performed with XP Solutions software package XP Storm, using a
two-dimensional floodplain analysis. The computer-generated output is provided in Appendix F.
An important consideration of the engineering effort has been to ensure that the project does not
exacerbate flood conditions. Consequently, there is no loss of storage capacity within the
floodplain; rather there is a net increase in capacity both above and below Tank Farm Road. Fill
within the floodplain is offset by expanding the boundaries of the floodplain or by providing
greater effective depth. Figures 14 and 20 illustrate the floodplains corresponding to the existing
and post-remediation conditions and are discussed in Section 3.0.
2.4 LONG-TERM WATER BALANCE
The long-term water balance evaluates changes in water storage within a defined control volume
over a period of approximately 12 months. Of particular interest are the monthly changes in
surface water storage in the North Marsh. This is used in the restoration effort to gauge the
viability of certain floral species based on their sensitivity to water supply.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 13
San Luis Obispo, California August 13, 2014
A detailed description of the calculation methodology for the water balance evaluation is
provided in Appendix G. In general, the analysis accounts for gains into and losses from the
control volume. Gains include precipitation, runon and rising groundwater, whereas losses
include runoff, infiltration, and evapotranspiration. Changes are calculated on a daily basis but
are reported as monthly totals.
The calculations for this report are based on site-specific climate data that were collected from
June 1999 to April 2002 using an onsite weather station. Those data are presented in
Appendix B, Climate Data. The analysis included in this report was originally published in
Hydrology Data Report and Water Balance Analysis (June 1999-April 2002) (England
Geosystem, Inc., May 2002), but is updated to reflect changes in the upstream watershed.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 14
San Luis Obispo, California August 13, 2014
3.0 TANK FARM CREEK WATERSHED
This section addresses the Tank Farm Creek Watershed. The first part describes delineation of
the watershed in detail. After defining the existing watershed, the section lists the changes that
will occur due to remediation and restoration, and then illustrates the additional incremental
changes that will occur due to either onsite or upstream development. The sections following the
watershed discussion present the results of the hydrology, hydraulic, flood stage, and water
balance analyses, including the differences that occur due to either remediation or development.
3.1 EXISTING CONDITION
An overview of the Tank Farm Creek watershed was provided in Section 1.3.2. As noted in the
overview discussion, Tank Farm Creek is an approximately two-mile long tributary of the East
Fork of San Luis Obispo Creek, which is, in turn, a tributary to San Luis Obispo Creek. Tank
Farm Creek splits into two separate reaches onsite (East Fork and West Fork), north of Tank
Farm Road (Figure 7). The West Fork of Tank Farm Creek extends almost due north and is, for
the most part, channelized and serves both urban and natural catchments. The East Fork of Tank
Farm Creek is a southwest trending reach that begins in the South Hills that border Broad Street.
Of particular interest to this project is the point at which Tank Farm Creek discharges off the
Tank Farm property (the headwall) and the contributing watershed to that point (Figure 7).
Approximately 4,000 feet of Tank Farm Creek, slightly more than 30 percent of its total length,
is upstream of the discharge point and is fed by a total runoff area of approximately 628 acres.
This is the principal watershed for this project. Changes that occur due to remediation or
development alter the watershed internally, but no portion of the watershed is diverted to a
separate discharge point.
3.1.1 Watershed Delineation
Figure 7 is a detailed illustration of the Tank Farm Creek watershed in its existing condition. It
has been divided into 17 separate subcatchments that range in size from 5.3 acres to 304 acres.
Of the total watershed area, 150 acres are onsite, while the remaining 478.3 acres are offsite.
The high point and low point of the watershed are at 595 feet above mean sea level (amsl) and
114 feet amsl, respectively. This is an overall slope of 6 percent. However, onsite the watershed
is relatively flat with a slope of only 0.3 percent. It is estimated that 13 percent of the watershed
is urban and relatively impervious. Unpaved areas are characterized by pasture or naturally
vegetated ground consisting of sands, silts, and clays. Appendix C provides a detailed
breakdown of each catchment, including size, SCS runoff coefficient, time of concentration, and
a brief discussion of relevant characteristics or special considerations.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 15
San Luis Obispo, California August 13, 2014
For the purpose of this analysis, the
headwall where Tank Farm Creek
discharges from the site has been
designated as the downstream design
point. The adjacent image shows the
condition of the headwall in the spring of
1999. The headwall with its two 36-inch-
diameter gate valves was constructed as
part of the petroleum storage operation,
and provided a means of retaining a
catastrophic release of oil onsite.
Union Oil sold the land immediately
upstream and downstream of the discharge
structure in 2001; subsequently, the eastern channel, shown on the left in the photo above, has
been filled to the headwall. This has the practical effect of halving the discharge capacity of
Tank Farm Creek at this location, which, other factors being equal, increases the likelihood of
local flooding in this area. The current invert elevation of the remaining valve is 115.9 feet amsl.
The top of the headwall is at elevation 122.4 feet amsl, which provides a total flood detention
capacity of approximately 118.5 acre-feet total onsite north and south of Tank Farm Road. In an
effort to maintain control of stormwater discharge from the property, it has been deemed prudent
for Chevron to construct a new discharge structure within its property boundary to mimic
discharge conditions of the current headwall structure.
On this portion of the site, Tank Farm Creek runs along the western property boundary in a
channel. It is connected to the greater part of the watershed by two 48-inch-diameter steel
culverts that cross beneath Tank Farm Road. Approximately 7 acres of this part of the watershed
south of Tank Farm Road (Catchment A22) is comprised of offsite land, primarily the adjacent
wrecking yard.
Another adjacent catchment, Catchment A21, is nearly 7 acres of commercial development.
Stormwater from this area collects in a basin that discharges onto the Tank Farm property
through a small PVC pipe under the perimeter berm. Although the pipe was likely constructed
concurrently with the basin, Chevron has no documentation or agreements (e.g., easement, right-
of-entry) regarding this structure.
A secondary channel joins Tank Farm Creek just upstream of the headwall. It drains
approximately 55.2 acres (Catchments A14, A24, and A25). The greater portion of this area,
35 acres, is north of Tank Farm Road. Conveyance for this subdrainage under Tank Farm Road
is provided by two 4-foot-high by 6-foot-wide (48 square feet total flow area) box culverts.
Although the topography north of Tank Farm Road is flat, the relatively low invert elevation of
the box culverts (118.85 feet amsl) prevents the structure from effectively decreasing peak
stormwater flows. Further, high groundwater levels can reach 117.9 feet amsl, the combination
of which is estimated to limit detention capacity at this location to a meager 1 acre-foot.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 16
San Luis Obispo, California August 13, 2014
This geometry prevents the North Marsh from providing any detention service to the remaining
portion of the watershed (nearly 580 acres). Approximately 36.2 acres (Catchments A17, A19,
and A20) drain to Tank Farm Creek from the west. Catchment A18 is the northwest operations
area of the site, where the local headquarters is currently housed. This area is partially paved and
the remaining land has a hardpack surface that is remnant of the operational period of the site.
The other two catchments (Catchments A19 and A20) are offsite and drain via sheet flow onto
the former operations area. A new sewer lift station has been constructed immediately west of
the former operations area by the City of San Luis Obispo. Stormwater runoff is routed around
the lift station and into Tank Farm Creek by passing through the former operations area.
At the eastern point of the operations area (Figure 7), Tank Farm Creek splits into its east and
west forks. Following the forks upstream, the West Fork runs nearly due north, while the East
Fork branches to the northeast. The onsite configuration of the forks has been altered from the
pre-development past through the use of berms and containment structures that were used to
minimize the migration of petroleum spills.
The East Fork drains an area of approximately 169 acres, which is shown in Figure 7. This
portion of the Tank Farm Creek Watershed is almost entirely overland flow; only 0.6 acre
(0.4 percent) is paved. In the lower part of the basin, the slope is relatively flat and consists of a
greater proportion of silt or clay soils. Offsite, the slope is generally steeper (about 16 percent)
as this (Catchments A11 and A12) is the local headland of the watershed. The ground is also
somewhat more impervious, as the bedrock daylights in this area forming the upland hills.
The West Fork drains approximately 336 acres, as shown in Figure 7. From its branch off the
Tank Farm Creek to the point where it enters the site along the north property line, the West Fork
follows a poorly defined channel that has a slope of just 0.4 percent and only 4 feet of relief.
Offsite the main course of the West Fork is channelized in one form or another and is ultimately
managed within a detention basin with a weir outlet control structure before being discharged
onto the site. Approximately 70 acres of the West Fork watershed are urbanized. Overland
conditions (land use and soil type) are similar to the East Fork, with silt and clay soils in the
relatively flat downstream areas, changing to steeper exposed bedrock in the headwater areas.
Another feature of the onsite portion of the Tank Farm Creek watershed is the series of closed
circular berms north of Tank Farm Road. These seven berms surrounded 55,000-barrel ASTs
during the operational past of the facility. In total, there are approximately 9.2 acres within the
berms that are isolated from the greater catchment. Without ongoing operations or maintenance
occurring, wetlands of low ecologic function (WSP, 2008) have established themselves in these
areas. Some of the wetlands support vernal pool fairy shrimp and are, therefore, federally
protected habitat.
3.1.2 Hydrology and Hydraulics
The existing site conditions were evaluated for a 24-hour storm with recurrence intervals of 2,
10, 25, 50, and 100 years. The key design points and pond features considered in the Existing
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 17
San Luis Obispo, California August 13, 2014
Conditions analysis are shown in Figure 7. Design Point DP-1 is where the outlet of the
Margarita Area Detention Basin enters the SLO Tank Farm along the western portion of the
north property line. DP-2 is where the East Fork of Tank Farm Creek crosses the north property
line. DP-3 is the confluence of the East Fork and the reach that drains former Reservoir 4. This
point is later used to assess the transition between development and the restored Tank Farm
Creek. DP-4 and DP-5 represent the two 48-inch-diameter round culverts and the two 4-foot by
6-foot box culverts, respectively. Design point DP-6 is the discharge point through the headwall
for Tank Farm Creek.
