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+0.27lb/day

Average total phosphorus concentration of runoff = 350 + 84 µg/L

  • Average annual rainfall inSpokane, 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 / Impervoius Acres / Impervoius
Fraction / Rainfall / Load
#/day
Washington / 20086 / .217 / 9.77 / 12.4
Idaho / 2739 / .217 / 13.6 / 2.3
  • From the Ecology 2007 PCB stormwater load reportloads from sampled CSOsand 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 / Location ID / Drainage Area (acres) / Area Impervious Off-Street (acres) / Area Impervious Roads (acres) / Total Impervious (acres) / Impervious Fraction†
TOTAL / 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 1970-2000average annual precipitation data
  • The Simple Method, applied to USGS estimates of impervious surfaces (22825 acres with an impervious fraction of 0.217) , and using 0.31mg/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

Thereforethecurrent stormwater load is estimated at 9.4 lb/day

Stormwater/CSO Summary

TP / BOD / ammonia
Flow
gal/day / Conc
mg/L / Load
lb/day / Conc
mg/L / Load
lb/day / Conc
mg/L / Load
lb/day
CSO Spokane / 118000 / 0.95 / .94 / 30 / 29.7 / 1 / 1
Stormwater WA / 2360000 / 0.31 / 6.10 / 3 / 59 / 0.05 / 1
Stormwater ID / 930000 / 0.31 / 2.41 / 3 / 23.3 / 0.05 / 0.4
Total / 3408000 / 9.45 / 112 / 2.4

Spokane runoff areas

Light Blue area storm runoff is serviced by CSO