HomeMy WebLinkAboutWPO202100007 Correspondence 2021-06-24 (2)M COLLINS ENGINEERING
June 9, 2021
Emily Cox, P.E.
Albemarle County
Department of Community Development
County Engineering
401 McIntire Road, Room 227
Charlottesville, VA 22902
200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902
(�434.293.3719 PH 434.293.2813 FX
www.coll i ns-engineering.com
RE: Brookhill Blocks 16 and 17 VSMP Plan (WPO 20210007) Request for Exception
Please allow this letter to serve as an official request for an exception from the regulations to discharge more than
what the energy balance allows, so that the stormwater management (SWM) facilities will be less likely to be clogged.
A calculations packet has been submitted with the Brookhill Blocks 16 & 17 VSMP plans. This packet provides SCS 1-
year energy balance calculations for subareas A and B. Subarea A's energy balance computation requires a maximum
flow rate of 0.11 cfs for the proposed plan, and subarea B's energy balance calculation requires a maximum flow rate of
0.18 cfs. To obtain these maximum thresholds, the SWM facility detaining subarea A necessitates a low -flow orifice
diameter of 1.33" and the SWM facility detaining subarea B requires a low -flow orifice diameter of 1.75". Routing
computations showing these values have been provided in the calculations packet as well.
Increasing the 1.33" and 1.75" low -flow orifice diameters to 3" would decrease the chances of clogging. The resultant
routed 1-year SCS outflows with an increased 3" diameter are 0.44 cfs and 0.41 cfs for subareas A and B, respectfully,
for the proposed development. The full buildout of subareas A and B would result in 1-year SCS routed outflows of 0.49
cfs and 1.09 cfs, respectfully, with an increased diameter of 3". The submitted calculations packet also provides these
corresponding routing calculations.
It is the applicant's belief that the requested exception is consistent with other projects throughout the County, and
that a minimum orifice diameter of 3" will reduce the chances of clogging and expensive retrofitting and maintenance
requirements that would otherwise be associated with 1.33" and 1.75" diameter orifices. If County Engineering is in
agreement with this request, the applicant can provide a fee of $240 to process this letter.
Sincerely,
Graham Murray, PE