D.C. Mun. Regs. tit. 21, r. 21-520
Current through Register 71, No. 45, November 7, 2024
Rule 21-520 - STORMWATER MANAGEMENT: PERFORMANCE REQUIREMENTS FOR MAJOR LAND-DISTURBING ACTIVITY520.1A 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.
520.2A 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 Storm water Retention Volume (SWRv), calculated as follows: SWRv=P × [(RvI × I) + (RvC × C) + (RvN × N)] × 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 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:(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, th e 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.520.6A 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.
D.C. Mun. Regs. tit. 21, r. 21-520
Final Rulemaking published at 60 DCR 10640 (July 19, 2013); amended by Final Rulemaking published at 67 DCR 844 (1/31/2020)Authority: Department of Consumer and Regulatory Affairs Civil Infractions Act of 1985, effective October 5, 1985, as amended (D.C. Law 6-42; D.C. Official Code §§ 2-1801.01 et seq. (2007 Repl. & 2012 Supp.)); District Department of the Environment Establishment Act of 2005, §§ 101 et seq., effective February 15, 2006, as amended (D.C. Law 16-51; D.C. Official Code §§ 8-151.01 et seq. (2008 Repl. & 2012 Supp.)); National Capital Revitalization Corporation and Anacostia Waterfront Corporation Reorganization Act of 2008, effective March 26, 2008 (D.C. Law 17-138; 55 DCR 1689), as amended by the Anacostia Waterfront Environmental Standards Amendment Act of 2012, effective October 23, 2012 (D.C. Law 19-192; D.C. Official Code §§ 2-1226.31 et seq.) (2012 Supp.)); The Soil Erosion and Sedimentation Control Act of 1977, effective September 28, 1977 (D.C. Law 2-23; 24 DCR 792), as amended by the Soil Erosion and Sedimentation Control Amendment Act of 1994, effective August 26, 1994, (D.C. Law 10-166; 41 DCR 4892; 21 DCMR §§ 500 -15); Uniform Environmental Covenants Act of 2005, effective May 12, 2006, as amended (D.C. Law 16-95; D.C. Official Code §§ 8-671.01 et seq. (2008 Repl.)); Water Pollution Control Act of 1984, effective March 16, 1985, as amended (D.C. Law 5-188; D.C. Official Code §§ 8-103.01 et seq. (2008 Repl. & 2012 Supp.)); and Mayor's Order 2006-61, dated June 14, 2006, and its delegations of authority.