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 For 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), the property owner 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:

(1) The Department provides a waiver, which shall be granted if:

(A) The site retains less than fifty percent (50%) of the Stormwater Retention Volume (SWRv) requirement on-site in the Gray Combined Sewer System (Gray CSS) area from a drainage area that is not targeted for sewer separation as documented in capital improvement budgets; and

(B) The property owner signs an agreement committing the project to use only High-Impact Stormwater Retention Credits (SRCs) to comply with its Off-Site Retention Volume (Offv) requirement pursuant to § 527; or

(2) The site’s discharge:

(A) 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;

(B) Does not flow into or through a tributary to those waterbodies that run above ground or for which the Department has created or received plans or documentation stating the waterbody will be daylighted to run above ground; and

(C) 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 a Special Flood Hazard Area as delineated on the effective Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map; or

(2) Meets the following two (2) 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 For a site that undergoes a major land-disturbing activity, the property owner shall retain the rainfall from a one and two-tenths (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 one and two-tenths (1.2) inch SWRv requirement, calculated as follows:

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

where:

SWRv = volume, in gallons, required to be retained,

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

RvN = .00 (runoff coefficient for natural cover),

RvC = .25 (runoff coefficient for compacted cover),

RvI = .95 (runoff coefficient for impervious cover),

RvB = .95 (runoff coefficient for BMP cover),

N = post-development natural cover surface area,

C = post-development compacted cover surface area,

I = post-development impervious cover surface area,

B = post-development BMP cover surface area;

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

(c) Calculating separately and complying with the SWRv requirement, with P equal to one and two-tenths (1.2) inches, for the portion of land-disturbing activity that is in the existing PROW, pursuant to § 521.

520.4 For a site that undergoes a major land-disturbing activity, the property owner may comply with the one and two-tenths (1.2) inch SWRv requirement on-site, off-site when not otherwise prohibited by this chapter, 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 one and two-tenths (1.2) inch SWRv requirement, calculated for the entire site, unless the Department:

(b)

(1) Approves an application for relief from extraordinarily difficult site conditions; or

(2) Provides a waiver, which shall be granted if:

(A) The site drains into the Gray CSS from a drainage area that is not targeted for sewer separation as documented in capital improvement budgets; and

(B) The property owner signs an agreement committing the project to use only High-Impact SRCs or Low-Impact Voluntary SRCs to comply with its Offv requirement pursuant to § 527; and

(c) Pursuant to § 527, the property owner shall use off-site retention for any portion of the SWRv requirement that is not retained on-site.

520.5 For a site that undergoes a major land-disturbing activity, the property owner may comply with on-site retention requirements by retaining more than the one and two-tenths (1.2) inch SWRv requirement for an area of the site, subject to the

following conditions:

(a) Unless a Site Drainage Area (SDA) drains into the Combined Sewer System (CSS) or the Department approves an application for relief from extraordinarily difficult site conditions, the property owner shall comply with at least fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement from the SDA through:

(1) Retention; or

(2) Treatment 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 to comply with at least fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement flowing from that entire area through retention or treatment;

(c) Retention in excess of a one and two-tenths (1.2) inch SWRv requirement 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 or waives the minimum on-site retention requirement pursuant to § 520.4(a)(2), the property owner shall comply with a minimum of fifty percent (50%) of the one and two-tenths (1.2) inch SWRv requirement for the entire site; and

(e) Retention of volume greater than that from a one and seven-tenths (1.7) inch rainfall event, calculated using the SWRv equation as stated in § 520.3(a) with a P equal to one and seven-tenths (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); as amended by Final Rulemaking published at 72 DCR 012101 (October 31, 2025).