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 a 1.2 inch rainfall event, which is 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 by:

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

$$\text{SWRv} = P \times [(Rv_I \times I) + (Rv_C \times C) + (Rv_N \times N)] \times 7.48 / 12$$

SWRv = volume, in gallons, required to be retained

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

Rv_I = 0.95 (runoff coefficient for impervious cover)

Rv_C = 0.25 (runoff coefficient for compacted cover)

Rv_N = 0.00 (runoff coefficient for natural cover)

I = post-development site area in impervious cover

C = post-development site area in compacted cover

N = post-development site area in natural cover

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 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:

(1) The Department approves an application for relief from extraordinarily difficult site conditions; or

(2) The site drains into the combined sewer system (CSS) from a drainage area that is not:

(A) Targeted for green infrastructure implementation under a court-approved consent decree, as determined using the tools available in the Department’s submittal database; or

(B) Targeted for sewer separation under a court-approved consent decree, as documented in capital improvement budgets; 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) Unless a Site Drainage Area (SDA) drains into the CSS or the Department approves an application for relief from extraordinarily difficult site conditions, at least fifty percent (50%) of the 1.2 inch SWRv from the SDA shall be:

(1) Retained; or

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

(b) Unless an SDA drains into the CSS or the Department approves an application for relief from extraordinarily difficult site conditions, 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;

(c) 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;

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

(e) Retention of volume greater than that from a 1.7 inch rainfall event, calculated using the SWRv equation as stated in § 520.3(a) 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 35 DCR 21 (January 1, 1988); as amended by Final Rulemaking published at 60 DCR 10640 (July 19, 2013); as amended by Final Rulemaking published at 67 DCR 818 (January 31, 2020).