Individual hydrographs for each catchment, pond, or outlet structure used in the model are found
in Appendix D. Supporting calculations for the hydraulic structures regarding capacity and
velocity are presented in Appendix E. Table 3 provides the peak flow from the design points
shown in Figure 7. Table 4 summarizes the available and engaged capacities of the selected
onsite ponds and detention features, and Table 5 lists the peak flows and velocities of selected
onsite channels for the 2-year precipitation event. Figure 8 through Figure 12 present flow or
storage hydrographs for the key design points and the North Marsh. Figure 9 illustrates the
relationship between storage and outflow at the headwall.
A quick review of the tables and hydrographs illustrates the inadequacy of the existing storm
water management infrastructure for storms exceeding the 10-year recurrence interval
(4.7 inches in a 24-hour period). As shown in Figure 8, the hydrograph for Design Point DP-6
(the offsite discharge point for Tank Farm Creek), approximately 10.2 hours into the 100-year
design, stormwater levels rise above the spill elevation for the headwall structure and begin an
uncontrolled discharge. The table below summarizes the onsite storage volume and peak flow
from the site discharge (outlet and overtopping).
Precipitation Event Storage Volume
(acre-feet)
Peak Flow
(cfs)
2-Year 24-Hour Storm 31.4 60.1
10-Year 24-Hour Storm 94.6 81.8
25-Year 24-Hour Storm 111.4 229.7
50-Year 24-Hour Storm 114.5 377.0
100-Year 24-Hour Storm 118.5 469.2
As a system, the existing infrastructure functions poorly. Individual components are undersized,
or grossly oversized, and generally do not complement one another. The majority of stormwater
is routed directly to Design Point DP-6 to discharge offsite. Consequently, the limited storage
capacity south of Tank Farm Road is quickly exceeded, and the substantially greater storage
capacity north of Tank Farm Road is never mobilized.
Both forks of Tank Farm Creek are nominally routed to Design Point DP-4, the pair of round
48-inch-diameter culverts (Figure 7). The flow of stormwater through the East Fork is
constrained by a 12-inch culvert under an onsite access road. Water in excess of the capacity of
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 18
San Luis Obispo, California August 13, 2014
this pipe overtops the road, with some of that flow discharging into the North Marsh. Once the
flood stage reaches an elevation of 122.9 feet amsl, the conduits are fully engaged and are
surcharged. As shown in Figure 10, peak flows at this location range between 131 cfs for the
2-year storm and 556 cfs for the 100-year storm. It is expected that the conduits will be fully
engaged for any storm at or above the 25-year intensity.
From this point, stormwater flows towards the offsite outlet. Before discharging, however, it is
forced to pond in a series of awkwardly connected ersatz storage basins. It seems by their layout
and function that the basins were not engineered to manage stormwater, but are, rather, an
artifact of the operational history of the site. The relationship of these basins and the offsite
discharge are illustrated in Figure 9.
In the past, the invert elevation of the headwall outlet (115.9 feet amsl) forced water to
accumulate in a small-capacity pond within Tank Farm Creek, but the storage volume was only
about 0.03 acre-foot before water was discharged through the headwall outlet. Recent grading
within the area has filled in this small pond to where no water is stored behind the headwall. As
the flood stage passes elevation 117.9 feet amsl, water is not only discharging through the
headwall outlet but also spilling into the pond feature within the parallel channel that drains the
North Marsh (PA-5). However, only 0.6 acre-feet of the first pond’s capacity has been used at
this point. Once the water level in the second pond reaches an elevation of 117.9 feet amsl, it is
hydraulically continuous with the first pond.
Once the flood stage reaches an elevation of 118.9 feet amsl, the outlet is fully engaged. Water
begins to spill to the South Pond, PA-7 (Figure 7), as the flood stage passes an elevation of
119.9 feet amsl. And, at an elevation of 122.4 feet amsl the ponds are completely full and the
headwall is subject to overtopping. Although inefficient, the existing stormwater infrastructure
will accommodate storms up to the 10-year event (4.7 inches over 24 hours). Storms of greater
intensity, however, induce overtopping within 10 to 11 hours of the start of precipitation.
As it is used currently, and in its existing configuration, the North Marsh (Figure 7) is an
ineffective hydraulic structure. Its storage capacity is not used for relatively small storms since
flow is routed directly to the round culverts. However, even if stormwater was routed into the
North Marsh, its utility would be impaired by the relatively low invert elevation of the box
culverts and the high level of groundwater. This relationship is illustrated in Figure 13. As
shown in the figure, the North Marsh in its current configuration provides approximately
2.5 acre-feet of retention capacity prior to passing water through the box culverts.
The box culverts themselves (DP-5) are vastly oversized. As will be discussed further in
Section 3.2.2, even if all stormwater is routed through them, they do not function as culverts, and
at best just flow full. Consequently they do not slow the passage of the peak flow, which
exacerbates the limited storage capacity south of Tank Farm Road. As the system functions
currently, the box culverts pass between 90 cfs for the 2-year storm and 518 cfs for the 100-year
storm (Figure 11). At the same time, maximum storage in the North Marsh ranges between
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 19
San Luis Obispo, California August 13, 2014
10.85 acre-feet and 53.97 acre-feet for the 2-year and 100-year storms, respectively (Figure 12).
Full capacity of the North Marsh is engaged by runoff from the 10-year storm.
The existing round culverts (DP-4) are positioned at approximately the same elevation as the
existing box culverts. Runoff is routed through the round culverts via Tank Farm Creek;
however, during a moderately sized storm, as runoff accumulates north of Tank Farm Road,
runoff overtops the low flow capacity of the existing East Fork of Tank Farm Creek and spills
into the North Marsh. From this point, runoff is passed immediately south of Tank Farm Road
due to the current design of the existing box culverts. During large flood events, the entire North
Marsh and surrounding areas are inundated. Currently, 100-year flood waters around both of the
round and box culverts reach an elevation approximately 2.5 to 3-feet lower than that of Tank
Farm Road and subsequently, when the culverts are properly maintained, never overtop the road.
Inspection of the site does not suggest that erosion is a major concern under the existing
condition. Observations during March 2008 precipitation events indicate that some erosion
occurs in the flat areas between the East and West Forks of Tank Farm Creek. However, these
are quickly attenuated by the heavy vegetation in the North Marsh. During large storm events,
the channels quickly spread across the relatively flat site. Consequently, velocity calculations
are limited to the 2-year storm event and are summarized in Table 5. Review of the table shows
that the calculated velocities for the various reaches do not appear to exceed 5 feet per second,
and as such, erosion will likely not be of concern.
3.1.3 Floodplain Analysis
A 100-year floodplain analysis was performed using XP Storm software package by XP
Solutions. Figure 14 shows the extent of the 100-year floodplain. The computer-generated
results are provided in Appendix F.
The entire flood plain within the property boundary occupies approximately 63.8 acres. At the
northern property boundary, the flood stage is at an approximate elevation of 126.7 feet amsl. At
the upstream side of Tank Farm Road, it is estimated that the flood stage is at 124.8 feet amsl,
and at the headwall, the flood stage has decreased to an estimated 121.0 feet amsl.
North of Tank Farm Road, it appears that the flood stays primarily within the limits of the SLO
Tank Farm property boundary. South of Tank Farm Road, the flood stage is nominally within
the property boundary. However, the hydrology analysis also shows that 56 acre-feet of water
overtop the headwall during a 100-year storm. However, without detailed downstream analysis,
it is not possible to state what the impacts of this uncontrolled discharge are relative to offsite
flooding.
3.1.4 Water Balance
In May 2002, England Geosystem, Inc. (EGI) issued a report titled Water Balance Analysis, San
Luis Obispo Tank Farm. It was a comprehensive evaluation of site-wide hydrologic issues,
including variations in groundwater storage. It also included, however, a focused analysis of the
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 20
San Luis Obispo, California August 13, 2014
changes in storage of the North Marsh over a typical water cycle. An overview of this analysis is
included in this section, with relevant background information supplied in Appendix G.
The water balance analysis considered the movement of water into and out of the nominal
10 acres that form the North Marsh in its current condition (Figure 7) that result in changes to the
volume stored. The control volume was formed by that lateral boundary, the nominal bottom of
the pond at an elevation of 120 feet amsl and the top of the pond at an elevation of 126 feet amsl.
Although specific contributions occurred daily (e.g., precipitation, evapotranspiration), changes
were reported on a monthly basis over a 12-month period.
Inflows to the control volume were direct precipitation, runon, and seepage from rising
groundwater. Losses from the control volume were runoff, evapotranspiration, and infiltration.
The monthly balances between these gains and losses are illustrated in Figure 15, which is based
on the climate data collected between August 1999 and June 2000. Figure 16 shows the volume
in storage in the North Marsh based on extending average monthly water levels across the
nominal storage area of the pond. The measurement or calculation of the inflows and losses is
described in detail in the EGI report (May 2002).
The hydrologic operation of the North Marsh portrayed in Figure 15 and Figure 16 is considered
typical. Storage occurs in relation to precipitation events, but is limited in overall volume by the
geometry of the pond. Storage is further limited by groundwater seepage into the pond. Figure
17 shows transducer readings from the August 1999-June 2000 climate data juxtaposed with
precipitation data from the same period. Water levels respond rapidly to precipitation events.
Even between precipitation events, or the end of the rainy season, groundwater levels clearly
sustain surface water elevations within the pond, essentially offsetting losses due to
evapotranspiration.
This condition bears upon the selection of plants that will be used in the restoration project, as
the North Marsh will retain its nominal vertical profile (i.e., bottom of pond relative to top of
pond). Second, additional storage capacity cannot be developed for the pond by excavating
deeper, as it would be occupied by rising groundwater.
