HomeMy WebLinkAboutWPO201200025 Review Comments Stormwater Management Plan 2012-05-04Ar,B���r
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: Monticello High School Parking Lot Improvements; WPO- 2012 -00025
Plan preparer: Mr. Darin Miller, PE; A. Morton Thomas and Associates
Owner or rep.: County of Albemarle School Board
Date received: 25 April 2012
Date of Comment: 4 May 2012
Engineer: Phil Custer
The first submittal of the SWM plan for the parking lot improvements at Monticello High School (WPO-
2012- 00025), received on 25 April 2012, has been reviewed. The plan can be approved after the following
items are addressed.
C. Stormwater Management Plan Review Comments (WPO- 2012 - 00025)
1. The applicant has provided a drainage area map from the approved final site plan set that
indicates the design of the drainage system downstream of the gravel lot had considered the lot to
be completely impervious. Upon hearing this, I first believed that this was enough information,
lacking actual SWM calculations, to conclude that the downstream stormwater facilities approved
with the original site plan had considered the gravel lot to be paved and the need for stormwater
management facilities for the upgrade from gravel to pavement would be eliminated. After
further review of the older files, however, it seems that the downstream wetland improvements
was not approved as a stormwater facility but as mitigation for wetland impacts associated with
the construction of the athletic fields. In fact, the county did not have a stormwater quality
requirement until 1998, 2 years after this plan was approved. This changes how the conversion of
this gravel lot is considered.
When the overflow gravel parking lot was installed in 1998, county staff indicated that what was
approved in 1996 for this phase II lot was prime and double seal. From a stormwater
management perspective, I'm not sure whether the original approval in 1996 vests the impervious
area perpetually, especially since a gravel lot was installed and possibly superseded the original
proposal. On Monday, I will discuss this issue with the county engineer to get a determination.
The worst case scenario for the applicant is that an inline curb facility (sized to treat — 0.50acre)
just uphill of the 8ft curb cut would be needed.
2. The concept for the proposed stormwater treatment solution for the modifications to the dropoff
area is acceptable. However, the proposed stormwater facility is not a biofilter [VSMH 3.11], but
a water quality swale [VSMH 3.13]. Please modify all references to this facility to reflect this.
3. Please provide more detailed grading information (contours and spot elevations) on the plan and
crosssection detail. In the crosssection, please specify a depth of the swale from one of the curbs.
Please show that the western curb is about 6" lower than the eastern curb. Please provide
reasonable freeboard from the top of the railroad tie to the top of the lowest curb and make this
clear in the cross - section detail (it looks as the northern end of the channel may need to be
lowered). A bottom width of closer to 12ft can be achieved assuming lft flat areas off of the
curbs, 2:1 slopes, and a depth of lft from the lower curb.
4. As designed, the plan only ponds about 268cf of water. The VSMH requires that water quality
swales are designed for the Water Quality Volume, which in this case is a little over 800cf.
Considering the downstream wetland and existing water quality swale, the full 800 +cf will not be
required. However, please modify the swale to provide more ponding. For instance, by my
calculations, providing the railroad tie check dams every 5ft provides about 420cf of ponding.
This would be acceptable, but more improvements are recommended.
In the profile, please provide at least 9in of gravel below the soil media. In the gravel layer,
please show the 6" perforated underdrains.
Please show riprap and provide specifications for the apron at the end of the concrete spillway
from the dropoff area.
It is recommended that the trees and shrubs are located on the banks for the channel to minimize
longterm erosion potential. This is not a requirement.
File: E1_swm_PBC_Monticello High Parking Lot Modifications.doc