Composite Report & Appendices

226 Pages·2009·13.69 MB·English

by  Information Technology Team

City of La Quinta

MASTER DRAINAGE
PLAN

Prepared by
3187 Red Hill Avenue, Suite 250
Costa Mesa CA 92626

March 2009
City of La Quinta Table of Contents
Master Drainage Plan

TABLE OF CONTENTS

EXECUTIVE SUMMARY.......................................................................................................ES-1

1.0 INTRODUCTION...........................................................................................................1-1
1.1 SCOPE OF STUDY.............................................................................................1-1
1.2 PROJECT DESCRIPTION..................................................................................1-1
1.2.1 Location...................................................................................................1-1
1.2.2 Topography and Soils..............................................................................1-3
1.2.3 Climate.....................................................................................................1-4
1.2.4 Existing Population and Land Uses.........................................................1-4
1.2.5 Infrastructure............................................................................................1-4

2.0 BASE MAP OF EXISTING FACILITIES......................................................................2-1
2.1 GENERAL INFORMATION..............................................................................2-1
2.1.1 Aerial Photo and Topography..................................................................2-1
2.1.2 Boundary, Parcel and Land Use Data......................................................2-1
2.1.3 FEMA Flood Zone Data..........................................................................2-2
2.2 DRAINAGE FACILITIES..................................................................................2-2
2.2.1 Regional Facilities...................................................................................2-2
2.2.2 On-Site Facilities.....................................................................................2-5

3.0 ENGINEERING DESIGN CRITERIA...........................................................................3-1
3.1 COUNTY REQUIREMENTS (FOR PUBLIC PROJECTS)..............................3-1
3.2 CITY REQUIREMENTS (FOR PRIVATE PROJECTS)...................................3-2
3.3 ‘HIGH CONFIDENCE’ VS. ‘EXPECTED VALUE’ LEVELS.........................3-3
3.4 RECOMMENDED REQUIREMENTS..............................................................3-4
3.5 WATER QUALITY PERMITTING REQUIREMENTS...................................3-4
3.5.1 New Development / Redevelopment.......................................................3-4
3.5.2 Construction Activities............................................................................3-6
3.5.3 Municipal Agency Activities...................................................................3-8

4.0 DRAINAGE EVALUATION AND ANALYSIS...........................................................4-1
4.1 FEMA ANALYSIS..............................................................................................4-1
4.1.1 Methodology............................................................................................4-2
4.1.2 Results......................................................................................................4-3
4.1.3 Recommendations....................................................................................4-5
4.2 PERCOLATION TESTING................................................................................4-6
4.2.1 Overview..................................................................................................4-6
4.2.2 Study Methods and Results......................................................................4-6
4.2.3 Conclusions and Recommendations........................................................4-7
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City of La Quinta Table of Contents
Master Drainage Plan

5.0 CONCLUSIONS AND RECOMMENDATIONS..........................................................5-1
5.1 CONCLUSIONS..................................................................................................5-1
5.2 FUNDING MECHANISMS FOR FURTHER RECOMMENDATIONS..........5-2
5.2.1 Federal Programs.....................................................................................5-3
5.2.2 State Programs.........................................................................................5-3
5.2.3 Loan Financing Programs........................................................................5-4

6.0 LIMITATIONS................................................................................................................6-1

7.0 REFERENCES................................................................................................................7-1
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Master Drainage Plan

LIST OF TABLES, FIGURES, APPENDICES

L.1 LIST OF TABLES
1-1 Climate Summary: Annual Averages..............................................................................1-4
2-1 Existing Land Uses..........................................................................................................2-1
2-2 FEMA Flood Designations..............................................................................................2-2
2-3 Other Major Regional Facilities.......................................................................................2-5
4-1 Percolation Test Locations...............................................................................................4-7
4-2 Percolation Test Results...................................................................................................4-7