3.2 CHANGES DUE TO REMEDIATION AND RESTORATION
The conceptual project plan shown in Figure 2 helps to illustrate the proposed remediation and
restoration activities. The principal activities considered in this section are capping of the
Northwest Operations Area, remediation of the North Marsh, and restoration of the riparian
function of Tank Farm Creek. Additional hydrologic analyses for caps, and other parts of the
remediation, are found in the Remedial Action Plan. In evaluating changes due to remediation
and restoration, this report also incorporates effects that are expected to occur due to upstream
development. From a hydrologic perspective, this significantly alters the watershed, but it also
provides an opportunity to address many of the hydrologic shortcomings of the onsite
stormwater management system.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 21
San Luis Obispo, California August 13, 2014
3.2.1 Watershed Delineation
One objective of remediation will be to restore as much of the original ecological function to the
Tank Farm Creek corridor as possible. This will entail substantial changes to the topography, as
well as to the stormwater management infrastructure. It is also anticipated that upstream
development will increase runon and will also occur within a similar timeframe as remediation,
and, therefore, must be considered as part of the post-remediation analysis. Another objective of
the remediation and restoration effort is to ensure that the site effectively manages flood waters
onsite to the maximum degree possible.
These objectives are achieved by the proposed changes to the watershed illustrated in Figure 18.
The modifications more closely replicate a natural channel and, to the degree possible, the
original channel that existed prior to development in the early 1900s. The restored channel will
also be more effective in managing flood waters. Additional detail regarding restoration is
provided in the Restoration Plan.
South of Tank Farm Road, the existing property line channel and auxiliary channel are combined
into a single sinuous streambed. This has the advantage of providing a greater effective channel
area and more closely replicating existing natural channels. A typical cross section of the
channel is shown in Figure 19. The channel has an overall slope of 0.2 percent and about 3 feet
of relief. Channel hydraulic performance is discussed in Section 3.2.2.
It was previously proposed to bore two 12-inch-diameter outlets near the bottom of the existing
headwall to provide low-flow drainage of the small permanent onsite pond upstream. However,
recent grading in the area by the current land owner has filled in this small pond and eliminated
the need for these two outlets. As such, for the purpose of this study, the existing 36-inch-
diameter outlet will be treated as the sole point of stormwater discharge from the site. Chevron
will construct a new site discharge structure immediately upstream of the existing headwall
within its property boundary to maintain control of stormwater discharge from the site. The
structure will be designed in such a way as to mimic the discharge conditions of the existing
headwall. It is also assumed that the current landowner will remove the existing headwall
structure to improve downstream channel hydraulics. Conceptually, the discharge structure will
be a two-stage weir as shown in Figure 9 with two 12-inch-diameter low-flow culverts, or a
single culvert of equivalent discharge, positioned at the low point to provide controlled site
drainage of Tank Farm Creek.
The area south of Tank Farm Creek (Figure 18) will be used as a borrow source for up to
41,000 cubic yards of soil. Borrowing this soil, and consolidation and expansion of the channels,
also consolidates the three small basins identified as PA-4, PA-5, and PA-7 on Figure 7 into a
single basin referred to as PA-7 on Figure 18. The overall detention capacity south of Tank
Farm Road is thereby increased by approximately 14 acre-feet, a 19 percent increase over the
existing condition.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 22
San Luis Obispo, California August 13, 2014
A cap will be constructed over the former operations area as part of remediation, but will also
serve as a pad for future development. As shown in Figure 18, a channel running parallel to
Tank Farm Road will capture stormwater from offsite catchments (A19 and A20) that used to
flow across the operations area and discharge it into the North Marsh. Runoff from the cap will
be routed into ditches along the upper perimeter and discharged to adjacent drainage features via
hardened drops. Detailed descriptions, including engineering drawings, for these components
are found in the Remedial Action Plan.
One of the most significant restoration-driven changes is the reconfiguration of the North Marsh
and Tank Farm Creek. As depicted, the East and West Forks of Tank Farm Creek will merge in
the North Marsh rather than being routed around it. The typical channel sections will be similar
to the illustration shown in Figure 19. Flows associated with storms between the 2-year and
10-year intensity are routed in the bottom portion of the channel. As discussed in the Restoration
Plan (WSP, December 2008), the bottom is sinuous, and supplied with natural features such as
wood and stone to limit water velocity and thereby control erosion. Larger storms utilize
proportionally higher sections of the surrounding bank until maximum flood stage is reached.
The North Marsh increases in area by approximately 8 acres, mostly through removing historical
debris. The creek is routed through a single barrel of the box culvert beneath Tank Farm Road.
The two 48-inch-diameter round culverts to the west will be abandoned and removed. A
V-notch or slotted weir will be constructed in one of the box culvert barrels, raising its invert
elevation by 2 feet (Figure 13). The box culverts will also be lengthened to facilitate anticipated
future widening of Tank Farm Road. These modifications create an additional 17.5 acre-feet of
storage capacity within the North Marsh that is engaged as a storm begins. The peak flow of a
given storm is both reduced and delayed (Figure 11), easing demand on the limited storage and
discharge capacities of the creek system south of Tank Farm Road.
The unused barrel of the box culvert will be retained as an emergency outlet for the North Marsh.
An overflow structure will be built around the barrel as shown conceptually in Figure 13. Water
will enter the structure at the estimated 100-year flood stage at elevation 122.85 amsl. The top of
the overflow structure would be provided with a trash rack to screen debris and to prevent
accidental falls into the structure.
Another significant topographic alteration will be capping of former Reservoir 4, and the rock
and soil that will be borrowed from the Flower Mound. As shown in Figure 2, that area is
proposed for future development. Remediation is, therefore, designed to facilitate that end use.
Grading of the area will produce large, gently sloped preliminary pads. It will also establish the
alignment and rough grade of the Santa Fe Road extension and the interior local roads. New
ditches are established in the area to manage runoff. These are shown and discussed in detail in
the Remedial Action Plan. The new ditches are routed to a small detention pond, which is used
to define the beginning of the riparian corridor. Treatment (e.g., sediment removal) of runoff
from the cap and the future development area will occur at or before this pond, in conjunction
with mass grading erosion control features, and development-phase Best Management Practices
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 23
San Luis Obispo, California August 13, 2014
(BMPs). Grading also temporarily increases the size of Subcatchment A12 and reduces the size
of Subcatchment A13, as discussed in Section 6.0.
Future development is anticipated to occur west, north, and east of the SLO Tank Farm north of
Tank Farm Road. The particular areas of development are described in the Margarita Area
Specific Plan or the Airport Area Specific Plan, both of which were prepared by the City of San
Luis Obispo (October 12, 2004 and August 23, 2005, respectively). As this study was being
prepared, we were aware of only one project, the King Venture and Serra Meadows (formerly
Sierra Gardens) development (see Figure 2), that had received approval. However, three other
projects adjacent to the SLO Tank Farm (referred to as Future Development), also shown in
Figure 2, are in the process of preliminary design.
This study has used the output hydrograph for the proposed detention basin that was defined in
the hydrology study prepared for King Ventures and Serra Meadows (formerly Sierra Gardens).
This hydrograph is used in place of assessing runoff from the greater watershed and making
assumptions regarding the development. It is why the watershed shown in Figure 18 has been
truncated north of the SLO Tank Farm property line. Details of this input hydrograph are found
in Appendix D.
3.2.2 Hydrology and Hydraulics
Details of the hydrologic and hydraulic analyses for the post-remediation condition are provided
in Appendix D and Appendix E, respectively. Summaries of peak flow and storage volumes are
presented in Tables 3 and 4, respectively. Figure 8 and Figure 11 are hydrographs of selected
design points following remediation (including restoration of Tank Farm Creek).
The most notable change relative to the existing conditions is the ability of the site to internally
detain the 100-year storm event. As shown in Table 2, under the existing condition it is
estimated that more than 56 acre-feet of water overflow the outlet headwall in an uncontrolled
manner during the 100-year storm event. Peak flow discharge for the 100-year storm for the
existing condition was estimated at 469 cfs (including overflow). Assuming that the remediation
changes are effective, peak flow out of the headwall (DP-6) will be approximately 78.8 cfs with
no uncontrolled overflow during the 100-year storm event. It will, however, take longer to fully
evacuate the storage capacity of the system. It is estimated that approximately six days is
required to recover most of the storage capacity of the system following a 100-year storm.
Overall, the system appears to efficiently use discharge capacity and available storage volume.
Peak flows are reduced, relative to the existing condition for all storm events (Table 3).
However, this requires substantial increases in onsite storage, as shown in Table 4. Overall
storage within the onsite Tank Farm Creek Watershed increases from approximately 118.5 acre-
feet to 211.5 acre-feet for the 100-year storm, nearly 79 percent. In the short term, after a large
storm event, surface water area will be larger, though the specific area will vary depending on
the size of the storm. Low-flow outlets in the box culvert weir will ensure that, over the long
term, inundated areas remain no larger than their present size.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 24
San Luis Obispo, California August 13, 2014
Contrasting the hydrographs in Figure 9, one observes that the new Tank Farm Creek corridor is
much more effective in managing flood waters than the poorly connected series of ponds it
replaced. Controlled discharge delays filling of the pond, and the increased storage capacity
prevents overtopping as the delayed peak from the North Marsh finally arrives.
The box culvert, with its new weir, and the enlarged North Marsh detention area also work to
more efficiently manage stormwater. The weir forces the North Marsh to begin impounding
water at the storm onset, rather than immediately conveying to the outlet. This initial storage not
only delays the peak flow, but also decreases its intensity as seen by comparing the hydrographs
in Figure 11. As noted above, the reductions in peak flow and preventing overtopping require
substantial increases in storage (see Tables 2 and 4).
Capping the northwest operations area prevents run-on from sheet flowing into the culvert inlet
as currently occurs. The design includes a frontage channel that conveys water from the offsite
properties to the North Marsh. It also includes an 18-inch-diameter culvert that allows access to
the capped Northwest Operations area at its current ingress point. The original engineering work
was performed in the Remedial Action Plan, but was validated as part of this study (see
Appendix E).
The East Fork continues to collect runoff from the easternmost portions of the Tank Farm Creek
Watershed, including the future development area over Reservoir 4 and the Flower Mound. It
also collects runoff from Catchment A14, which used to discharge directly into the North Marsh.
Overall, the tributary area of the East Fork has increased by 30 acres (17 percent).
