HomeMy WebLinkAboutWPO202200028 Correspondence 2023-03-17 (2)SHIMP ENGINEERING, P.C.
Design Focused Engineering
November 14, 2022
John Anderson, PE,CFM
County of Albemarle
Department of Community Development
401 McIntire Road, North Wing
Charlottesville, Virginia 22902
RE: Exception Request Letter - River's Edge — VSMP - WPO2022-00028
Dear Mr. Anderson,
Pursuant to Virginia state code, Sec. 9VAC25-870-122 Exceptions, `A VSMP authority may grant
exceptions to the provisions of Part H B or Part H C of this chapter." Pursuant to Albemarle County
Code, Sec. 17-408-Exceptions from the requirements of the VMSP, "The administrator may grant
exceptions from the requirements of the VAER." & "an owner may request an exception from any
technical criteria in 9VAC25-870-62 through 9VAC25-870-92 ......
We request that the VSMP authority (Albemarle County) grant an exception to Sec. 9VAC25-870-66.B.3
for the location of the Point of Analysis in the natural stormwater conveyance system (North Fork
Rivanna River). Per Sec. 9VAC25-870.B, "Concentrated stormwater flow shall be released into a
stormwater conveyance system and shall meet the criteria in subdivision 1, 2, or 3 of this subsection,
where applicable, from the point of discharge to a point to the limits of analysis in subdivision 4 of this
subsection."
The stormwater management plan for this project proposes the Point of Analysis to be located in the
North Fork Rivanna River, 970-ft downstream of the proposed post -development discharge location. The
Energy Balance Equation is not obtainable at the point of discharge due to the existing site conditions.
The site is located at the top of a ridge that is completely encompassed by the river and does not have any
point of concentrated flow to the river in the existing conditions. Because of the nature of the site, the
new development will capture runoff from areas that did not contribute to the proposed point of discharge
in pre -development conditions. As a result, the Energy Balance Equation cannot be met at the proposed
point of discharge with reasonable stormwater detention measures. Therefore, we have proposed to meet
the Energy Balance Equation at the last point where the pre -development drainage area discharges to the
river. While it is possible for the detention system to discharge at this Point of Analysis, it would require
significantly more disturbance in the flood plain and steep slopes. This would be at a detriment to the
912 E. High St. Charlottesville, VA 22902 1434.227.5140 1 shimp-engineering.com
design, construction and cost of the project while providing no real benefit to the prevention of erosion as
is explained below.
The discharge is proposed to be within the North Fork Rivanna River at a location and elevation that
allows the river to provide backwater flow in the final section of pipe which will have a minimal slope of
1% to promote lower velocity and more backflow. This will significantly reduce the discharge velocity to
the degree that it is negligible. Additionally, the river is made up of bedrock at the point of discharge
which significantly reduces the possibility of erosion.
While the runoff flowrate from the site is insignificant compared to the flowrate in the North Fork
Rivanna River, the detention system will still provide for attenuated release of runoff so the peak flowrate
in the river will occur before the peak flowrate of discharge from the site. Therefore, any increase of
flowrate in the river caused by the development is reduced to the point that it is negligible.
Please find attached with this letter supporting exhibits and calculations that support the explanations
above.
P.C.
912 E High St Charlottesville, VA 22902 1 434 227 5140 1 shimp engineering com