Hydrology Report

Including Offsite Drainage Areas

For

El Centro Aquatic Center

Park Avenue between 4th Street and 6th Street

El Centro, California 92243

September 29, 2016

Prepared by

Arsalan Dadkhah, Ph.D., PE (RCE 41560) D-MAX Engineering, Inc.

7220 Trade Street, Suite 119 San Diego, CA 92121

858.586.6600

Table of Contents 1 Introduction .......................................................................................................................................... 1

2 Existing Conditions ................................................................................................................................ 1

3 Proposed Conditions ............................................................................................................................. 6

4 Conclusion ............................................................................................................................................. 9

5 Project Site Maps ................................................................................................................................ 11

Tables Table 1. Existing Condition Runoff Coefficients for Onsite Drainage Areas. ................................................ 1

Table 2a. Existing Condition Surface Type Composition (acres) for Offsite Drainage Areas. ....................... 2

Table 2b. Existing Condition Runoff Coefficients for Offsite Drainage Areas. .............................................. 2

Table 3a. Existing Condition Times of Concentration for Onsite Drainage Areas......................................... 3

Table 3b. Existing Condition Times of Concentration for Offsite Drainage Areas. ....................................... 4

Table 4a. Existing Condition Onsite 25- and 100-Year Storm Peak Flows. ................................................... 5

Table 4b. Existing Condition Offsite 25- and 100-Year Storm Peak Flows.. .................................................. 5

Table 5. Total Flow for the Existing Condition for the 25- and 100-Year Storms.. ....................................... 6

Table 6a. Proposed Condition Onsite South DMA Runoff Coefficients.. ...................................................... 6

Table 6b. Proposed Condition Onsite South DMA Time of Concentration.. ................................................. 7

Table 6c. Proposed Condition Onsite South DMA 25- and 100-Year Storm Peak Flows.. ............................ 7

Table 7a. Proposed Condition Surface Type Composition (acres) for Offsite Drainage Areas.. ................... 7

Table 7b. Proposed Condition Runoff Coefficients for Offsite Drainage Areas.. .......................................... 7

Table 7c. Proposed Condition Times of Concentration for Offsite Drainage Areas.. ................................... 8

Table 7d. Proposed Condition Offsite 25- and 100-Year Storm Peak Flows.. ............................................... 8

Table 8. Orifice Flow Rate Calculations for the Proposed Onsite East and West Basins... ........................... 8

Table 9a. 25-Year Peak Flow Summary for the Proposed Onsite Drainage Areas... ..................................... 9

Table 9b. 100-Year Peak Flow Summary for the Proposed Onsite Drainage Areas... .................................. 9

Table 10. Total Flow for the Proposed Condition for the 25- and 100-Year Storms... ................................. 9

Table 11. Overall Changes in Peak Flows Entering the Caltrans Storm Drain System on Highway 86.... ... 10

Figures

Figure 1. Intensity Duration Frequency (IDF) Curve for the 25-Year Storm.. ............................................... 4

Figure 2. Intensity Duration Frequency (IDF) Curve for the 100-Year Storm.. ............................................. 5

Maps Map 1. Existing Condition Onsite Drainage Areas.. .................................................................................... 11

Map 2. Existing Condition Offsite Drainage Areas.. .................................................................................... 11

Map 3. Proposed Condition Onsite Drainage Areas (DMAs, or Drainage Management Areas).. ............... 12

Map 4. Proposed Condition Offsite Drainage Areas.. ................................................................................. 12

Page | 1

1. Introduction The proposed site for the El Centro Aquatic Center includes Adams Park between N 6th Street and N 4th

Street and the section of N 5th Street from Park Avenue to Adams Avenue. The segment of Highway 86

analyzed in this report includes Adams Avenue and N 4th Street, located adjacent to the north and east

of the project site. To assess the impacts of the proposed Aquatic Center on the Caltrans drainage

system, hydrology calculations were performed for onsite and offsite drainage areas for the existing and

proposed conditions. Offsite peak flows were added to onsite peak flows for both the existing and

proposed conditions to quantify the result of the development on the total peak flows entering the