Grading of the Flower Mound and Reservoir 4 slightly increases peak flow from these areas over
the current conditions. The polishing/sedimentation water quality basin is used to reduce these
peak flows to just below their existing condition values, and to delay arrival of that peak into the
North Marsh (see Table 3 and reference Design Point DP-3). Details of the grades, swales, and
BMPs for the Reservoir 4 and Flower Mound areas are provided in the Remedial Action Plan and
will be addressed in the Storm Water Pollution Prevention Plan (SWPPP).
The widened low-flow channel lowers flow velocities (Table 5). It is anticipated that overall
erosion will be minimal, although various BMPs will be used, as described in the SWPPP, to
ensure that erosion is limited while vegetation is allowed to establish itself.
The catchment area tributary to the West Fork remains approximately the same compared to the
existing condition. The West Fork includes several tributaries that ensure runoff from separated
parts of the watershed flow efficiently to the new channel and then into the North Marsh. The
hydrograph provided by RRM Design Group for the update to the King and Sierra Gardens
(Serra Meadows) development identifies an increased peak runon to the site ranging between
252 cfs to 997 cfs for the 2-year and 100-year storms, respectively. The increased channel width
maintains flow velocities to within 0.83 ft/sec of the existing condition (see Table 5) for the
2-year storm.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 25
San Luis Obispo, California August 13, 2014
Performance of the channels and ponds is described further in the following section on the
100-year flood analysis.
3.2.2.1 Downstream Peak Runoff Timing Analysis
As shown in Figure 8, the peak discharge from the site has been greatly reduced due to optimized
flood management of stormwater onsite from remediation and restoration activities. However,
the timeframe of stormwater discharge from the site is lengthened as stormwater runoff is
discharged through the site outlet structure at a controlled rate. The City of San Luis Obispo
requested that an analysis be performed to determine the effect of modifying the discharge
hydrograph from the site on downstream receptors. To determine these effects on downstream
systems by lengthening the discharge period for respective storm events, it was necessary to
analyze estimated peak flow times downstream of the site. The confluence of the East Fork San
Luis Obispo Creek and Tank Farm Creek was chosen as the focal point for this analysis because
discharge from the site is expected to have little effect on peak flow rates further downstream in
the system.
Using time of concentration (TOC) calculation procedures outlined in the Waterway
Management Plan (City of San Luis Obispo, 2003), TOC values for both the East Fork of San
Luis Obispo Creek and Tank Farm Creek were calculated for each respective watershed
upstream of the confluence of the two creeks. The calculated TOC for the East Fork San Luis
Obispo Creek watershed is approximately 30 minutes while the TOC for Tank Farm Creek is
approximately 12.5 minutes. Both watersheds tend to respond quickly with regards to
stormwater runoff due to fairly steep grades and low permeability soils present in the headwaters
for each system. Therefore, they tend to produce high peak flows for a fairly short duration and
tail off quickly. Given the two watersheds calculated TOC, the timing of the peak flows from
each watershed will in their current configuration is expected to be fairly close to one another at
the confluence of the two creeks. With this in mind, it is important to understand the magnitude
of the expected peak flows to determine if modification of the timing will affect downstream
hydrology.
The Rational Method was used to estimate peak flow from the East Fork San Luis Obispo Creek
watershed, while the hydrologic model for the site was used to estimate peak flow from Tank
Farm Creek watershed. Assuming an average runoff coefficient (C) of 0.3 and using rainfall
intensity data from the Waterway Management Plan with a TOC of 30 minutes, the peak flow
rate for the East Fork San Luis Obispo Creek is estimated to be approximately 2,300 cfs for the
2-year storm and 5,700 cfs for the 100-year storm. The site’s hydrology model estimates peak
flow from Tank Farm Creek leaving the site to be approximately 60 cfs for the 2-year storm and
469 cfs for the 100-year storm in its current configuration. In the post-development
configuration, the peak flow from Tank Farm Creek is estimated to be approximately 18 cfs for
the 2-year storm and 79 cfs for the 100-year storm. It can be seen in Figure 8 that the peak flow
is greatly reduced due to onsite stormwater management improvements. Even though the
sustained timeframe of peak flow discharge is extended during the post-development conditions,
the percentage of overall flow that the post-development Tank Farm Creek watershed contributes
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 26
San Luis Obispo, California August 13, 2014
to the East Fork San Luis Obispo Creek at their confluence during the 100-year storm event is
estimated to be only 2 percent. This minimal contribution during the sustained peak flow
timeframe is, thus, considered to have negligible effect on downstream hydrologic conditions
and the reduced peak flow leaving the site presents a net benefit for stormwater management
infrastructure downstream.
3.2.3 Floodplain Analysis
Figure 20 depicts the expected alterations to the 100-year floodplain from the changes discussed
in Section 3.2.1. In the post-remediation condition, the 100-year floodplain covers 78.9 acres
onsite, a 23 percent increase over the existing condition. Although approximately 6 acre-feet of
floodplain capacity was lost due to capping the northwest operations area, approximately
93 acre-feet of capacity was gained through remediation of the North Marsh, realignment of
Tank Farm Creek, and restoration and expansion of the riparian corridor. Further, the analysis
indicates that the site can now manage the entire 100-year flood, whereas overtopping and
uncontrolled flooding previously occurred for any event in excess of the 10-year flood.
At the northern property boundary, the flood stage is at an approximate elevation of 126.1 feet
amsl. At the upstream side of Tank Farm Road, it is estimated that the flood stage is at
125.2 feet amsl, and at the headwall, the flood stage has decreased to an estimated 120.0 feet
amsl.
3.3 CHANGES DUE TO ONSITE DEVELOPMENT
Development will not alter watershed area. It will, however, alter the character of portions of the
watershed and the means by how some of the water is conveyed across the site. The two most
significant changes will be construction of a storm drain system with drop inlets, catch basins,
and other urban type infrastructure, and the degree to which the 43 acres zoned for development
within this watershed are paved.
3.3.1 Watershed Delineation
Figure 2 shows the proposed development areas for the SLO Tank Farm. The former Northwest
Operations Area and the eastern development (former Reservoir 4 and Flower Mound) north of
Tank Farm Road are relevant to the Tank Farm Creek Watershed. The Northwest Operations
Area development will likely include modification of the grades and the installation of catch
basins and storm drains. These changes are expected to be minor relative to the degree of paving
that will occur. For modeling purposes, it is assumed that 80 percent of the development area is
paved or converted to impervious rooftop, and 20 percent remains as landscaping.
Figure 21 shows the proposed development-related infrastructure to manage stormwater. Only
backbone or tract-level infrastructure is shown. Lot-level infrastructure has not been developed
at this time. For modeling purposes, the same proportion of pavement and landscaping as was
assumed for the Northwest Operations Area is used here. Although the naturally vegetated
channel along Tank Farm Road is still used to manage runoff from the southernmost parcels and
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 27
San Luis Obispo, California August 13, 2014
the road, approximately 48 acres of stormwater catchment will be collected into catch basins and
storm drains.
3.3.2 Hydrology and Hydraulics
Paving the eastern development area will substantially increase runoff through DP3 (Table 3)
relative to the post-remediation conditions. However, the retention and sedimentation pond at
this location will be engineered as part of remediation to accommodate this volume and flow
rate. Its purpose is to augment other BMPs such as the vegetated swale along Tank Farm Road
or the lot-level stormwater infrastructure (e.g., catch basin trash and oil/water separators) that
will be designed during development to help meet LID standards.
The increased runoff from the development area following paving is, however, easily absorbed
by the improvements to the detention capacity of the North Marsh. Consequently, flow rates at
DP5 and DP6 remain substantially less than the existing condition and are similar to those for the
post-remediation condition, especially for larger storms up to the 100-year storm event (Table 3).
3.3.3 Floodplain Analysis
While there are some changes in peak flow resulting from development, it is not anticipated that
there will be any significant alteration of the 100-year floodplain relative to the post-remediation
condition. Remediation-established cap or pad grades are well out of the floodplain and more
than replaced the lost capacity with additional capacity elsewhere in the onsite system. Further,
the pads are sufficiently elevated that in the event of a catastrophic flood or poor performance by
the culverts or outlets, buildings would not be inundated.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 28
San Luis Obispo, California August 13, 2014
4.0 ONSITE CATCHMENTS TRIBUTARY TO THE
EAST FORK OF SAN LUIS OBISPO CREEK
As described in Section 2.0, there are three primary watersheds of interest at the SLO Tank Farm
in its existing condition. This section addresses the small onsite watershed that is tributary to an
old gate valve that has rusted open in the berm adjacent to the East Fork of San Luis Obispo
Creek.
None of the planned remediation activities affect this area, shown in Figure 5. However, two
interesting features of this catchment are the auxiliary channel and oxbow that were cut-off from
the East Fork of San Luis Obispo Creek by construction of the containment berm along the
southeast property line. As part of the site-wide restoration efforts, it is proposed to restore flow
to these features by breaching the berm in two locations.
The following section describes the existing catchment, storage characteristics, and the estimated
outflow through the gate valve. The subsequent section discusses the proposed breaching of the
berm and what changes are expected to occur. The discussion is qualitative, given the small
scope of the changes relative to the large watershed tributary to the East Fork of San Luis Obispo
Creek.
4.1 EXISTING CONDITIONS
Figure 22 is a detailed depiction of this watershed, referred to as Watershed B in Figure 5. The
entire watershed is 19 acres, wholly within the SLO Tank Farm property. It is divided into four
subcatchments, ranging between 1.8 and 7.1 acres. The subcatchments generally define the
limits of ponds within the watershed that will temporarily store water. The upstream ponds spill
into the adjacent downstream ponds during large storm events and then ultimately discharge to
the East Fork of San Luis Obispo Creek through the old gate valve.
Access to the area is difficult, generally limited to foot traffic. It is also heavily overgrown with
thick brush. The heavy vegetation tends to slow water flow and limits erosion. Inspections of
these ponds did not detect any significant erosion or geotechnical issues.
The ponds are able to store between 0.7 and 4 acre-feet of water (Table 4) collectively for the
2-year and 100-year storm events, respectively. It is estimated that peak discharge from the gate
valve (Design Point DP-7) ranges between 0.4 cfs and 5 cfs for the 2-year and 100-year storms,
respectively (Table 3).
