DC Department of Environment Proposed Stormwater Regulation

Discount For Stormwater Retention

The stormwater fee is based on the “Equivalent Residential Unit” (ERU), a unit of measurement established in § 556. Each ERU is equal to one thousand square feet (1,000 sq. ft.) of impervious surface. For a 1.2 in. rainfall event, each ERU produces seven hundred ten and seventy-five hundredths (710.75) gallons of stormwater runoff. Currently, a property pays $2.67 per month per ERU.


The following sample calculation shows how a customer’s use of a stormwater retention practice can reduce the fee. Assume that a property has twenty thousand square feet (20,000 sq. ft.) of impervious surface, or twenty (20) ERUs. Assume also that the impervious area consists of seventeen thousand five hundred square feet (17,500 sq. ft.) of roof area and two thousand five hundred square feet (2,500 sq. ft.) of a concrete driveway. At present rates, $2.67/ERU, the property must pay a monthly stormwater fee of $53.40 for this impervious area. ($2.67 X 20 ERUs)

Now assume that the owner installs a green roof that has a DDOE-approved design, and that engineering calculations have demonstrated will retain a maximum of ten thousand three hundred sixty-two gallons (10,362 gal.) of stormwater runoff per storm event. As an approved retention practice, the green roof entitles the customer to a stormwater fee discount.

The Department will calculate the monthly stormwater fee discount as follows:

Step 1: Input the maximum volume of stormwater runoff retained by the DDOE-approved stormwater retention practice per storm event, in gallons:

10,362 gallons

Step 2: Convert the retention volume from Step 1 to ERUs by dividing it by seven hundred ten and seventy-five hundredths gallons per ERU (710.75 gallons/ERU) (rounding the quotient up to the nearest tenth):

10,362 gallons ÷ 710.75 gallons/ERU ═ 14.6 ERUs

Step 3: Multiply the number of ERUs from Step 2 by the maximum allowable discount percentage of fifty-five percent (55%) (rounding the result up to the nearest tenth):

14.6 ERUs × 55% ═ 8.1 ERUs

Step 4: Determine the dollar amount of the monthly stormwater fee discount by multiplying the number of ERUs from Step 3 by the current monthly fee, two dollars and sixty-seven cents ($2.67) per ERU:

$2.67/ERU × 8.1 ERU ═ $21.63 monthly discount

(DDOE)