HomeMy WebLinkAboutWPO200700082 Review Comments Erosion Control Plan 2009-11-30� 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
(Rev. 2) 5 November 2009
Date of comments:
19 August 2009
(Rev. 1) 13 October 2009
(Rev. 2) 30 November 2009
Reviewer:
Phil Custer
The Erosion and Sediment Control Plan for Ashcroft West, submitted on 5 November 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
Albemarle County Community Development
Engineering Review Comments
Page 2 of 3
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 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 constructionlimprovements 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.
(Rev. 2) The applicant's latest proposal to address MS -19 concerns is conceptually acceptable to
the County. However, please provide the following minor technical modifications to the plan:
a. Remove all contradictory trap conversion details on sheet C -7.
b. The depth of the concrete level spreader structure must be a minimum of 18 ".
c. Please provide a trashrack on the 6" pipe in the detention facility on TMP 78A -1.
d. Please clearly indicate that the fill above and around the 6" pipe in the trap to be
converted into a detention facility will be soil. The detail appears to simply specify
riprap. The soil must be relatively impervious clay that is approved by an engineer.
The soil must be compacted to within 95% dry density and 2% of optimum moisture
content.
e. The downstream slope of the embankment of the converted trap must be filled to meet a
2.5:1 minimum grade.
f. The spillway should be 2.5ft above the top of the 6" pipe, rather than 1.5ft.
g. The owner of TMP 78A -1 must provide a stormwater facility maintenance agreement
and fee for the detention facility.
As agreed upon, the County will hold a small bond on the outlet of each facility (1 -3) for a
period of two years to evaluate the effectiveness of the proposed level spreaders. At the end of
the term, if failure to the outlets are found and erosion has occurred or likely to occur in the
opinion of county personnel, the owner will be required to modify the outlets to the satisfaction
of the county.
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.
(Rev. 2) This trap has been relocated to an acceptable location. For comments on the existing
trap conversion to a detention facility, please see the previous comment.
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.
Albemarle County Community Development
Engineering Review Comments
Page 3 of 3
4. Staging and construction areas do not appear to be protected by ESC measures at Sta. 14 +00 and
34 +00.
(Rev. l) The staging area at Sta. 34 +00 has not been protected with silt fence.
(Rev. 2) Comment has been addressed.
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. I) 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. l) Please provide rip -rap where the channels enter the facilities down the 3:1 cut slopes.
(Rev. 2) Comment has been addressed.
7. A variance has been given by the program authority for the height of the sediment traps given the
steep topography.
E3_esc_PBC_Ashcroft West ESC Plan.doc