Development is not proposed for this area, and the planned restoration efforts (see next section)
are anticipated to have a small, though positive, effect on a very large watershed. Consequently,
the effort required to perform a flood stage analysis did not seem warranted. However, a few
observations can be made. A review of nearby hydrology studies suggests that the connection to
the SLO Tank Farm through the valve is not generally understood. Nevertheless, during a
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 29
San Luis Obispo, California August 13, 2014
100-year storm, this watershed is open to flooding since the gate valve cannot be closed. It is
also true, however, that the availability of this storage capacity is limited due to the relatively
small size (approximately 12 inches) of the valve.
4.2 CHANGES DUE TO REMEDIATION OR RESTORATION
The proposed changes to this watershed are shown in Figure 23, which illustrates the grading
that would breach the berm and fully open the area to flows from the East Fork of San Luis
Obispo Creek. Openings would be made at the upstream and downstream end of the northern
oxbow. For the southern oxbow channel, one opening occurs at the upstream point of the
southern oxbow. The second opening occurs at the current location of the gate valve. Each is
approximately 100 feet wide. Invert elevations for the breaches are set at 137.9 and 132.9 feet
amsl for the northern oxbow channel and 129.9 and 121.9 feet amsl for the southern oxbow
channel, which are approximately 2 to 3 feet above the adjacent invert elevations of the East
Fork of San Luis Obispo Creek. Consequently, the channels are not anticipated to engage during
non-storm events, but rather at times when the creek is flowing at approximately 40 percent
capacity.
Clearly, runoff from this watershed will contribute quickly and directly to the East Fork of San
Luis Obispo Creek. However, as observed before, the contribution from this 19-acre catchment
has insignificant effect on a watershed that is measured in square miles. The direct connection
will increase the potential floodplain storage. As above, the total amount of storage gained is
small compared to the overall storage required for the entire drainage. We believe that on
balance, the changes are a net hydrologic benefit to the larger East Fork watershed. In addition,
the changes are expected to improve the ecologic function of the oxbow and the auxiliary
channel. These benefits are discussed more fully in the Restoration Plan.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 30
San Luis Obispo, California August 13, 2014
5.0 ONSITE ISOLATED CATCHMENTS
One of the significant physical characteristics of the SLO Tank Farm that is a relic of its
operational past is the degree to which much of the site is hydrologically isolated from the
surrounding land. Nearly 42 percent of the site consists of closed catchments. The proposed
extension of Santa Fe Road across the site will alter this condition. Since the finished remedial
grading includes establishing the rough grade for this road, those alterations will occur as part of
remediation.
5.1 EXISTING CONDITION
The existing condition of the isolated onsite catchments is depicted in Figure 24. Isolated
catchments occur both north and south of Tank Farm Road, 9 and 131 acres, respectively. The
relatively small area of closed catchments north of Tank Farm Road is formed from the remnants
of the containment berms of the seven most northwestern of the old ASTs. The larger area south
of Tank Farm Road consists of the various containment features constructed to ensure petroleum
releases from the large reservoirs were contained onsite.
Within the overall bounds of the isolated catchments (Watershed C in Figure 5) there is no
hydrologic communication with other watersheds, although there is some communication
between certain subcatchments. Further details on the subcatchment delineation and their
internal connections are available in Appendix C.
Water is stored in the closed catchments in proportion to the total depth of precipitation. The
nominal storage values for the isolated catchment ponds analyzed in this study are listed in
Table 4. There are no hydraulic structures within the isolated catchments. North of Tank Farm
Road, the storage capacity of the former AST containment rings is not available to Tank Farm
Creek during a 100-year flood. Similarly, the storage capacity south of Tank Farm Road is not
available to the East Fork of San Luis Obispo Creek.
5.2 CHANGES DUE TO REMEDIATION AND RESTORATION
Some of the isolated catchments will be disturbed by remediation and restoration efforts. Most
of that work will not significantly alter the overall hydrologic function within the closed
watershed. A thorough discussion of particular hydrologic and hydraulic features of the
remediation work in this area is provided in the Remedial Action Plan. However, there are a few
actions that will result in changes to the hydrologic character of portions of the isolated
catchments and, therefore, warrant more detailed discussion.
5.2.1 Watershed Delineation
The anticipated configuration of the isolated catchments after remediation is shown in Figure 25.
North of Tank Farm Road, the seven containment rings for the former ASTs will remain intact
and isolated from the Tank Farm Creek Watershed.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 31
San Luis Obispo, California August 13, 2014
As was first shown in Figure 2, the eastern portion of the isolated catchments, that part south of
Tank Farm Road, has been dedicated for development. It will also be used as a borrow source
for topsoil during remediation.
One of the final tasks during remediation will be restoration of the borrow areas. In this case,
that activity specifically includes backfilling to establish rough grade pads for future
development. This area has been broken into separate watersheds that will be addressed
individually in this section (Figure 25). The larger section is a portion of Watershed C
(Subcatchment C3, 15.9 acres) in the future development area. Watershed E2, after remediation
and restoration grading activities, consists of a portion of the future development area just east
and west of the extension to Santa Fe Road and is comprised of approximately 10 acres
(Figure 6).
5.2.2 Hydrology and Hydraulics
The developed portion of Watershed C will remain, at least initially, an isolated catchment.
However, the altered grades and its future development warrant constructing a detention and
sedimentation basin to minimize erosion of the soil cover. Runoff from the pads will occur as
sheet flow, but will be concentrated into small channels and directed to the stormwater
management basin. The sizing of the basin and the hydraulic function of the ditches are
presented in the Remedial Action Plan. Water in the basin will be allowed to infiltrate, or may
be discharged to the wetland portion of the isolated catchment after treatment. In conjunction
with stormwater BMPs presented in the SWPPP, the basin will provide water quality control for
stormwater runoff from the development pads.
Alternatively, water in the basin could be discharged into the oxbow in a controlled manner.
There are no engineering factors that would prevent this, and any pollutants (e.g., sediment)
could be readily removed prior to discharge into the oxbow. Selection of this alternative is,
therefore, a regulatory consideration based on the most beneficial use of the collected
stormwater.
Watershed E2 is separated from its original isolated catchment by the extension of Santa Fe
Road. With this change in grade, this 10-acre catchment will now drain, via a naturally
vegetated perimeter ditch, to Acacia Creek. Peak flows will range between 14 and 43 cfs, for the
2-year and 100-year storms, respectively, for post-remediation conditions. Total discharge for
the 100-year storm is estimated to be 5 acre-feet, while the total discharge from the 2-year storm
is anticipated to be less than 2 acre-feet. These volumes and flow rates are insignificant
compared to those for Acacia Creek and the East Fork of San Luis Obispo Creek. Consequently,
no extraordinary measures for detention or flow control are recommended at this time.
5.2.3 Floodplain Analysis
Neither Watershed C nor Watershed E is currently within the estimated 100-year floodplain.
The grading changes within Watershed C will not alter that condition. With the new hydraulic
connection to Acacia Creek, Watershed E could potentially be impacted by a 100-year flood.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 32
San Luis Obispo, California August 13, 2014
However, the future building pads within each watershed have been set at elevations believed to
be above the 100-year flood, based on the analysis performed by Mead & Hunt (March 2006) for
the San Luis Obispo County Runway Expansion. Consequently, neither watershed is removing
storage capacity that would otherwise be available to the floodplain, and the design ensures that
any structures will be well above the potential flood stage in the event the watersheds are
engaged.
5.3 CHANGES DUE TO DEVELOPMENT
Development within the isolated catchments is limited to those catchments located on the eastern
portion of the site just south of Tank Farm Road. Similar to development north of Tank Farm
Road, development in this area will consist of the construction of a storm drain system with drop
inlets, catch basins and other urban-type infrastructure, a detention/sedimentation basin, and a
particular amount of pavement and other impermeable surfaces.
5.3.1 Watershed Delineation
Figure 2 shows the proposed development areas for the SLO Tank Farm. The areas designated
for development south of Tank Farm Road on the eastern portion of the site are relevant to the
isolated catchments and the Acacia Creek watershed. Figure 21 shows the proposed extent of
developmental grading and related infrastructure to manage stormwater. For modeling purposes,
it is assumed, as with development elsewhere onsite, that 80 percent of the development area is
paved or converted to impervious rooftop and 20 percent remains as landscaping.
As part of the proposed development in this area, a new detention/sedimentation basin will be
constructed for the treatment of runoff before discharging to the existing isolated ponds onsite.
As an alternative, treated runoff can also be routed directly into the existing oxbow and
ultimately discharged into the East Fork of San Luis Obispo Creek. However, this scenario has
not been addressed in this study. The basin will collect runoff from approximately 8.5 acres of
developed property west of the proposed Santa Fe Road extension. The remaining development
east of the Santa Fe Road extension (approximately 2 acres) drains via sheet flow to Acacia
Creek. Stormwater BMPs and site drainage will be addressed as development occurs.
5.3.2 Hydrology and Hydraulics
Paving the eastern development area will substantially increase runoff collecting within Pond
PC1 (Table 4) relative to existing conditions. Currently, Pond PC1 stores approximately
0.06 and 2.5 acre-feet of runoff for the 2-year and 100-year storms, respectively. After
development, Pond PC1 is anticipated to store approximately 2 and 6.1 acre-feet of runoff for the
2-year and 100-year storms, respectively. The added runoff is a direct result of paving and other
impermeable surfaces that coincide with development activities and a slightly larger catchment
area generated as a result of developmental grading.
Runoff discharging to Acacia Creek from Watershed E will be greatly reduced (Table 3, Design
Point DP9) relative to the post-remediation condition due to catchment reconfiguration from
grading and storm drain construction activities. Runoff is expected to discharge to Acacia Creek
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 33
San Luis Obispo, California August 13, 2014
via sheet flow at a rate of approximately 3.6 and 9.8 cfs for the 2-year and 100-year storms,
respectively, for post-development conditions.