L.2 LIST OF FIGURES
1-1 General Vicinity Map: City of La Quinta........................................................................1-2
1-2 Satellite Photo: Salton Sink.............................................................................................1-3
2-1 Main Regional Facilities: Aerial Photo............................................................................2-3
2-2 Whitewater River: Watershed Map.................................................................................2-4
2-3 Whitewater River Storm Channel: USGS Map...............................................................2-4
4-1 La Quinta Flood Zones: 1991 FIRM Panel.....................................................................4-2
4-2 Updated Flood Plain Analysis: Overall Area..................................................................4-3
4-3 Updated Flood Plain Analysis: Zone AO.......................................................................4-4
4-4 Updated Flood Plain Analysis: Zone X..........................................................................4-5
4-5 La Quinta Flood Zones: Dec. 2007 FIRM Panel............................................................4-6

L.3 LIST OF APPENDICES

Appendix A. Exhibits
Exhibit 1A – General Base Map – Overall
Exhibit 1B – General Base Map – Cove Area
Exhibit 2 – Land Use Map – Overall
Exhibit 3A – FEMA Flood Zone Map – Overall
Exhibit 3B – FEMA Flood Zone Map – Cove Area
Exhibit 4A – Drainage Map – Overall
Exhibit 4B – Drainage Map – Cove Area

Appendix B. Portions of 1991 FEMA Flood Insurance Rate Map (FIRM) for City of La Quinta

Appendix C. Supplement ‘A’ to Riverside County Drainage Area Management Plans (DAMP)
New Development Guideline

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City of La Quinta Table of Contents
Master Drainage Plan

Appendix D. Requests for Letter of Map Revisions (LOMR)
Initial Submittal (5/24/04)
Supplemental Submittal 1 (6/9/04)
Supplemental Submittal 2 (10/04)
Additional Submittal (10/07)
Dec. 18, 2007 FEMA Approval with Current FIRM Panel

Appendix E. Percolation Testing Technical Memorandum from GeoPentech (9/22/04)

Appendix F. Downtown Area Drainage Study

Drainage System Atlas Maps – Bound Separately
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City of La Quinta Executive Summary
Master Drainage Plan
EXECUTIVE SUMMARY

This Master Drainage Plan update has been prepared to inventory and analyze the existing storm
drainage system within the City of La Quinta. The scope of the update to the Master Drainage
Plan is detailed in Section 1.1 but, to summarize, it included base mapping, review of
engineering design criteria, analysis of existing facilities and percolation testing to establish
retention basin criteria. Analyses focused on the existing FEMA Flood Zones in the northeast
area of the ‘Cove’ in order to attempt to remove the flood plain from this portion of the City.
Facility deficiencies were to be identified and prioritized, if any, and potential funding sources
identified. The system inventory included research of existing plans as well as field work and
this data was all compiled into a Geographic Information System (GIS). Lastly, a Storm Drain
Atlas Map System was generated from the GIS.

Through analyses and diligent written and verbal communication with FEMA, two flood plain
map revisions were ultimately approved by FEMA that have the effect of removing all developed
areas of the City from the 100-year flood plain. The end result of these map revisions is any of
these areas that were previously in the flood plain will now be removed from the requirement to
purchase flood insurance.

Because of the successful hydraulic/hydrologic analyses involved with the FEMA submittal and
the removal of the 100-year flood plain, no substantial drainage deficiencies exist within the City
and, therefore, no major capital improvement program is needed. However, a detailed drainage
study of the Downtown area was conducted primarily due to the fact that no retention facilities
have been included in the development of this area as are typically required in residential
developments. City staff voiced a concern there would be potential local flooding problems if
this area continues to build out without any on-site retention.

This Downtown Drainage Study did determine that local flooding will occur at buildout along
Calle Tampico in the vicinity of Avenida Bermudas due primarily to the lack of retention
facilities and a capacity constraint in the existing storm drain along Calle Tampico east of
Avenida Bermudas. Alternatives are available to resolve this localized issue and a design report
to determine the optimum solution is recommended. Among the alternatives to consider is
diversion of flow north at Avenida Bermudas and Calle Tampico up to an existing retention
facility, if there are no major utility conflicts that would preclude this alternative hydraulically.
Paralleling the existing reinforced concrete box culvert along Calle Tampico downstream of the
existing capacity constraint at Avenida Bermudas to provide the required hydraulic capacity is
another feasible, but likely more expensive, alternative if the diversion proves infeasible. In any
case, some additional catch basins along Calle Tampico westerly from Eisenhower Drive and on
both sides of Avenida Bermudas just south of the Calle Tampico intersection are recommended
to get as much flow off the street and into the existing box culvert as possible, as detailed in this
report included in Appendix F.

