HomeMy WebLinkAboutWPO202000055 Correspondence 2021-07-01 (2)I® COLLINS ENGINEERING
May 14, 2021
Emily Cox
Department of Community Development
401 McIntire Road
Charlottesville, VA 22902
RE: White Gables VSMP (WP0202000053) Energy Balance Exception Letter
200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902
��434.293.3719 PH 434.293.2813 FX
www.coll ns-eng ineeri ng.com
The purpose of this letter is to request an exception to allow a greater discharge than what the energy balance
equation (9VAC25-870-66.B.3.a) requires of this site. In order to meet energy balance requirements, the proposed
detention basin must release runoff at or below a peak flow rate of 0.17 cfs for the 1-year 24-hour storm. The designed
detention basin has the ability to release runoff from the proposed development at a peak flow rate of 0.17 cfs by
utilizing a 1.75-inch low -flow orifice in the riser. However, this site proposes a 3-inch diameter low -flow orifice. The
peak release flow rate of a 3-inch orifice for the 1-year 24-hour storm is 0.45 cfs, which is greater than the allowable
release rate of the energy balance equation. It should also be noted that this higher release rate is still a sizeable
improvement of the existing predeveloped condition, which is 0.73 cfs for the 1-year 24-hour storm. According to DEQ
Spec. 15 under Non -Clogging Low Flow Orifice, "orifices less than 3 inches in diameter may require extra attention
during design to minimize the potential for clogging" To avoid clogging and promote optimal performance of the
detention basin, it is requested that the higher release rate from a 3-inch low -flow orifice be utilized and a greater
discharge than the energy balance equation requires is allowed.
Should you have any questions, please feel free to contact Scott Collins at Scott@collins-engineering.com.
Sincerely,
Scott Collins, PE