5.3.3 Floodplain Analysis
As with the post-remediation and post-restoration conditions, after development neither
Watershed C nor Watershed E will be within the estimated 100-year floodplain as a result of
grading changes. Future building pads within each watershed have been set at elevations
believed to be above the 100-year flood, based on the analysis performed for the San Luis
Obispo County Runway Expansion (Mead & Hunt, March 2006). Consequently, neither
watershed is removing storage capacity that would otherwise be available to the floodplain, and
the design ensures that any structures will be well above the potential flood stage in the event the
watersheds are engaged.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 34
San Luis Obispo, California August 13, 2014
6.0 MINOR WATERSHEDS
Minor watersheds are those catchments at the site that are relatively small, do not possess clearly
defined channels, or are not otherwise easily addressed in the previous sections. There are two
minor watersheds described in this study. The first includes approximately 11 acres of berm
slope that drains into Acacia Creek and the East Fork of San Luis Obispo Creek. This area is
negligible with respect to the overall watershed and is not changed by either remediation and
restoration or development, and is not discussed further in this study.
The second minor watershed is referred to as Watershed D in Figure 5 and 6. In its existing
condition, it is approximately 4 acres that drains east via sheet flow to the adjacent property.
Peak runoff flow rates range from 3.5 cfs for the 2-year storm up to 14 cfs for the 100-year
storm. Total runoff volume is estimated to be between 0.4 acre-feet and 1.5 acre-feet for the 2-
year and 100-year storms, respectively (Figure 26). Extensive hydrographs for the watershed are
provided in Appendix D. Watershed D is not within the 100-year floodplain.
Remediation includes substantial grading of the Flower Mound and Reservoir 4. At the
conclusion of these activities, Watershed D will have increased to 7 acres (Figure 6). Runoff
will no longer be allowed to sheet-flow across the property line. Rather, it will be collected in a
ditch along the property line that will discharge into the shoulder ditch that runs along Tank
Farm Creek and from there into Acacia Creek. A small basin located just before the ditch along
Tank Farm Road will be used to attenuate the peak flow and to reduce sediment load.
This is, however, a temporary condition. It is anticipated that development of this area will
occur within a few years following remediation. As is shown in Figure 21 the finished grades in
Watershed D will drain to catch basins connected to a storm drain system. That storm drain will
discharge to the natural swale along the west side of the development area. The hydrology and
hydraulics of Watershed D in the post development condition are included in the Tank Farm
Creek Watershed discussion in Section 3.3.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 35
San Luis Obispo, California August 13, 2014
7.0 CLOSING REMARKS
Avocet appreciates the opportunity to assist Chevron and the project team on this project. As
mentioned previously, this document has been prepared in conjunction with the Restoration
Plan, and that these documents support the proposed remediation and development of the SLO
Tank Farm.
Given that the project is in the early stages of permitting it is recognized that changes may be
necessary as the project is refined. This includes the potential routing of Tank Farm Creek
through the southern property line, assuming that a regional working group can be established to
address the various administrative challenges associated with that proposition. It is also
understood that further evaluation will be needed for particular design elements such as the Santa
Fe Bridge, which are not included in this document.
We believe, however, that this study provides a detailed evaluation of the existing and proposed
conditions of the site, but also presents realistic solutions to address hydrologic and hydraulic
issues associated with the remediation, restoration and development aspects of this project.
Respectfully submitted,
AVOCET ENVIRONMENTAL, INC.
Robert Van Hyning, P.E.
Principal
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 36
San Luis Obispo, California August 13, 2014
REFERENCES
Avocet Environmental, Inc., March 15, 2007, Feasibility Study, San Luis Obispo Tank Farm,
276 Tank Farm Road, San Luis Obispo, California, Project No. 1185.001.
Avocet Environmental, Inc., December 18, 2007, Remedial Action Plan, San Luis Obispo Tank
Farm, 276 Tank Farm Road, San Luis Obispo, California, Project No. 1212.001.
Bouwer, H., and R.C. Rice, 1976, "A Slug Test for Determining Hydraulic Conductivity of
Unconfined Aquifers with Completely or Partially Penetrating Wells," Water Resources
Res., Vol. 12, pp. 423-428.
California Department of Water Resources, 1958, San Luis Obispo County investigation: State
Water Resources Board Bulletin 18, vols. 1 and 2.
City of San Luis Obispo, Public Works Department, County of San Luis Obispo Flood Control
District, 2003, San Luis Obispo Creek Waterway Management Plan Volume III Drainage
Design Manual, February 2003.
City of San Luis Obispo, Community Development Department, August 23, 2005, City of San
Luis Obispo Airport Area Specific Plan.
City of San Luis Obispo, Community Development Department, October 12, 2004, City of San
Luis Obispo Margarita Area Specific Plan A Transit-Oriented Development.
England Geosystem, Inc. (EGI), December 12, 2000, Hydrology Data Report (June 1999-
September 2000), San Luis Obispo Tank Farm.
England Geosystem, Inc. (EGI), January 29, 2001, Water Balance Analysis, San Luis Obispo
Tank Farm.
England Geosystem, Inc. (EGI), May 2002, Hydrology Data Report and Water Balance Analysis
(June 1999-April 2002) San Luis Obispo Tank Farm 276 Tank Farm Road San Luis
Obispo, California, Project No. 147.009.
Mead & Hunt, Inc., March 2006, Drainage Study San Luis Obispo County Regional Airport
Runway 11 Extension.
Padre Associates, Inc. (Padre), December 2007, Final Project Execution Plan
Regional Water Quality Control Board Central Coast Region, September 8, 1994, Water Quality
Control Plan for the Central Coast Basin.
Revised Hydrology Study
Former San Luis Obispo Tank Farm
276 Tank Farm Road Page 37
San Luis Obispo, California August 13, 2014
RRM Design Group, November 19, 2008, Vesting Tentative Tract Map No. 2981 Chevron SLO
Tank Farm, City of San Luis Obispo, California
San Luis Obispo County Department of Public Works, June 30, 2006, National Pollution
Discharge Elimination System Phase II Stormwater Management Program County of San
Luis Obipso
TEC Civil Engineering Consultants, Revised January 30, 2008, Addendum and Update to the
Hydrologic and Hydraulic Analysis Report for the Margarita Area City of San Luis
Obispo Dated June 28, 2005, Prepared for King Ventures & Sierra Gardens, VTM 2353-
DeBlauw, VTM 2428-King, VTM 2342-Cowan.
U.S. Department of Agriculture Soil Conservation Service (SCS), June 1986, Technical Release
55, Second Edition, Urban Hydrology for Small Watersheds.
Viessman, W., Jr., and G.L. Lewis, 1996, Introduction to Hydrology, Fourth Edition.
WSP Environmental Strategies (WSP), December 2008, Landscape Restoration for the San Luis
Obispo Tank Farm.
COLLECTOR ROAD
S
A
N
T
A F
E R
O
A
D (E
XT
E
N
SIO
N)
FUTURE DEVELOPMENT
FUTURE
DEVELOPMENT
KING VENTURES AND SIERRA GARDENS
DEVELOPMENT
FUTURE
DEVELOPMENT
TANK FARM ROAD
SANTAFEROAD
NORTH WEST
OPERATIONS
AREA
RESERVOIR
5
RESERVOIR
7
RESERVOIR
6
RESERVOIR
3
RESERVOIR
2
RESERVOIR
4
FLOWER
MOUND
E A S T F O R K S A N L U I S O BIS P O CREEK
TA N K FA R M CREEK
WE
S
T
F
O
R
KTANKFAR
M
C
R
E
E
K
E A S T F O R K TANK FARM C R E E K
S:
\
G
I
S
\
1
2
1
2
_
C
h
e
v
r
o
n
_
P
a
d
r
e
_
S
L
O
T
F
\
0
0
5
_
H
y
d
r
o
l
o
g
y
_
S
t
u
d
y
\
A
r
c
M
a
p
D
o
c
u
m
e
n
t
s
\
0
1
7
_
1
2
1
2
.
0
0
5
_
P
r
o
p
o
s
e
d
_
L
a
n
d
_
U
s
e
.
m
x
d
\
0
8
/
1
4
/
1
4
FIGURE 2
PROPOSED LAND USE ACTIVITIES
HYDROLOGY STUDY
SAN LUIS OBISPO TANK FARM
PREPARED FOR
CHEVRON EMC
SAN LUIS OBISPO, CALIFORNIA
REFERENCE:
AERIAL PHOTOGRAPH PROVIDED BY WSP.
DATED FEBUARY 4, 2008.
APPROXIMATE
SCALE
FEET
0 500 1,000250
LEGEND
PRIMARY CREEK
SECONDARY CREEK
PROPOSED ROADS
LIMITS OF GRADING TO SUPPORT DEVELOPMENT
REMEDIATION
RESTORATION
DEVELOPMENT
PROPERTY BOUNDARY
PARCEL BOUNDARY
NOTE:
RESTORATION DISTURBANCE AREA IS THAT
WHICH DOES NOT FALL WITHIN REMEDIATION
AREAS. DEVELOPMENT DISTURBANCE AREA IS
THAT WHICH DOES NOT FALL WITHIN
REMEDIATION AND RESTORATION AREAS.
ONSITE LAND USE AREA
OPEN SPACE 252.5 ac
DEVELOPMENT 64.9 ac
PUBLIC DEDICATION 15 ac
(I.E. ROADS)
TOTAL 332.4 ac
ONSITE DISTURBANCE AREA
REMEDIATION 146.4 ac
RESTORATION 41.6 ac
DEVELOPMENT 16 ac
UNDISTURBED 128.4 ac
TOTAL 332.4 ac
!!!!!
!
!
!!!!
!
!
!
!!!!
!
!
!!!!
!
!
!
!!!!!!
!
!
!!!!!
!
!
!!!!!
!
!!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!
!!!!!
!
!
!!!!
!
!
!
!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!!!!
!
!
!
!!!!
!
!
!!!!!
!
!
!!!!!!
!
!
!!!!!
!
!
!!!!
!
!
!!!!!
!
!
!
!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!!!!
!
!
!
!!!!
!
!
!!!!
!
!
!
!!!!!!
!
!
!!!!!
!
!
!!!!
!
!
!!!!!
!
!
!
!!!!
!
!
!!!!!
!
!
!!!!!!
!
!
!!!!
!
!
!
!!!!
!
!
!!!!!
!
!
!!!!!
!
