1876 Drainage Study - Lee & Associates

PRELIMINARY

DRAINAGE STUDY For

34th & J Residences

3402 J St.

San Diego, CA 92102 A.P.N 545-250-08

Prepared By:

Kenneth J. Discenza, P.E.

Site Design Associates, Inc.

1016 Broadway, Suite A

El Cajon, CA 92021

619-442-8467

September 12, 2011

J-1876

TABLE OF CONTENTS

1. INTRODUCTION............................................................................................................... 1

2. VICINITY MAP.................................................................................................................. 3

3. RATIONAL METHOD ...................................................................................................... 4

4. HYDROLOGIC RESULTS................................................................................................ 9

5. DETENTION REQUIREMENTS ................................................................................... 11

6. CONCLUSION.................................................................................................................. 12

7. DECLARATION OF RESPONSIBLE CHARGE......................................................... 13

FIGURES

FIGURE 1 - OVERLAY OF THE SOIL RUNOFF POTENTIAL MAP ON THE PROJECT SITE...................... 6

FIGURE 2 - SIX-HOUR PRECIPITATION FOR 100-YEAR STORM FREQUENCY ................................... 7

FIGURE 3 - 24-HOUR PRECIPITATION FOR 100-YEAR STORM FREQUENCY ..................................... 8

TABLES

TABLE 1 - RUNOFF COEFFICIENTS, MAXIMUM OVERLAND FLOW LENGTHS & INITIAL TC ............. 5

TABLE 2: SUMMARY OF PRE-PROJECT PEAK DISCHARGE RATES.............................................. 9

TABLE 3: IMPERVIOUS AREA PERCENTAGES............................................................................. 10

TABLE 4: SUMMARY OF POST-PROJECT PEAK DISCHARGE RATES.......................................... 10

APPENDICES

Appendix A: Pre-Project Drainage Map

Appendix B: Post-Project Drainage Map


34th & J Residences Page 1 J-1876 July 25, 2011

1. INTRODUCTION

This drainage report presents the pre and post-project condition hydrologic analyses for the

proposed 34th & J Residences project (hereafter referred to as project). The project is located at

3402 J St., in the City of San Diego, County of San Diego. The drainage area associated with the

proposed project and adjacent off-site slopes is approximately 0.91 acres. See the Vicinity Map,

located in Section 2 of this report, for the approximate location of the project.

Currently, there is one home constructed on the western portion of the lot, a large asphalt driveway,

and a separate asphalt area on the northern boundary of the site. The remainder of the lot is

covered with annual grasses and a few large trees. Stormwater runoff sheet drains in a westerly

direction and eventually flows west down J St. From there it enters a curb inlet which empties

directly into Chollas Creek, a concrete-lined channel.

The land use of the site is currently designated as residential. The proposed project includes the

construction of 12 2-story condominium homes, a concrete driveway, 27 parking spaces (including

2 per garage and 3 guest spaces), and associated walks and landscaping. The storm drain

improvements for the post-project condition consist of inlets and pipes throughout the site to collect

runoff from roofs and landscaped areas. The inlets within the driveway will include filtration

devices to treat the water before it gets discharged to the street and then to Chollas Creek. Off-site

run-on from the 0.11 acres to the east of the site will be collected in a concrete brow ditch and

discharged along with the on-site runoff. Run-on is expected to be minimal as the drainage area is

quite small and is heavily vegetated with ice plant.

The majority of run-off will flow to an underground detention basin and then be released at the pre-

project flow rate to a bioretention area for further treatment. Inlets in the northwest and southwest

corners of the site discharge directly to the bioretention area (see the Post-Project Drainage Map in

Appendix B for details). All runoff will overflow from the bioretention area through a curb outlet

onto 34th street, and will flow down J St to an existing inlet and then to Chollas Creek as it

currently does.



The western portion of the driveway slopes steeply to 34th St. The runoff from this portion will be

collected by a trench drain at the end of the driveway. The trench drain will slope north and

drainage will flow into the bioretention area for treatment before being released through the curb

outlet.

The City of San Diego Stormwater Standards, dated January 14, 2011 conforms to the County of

San Diego Standard Urban Storm Water Mitigation Plan (herein referred to as SUSMP), which

requires that all Priority Development Projects must be designed so that runoff rates and durations

are controlled to maintain or reduce pre-project downstream erosion conditions and protect stream

habitat. All post-project flows will be treated and managed per the SUSMP.

Final inlet sizing and hydraulic calculations for the proposed storm drain system will take place

during final engineering of this project, and are not included in this report.



2. VICINITY MAP



3. RATIONAL METHOD

The watershed is relatively small; therefore the rational method (RM) is selected to calculate

runoff.

Q = CIA

Where,

Q = Peak Rate of Flow (cfs)

C = Runoff Coefficient

I = Average rainfall intensity (in/hr)

A = Drainage Area (acres)

The soil runoff potential for this site is determined by overlaying the site on the Soil Runoff

Potential Map, published by the County of San Diego, DPW. The Soil Runoff Potential Overlay

Map is shown in Figure 1. Soil Group ‘D’ and undetermined soils are present near this site. Soil

group ‘D’ is conservatively chosen for the purpose of calculations in this report as the map shows

the soil type to be unknown.

The storm frequency chosen for design purposes this study has a 100-year recurrence interval. The

six-hour anticipated precipitation for the project site subject to the design storm frequency, 6P , is

shown in Figure 2. The twenty-four hour anticipated precipitation for the project site subjected to

the design storm frequency; 24P is shown in Figure 3.

6P = 2.5 inches

24P = 4.0 inches

6P is in the range of 45% to 65% (62.5%) of 24P and therefore needs no adjustment.

