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Flood Routing and Groundwater Recharge Benefit Calculations Three Case Studies
Steven M. Bell, PE, QSD
Why are we talking about groundwater projects at a floodplain management conference anyway?
Urbanization and Floodplains Natural floodplains
enhance groundwater recharge
Infiltration removes pollutants from water bodies
Concrete channelization protects against flooding, but lessens recharge
Pollutants remain within surface water
Urbanization and Floodplains Depletion of aquifers Most aquifers in
southern California have less water now than they did 20 years ago
Falling water levels means more expensive to drill and to pump
Urbanization and Floodplains Nuisance flows
Overwatering of landscaping
Water quality issues Motor oil Fertilizer Pet waste Trash
Wasted water Particularly poor
optics during droughts
Recharge Projects In southern California, we do
Spreading Grounds
Infiltration Basins
Subsurface Infiltration Chambers
Recharge Project
Location
Hydrology
Soils Design How to
Key design factors: Rainfall Primary purpose Size of watershed Jurisdiction
Rainfall
More rainfall intensity = larger BMP
Primary Purpose
Low Impact Development (LID)
(Water quality)
LID + Hydromodification
(Water quality)
Stormwater Recharge
(Water quality) (Water supply)
Flood Storage
(Water quality) (Water supply)
(Flood protection)
Watershed Size of
Neighborhood project
Small regional project
Large regional project
Jurisdiction
Los Angeles County • Runoff method: MODRAT • Runoff losses based on
impervious area • Unit hydrograph peak at
80% of storm
Santa Barbara • Runoff method: SBUH • Runoff losses based on
SCS curve numbers • Unit hydrograph peak at
42% of storm
San Bernardino County • Runoff method: Rational
Method or Unit Hydrograph Method
• Runoff losses based on SCS curve numbers
• Unit hydrograph peak at 67% of storm
Three projects
Bohnett Park
Area 1: 4.20 ac. Area 2:
1.65 ac.
Drainage Areas
SCS Type I rainfall distribution
Santa Barbara Urban Hydrograph
Rainfall-Runoff City of Santa Barbara Requirements
Initial Abstraction
Ia only a function of CN
Small P (WQ design storm) = Almost no Q
VWQ = 5,686 cf 75% of WQ rainfall lost due to Ia
MODRAT Modified Rational Method
0
5,000
10,000
15,000
20,000
Area 1 Area 2 Total
Runo
ff Vo
lum
e (c
f)
SBUHMODRAT
Q=ciA
Increase of 174% over SBUH/SCS
Infiltration Factoring in
0
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
0 500 1000 1500Vo
lum
e (c
f) Time (min)
Storage Volume, NotFactoring InfiltrationStorage Volume,Factoring Infiltration
Over 24 Hours
Design
Chamber system Diverts from storm drain Captures low flows Captures the one-inch, 24-hour storm Flood flows bypass upstream Fully infiltrates within 48 hours Pretreatment system for trash and
sediment
Cactus Basins No. 4 & 5
How much water will recharge the
Rialto-Colton groundwater subbasin in a
year?
Drai
nage
A
reas
4.63 sq. mi.
2.85 sq. mi.
Rainfall
05
101520253035404550
1926
-192
719
29-1
930
1932
-193
319
35-1
936
1938
-193
919
41-1
942
1944
-194
519
47-1
948
1950
-195
119
53-1
954
1956
-195
719
60-1
961
1963
-196
419
66-1
967
1969
-197
019
72-1
973
1975
-197
619
78-1
979
1981
-198
219
84-1
985
1987
-198
819
90-1
991
1993
-199
419
96-1
997
1999
-200
020
02-2
003
2005
-200
620
08-2
009
2011
-201
220
14-2
015
Ann
ual R
ainf
all,
in.
5 rain gauges, 90 years of daily rainfall data
Runoff Yield (fraction of
daily precipitation that becomes runoff) calculated for each subarea and each day
Yield * Precipitation = Runoff Volume
(from San Bernardino County Hydrology Manual)
Land
Use
an
d So
ils
Antecedent Moisture Condition CN varies depending
on amount of moisture in the soil
CN calculated as a function of precipitation in previous week <0.1” = AMC I >0.1” = AMC II >1.0” = AMC III
Calculation Existing Model Run
Runoff from upper DA
Runoff from lower DA
Storage (previous/next day)
Infiltration
Outflow to Rialto Channel
1 2 3
Calculation Proposed Model Run
Runoff from upper DA
Runoff from lower DA
Storage (previous/next day)
Infiltration
Outflow to Rialto Channel
1 2 3 4 5
Incidental Infiltration
Storage (previous/next day)
Infiltration Summary
0500
1,0001,5002,0002,5003,0003,5004,0004,500
1926
-27
1929
-30
1932
-33
1935
-36
1938
-39
1941
-42
1944
-45
1947
-48
1950
-51
1953
-54
1956
-57
1960
-61
1963
-64
1966
-67
1969
-70
1972
-73
1975
-76
1978
-79
1981
-82
1984
-85
1987
-88
1990
-91
1993
-94
1996
-97
1999
-00
2002
-03
2005
-06
2008
-09
2011
-12
2014
-15
Infil
trate
d Ru
noff
(ac-
ft/yr
)
Existing Conditions Proposed Conditions
Groundwater Recharge Benefit Metric
Existing Conditions (no build)
Proposed Conditions
(build) Average Annual
Infiltration 1,365 ac-ft/yr 1,535 ac-ft/yr
Average Annual Runoff to
Rialto Channel 1,281 ac-ft/yr 1,110 ac-ft/yr
Difference of 170 ac-ft/yr
Enough water to supply 1,800 Southern Californians per year
John Anson Ford Park
Design Schematic Drainage Area: 3.6 Sq. Mi.
Continuous Hydrograph
0
10
20
30
40
50
60
70
80
90
100
0
100
200
300
400
500
600
0:00
:00
2:00
:00
4:00
:00
6:00
:00
8:00
:00
10:0
0:00
12:0
0:00
14:0
0:00
16:0
0:00
18:0
0:00
20:0
0:00
22:0
0:00
0:00
:00
Stor
age
(ac-
ft)
Flow
Rat
e (c
fs)
Runoff Inflow Infiltration Storage
0
100
200
300
400
500
6000.000.020.040.060.080.100.120.140.160.180.20
00:00:00
02:00:00
04:00:00
06:00:00
08:00:00
10:00:00
12:00:00
14:00:00
16:00:00
18:00:00
20:00:00
22:00:00
00:00:00
Runo
ff (c
fs)
Rain
fall
(in)
Rainfall Runoff
1,100 ac-ft per year on average: 11,500 Southern Californians
Comparison Bohnett Park Project
Cactus Basins 4 & 5
J.A. Ford Park Cistern Project
County Santa Barbara San Bernardino Los Angeles
Purpose Water Quality Flood Protection + Water Supply
Water Quality + Water Supply
Watershed Size 6 acres 2,963 acres 2,295 acres
Rainfall
Design Methodology Single-Event Design Storm
Continuous Hydrograph – 1-Day Increment
Continuous Hydrograph – 5-Min. Increment
Storage 0.26 ac-ft 1,200 ac-ft 100 ac-ft*
Annual Groundwater Recharge
~3 ac-ft 170 ac-ft 1,100 ac-ft* *Ultimate condition