!
!!!!!
!
!
RESERVOIR 4
NORTHWEST
AREA
RESERVOIR
5
RESERVOIR
6
RESERVOIR
7
RESERVOIR
3
RESERVOIR
2
FORMER
RECYCLING
AREA
FIRE SCHOOL AREA
EXISTING
BUILDINGS
TANK FARM ROAD
NORTH
MARSH
SAN LUIS OBISPO COUNTY
REGIONAL AIRPORT
TRAILER
PARK
FLOWER MOUND
Tank Farm Creek
E a st ForkSan Luis
Obispo
Creek
SANTA FE R OAD
Acacia Creek
E a s t F o r k T a n k F a r m C r e e k
West Fork Ta
nk Farm Creek
OUTLET STRUCTURE
CUT OFF OXBOW
CUT OFF OXBOW
C r e e k
Obispo
San Luis
F o r k
E a s t
M e a n d er O f
S:
\
G
I
S
\
1
2
1
2
_
C
h
e
v
r
o
n
_
P
a
d
r
e
_
S
L
O
T
F
\
0
0
5
_
H
y
d
r
o
l
o
g
y
_
S
t
u
d
y
\
A
r
c
M
a
p
Do
c
u
m
e
n
t
s
\
0
1
6
_
1
2
1
2
.
0
0
5
_
E
x
i
s
t
i
n
g
_
S
i
t
e
_
C
o
n
d
i
t
i
o
n
.
m
x
d
\
1
1
/
1
9
/
0
8
FIGURE 3
EXISTING SITE CONDITIONS
HYDROLOGY STUDY
SAN LUIS OBISPO TANK FARM
PREPARED FOR
CHEVRON EMC
SAN LUIS OBISPO, CALIFORNIA
EXPLANATION
PROJECT AREA BOUNDARY
APPROXIMATE
SCALE
FEET
0 500 1,000250
REFERENCE:
AERIAL PHOTOGRAPH BY CENTRAL COAST AERIAL MAPPING.
DATED MARCH 2, 2007.
FORMER AST LOCATION!
!!!!
!
!
!
!!!!
!
!
REFERENCE:
CENTRAL COAST REGION - BASIN PLAN,
CALIFORNIA REGIONAL WATER QUALITY CONTROL BOARD
CENTRAL COAST REGION, AUGUST 2006.
V:
\
1
2
1
2
_
P
a
d
r
e
-
C
h
e
v
r
o
n
_
S
L
O
T
F
\
1
2
1
2
.
0
0
5
_
H
y
d
r
o
l
o
g
y
_
S
t
u
d
y
\0
3
7
_
1
2
1
2
.
0
0
5
_
R
e
g
i
o
n
a
l
_
H
y
d
r
o
l
o
g
i
c
_
B
a
s
i
n
s
.
a
i
\
1
2
/
2
2
/
0
8
FIGURE 4
REGIONAL HYDROLOGIC BASINS
HYDROLOGY STUDY
SAN LUIS OBISPO TANK FARM
PREPARED FOR
CHEVRON EMC
SAN LUIS OBISPO, CALIFORNIA
S. Higuera Street
Tank Farm Road
Prado Road
Santa Fe Road
B
r
o
a
d
S
tr
e
e
t
EastFork TankFarmCreek
West
Fork
Tan
k
F ar mCreek
Acacia
Creek
Orcutt
Creek
San
L
u
i
s
O
bi
spo Creek
TankFarmCreek
E a st F o rk S an
Luis Obisp o C r e e k
DP-8
DP-7
DP-3
DP-6
DP-4DP-5
DP-2
DP-1
S:\GIS\1212_Chevron_Padre_SLOTF\005_Hydrology_Study\ArcMapDocuments\013_1212.005_Watersheds_Overview_Existing_Conditions.mxd\08/14/14
FIGURE 5
WATERSHEDS OVERVIEWEXISTING COND ITIONS
HYDROLOGY STUDYSAN LUIS OBISPO TANK FARMPREPARED FORCHEVRON EMCSAN LUIS OBISPO, CALIFO RNIA
UVA
UVB
UVC
0 1,40 0 2,80 0700Feet
(2) 4'x6' BOX CULVERTS
(2) 48" DIA.CONCRETE CULVERTS
36" DIA. GATEDDISCHARGE
UVD
NOTES:1) DIAGONAL HATCHED AREAS ARE TR IBUTARY TO THE NORTH MARSH AND AUXILIAR Y TANK FARM CREEK AN D ARE NOT ENGAGED IMMEDIATELY DURING A STORM,
WATERSHED ID ONSITE (ac)OFFSITE (ac)TOTAL (ac)WATERSHED ATANK FARM CREEK WATERSHEDWATERSHED BONSITE TRIBUTARY TO THE EAST FORK OF SAN LUIS OBISPO CREEKWATERSHED CONSITE HYDROLICALLY ISOLATED DEPRESSIONSWATERSHED DONSITE AREA TO THE EAST WHICH DRAINS OFFSITEOTHEROTHER MINOR OFFSITE DRAINAGETOTAL 333.3 477.3 810.6
160.1 476.5 636.6
19.1 0.0 19.1
0.8 139.9
4.00.04.0
11.0 0.0 11.0
139.1
Legen d
!H Hydraulic Design Point
Watershed Catc hment AreaTB
Creek/Stream
EXISTING DETENTIONBASIN
!H !H
!H
!H
!H
!H
!H
!H
S. Higuera Street
Tank Farm Road
Prado Road
Santa Fe Road
B
r
o
a
d
S
tr
e
e
t
West
Fork
Tan
k
F ar mCreek
Acacia
Creek
Orcutt
Creek
San
L
u
i
s
O
bi
spo Creek
E a st F o r k S an
Luis Obisp o C r e e k
EastFo r k Tank F ar m C reek
TankFarmCreek
!A
DP-8
DP-9
DP-7
DP-3
DP-6
DP-5
DP-1 DP-2
S:\GIS\1306_SLOTF-EMC_Work_Activities\1306.003_2013_Work_Activities\ArcMapDocuments\063_1306.003_Watersheds_Overview_PostDev.mxd
FIGURE 6
WATERSHEDS OVERVIEWPOST-REMEDIATION ANDPOST-DEVELOPMENTCONDITIONS
HYDROLOGY STUDYSAN LUIS OBISPO TANK FARMPREPARED FORCHEVRON EMCSAN LUIS OBISPO, CALIFORNIA
UVA
UVB
UVC
0 1,400 2,800700Feet
(2)4'x6' BOX CULVERT
(1) 36" DIA.DISCHARGEOPENING
UVD
UVC UVE
2
NEW DETENTIONBASIN
EXISTING DETENTIONBASIN
WATERSHED ID ON-SITE (ac)OFF-SITE (ac)TOTAL (ac)WATERSHED ATANK FARM CREEK WATERSHEDWATERSHED BON-SITE TRIBUTARY TO THE EAST FORK OF SAN LUIS OBISPO CREEKWATERSHED CON-SITE HYDROLICALLY ISOLATED DEPRESSIONSWATERSHED C2EAST FORK SAN LUIS OBISPO CREEK WATERSHEDWATERSHED DON-SITE AREA TO THE EAST WHICH DRAINS OFF-SITEWATERSHED EEAST FORK SAN LUIS OBISPO CREEK WATERSHEDOTHEROTHER MINOR OFFSITE DRAINAGE
TOTAL 333.3 477.3 810.6
107.9 0.0 107.9
10.1 0.0 10.1
2.8 0.2 3.0
22.3 0.6 22.9
7.0 0.0 7.0
164.1 476.5 640.7
19.1 0.0 19.1
Legend
!H Hydraulic Design Point
Watershed Catchment AreaTB
Creek/Stream
POTENTIAL TANK FARM CREEK OUTLET(REQUIRES REGIONAL WORKING GROUP)
NOTES:1) DIAGONAL HATCHED AREAS ARE AREAS DESIGNATED FOR DEVELOPMENT.2) CATCHMENT E2 (GREEN HATCHED AREA, 10 ACRES) IS PRESENT IN THE POST-REMEDIATION CASE ONLY. IT IS DIVED INTO CATCHMENT C2 AND E IN THE POST DEVELOPMENT CASE.
UVE2
Tables
Catchment
ID
Area
(acres)
SCS Curve Number
(Composite)
Time of
Concentration
(Tc )
(minutes)
Catchment
ID
Area
(acres)
SCS Curve
Number
(Composite)
Time of
Concentration
(Tc )
(minutes)
Existing Conditions Post-Remediation Conditions
303.5 - - 303.5 - -
A10 19.5 65.0 39.3 A10 17.8 65.0 39.3
A11 61.3 83.0 24.0 A11 61.3 81.2 24.0
A12 51.4 84.2 19.6 A12 57.3 85.6 19.6
A13 56.5 77.0 32.5 A13 49.1 84.9 57.0
A14 35.0 48.0 53.0 A14 35.0 48.0 53.0
A15 11.1 48.0 50.0 A15 8.2 48.0 50.8
A16 7.7 55.5 60.2 A16 13.6 48.0 60.2
A17 5.3 63.0 48.1 A17 9.9 63.0 48.1
A18 7.4 79.9 65.0 A18 7.4 91.0 65.0
A19 25.2 79.0 54.2 A19 25.2 79.0 54.2
A20 5.7 98.0 4.9 A20 5.7 98.0 4.9
A21 6.9 91.0 5.9 A21 6.9 91.0 5.9
A22 11.3 69.8 37.4
A23 9.8 48.0 33.0
A24 10.5 51.5 63.3
A25 9.7 48.0 57.1 A25 14.7 48.0 57.1
B1 4.4 71.0 9.8 B1 4.4 71.0 9.8
B2 5.8 71.0 28.3 B2 5.8 71.0 28.3
B3 1.8 71.0 3.2 B3 1.8 71.0 3.2
B4 7.1 65.0 58.0 B4 7.1 65.0 58.0
C1 1.8 48.0 18.3 --- -
C2 2.4 48.0 27.0
C3 21.0 48.0 48.5
C4 9.3 60.8 39.4 C4 9.3 60.8 39.4
C5 10.9 48.0 39.4 C5 11.4 48.0 39.4
D1 3.6 83.0 5.5
D1 7.8 83.0 48.1
E1 3.0 91.0 8.4
Notes:
(1) Existing catchments A22 through A24 are combined (A22) in the post-remediation condition.