Caltrans storm drain along Highway 86. Runoff calculations were performed for peak flows for the 25-

and 100-year 6-hour storms using the rational method. The equation for peak flow calculations is as

follows:

Q = CIA, where

Q – Peak flow rate (cfs)

C – Runoff coefficient (dimensionless)

I – Intensity (in/hr)

A – Area (acres)

2. Existing Conditions Onsite drainage areas were delineated based on existing topography (see Map 1). Runoff coefficients

(“C” values) for pervious and impervious areas were assigned based on the standard runoff coefficient

ranges in the Caltrans Highway Design Manual, and a weighted average runoff coefficient was calculated

based on the relative amounts of pervious and impervious area in each drainage area. As dictated by the

procedure in the Caltrans Highway Design Manual, runoff coefficients need to be adjusted based on the

storm frequency intervals. The 25-yr storm C value is 1.1 times the weighted C value or 0.95, whichever

is less, and the 100-year storm C value is 1.25 times the weighted C value.

Table 1. Existing Condition Runoff Coefficients for Onsite Drainage Areas.

Existing Drainage

Area Drains to Area

(acres) %

Impervious

C (Pervious)

C (Impervious)

Weighted Average C

25-Year Storm C

100-Year Storm C

West Inlet at SE side of 6th & Adams 0.51 6% 0.17 0.90 0.22 0.24 0.28

New West Central

Inlet at SW side of 5th & Adams 1.73 3% 0.17 0.90 0.19 0.21 0.24

East Central

Inlet at SE side of 5th & Adams 0.20 48% 0.17 0.90 0.52 0.57 0.65

East Inlet at NW side of 4th & Park 0.62 0% 0.17 0.90 0.17 0.19 0.21

Page | 2

Offsite areas were delineated based on Sheet 22-14 of the Drainage Facilities Maps provided by the City

of El Centro and adjusted using survey data from Kimley-Horn (see Map 2). For the offsite drainage

areas, runoff coefficients were calculated based on each specific surface type according to the Caltrans

Highway Design Manual and weighted average based on acreage.

Table 2a. Existing Condition Surface Type Composition (acres) for Offsite Drainage Areas.

Existing Drainage

Area

Asphaltic

Road/Parking

lot (C=0.9)

Roof, Assorted

Types (C=0.85)

Concrete,

Average

(C=0.875)

Unimproved,

Low Infiltration

(C=0.30)

Lawn, Heavy

Soil Flat

(C=0.13)

Brick Pavers

(C=0.70)

E-1 0.512 0.092 0.048 0.255 0 0

E-2A 0.531 0 0.149 0 0 0

E-2B 0.750 0.241 0.121 0 0.338 0

E-3A 0.349 0 0.075 0.064 0 0

E-3B 0.264 0 0.007 0.010 0 0

E-4 0.288 0.110 0.120 0.207 0.182 0.006

Table 2b. Existing Condition Runoff Coefficients for Offsite Drainage Areas.

Existing Drainage Area Drains to

Area (acres) Weighted C 25-Year C 100-Year C

E-1 Inlet at SE side of 6th &

Adams 0.907 0.725 0.797 0.906

E-2A Inlet at SW side of 5th & Adams

0.680 0.895 0.950 0.950

E-2B 1.450 0.710 0.781 0.888

E-3A Inlet at NW side of 4th & Park

0.488 0.817 0.899 0.950

E-3B 0.281 0.878 0.950 0.950

E-4 Inlet on SW side of 4th &

Park 0.913 0.600 0.660 0.750

Intensity was calculated based on local rainfall data and time of concentration. For all drainage areas,

the travel path includes sheet flow until the flow reaches the curb gutter and becomes concentrated

flow in the curb gutter. Sheet flow duration was calculated using the following formula, which is

provided in the Caltrans Highway Design Manual:

where,

Tt – Travel time (min)

