Section 21-520. STORMWATER MANAGEMENT: PERFORMANCE REQUIREMENTS FOR MAJOR LAND-DISTURBING ACTIVITY

 

520.1A 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.

 

520.3A 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:

 

SWRv = [P × [(RvI × %I) + (RvC × %C) + (RvN × %N)] × SA] × 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.

 

520.4A 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.

 

520.5A 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.

 

520.6A 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.

 

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.

source

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