(2) Existing closed catchments C2 and C3 are combined (C3) in the post-remediation condition.
A1
Catchment A1 represents the subcatchment that
drains to a detention basin that was constructed to
manage stormwater runoff from new development
north of the property. Onsite flow is assumed to be
that which is discharged from the detention basin and
was provided in the Drainage Calculations report
prepared for MD2 Communities, Inc (RRM, 2011).
Table 1
Summary of Current and Post-Remediation Catchment Areas
San Luis Obispo Tank Farm
San Luis Obispo, California
Catchment A1 represents the subcatchment that drains
to a detention basin that was constructed to manage
stormwater runoff from new development north of the
property. Onsite flow is assumed to be that which is
discharged from the detention basin and was provided
in the Drainage Calculations report prepared for MD2
Communities, Inc (RRM, 2011).
A1
C3(2)22.9 79.0 42.1
A22(1)32.6 80.1 63.3
Table 2
Comparison of Tank Farm Creek Watershed Offsite Discharge Peak Flows and Volumes, and
Onsite Storage for the Existing and Post-Remediation Conditions
San Luis Obispo Tank Farm
San Luis Obispo, California
Discharge(2) Storage(3)Overflow Total Existing 36" Overflow Total
2-Year 24-Hour Storm
Existing Conditions 60.3 31.4 0.0 91.7 60.1 0.0 60.1
Post-Remediation 43.6 65.7 0.0 109.3 18.4 0.0 18.4
10-Year 24-Hour Storm
Existing Conditions 136.7 94.6 0.0 231.3 81.8 0.0 81.8
Post-Remediation 106.7 123.0 0.0 229.7 48.5 0.0 48.5
25-Year 24-hour Storm
Existing Conditions 156.4 111.4 16.6 284.4 84.5 145.2 229.7
Post-Remediation 145.7 157.4 0.0 303.1 62.6 0.0 62.6
50-Year 24-hour Storm
Existing Conditions 163.2 114.5 35.6 313.3 85.4 291.7 377.0
Post-Remediation 173.4 182.8 0.0 356.2 70.8 0.0 70.8
100-Yer 24-hour Storm
Existing Conditions 168.2 118.5 56.0 342.7 86.0 383.1 469.2
Post-Remediation 200.0 211.5 0.0 411.5 78.8 0.0 78.8
Notes:
ac-ft = acre feet
cfs = cubic feet per second
(1) Table compares existing and post-remediation conditions at the offsite outlet for Tank Farm Creek.
(2) Discharge volumes are time-integrated flow through the engineered orifice only.
(3) Storage includes all features capable of storing water hydraulically upstream of the outlet.
Volumes (ac-ft)
Storm Event Peak Flow(1) (cfs)
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Watershed A
DP-1 251.83 251.83 251.83 562.97 562.97 562.97 733.58 733.58 733.58
DP-2 39.59 47.87 37.08 86.36 100.92 83.39 113.44 131.23 110.47
DP-3 58.17 22.97 52.24 137.62 90.36 140.73 184.87 127.98 177.82
DP-4(1)130.65 - - 323.95 - - 406.49 - -
DP-5 90.02 20.81 35.81 263.94 58.33 80.41 372.77 82.66 103.65
DP-6 60.13 18.36 29.79 81.82 48.48 60.29 229.68 62.55 71.12
Watershed B
DP-70.4- -2.06- -3.19- -
Watershed D
DP-8 3.63 3.46 - 8.12 7.85 - 10.76 10.42 -
Watershed E
DP-9 - 14.35 3.63 - 26.74 6.31 - 33.58 7.78
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Watershed A
DP-1 865.38 865.38 865.38 997.43 997.43 997.43
DP-2 132.6 152.52 129.62 151.8 173.81 148.85
DP-3 218.52 151.05 202.38 252.62 174.7 226.6
DP-4(1)479.88 - - 555.91 - -
DP-5 446.24 103.26 124.87 518.37 124.26 132.55
DP-6 377.0 70.83 78.37 469.15 78.75 81.13
Watershed B
DP-7 4.0 - - 4.82 - -
Watershed D
DP-8 12.62 12.25 - 14.5 14.08 -
Watershed E
DP-9 - 38.35 8.8 - 43.1 9.82
Notes:
cfs = cubic feet per second
(1) The round culverts beneath Tank Farm Road, represented as design point DP4, are removed in the Post-Remediation and Post-Development conditions.
Table 3
Summary of Peak Flows
San Luis Obispo Tank Farm
San Luis Obispo, California
Design
Point ID
Design
Point ID
2-Year Peak Flow
(cfs)
10-Year Peak Flow
(cfs)
25-Year Peak Flow
(cfs)
50-Year Peak Flow
(cfs)
100-Year Peak Flow
(cfs)
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Watershed A
PA1 10.85 49.84 44.13 33.43 88.1 81.6 43.59 110.1 106.03
PA2 3.74 - - 6.32 - - 7.44 - -
PA3 0.03 0.05 0.05 0.34 0.53 0.53 0.64 0.99 2.58
PA4(1)8.98 - - 23.26 - - 27.72 - -
PA5(1)13.55 - - 31.43 - - 34.15 - -
PA6 1.11 0.62 0.62 1.42 1.15 1.15 1.42 1.42 1.42
PA7(1)3.34 12.04 19.31 7.61 33.69 45.78 8.35 49.41 64.83
PA8 4.43 5.1 6.03 7.11 6.84 7.88
Watershed B
PB1 0.24 - - 0.65 - - 0.93 - -
PB2 0.32 - - 0.88 - - 1.25 - -
PB3 0.09 - - 0.26 - - 0.38 - -
PB4 0.01 - - 0.06 - - 0.11 - -
Watershed C
PC1 0.06 1.48 2.01 0.73 3.38 3.63 1.38 4.54 4.64
Existing
Condition
Post-
Remediation
Post-
Development
Existing
Condition
Post-
Remediation
Post-
Development
Watershed A
PA1 47.67 125.03 122.91 53.97 140.56 146.77
PA2 8.19 - - 8.94 - -
PA3 0.89 2.5 5.02 1.17 5.16 6.09
PA4(1)29.17 - - 31.14 - -
PA5(1)34.29 - - 34.46 - -
PA6 1.42 1.42 1.42 1.42 1.42 1.42
PA7(1)8.71 64.24 79.78 8.92 80.63 86.07
PA8 7.34 8.37 7.81 8.85
Watershed B
PB1 1.13 - - 1.34 - -
PB2 1.52 - - 1.79 - -
PB3 0.46 - - 0.54 - -
PB4 0.14 - - 0.19 - -
Watershed C
PC1 1.92 5.37 5.38 2.52 6.22 6.14
Notes:
ac-ft = acre feet
(1) Ponds PA4, PA5, and PA7 are combined into one pond (Pond PA7) for the Post-Remediation and Post-Development scenarios.
Table 4
Summary of 24-Hour Peak Storage Volumes at Selected Ponds
San Luis Obispo Tank Farm
San Luis Obispo, California
Pond or
Storage
Basin ID
50-Year Storage Volume
(ac-ft)
100-Year Storage Volume
(ac-ft)
Pond or
Storage
Basin ID
2-Year Storage Volume
(ac-ft)
10-Year Storage Volume
(ac-ft)
25-Year Storage Volume
(ac-ft)
Flow Volume
(cfs)
Velocity
(ft/sec)
Flow Volume
(cfs)
Velocity
(ft/sec)
Flow Volume
(cfs)
Velocity
(ft/sec)
West Fork Tank Farm Creek
Reach 1 217.5 2.14 245.67 2.97 247.49 2.98
East Fork Tank Farm Creek
Reach 1 57.88 3.08 47.87 3.24 36.73 2.92
Reach 2(1)14.89 0.82 22.97 1.24 51.92 1.61
Tank Farm Creek South of Tank Farm Road
Reach 1 128.4 1.6 20.81 2.15 35.81 3.36
Notes:
cfs = cubic feet per second
ft/sec = feet per second
Channel Reach
Post-Remediation 2-Year Storm Post-Development 2-Year StormExisting Condition 2-Year Storm
(1) Flow through Reach 2 of the East Fork Tank Farm Creek for the existing condition is limited by the size of the existing culvert
beneath the access road. Excess flow not passing through the culvert is routed directly into the North Marsh.
Table 5
Summary of Peak Flows and Velocities in
Selected Tank Farm Creek Watershed Channel Reaches
San Luis Obispo Tank Farm
San Luis Obispo, California
Attachment 2
(Letter from Chevron)
1
Camacho, Robert A.
From:Smith, Andrew Joseph <smith.andy@chevron.com>
Sent:Wednesday, May 16, 2018 2:08 PM
To:Walters, Tim J.
Cc:Snelling, Eric; Camacho, Robert A.
Subject:Avila Ranch
Tim,
I understand that you and Eric Snelling met with Hal Hannula from the City of San Luis Obispo on April 13th and that Hal
has asked for an update from Chevron on the current remediation project at the San Luis Obispo Tank Farm in relation
to the planned storm water improvements on Tank Farm Creek. At this time, Chevron is planning to complete the North
Marsh remediation and restoration work by the end of 2019. Grading permits have been obtained from the County of
San Luis Obispo Planning and Building Department and Chevron has budgeted for the work scope. This work will include
increasing the storm water flow detention capacity of the North Marsh and will also include installation of a concrete
weir at Tank Farm Road that will result in reduced peak flows, as documented in the Hydrology Study prepared for the
San Luis Obispo Tank Farm project by Avocet Environmental, Inc. and dated August 13, 2014.
Please contact me if you have any questions.
Thanks,
Andy Smith
Project Manager
Chevron Environmental Management Company
4000 Highway One
Morro Bay, CA 93442
SLO Office: 805-546-6915
MB Office: 805-788-6282
Mobile: 805-540-9812
smith.andy@chevron.com
Attachment 3
(Permitted Remediation Plans)