HomeMy WebLinkAboutWPO201800044 Correspondence 2022-01-28SHIMP ENGINEERING, P.C.
Design Focused Engineering
January 27, 2022
John Anderson, PE
County of Albemarle
Department of Community Development
401 McIntire Road, North Wing
Charlottesville, Virginia 22902
Regarding: ABC Block 1 Mass Grading -Major Amendment For Multifamily
WP02018-00044 Major Amendment 92
Request for Exception of the Energy Balance Equation Dictated Orifice Size
Dear John,
The Stormwater Management System for ABC Block 1 Multifamily was designed to comply with Virginia State
Channel Protection and Flood Protection requirements, specifically 9VAC25-870-66-B.3.a and 9VAC25-870-66-
C.2,b. However, compliance with the Energy Balance Equation to meet channel protection requirement 9VAC25-
870-66-B.3.a at Point of Analysis 2 requires a I -year control orifice which is less than 3 inches in diameter.
Shimp Engineering requests an exception to the Energy Balance Equation based on the following findings.
Predev Flow Rates Maximum Regulatory Flow Rates
Q1.yr= 1.59 cfs Qi_y, = 0.72 cfs (per Energy Balance)
QIO-yr= 10.23 cfs Qio.,„= 10.23 (per Flood Protection)
The Energy Balance Equation can be achieved with a maximum 2.9" orifice size.
Flows with 2.9" orifice
Qi, = 0.72 cfs
Qto-, = 9.26 cfs
Reduction = 0.87 cfs
Flows with 3.0" orifice
Qi.sr = 0.75 cfs
Qio.yr = 8.97 cfs
Reduction = 0.84 cfs
Shimp Engineering recommends that that the BMP's 1-yr control orifice be sized at 3.0 inches to better prevent
BMP failure due to an increased risk of clogging from a smaller orifice size. Conventional design guidelines
suggest that orifices less than 3 inches are more prone to clogging and should be avoided. For this project, a 3.0
inch orifice provides the following:
1. Post -development peak runoff is less than 1.0 cfs. Achieved total peak flow rate to POA 2 is 0.75 cfs,
compared to the 1.59 cfs pre -development peak flow rate.
2. Total Post -development peak runoff to POA 2 is reduced by 0.84 cfs from predevelopment runoff.
The required reduction is 0.87 cfs.
3. At POA 2, the postdevelopment flow rate is 53% lower than the predevelopment flow rate,
4. Use of a 3.0" orifice achieves 97% of the target EBE-generated reduction.
5. All 1-year storm runoff is controlled by this 3.0" orifice.
6. Alternative BMP's were not effective options based on site constraints.
912 E High St Charlottesville, VA 229021434 227.51401 shimp engineering cam
Shimp Engineering selected the ADS Stormtech Chambers and Isolator Row as the development's BMP's. These
BMP's are underground detention facilities which provide detention and sediment filtration to achieve a 40%
phosphorous reduction. Several other options were considered, including infiltration basins, large corrugated
metal pipe (CMP) for underground detention, and biofiltration basins. Site constraints were not favorable to these
other options. Insufficient soil permeability prohibited the use of an infiltration system. Lack of usable surface
area and steep topography at the low points of the site are not favorable for the use of bioretention basins. Utility
density, excavation depth, and separation constraints from the multifamily development did not promote the use
of underground CMP detention.
The Stormtech Chamber BMP's have been designed to detain 1-yr storm runoff so that the 3.0" orifice is the only
control outlet for the 1-yr storm. The system was upsized to provide lower head in the system to minimize peak
flow rates. Despite these provisions, it was concluded that a 3.0" orifice was too large to achieve the reduction
rate required by the energy balance equation. However, the use of a smaller orifice increases the risk of clogging,
and if the 1-yr control orifice clogs, 1-yr runoff would discharge through the 10-yr outlet control orifices at a flow
rates higher than the pre -developed peak flow. This would counteract the design goal of channel protection.
We appreciate Albemarle County Engineering's consideration for this exception request. Shimp Engineering would
be glad to answer any questions about this request and can be contacted at: iustin'ii shimp-engineering coin or by
phone at 434-227-5140.