HomeMy WebLinkAboutWPO202000034 Approval - County 2021-05-11ROUDABUSH, GALE & ASSOC. , Inc.
A PROFESSIONAL CORPORATION Serving Virginia Since 1956
15 April 2021
Mr. Frank Pohl
APPROVED
by the Albemarle County
Community Development Department
nnto 05/11/2021
Department of Community Development File vvPo9mnnonu
401 McIntire Road Amendment 1
Charlottesville, VA 22902 Waiver approved
for 3" orifice.
Re: Belvedere Phase 5 VSMP, WPO-2020-00034, Amendment #1
Request for Waiver of Energy Balance Orifice Size
Dear Mr. Pohl,
Per Energy Balance standards of the Virginia Stormwater Management regulations, the orifice
for the Dry Pond was developed to limit hydraulic release from the stormwater facility. The
allowable flow, 0.144 CFS, was reached using an orifice (bored into the riser) of approximately
1.5-inch diameter.
Additional revisions, due to the presence of bedrock, were required:
• The top of the pond was raised by 0.3-feet to provide capacity
• The top of the riser was increased by 0.1-feet and a 10-inch orifice, at elevation 420.2,
was added to control the 10-year flow
• The emergency spillway was widened from 10-feet to 30-feet to provide sufficient
freeboard
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-year flow will increase from 0.144 CFS to 0.512 CFS
• The riser height can remain as previously approved and the 10-inch orifice can be
omitted. The 10-year flow is 3.88 CFS, which is below the required flow of 4.37 CFS
• The emergency spillway will be 15-feet
Updated stage storage reports and hydrographs are included with this submission showing both
the 1.5-inch and 3-inch orifice scenarios.
Thank you and we look forward to your consideration in this matter.
Sincerely,
di'm Taggart, P.E.
For Roudabush, Gale and Associates, Inc.