522 STORMWATER MANAGEMENT: PERFORMANCE REQUIREMENTS FOR MAJOR SUBSTANTIAL IMPROVEMENT ACTIVITY

522.1 If land disturbance associated with a major substantial improvement activity constitutes a major land disturbing activity, or is part of a common plan of development with a major land disturbing activity, then it shall comply with the performance requirements for a major land disturbing activity; otherwise it shall comply with the provisions of this section.

522.2 For the purposes of calculating the cost of a major substantial improvement to a building or structure, an applicant may exclude the cost of replacing manufacturing and industrial equipment, including pumps, valve chambers, and wastewater treatment facilities, but may not exclude the cost of replacing boilers, furnaces, and other equipment that is part of the heating and cooling system or other infrastructure commonly found in a building or structure.

522.3 A site that undergoes a major substantial improvement activity shall employ each Best Management Practice (BMP) and land cover necessary to meet the requirements of this section until the property is redeveloped in compliance with these regulations.

522.4 A site that undergoes a major substantial improvement activity shall achieve retention of the rainfall from a 0.8 inch rainfall event, which is the eightieth (80th) percentile rainfall event for the District of Columbia, measured for a twenty-four (24)-hour storm with a seventy-two (72)-hour antecedent dry period by:

(a) Employing each BMP necessary to retain the 0.8 inch 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 = 80^th percentile rainfall event for the District (0.8 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 site area includes substantially improved building footprint plus land disturbance and 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 § 521.

522.5 A site that undergoes a major substantial improvement activity may achieve the 0.8 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 0.8 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 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.

522.6 A site that undergoes a major substantial improvement activity may achieve on-site retention by retaining more than the 0.8 inch SWRv for an area of the site, subject to the following conditions:

(a) Unless an 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 0.8 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 0.8 inch SWRv flowing from that entire area is retained or treated;

(c) Retention in excess of a 0.8 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 0.8 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 with a P equal to 1.7 inches, shall not be counted toward on-site retention.

522.7 A major substantial improvement 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).