520 STORMWATER MANAGEMENT: PERFORMANCE REQUIREMENTS FOR MAJOR LAND-DISTURBING ACTIVITY

520.1 A site that undergoes a major land-disturbing activity shall employ each Best Management Practice (BMP) and land cover necessary to meet the requirements of this section until site redevelopment that follows a Department-approved Stormwater Management Plan (SWMP) occurs.

520.2 A site that undergoes a major land-disturbing activity, except the area of a site that is in the existing Public Right of Way (PROW), shall maintain the following:

(a) Post-development peak discharge rate for a twenty-four (24) hour, two (2)-year frequency storm event at a level that is equal to or less than the storm event's pre-development peak discharge rate unless the site's discharge:

(1) Flows directly or through the separate sewer system to the main stem of the tidal Potomac or Anacostia Rivers, the Washington Channel, or the Chesapeake and Ohio Canal;

(2) Does not flow into or through a tributary to those waterbodies that runs above ground or that the Department expects to be daylighted to run above ground; and

(3) Will not cause erosion of land or transport of sediment.

(b) Post-development peak discharge rate for a twenty-four (24) hour, fifteen (15)-year frequency storm event at a level that is equal to or less than the storm event's pre-project peak discharge rate; and

(c) Post-development peak discharge rate from a twenty-four (24) hour, one hundred (100)-year storm event at a level that is equal to or less than the storm event's pre-project peak discharge rate if the site:

(1) Increases the size of Special Flood Hazard Area as delineated on the effective Flood Insurance Rate Map; or

(2) Meets the following two conditions:

(A) Does not discharge to the sewer system and

(B) Has a post-development peak discharge rate for a one hundred (100)-year storm event that will cause flooding to a building.

520.3 A site that undergoes a major land-disturbing activity shall achieve retention of the rainfall from the ninetieth (90th) percentile rainfall event for the District of

Columbia, measured for a twenty-four (24)-hour rainfall event with a seventy-two (72)-hour antecedent dry period (1.2 inch rainfall event) by:

(a) Employing each BMP necessary to retain the 1.2 inch Stormwater Retention Volume (SWRv), calculated as follows:

$$\text{SWRv} = [\text{P} \times [(\text{Rv}\text{I} \times \%\text{I}) + (\text{Rv}\text{C} \times \%\text{C}) + (\text{Rv}_\text{N} \times \%\text{N})] \times \text{SA}] \times 7.48 / 12$$

SWRv = volume, in gallons, required to be retained

P = 90th percentile rainfall event for the District (1.2 inches)

RvI = 0.95 (runoff coefficient for impervious cover)

RvC = 0.25 (runoff coefficient for compacted cover)

RvN = 0.00 (runoff coefficient for natural cover)

%I = post-development percent of site in impervious cover

%C = post-development percent of site in compacted cover

%N = post-development percent of site in natural cover

SA = surface area, in square feet, of land-disturbing activity

where the surface area under a BMP shall be calculated as part of the impervious cover (%I);

(b) Employing each post-development land cover factored into the SWRv; and

(c) Calculating separately and achieving the SWRv, with P equal to 1.2 inches, for the portion of land-disturbing activity that is in the existing Public Right of Way (PROW), in compliance with the section of this chapter pertaining to performance requirements in the existing PROW.

520.4

A site that undergoes a major land-disturbing activity may achieve the 1.2 inch SWRv on site or through a combination of on-site retention and off-site retention, under the following conditions:

(a) The site shall retain on site a minimum of fifty percent (50%) of the 1.2 inch SWRv, calculated for the entire site, unless the Department approves an application for relief from extraordinarily difficult site conditions; and

(b) The site shall use off-site retention for the portion of the SWRv that is not retained on site.

520.5 A site that undergoes a major land-disturbing activity may achieve on-site retention by retaining more than the 1.2 inch SWRv for an area of the site, subject to the following conditions:

(a) At least fifty percent (50%) of the 1.2 inch SWRv from each Site Drainage Area (SDA), unless it drains into the combined sewer system, shall be:

(1) Retained; or

(2) Treated to remove eighty percent (80%) of total suspended solids; and

(3) The entirety of an area intended for use or storage of motor vehicles shall drain to each necessary BMP so that at least fifty percent (50%) of the 1.2 inch SWRv flowing from that entire area is retained or treated;

(b) Retention in excess of a 1.2 inch SWRv for one area of the site may be applied to the volume required for another area of the site;

(c) The requirement for retention of a minimum of fifty percent (50%) of the 1.2 inch SWRv for the entire site shall be achieved, unless the Department approves an application for relief from extraordinarily difficult site conditions; and

(d) Retention of volume greater than that from a 1.7 inch rainfall event, calculated using the SWRv equation with a P equal to 1.7 inches, shall not be counted toward on-site retention.

520.6 A major land-disturbing activity may achieve on-site retention by directly conveying volume from the regulated site to a shared BMP with available retention capacity.

SOURCE: Final Rulemaking published at 60 DCR 10640 (July 19, 2013).