522.1 If a major substantial improvement activity demonstrates that it is not part of a common plan of development with a major land-disturbing activity, then it shall comply with the provisions of this section; otherwise, it shall comply with the requirements for a major land-disturbing activity.

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 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 (0.8 inch rainfall event) by:

(a) Employing each BMP necessary to retain the 0.8 inch Stormwater Retention Volume (SWRv), calculated as follows:

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

SWRv = volume, in gallons, required to be retained

P = 80th percentile rainfall event for the District (0.8 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 substantially improved building footprint plus land disturbance where the surface area under a BMP shall be calculated as part of the impervious cover (%I); and

(b) Employing each post-development land cover factored into the SWRv.

(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.

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 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.

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) At least fifty percent (50%) of the 0.8 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 0.8 inch SWRv flowing from that entire area is retained or treated;

(b) 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;

(c) The requirement for retention of a minimum of fifty percent (50%) of the 0.8 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.

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 60 DCR 10640 (July 19, 2013).