HomeMy WebLinkAboutWPO201900061 Correspondence 2020-12-22 (2)ROUDABUSH, GALE & ASSOCIATES, INC.
County of Albemarle
401 McIntire Road
Charlottesville, VA 22902
Mr. Matthew Wentland, P.E.
December 22, 2020
Re: Variance Request
Project title: Georgetown Hydraulic Professional Office building - VSMP
Project file number: WPO-2019-0061
Stormwater facility orifice size
Mr. Wentland,
In our preparation of Georgetown/Hydraulic plans for construction, we discovered an error in
coordination with the ADS (stormwater management facility) provider.
Our depiction, on the approved plans, of a sump within the control structure does not properly keep the
system from retaining water. Further, the release modeling seems to be flawed with the apparent open
bottom to the weir plate. By omitting the sump and open bottom weir plate, water will be retained up
to 3-feet before it can drain between storm events.
Therefore, we have adjusted the vertical elevations of the system and reworked the control structure to
address these issues. The control structure has been changed as follows:
1. Remove the sump, eliminate the open bottom weir plate, sealing the bottom of the structure
2. Raise the aggregate collection system to not retain water and add an underdrain
3. Move the orifice to bottom of the weir plate to allow the system to fully drain, and
4. Adjust the orifice size to 1" diameter at the new elevation to meet the energy balance
requirement.
The system is designed to meet Energy Balance and Flood protection standards. However, it is our
recommendation that the orifice be enlarged from 1-inch to 3-inches in an effort to protect the facility
from failure due to likely clogging of a 1-inch orifice. In our experience, limiting such constrictions to 3-
inches helps minimize potential maintenance issues while still providing substantial attenuation to the
flows.
Enlarging the orifice to 3-inches will increase the potential 1-year flow from 0.038 CFS to 0.268 CFS. The
10-year flow will be 0.41 CFS which is within the permissible range for Flood protection standards.
Please see the attached
lim Taggart, P.E.
For Roudabush, Gale and Associates, Inc.
999 Second Street SE. Charlottesville. VA 22902 www.Roudabush.com