After a review of County and other local municipalities in the Coachella Valley, it was
concluded that Riverside County Flood Control and Water Conservation District requirements
are the most appropriate for use for all public projects with the ability to add requirements for
March 9, 2009 Page ES-1
City of La Quinta Executive Summary
Master Drainage Plan
private projects as described in Section 3. This section also develops the rationale for use of
‘high confidence’ levels for design flows and soil type and infiltration rate testing for projects
that propose to incorporate infiltration features. Water quality permitting requirements are also
summarized in Section 3.

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City of La Quinta Section 1.0
Master Drainage Plan
SECTION 1.0
INTRODUCTION

1.1 SCOPE OF STUDY

The scope of the update to the Master Drainage Plan consists of the following:
• Developing a base map consisting of aerial photography, topography, parcel and
centerline data and existing backbone storm drain systems within the City (Section 2)
• Reviewing basic engineering design criteria for drainage and flood control within the
City (Section 3)
• Analyzing existing public facilities, particularly addressing FEMA Flood Zones in the
northeast area of the ‘Cove’ (Section 4.1)
• Perform percolation testing to verify/establish infiltration rate criteria for future retention
basins (Section 4.2)
• Identifying hydraulic deficiencies in the existing storm drain infrastructure (if necessary)
• Determining potential improvements to the storm drain infrastructure (if necessary)
• Prioritizing and estimating cost of recommended projects (if necessary)
• Preparing a report presenting the findings of this project
• Preparing an Atlas Map System showing existing storm drain facilities.

1.2 PROJECT DESCRIPTION

1.2.1 LOCATION

The City of La Quinta is located approximately 125 miles east of Los Angeles in central
Riverside County. The City is within the Coachella Valley, which extends 50 miles from Palm
Springs to the Salton Sea. The City is bounded by the Santa Rosa Mountains to the west, the
City of Indian Wells to the northwest, the City of Indio to the northeast, and unincorporated areas
of Riverside County to the north and south. This report focuses particularly on the Cove area,
which is located in the southwest portion of the City and is bounded by Calle Tampico to the
north, Calle Tecate to the south, Avenida Montezuma to the west and Avenida Bermudas to the
east. A detailed drainage study was also conducted for the Downtown area, which for purposes
of this study is bounded by Calle Tampico to the north, Avenue 52 to the south, Avenida
Bermudas to the west and Desert Club Drive to the east, although the immediate area westerly to
Eisenhower Drive was included for tributary flows.

Figure 1-1 is a general vicinity map for the City of La Quinta. General Base Maps of the City
and the Cove area are provided in Appendix A as Exhibits 1A and 1B, respectively.

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City of La Quinta Section 1.0
Master Drainage Plan

FIGURE 1-1
GENERAL VICINITY MAP:
CITY OF LA QUINTA

Source: City of La Quinta
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City of La Quinta Section 1.0
Master Drainage Plan

1.2.2 TOPOGRAPHY AND SOILS

The City terrain ranges in elevation from 20 feet below sea level to approximately 1,600 feet
above sea level (BSI, 1987). The City is located in an area known as the Salton Sink, which
consists of a gradually sloping plain extending from San Gorgonio Pass (11,485 feet above sea
level at Mt. Gorgonio) to the Salton Sea (240 feet below sea level). The Salton Sink was once
part of the Gulf of California but was separated from the Pacific Ocean by the delta created from
silt deposits by the Colorado River (Indio, 2004). Figure 1-2 is a satellite photo of the Salton
Sink.

FIGURE 1-2
SATELLITE PHOTO:
SALTON SINK

Source: SDSU, 1994

The United States Department of Agriculture (USDA) mapped soils in the Coachella Valley in
1974 in conjunction with the University of California Agricultural Experiment Station (AES).
Soils are generally fine-grained sand with small amounts of fine clay and course sand (Indio,
2004). La Quinta is characterized by highly permeable alluvial deposits with some consolidated
sediments and geological structures (BSI, 1987).

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