Spokane River Basin Stormwater Loading
Background and historic data

From the 1981 (Spokane River Wasteload Allocation Study by URS
City Urban runoff = 5.01 CFS @ 0.89 mg/L TP = 24.05 #/day TP
North City = 0.8 CFS @ 0.89 mg/L = 3.84 #/day TP (to LSR)
Valley 2.78 CFS @ 0.89 mg/L = 13.35 lb/day TP

From the 1987 Spokane River Basin Allowable Phosphorus Loading report by CR Patmont of Harper-Owes
After CSO control complete = 2.6 + 0.68 lb/day
Spokane = 2.0 + 0.77 lb/day
Spokane Valley = 0.63 +0.27 lb/day
Average total phosphorus concentration of runoff = 350 + 84 µg/L

Average annual rainfall in Spokane, according to Prism, is 17.98”. Of that total, 9.77” falls from March 1 to
October 31.

Average rainfall in Coeur d’Alene is 26.06”. Of that total, 13.60” falls between March and October.

For the 2004 aquifer recharge model, the USGS made estimates of impervious areas (table below). 88% is
in the greater Spokane area, 12% in Coeur d’Alene.
Area
Washington
Idaho

Impervoius
Acres
20086
2739
Impervoius
Fraction
.217
.217
Rainfall
9.77
13.6
Load
#/day
12.4
2.3

From the Ecology 2007 PCB stormwater load report loads from sampled CSOs and stormwater basins were
calculated using two different discharge estimates:
1. calculated by the Simple Method only, and
2. the reported discharge volumes from the City of Spokane’s CSO Annual Report for fiscal year
2005 (City of Spokane 2006) combined with Simple Method calculations for stormwater basins.
Stormwater
Basin
TOTAL



Location ID
Drainage
Area
(acres)
Area
Impervious
Off-Street
(acres)
Area
Impervious
Roads
(acres)
Total
Impervious
(acres)
Impervious
Fraction†
17,282
2,000
2,700
4,700
0.272
Applying the Simple Method to the above table yields a total phosphorus load of 11.2 lb/day for Spokane
using 2001-2007 average annual precipitation data and 12.9 lb/day using 1970-2000 average annual
precipitation data
The Simple Method, applied to USGS estimates of impervious surfaces (22825 acres with an impervious
fraction of 0.217) , and using 0.31 mg/L as the average TP concentration (national average), gives a load of
14.7 lb/day. Of that, 2.3 lb/day is from the Coeur d’Alene area and the remainder from Spokane Metro.
Spokane county and North Idaho has over 11,000 drywells. Their widespread use and our porous soils
make Simple Method calculations overestimate stormwater loads.
Stormwater Load Allocation
For the TMDL stormwater load determination, a hybrid method was utilized that combines Spokane’s measured
data for CSO flows, TP concentrations, and average annual Prism precipitation data from 2001 through 2007,
combined with Simple Method calculations for areas in Spokane and Coeur d’Alene without flow data (non-CSOs).
In the largest non-CSO basin, there are over 1,100 drywells, as well as a number of catch basins and infiltration
basins. It is probable that the Simple Method is overestimating runoff from basins in this region.
CSO basins
Flow

The city of Spokane estimates an average annual 54.8 million gallons (MG) of stormwater are discharged
from their CSO’s.
Total phosphorus concentration

The estimated concentration of TP in the CSO discharge is 0.95 mg/L.
53% of 54.8 MG = 29 MG * .95 mg/L = 230 lbs total phosphorus in 245 days, or 0.94 lb/day TP from CSOs
Non CSO basins (Simple Method)
Area

The area in Spokane not connected to CSO’s is 9,330 acres. The impervious fraction of this area averages
.274.
Total phosphorus concentration

TP from runoff in these areas is estimated at 0.31 mg/L .
Precipitation

Spokane averaged about 16” of rain in 2001 8.5”. Of that total, 53% fell from March through October
Using the Simple Method for non-CSO runoff
L = 0.226 * R * C * A
Where: L = Annual load (lbs)
R = Annual runoff (inches) – see equation below
C = Pollutant concentration (mg/l) = 0.31
A = Area (acres) = 9,330
0.226 = Unit conversion factor
Annual Runoff
The Simple Method calculates annual runoff as a product of annual runoff volume, and a runoff coefficient (Rv).
Runoff volume is calculated as:
R = P * Pj * Rv
Where: R = Annual runoff (inches)
P = Annual rainfall (inches) = 8.5
Pj = Fraction of annual rainfall events that produce runoff = 0.9
Rv = Runoff coefficient (see equation below)
In the Simple Method, the runoff coefficient is calculated based on impervious cover in the subwatershed.
Although there is some scatter in the data, watershed imperviousness does appear to be a reasonable predictor of
Rv.
Rv = 0.05+0.9Ia
Where: Ia = Impervious fraction = 0.274
Rv = 0.05+( 0.9 * 0.274)
R=8.5 * 0.9 *0 .2966= 2.269”
L=0.226 * 0.31 * 9330 * 2.269= 1483 lb over 245 days, or 6.06 lb/day
For Spokane, the combined stormwater/CSO load is 6.06 + 0.95 = 7.0 lb/day
For Idaho, assuming the same impervious fraction and local precipitation
R=13.6*0.9*.2966=3.63
L= 0.226*0.31*2350*3.63=598 lb or 2.4 lb/day
Therefore the current stormwater load is estimated at 9.4 lb/day
Stormwater/CSO Summary
TP
CSO Spokane
Stormwater WA
Stormwater ID
Total
Flow
gal/day
118000
2360000
930000
3408000
Conc
mg/L
0.95
0.31
0.31
Load
lb/day
.94
6.10
2.41
9.45
BOD
Conc
Load
mg/L
lb/day
30
29.7
3
59
3
23.3
112
ammonia
Conc
Load
mg/L
lb/day
1
1
0.05
1
0.05
0.4
2.4
Spokane runoff areas
Light Blue area storm runoff is serviced by CSO