L – Flow path length (ft)

n – Manning’s roughness coefficient for sheet flow

P2 – 2-year, 24-hour rainfall depth (1.01 in per NOAA Atlas 14)

s – Longitudinal slope along flow path

Page | 3

In the existing condition onsite drainage areas, grass cover at the project site is sparse, so a Manning’s n

of 0.08 was used to account for fallow land (0.05) and short grass prairie (0.15). In the existing condition

offsite drainage areas, the following Manning’s n values were used:

0.030 for E-1 for asphalt (0.011 – 0.016) and fallow land (0.05) based on professional estimate

0.015 for E-2A, E-3A and E-3B for asphalt only (0.011 – 0.016)

0.054 for E-2B for asphalt (0.011 – 0.016), concrete (0.012 – 0.014), and lawns (0.41 for

Bermuda grass) based on professional estimate

0.110 for E-4 for asphalt (0.011 – 0.016), concrete (0.012 – 0.014), fallow land (0.05), and lawns

(0.41 for Bermuda grass) based on professional estimate

Curb gutter flow travel time was estimated by calculating a curb gutter velocity and applying that to the

length of flow in the gutter. The following equation was used to calculate curb gutter velocity:

For both sheet flow and gutter flow, the travel path length and longitudinal slope are based on

measurements from existing topography maps. The road pavement cross slope was set at 0.03 and the

gutter Manning’s n was set at 0.015 for rough asphalt. The width of flow was estimated to be 15 feet,

half of the width of a road with two-way single-lane traffic, to account for the road becoming full of

runoff during the 25- and 100-year storms due to the flatness of the area. This value was also used for

Highway 86 as a conservative estimate, since several additional inlets exist between the downstream

inlets analyzed in this report. The total time of concentration was calculated based on the sum of sheet

flow travel time and curb gutter travel time, as summarized in the table below.

Table 3a. Existing Condition Times of Concentration for Onsite Drainage Areas.

Sheet Flow Travel Time Curb Gutter Travel Time

Existing Drainage

Area L (ft) n s

Tc, Sheet Flow (min) L s v (fps)

Tc, Curb Gutter (min)

Total Tc (min)

West 80 0.08 0.005 15.4 250 0.005 3.1 1.4 16.7

New West Central 90 0.08 0.005 16.9 610 0.005 3.1 3.3 20.2

East Central 40 0.08 0.005 8.8 175 0.005 3.1 1.0 9.8

East 90 0.08 0.005 16.9 360 0.005 3.1 2.0 18.8

Page | 4

Table 3b. Existing Condition Times of Concentration for Offsite Drainage Areas.

Sheet Flow Travel Time Curb Gutter Travel Time

Existing Drainage

Area L (ft) n s

Tc, Sheet Flow (min) L s v (fps)

Tc, Curb Gutter (min)

Total Tc (min)

E-1 90 0.030 0.005 7.7 530 0.005 3.1 2.9 10.6

E-2A 67 0.015 0.005 3.5 580 0.005 3.1 3.2 6.6

E-2B 160 0.054 0.005 19.5 775 0.005 3.1 4.2 23.7

E-3A 55 0.015 0.005 3.0 400 0.005 3.1 2.2 5.2

E-3B 115 0.015 0.005 5.4 345 0.005 3.1 1.9 7.3

E-4 77 0.110 0.005 19.2 465 0.005 3.1 2.5 21.8

To calculate intensity, an intensity duration frequency (IDF) curve was developed by importing NOAA

Atlas 14 precipitation data1 for the El Centro rain gauge (EL CENTRO 2 SSW) into HydroCAD software,

which then developed an IDF table and curve. The IDF table from HydroCAD was plotted in Excel, and a

power equation was developed to calculate the intensity based on the time of concentration (duration).

The IDF curve and equation are shown in Figures 1 and 2 below.

Figure 1. Intensity Duration Frequency (IDF) Curve for the 25-Year Storm.

1 NOAA Atlas 14 Precipitation Data Frequency Server, http://hdsc.nws.noaa.gov/hdsc/pfds/

y = 10.821x-0.562 R² = 0.9949

0

1

2

3

4

5

0 20 40 60 80 100 120

Inte

nsi

ty (

in/h

r)

Duration (min)

25-Yr Intensity Duration Frequency

25-Yr Intensity Duration Frequency

Power (25-Yr Intensity Duration Frequency)

Page | 5

Figure 2. Intensity Duration Frequency (IDF) Curve for the 100-Year Storm.

