HomeMy WebLinkAboutWPO200700082 Review Comments Erosion Control Plan 2009-10-13� OF AL
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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: Ashcroft West ESC Plan [WP0200700082]
Plan preparer: Neal Wright; Dominion Engineering
Owner or rep.: Pantops - Lakeridge LLC
Plan received date: 14 July 2009
(Rev. 1) 2 September 2009
Date of comments: 19 August 2009
(Rev. 1) 13 October 2009
Reviewer: Phil Custer
The Erosion and Sediment Control Plan for Ashcroft West, submitted on 2 September 2009, has been
reviewed. The following comments must be addressed before approval of the set is given:
Adequate channel concerns have not been addressed with the addition of the level spreaders, as
proposed, at the basin outlets. The discharges these facilities will be subject to are too high and
will likely lead to reconcentration of the water immediately downhill of the facility and,
consequentially, long -term erosion. In addition, the VESCH also stipulates that the area
downstream should not be greater than 10%. To address the adequate channel issue, engineering
recommends that the facilities be modified to reduce the outlet flows of the 2 and 10 -year storms
as much as possible. For each facility, I managed to keep 10 -year flows under lcfs while keeping
the same geometry by using a flow limiting inlet.
Embankment spillways should be properly lined. Please specify the dimension and lining of the
spillway for each facility.
There are also adequate channel concerns at the ditch outlets for the section of the new road.
Erosion appears to be an issue before and after the old sediment trap across from Lego Dr.
(Rev. 1) Comment has not been addressed. The routings submitted with the latest revision do
not model the critical duration storm for the 2, 10, and 100 year storms. As a result, the actual
discharges will be higher than those computed. The original purpose of the over - detention
compromise offered by the county was to use as much of the facility volume as possible without
altering the grading of each BMP. The 8" drain proposed in each facility will not utilize
enough of the existing volume of the facility. Please reduce the riser to a smaller structure to
fill the pond as much as possible during the 2 and 10 year storms. I recommend using a 6"
orifice with an ADS barguard as a trash rack (though the riser should be of a larger diameter
to reduce the risk of clogging). Also, the original contours of trap 2 should remain to provide
as much detention as possible in facility 2.
Since our last correspondence, the county has contacted the regional DCR office to request
their opinion on the MS -19 issues this site is facing. Once the site begins construction DCR
will also have regulatory authority so it is beneficial to receive their perspective. Since they did
not receive a full plan, DCR could not provide an official determination but they were leery of
Albemarle County Community Development
Engineering Review Comments
Page 2 of 2
the proposed level spreaders upstream of forested critical slopes (level spreaders are specified in
the VESCH to be temporary measures provided uphill of a grassed area of no more than 10%).
DCR's first recommendation was adding more biofilters to provide additional release points to
decrease the discharges each level spreader was subjected to. In addition to that, they also
recommended that the county hold a bond for the cost of channel construction /improvements
in the case level spreading doesn't work and downstream erosion problems occur after
development. While the county believes both of these recommendations are reasonable, we
anticipate that the applicant is not comfortable with the idea of having to bond 3000- 5000ft of
channels down 25% slopes and reconfiguring the SWM plan. Please contact me to schedule a
meeting to discuss the resolution to the adequate channel concerns for this project. The county
has identified some alternative solutions that may be more acceptable to the applicant.
The channel downstream of the first section of new road must be analyzed. If improvements
are warranted they must be shown on this plan. After walking the site, it would be hard to
imagine that much of this channel section will be found to be adequate.
2. Sediment trap 4 needs to be relocated to an area where it can capture construction activity.
(Rev. 1) The trap should be located downstream of the proposed road culvert so both ditches
and the construction entrance drains to the trap. To address this comment, the blownout trap
downstream can be reconstructed. To help address adequate channel concerns in this area, a
permanent detention pond can be placed here to limit discharges down this drainageway.
3. ESC measures are needed on the east side of the road at Sta. 19 +00. A sediment trap appears to be
the most appropriate measure to be located there.
(Rev. 1) Comment has been addressed.
4. Staging and construction areas do not appear to be protected by ESC measures at Sta. 14 +00 and
34 +00.
(Rev. 1) The staging area at Sta. 34 +00 has not been protected with silt fence.
5. Please extend the silt fence on the west side of the road from Sta. 16 +50 to 18 +50 to more
adequately protect the area below the road embankment.
(Rev. 1) Comment has been addressed.
6. Please show the grading and design of the ditches from the two cul -de -sacs down to the
stormwater facilities. Please specify the dimension and lining of the ditches and detail how these
ditches will cross the access roads to reach the facilities.
(Rev. 1) Please provide rip -rap where the channels enter the facilities down the 3:1 cut slopes.
A variance has been given by the program authority for the height of the sediment traps given the
steep topography.
E2_esc_PBC_Ashcroft West ESC Plan.doc