HomeMy WebLinkAboutWPO201100076 Calculations 2011-09-12 WATER QUALITY ANALYSIS
FOR
RIVANNA PLAZA
June 13, 2008
Resubmission September 12, 2011
LOCATION:
Tax Map 45, Parcel 109 &112C1
Albemarle County, Virginia
OWNER:
Rivanna Plaza LLC
109 Robinson Woods
Charlottesville, Virginia 22903
DEVELOPER:
Rivanna Plaza LLC
109 Robinson Woods
Charlottesville, Virginia 22903
PREPARED BY:
Dominion Engineering and Design, LLC
172 South Pantops Road
Charlottesville, VA 22911
434.979.8121
(F) 434.979.1681
The performance-based water quality criteria states that for land development, the
calculated post-development non-point source pollutant runoff load shall be compared
to the calculated pre-development load based upon the average land cover condition or
the existing site condition. This approach requires the designer to calculate the pollutant
load to be removed, implement a BMP strategy, and then calculate the performance of
that strategy, based on the effectiveness or pollutant removal efficiency of the selected
BMP(s).
The simple method was used in determining the required pollutant removal for
Rivanna Plaza. The percent imperviousness for the pre-developed condition is 13%
putting the project in the New Development category. The parcel in question is not in a
drinking water watershed or other rural lands. The post-developed condition adds
101,898 SF of new impervious surfaces while removing 18,053 SF of the existing
impervious surface. The resultant removal rate for New Developments in Development
Areas is 66%. The BMP's proposed for removing pollutants from the drainage area are
a treatment train consisting of a 3 Filterra structures and a combination of an in-line
hydrodynamic separator in conjunction with an off-line filtration system. Both structures
are Contech Stormwater Solution products and have been sized accordingly(See
Sizing Summary).
The Filterra systems treat 65% of the stormwater. The hydrodynamic separator
has a capacity to treat 15% of the required removal rate for Rivanna Plaza while the
Stormfilter system has the capacity to treat 50% of the required removal rate.
Runoff from nearly 100% of the new impervious surface is captured and routed through
the treatment train. These three BMP's in series will remove 66% of the pollutants thus
meeting the 66% requirement.
66% of 2.76 Ac = effective treatment area of 1.82 Ac
0.86 Ac to Filterra
0.86*0.65 = 0.56 Ac effective area treated
2.76-0.56 = 2.2 Ac
2.2 Ac to Hydrodynamic Separator
2.2*0.15 = 0.33 Ac effective area treated
2.76-0.56-0.33 = 1.87 Ac
1.87*0.5 = 0.94 Ac effective area treated
Sum of effective treatment areas = 1.83 > 1.82