Based on the IDF curves and equations above, the intensity for each drainage area was calculated, and

the intensity was used to calculate the 25- and 100-year 6-hour existing condition peak flows.

Table 4a. Existing Condition Onsite 25- and 100-Year Storm Peak Flows.

Existing Drainage Area

Total Tc (min)

25-Year Storm C

100-Year Storm C

25-yr I (in/hr)

100-yr I (in/hr)

Area (acres)

Q25 (cfs)

Q100 (cfs)

West 16.7 .24 .28 2.22 3.22 0.51 0.27 0.45

New West Central 20.2 .21 .24 2.00 2.89 1.73 0.72 1.19

East Central 9.8 .57 .65 3.00 4.38 0.20 0.35 0.57

East 18.8 .19 .21 2.08 3.01 0.62 0.24 0.40

Table 4b. Existing Condition Offsite 25- and 100-Year Storm Peak Flows.

Existing Drainage Area

Total Tc (min)

25-Year Storm C

100-Year Storm C

25-yr I (in/hr)

100-yr I (in/hr)

Area (acres)

Q25 (cfs)

Q100 (cfs)

E-1 10.6 0.797 0.906 2.87 4.19 0.907 2.08 3.44

E-2A 6.6 0.950 0.950 3.73 5.47 0.680 2.41 3.53

E-2B 23.7 0.950 0.950 1.82 2.63 1.450 2.07 3.39

E-3A 5.2 0.899 0.950 4.30 6.33 0.488 1.89 2.93

E-3B 7.3 0.950 0.950 3.55 5.20 0.281 0.95 1.39

E-4 21.8 0.659 0.749 1.92 2.77 0.913 1.15 1.90

y = 16.224x-0.574 R² = 0.992

0

1

2

3

4

5

6

7

0 20 40 60 80 100 120

Inte

nsi

ty (

in/h

r)

Duration (min)

100-Yr Intensity Duration Frequency

100-Yr Intensity Duration Frequency

Power (100-Yr Intensity Duration Frequency)

Page | 6

Onsite and offsite peak flows were directly added without accounting for the differences in travel time

for the 25- and 100-year 6-hour storm. This is a conservative assumption. Table 5 summarizes the

addition of flows and provides the total flow for each major storm drain inlet (see Maps 1 and 2) and the

total flow entering the storm drain system along Highway 86 (Adams Avenue and N 4th Street) for the

existing condition. Offsite flow for the inlet at the SW side of 5th & Adams includes runoff from E-2A and

E-2B. Offsite flow for the inlet at the NW side of 4th & Park includes runoff from E-3A and E-3B.

Table 5. Total Flow for the Existing Condition for the 25- and 100-Year Storms.

25 Year 100 Year

Downstream Inlet Onsite Q (cfs)

Offsite Q (cfs)

Drainage Area Q25

(cfs)

Storm Drain Q25

(cfs) Onsite Q (cfs)

Offsite Q (cfs)

Drainage Area Q100

(cfs)

Storm Drain

Q100 (cfs)

SE - 6th & Adams 0.27 2.08 2.35 2.35 0.45 3.44 3.89 3.89

SW - 5th & Adams 0.72 4.48 5.20 7.55 1.19 6.92 8.11 12.00

SE - 5th & Adams 0.35 0.00 0.35 7.90 0.57 0.00 0.57 12.57

NW - 4th & Park 0.24 2.84 3.08 10.98 0.40 4.32 4.72 17.29

SW - 4th & Park 0.00 1.15 1.15 12.13 0.00 1.90 1.90 19.19

Total Flow 1.58 10.55 12.13 2.61 16.58 19.19 Note: Storm Drain flow rate is the cumulative flow in the underground storm drain system at the inlet location and

includes both the drainage area flow plus the flow from upstream project drainage areas discussed in this report.