The time of concentration for on-site basin, cT , is determined from the following formula [1].

3

)1.1(8.1S

DCTc−

=

Where,

cT = Time of Concentration in hours ( cT ≥ 5 min.)

D = Watercourse Distance (ft.)



S = Slope (%)

C = Runoff Coefficient

The average rainfall intensity is calculated from the following equation (San Diego County

Hydrology Manual, June 2003). 645.0

644.7 −××= DPI

Where,

I = Rainfall Intensity (in/hr)

6P = Six hour precipitation (inches)

D = Duration (min.)

Time of concentration ( cT ) is composed of two components: The initial time of concentration ( iT ),

and the travel time ( tT ).

The maximum overland flow length ( ML ) is used from Table 1 in calculating the initial time of

concentration.

Table 1 Runoff Coefficients, Maximum Overland Flow Lengths & Initial Tc



Figure 1 - Overlay of the Soil Runoff Potential Map on the Project Site

PROJECT SITE

SOIL GROUP



Figure 2 - Six-Hour Precipitation for 100-Year Storm Frequency

PROJECT SITE

P6 = 2.5 INCHES



Figure 3 - 24-Hour Precipitation for 100-Year Storm Frequency

PROJECT SITE

P24 = 4.0 INCHES



4. HYDROLOGIC RESULTS

The 100-year, 6-hour peak flow rates for the pre-project can be found in Table 2. Watershed

boundaries and drainage areas can be found on the, “Pre-Project Drainage Map,” located in

Appendix A. Area A represents all on-site runoff while Area B represents the small area of off-site

run-on. The C-values and initial times of concentration for both areas were taken from Table 1

based on impervious percentage.

Area B is entirely pervious and is therefore given a C-value of 0.35. But due to its steep slope and

short travel distance of 38 ft., the Tc is shown as the minimum of 5 minutes.

Area A has approximately 10,000 sf of impervious area of the total 34,970 sf. This gives an overall

impervious percentage of 28.7%, so the values in Table 2 were interpolated from the Table 1 values

listed under the 25% and 30% impervious.

There is no existing stormwater conveyance system on site, and therefore all existing drainage is

sheet flow. So the given Tc’s are comprised only of Ti as Tt = 0

Table 2: Summary of Pre-Project Peak Discharge Rates

The 100-year, 6-hour peak flow rates for the post-project can be found in Table 4. The outline of

Area B remains the same as do its hydraulic characteristics. The overall outline of Area A remains

the same but it is broken into three subareas A1, A2, and A3 as shown on the Post-Project Drainage

Map in Appendix B. Area A1 represents the majority of the site which will drain to the proposed

detention structure before draining to the bioretention area for treatment. Area A2 represents the

area draining directly to the bioretention area. And Area A3 represents the areas that will drain

directly to 34th St., the majority of which is part of the expanded right-of-way.

The C-values for all three subareas were again interpolated from Table 1 based on impervious



percentage. The breakdown of impervious percentages of each subarea is shown in Table 3.

Table 3: Impervious Area Percentages

In this case, Table 1 does not apply for the initial times of concentration as all of the initial travel

distances are shorter then the maximum (LM) given by the table. Therefore, Formula 1 (given in

section 3) was used to calculate Ti. In each case the Ti was less then the minimum of 5 min. and

therefore 5 min. was used in the calculation of the intensity (I) as described in section 3.3 of the San

Diego County Hydrology Manual. The Tt for area A1 was calculated from the travel time through

250 ft of pipe length, upstream of the detention structure, at a velocity of 2.5 ft/s. This gives a total

travel time off 100 sec. or 1.67 min.

Table 4: Summary of Post-Project Peak Discharge Rates



5. DETENTION REQUIREMENTS

The on-site 100-year pre-project peak flow rate is 2.01 cfs. The on-site 100-year post-project peak

flow rate totals approximately 3.31 cfs. Therefore, detention will be required for this storm event.

The project proposes an on-site, underground detention facility plus storage within the bioretention

area which will detain the post-project flows back to pre-project flow rates. The preliminary

storage volume needed to meet this requirement is approximately 1600 sq-ft. Details for the

detention basin will be completed with the final design.



6. CONCLUSION

This drainage report presents the 100-year, 6-hour pre and post-project hydrologic analyses for the

34th & J Residences project. Off-site flows are conveyed through the project site and are

discharged in the same location as they were in the pre-project condition. On-site runoff from the

proposed project will be detained, treated as necessary, discharged to the street and ultimately

drains into Chollas Creek.

The pre-project condition 100-year, 6-hour peak runoff rate on-site is 2.01 cfs. The post-project

condition 100-year, 6-hour peak runoff rate on-site is 3.31 cfs. Therefore, detention is required for

this storm event. The preliminary storage volume required to detain this event is 1,600 sq-ft.

Details for the detention basin will be included with the final grading plan design.

Inlet sizing and hydraulic calculations for the proposed storm drain system will take place during

final engineering of this project.

All post-project flows will be treated per the SUSMP, and will be addressed in a separate report.



7. DECLARATION OF RESPONSIBLE CHARGE

I, hereby declare that I am the engineer of work for this Drainage Report, that I have exercised

responsible charge over the design of the project with respect to this study as defined in section

6703 of the Business and Professions Code, and that the design is consistent with current standards.

I understand that the check of project drawings specifications, studies and reports by the City of

San Diego is confined to a review only and does not relieve me, as engineer in responsible charge,

of my responsibilities for project design.

Kenneth J. Discenza RCE 29930 Expires 06/30/13 Date



APPENDIX A

Pre-Project Drainage Map



APPENDIX B

Post-Project Drainage Map

1876 Drainage Study - Lee & Associates

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