HomeMy WebLinkAboutWPO201300032 Approval - County 2021-05-12®KOUDABUSH, GALE & ASSOC. , Inc.
A PROFESSIONAL CORPORATION Serving Virginia Since 1956
23 April 2021
Mr. John Anderson
Department of Community Development
401 McIntire Road
Charlottesville, VA 22902
Re: Cascadia blocks 4-7 VSMP Amendment #2, WPO-2013-00032
Request for Exception of Energy Balance Orifice Size
Dear Mr. Anderson,
Per Virginia Stormwater Management regulations, the orifice for the Dry Pond was developed to
limit hydraulic release from the stormwater facility. The original post -developed flows:
Q1.5 = 0.48 CFS
Q2 = 0.52 CFS
Q10 = 8.83 CFS
Flows with 2.8" orifice
Q1.5=0.43CFS
Q2 = 0.52 CFS
Q10 = 8.78 CFS
Flows with 3.0" orifice
Q1.5 = 0.49 CFS
Q2 = 0.55 CFS
Q10=8.77CFS
It is our recommendation that the orifice be enlarged to 3-inches in an effort to protect the facility
from failure due to likely clogging of the smaller orifice. In our experience, limiting such
constrictions to 3-inches helps minimize clogging while still providing substantial attenuation to
the flows.
With the County approval of the 3-inch orifice:
• The 1.5-year flow will increase from 0.43 CFS to 0.49 CFS
• The 2-year flow will increase from 0.52 CFS to 0.55 CFS
• The 10-year flow will decrease from 8.78 CFS to 8.77 CFS due to the WSE being lower
on the 75` V-notch weir.
The narrative has been updated to include all the updated information. All routing and WSE's in
the report and on the plans reflect the 3.0" orifice.
Thank you and we look forward to your consideration in this matter.
Sincerely,
APPROVED
�—>l by the Albemarle County
Dustin Greene, PE Community Development Department
Dustin Ghr
For Roudabush, Gale and Associates, Inc. Date 05/12/2021
File WP0201300032