3. Proposed Conditions Onsite proposed drainage areas are shown on the drainage management area (DMA) map (Map 3). The

East and West DMAs drain to detention basins. The Park Avenue DMAs “Park Ave, North” and “Park Ave,

South” are accounted for in the offsite drainage areas P-3B and P-4, respectively. These DMAs

represent improvements to Park Avenue and flow from adjacent segments of Park Avenue west of these

DMAs will comingle with runoff from these DMAs and drain to the east to 4th Street. The South DMA

drains to Park Avenue, where it enters the storm drain system at the same point as the discharge from

the East DMA detention basin (storm drain at NW side of 4th & Park). However, the South DMA is

located on the project site and therefore was treated as a separate, onsite drainage area. The peak flows

for the South DMA were calculated using the same methodology as the existing condition onsite

drainage areas, added to the orifice outflow from the East DMA detention basin, and the total

represents the onsite peak flow entering the storm drain at NW side of 4th & Park.

Table 6a. Proposed Condition Onsite South DMA Runoff Coefficients.

DMA Area (Ac)

% Impervious

C (Pervious)

C (Impervious)

Weighted Average C 25-Year C 100-Year C

South 0.30 87% 0.17 0.90 0.81 0.89 0.95

Page | 7

Table 6b. Proposed Condition Onsite South DMA Time of Concentration.

Sheet Flow Travel Time Curb Gutter Travel Time

DMA L (ft) n s

Tc, Sheet Flow (min) L s v (fps)

Tc, Curb Gutter (min)

Total Tc (min)

South 25 0.15 0.005 10.0 550 0.005 3.1 3.0 13.0

Table 6c. Proposed Condition Onsite South DMA 25- and 100-Year Storm Peak Flows.

DMA Total Tc (min)

25-Year C

100-Year C

25-Yr I (in/hr)

100-Yr I (in/hr)

Area (acres)

Q25 (cfs)

Q100 (cfs)

South 13.0 0.89 0.95 2.56 3.72 0.30 0.69 1.07

Offsite drainage areas for the proposed condition were calculated in the same fashion as in the existing

condition offsite drainage areas. The proposed offsite drainage areas were affected by the modifications

to the intersection of Park Avenue and N 5th Street. The existing cross gutter along the west side of N 5th

Ave will be replaced by a cross gutter along the south side of Park Avenue. The offsite drainage areas for

the proposed condition are illustrated in Map 4. For each drainage area, the Manning’s n values for

sheet flow remained the as the existing condition for each location with the exception of E-4. The n used

for E-4 was 0.092 for the combination of sheet flow from exiting drainage areas E-2B and E-4, weighted

based on professional estimate.

Table 7a. Proposed Condition Surface Type Composition (acres) for Offsite Drainage Areas.

Proposed Drainage

Area

Asphaltic Road/Parking

lot (C=0.9) Roof, Assorted Types (C=0.85)

Concrete, Average

(C=0.875)

Unimproved, Low Infiltration

(C=0.30)

Lawn, Heavy Soil, Flat (C=0.13)

Brick Pavers (C=0.70)

P-1 0.512 0.092 0.048 0.255 0 0

P-2 0.531 0 0.149 0 0 0

P-3A 0.349 0 0.075 0.064 0 0

P-3B 0.513 0 0.007 0.010 0 0

P-4 0.700 0.352 0.265 0.208 0.569 0.006

Table 7b. Proposed Condition Runoff Coefficients for Offsite Drainage Areas.

Proposed Drainage Area Drains to

Area (acres) Weighted C 25-Year C 100-Year C

P-1 Inlet at SE side of 6th &

Adams 0.907 0.725 0.797 0.906

P-2 Inlet at SW side of 5th &

Adams 0.680 0.895 0.950 0.950

P-3A Inlet at NW side of 4th & Park

0.488 0.817 0.899 0.950

P-3B 0.530 0.888 0.950 0.950

P-4 Inlet on SW side of 4th &

Park 2.100 0.620 0.682 0.775

Page | 8

Table 7c. Proposed Condition Times of Concentration for Offsite Drainage Areas.

Sheet Flow Travel Time Curb Gutter Travel Time

Proposed Drainage

Area L (ft) n s

Tc, Sheet Flow (min) L s v (fps)

Tc, Curb Gutter (min)

Total Tc (min)

P-1 90 0.030 0.005 7.7 530 0.005 3.1 2.9 10.6

P-2 67 0.015 0.005 3.5 580 0.005 3.1 3.2 6.6

P-3A 55 0.015 0.005 3.0 400 0.005 3.1 2.2 5.2

P-3B 115 0.015 0.005 4.8 945 0.005 3.1 6.3 11.1

P-4 160 0.095 0.005 30.7 1020 0.005 3.1 5.5 36.2

Table 7d. Proposed Condition Offsite 25- and 100-Year Storm Peak Flows.

Proposed Drainage Area

Total Tc (min)

25-Year Storm C

100-Year Storm C

25-yr I (in/hr)

100-yr I (in/hr)

Area (acres)

Q25 (cfs)

Q100 (cfs)

P-1 10.6 0.797 0.906 2.87 4.19 0.907 2.08 3.44

P-2 6.6 0.950 0.950 3.73 5.47 0.680 2.41 3.53

P-3A 5.2 0.899 0.950 4.30 6.33 0.488 1.89 2.93

P-3B 11.1 0.950 0.950 2.80 4.08 0.530 1.40 2.04

P-4 36.2 0.682 0.775 1.44 2.07 2.100 2.06 3.36

The East and West basins have the capacity to hold runoff from a 3 inch storm. Per NOAA Atlas 14, the

100-year, 6-hour storm is 2.50 inches. Therefore, the proposed basins have the capacity to capture all

runoff from the 25-year and 100-year 6-hour storms. This means the peak outflow from these basins in

the proposed condition can be calculated based on the orifice equation. The calculations in Table 8 are

based on the basins being completely full (head values of 6 feet and 6.2 feet). Therefore, the orifice

flow rates as calculated for these head values using the orifice equation are conservative estimates of

the outflows from the basins during 25-year and 100-year storms.

Table 8. Orifice Flow Rate Calculations for the Proposed Onsite East and West Basins.

West Basin

East Basin

Cv 0.6 0.6 0.6

0.6 0.6 0.6

Water Ponding above Outlet (feet) 6 4.5 1

6.2 3.2 1

Orifice Pipe Diameter (in) 1.5 1.5 1.5

2.5 2.5 2.5

Velocity from Orifice (ft/s) 11.79 10.21 4.81

11.99 8.61 4.81

Flow from Orifice (cfs) 0.14 0.13 0.06

0.41 0.29 0.16

Page | 9

Table 9a. 25-Year Peak Flow Summary for the Proposed Onsite Drainage Areas.

Proposed DMA(s) Drains to

Flow Directly to Storm Drain (cfs)

Max Orifice Flow (cfs)

25-yr Peak Flow, Post-Project Condition (cfs)

West Storm drain at SE side

of 6th & Adams 0 0.14 0.14*

East, South Storm drain at NW side of 4th & Park 0.69 0.41 1.10**

Notes:

* Maximum outflow from West Basin

** Maximum outflow from East Basin plus proposed 100-yr peak flow from South DMA.

Table 9b. 100-Year Peak Flow Summary for the Proposed Onsite Drainage Areas.

Proposed DMA(s) Drains to

Flow Directly to Storm Drain (cfs)

Max Orifice Flow (cfs)

100-yr Peak Flow, Post-Project Condition (cfs)

West Storm drain at SE side

of 6th & Adams 0 0.14 0.14*

East, South Storm drain at NW side of 4th & Park 1.07 0.41 1.48**

Notes:

* Maximum outflow from West Basin

** Maximum outflow from East Basin plus proposed 100-yr peak flow from South DMA.

As done for the existing condition, onsite and offsite peak flows for the proposed condition were directly

added without accounting for the differences in travel time for the 25- and 100-year 6-hour storm. Table

10 summarizes the addition of flows and provides the total flow for each major storm drain inlet (see

Maps 4) and the total flow entering the storm drain system along Highway 86 (Adams Avenue and N 4th

Street) for the proposed condition. Offsite flow for the Inlet at the NW side of 4th & Park includes runoff

from P-3A and P-3B.

Table 10. Total Flow for the Proposed Condition for the 25- and 100-Year Storms.

25 Year 100 Year

Downstream Inlet Onsite Q (cfs)

Offsite Q (cfs)

Drainage Area Q25

(cfs)

Storm Drain Q25

(cfs) Onsite Q (cfs)

Offsite Q (cfs)

Drainage Area Q100

(cfs)

Storm Drain

Q100 (cfs)

SE - 6th & Adams 0.14 2.08 2.22 2.22 0.14 3.44 3.58 3.58

SW - 5th & Adams 0.00 2.41 2.41 4.63 0.00 3.53 3.53 7.11

SE - 5th & Adams 0.00 0.00 0.00 4.63 0.00 0.00 0.00 7.11

NW - 4th & Park 1.10 3.29 4.39 9.02 1.48 4.97 6.45 13.56

SW - 4th & Park 0.00 2.06 2.06 11.08 0.00 3.36 3.36 16.92

Total Flow 1.24 9.84 11.08 1.62 15.31 16.92 Note: Storm Drain flow rate is the cumulative flow in the underground storm drain system at the inlet location and

includes both the drainage area flow plus the flow from upstream project drainage areas discussed in this report.

Page | 10

4. Conclusion Based on the Rational Method calculations performed for the onsite and offsite peak flows for the

existing and proposed conditions for the 25- and 100-year 6-hour storms, the El Centro Aquatic Center

will likely decrease the total peak flow entering the Caltrans storm drain line along Highway 86. The

results are presented in Table 11 below.

Table 11. Overall Changes in Peak Flows Entering the Caltrans Storm Drain System on Highway 86.

25 Year 100 Year

Storm Drain Onsite Q

(cfs) Offsite Q

(cfs) Total Q

(cfs) Onsite Q

(cfs) Offsite Q

(cfs) Total Q

(cfs)

Existing Total 1.58 10.55 12.13 2.61 16.58 19.19

Proposed Total 1.24 9.84 11.08 1.62 15.31 16.92

Difference -21.48% -6.72% -8.66% -37.93% -7.67% -11.83%

There will be a reduction in peak flows in the proposed condition for both the onsite and offsite

drainage areas. The most significant decrease in peak flows will be for the onsite drainage areas. This is

due to the detention basins that are designed to handle a large storm of 3 inches in depth, which

exceeds the depth of the 100-year 6-hour storm. The orifice will control the outflow from the basins and

therefore significantly reduce the onsite peak flows entering the storm drain along Highway 86. There is

a smaller decrease in peak flows in the proposed condition for the offsite drainage areas. This is likely

due to the total offsite drainage area remaining the same and the change in flow direction at the

intersection of Park Avenue and 5th Street. The existing cross-gutter along the west side of 5th Street at

the intersection will be removed and a new cross-gutter will be installed on the south side of Park

Avenue at the intersection. This will increase the time of concentration of flow from the tributary areas

west of the intersection (the time of concentration for P-4 is 36.2 minutes) and therefore decrease the

peak flows downstream. In conclusion, it can be expected that the El Centro Aquatic Center will

decrease the 25- and 100-year 6-hour storm peak flows entering the Caltrans storm drain line along

Highway 86 by 8.66% and 11.83%, respectively.

Page | 11

5. Project Site Maps. Map 1. Existing Condition Onsite Drainage Areas.

Note: Stars represent the downstream storm drain inlet for each drainage area.

Map 2. Existing Condition Offsite Drainage Areas

Note: Stars represent the downstream storm drain inlet for each drainage area. E-2A and E-2B have the same

downstream inlet. E-3A and E-3B have the same downstream inlet.

Page | 12

Map 3. Proposed Condition Onsite Drainage Areas (DMAs, or Drainage Management Areas)

Map 4. Proposed Condition Offsite Drainage Areas

Note: Stars represent the downstream storm drain inlet for each drainage area. P-3A and P-3B have the same

downstream inlet.