HomeMy WebLinkAboutWPO201200057 Review Comments Mitigation Plan 2012-07-25ALg�,��
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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: WPO- 2012 - 00057, King's Grant SWM Plan Amendment
Plan preparer: Mr. Alan Franklin, PE; Water Street Studios
Owner or rep.: Elizabeth Allen Revocable Trust
Date received: 18 June 2012
Date of Comment: 25 July 2012
Engineer: Phil Custer
The SWM Plan Amendment for Kings Grant, received on 18 June 2012, has been reviewed. This plan can
be approved after the following comments have been addressed.
1. The concept of replacing the additional stream buffer in some areas and providing two more
biofilters is acceptable. However, the additional stream buffers offered by the applicant on the
north side of lots 4 and 5 offer a minimal benefit. Please remove these areas (keeping the area on
the south side of lot 4) and retain the 75ft buffer west of the building site on Lot 7 from the limits
of the pond grading to the existing southern buffer line. The 225ft long additional buffer south of
the building site for lot 7 can be eliminated. Please see the drawing accompanying this comment
letter for clarification.
2. Please remove the amenity pond detail from the plan to avoid confusion as to what county
engineering is approving. An amenity pond would need its own ESC plan if the applicant wishes
to construct it.
3. Please replace all references to Luck Biofilter Mix with "State Approved Mix." The contractor
will be required to have a third party test the soil mix after construction to certify it meets state
standards.
4. The minimum depth of biofilter mix is 30 ", not 24 ".
5. Please modify the biofilter detail to clearly show the elevation of the spillway compared to the
elevation of the biofilter bed. Since stone spillways are porous, the detail must show the
impervious soil spillway underneath the stone that sets the ponding depth. Please refer to the latest
edition of the design manual (February 2010) for a better biofilter detail to modify and use in this
plan submittal.
6. The sizing ratio listed in the Virginia Stormater Management Handbook (biofilter bed
footprint =2.5% of the impervious area for a RR of 50 %, 4% or 5% of the impervious area for a
RR of 65 %) assumes a ponding depth of Ift. Please either change notes on the plan to show a
provided removal rate of 65% while increasing the ponding depth to I ft or clarify in the
calculations the extra step (change "5 %" into "2.5% x 2 (because only 6" of ponding provided) ".
The provided footprints are adequate for the current watersheds.
7. Please move the trees for the lot 4 biofilter out of the clear zone for the street.
8. Please provide computations for the maximum 2 -year velocity for the ditches just above the
biofilter to show that rip rap is not needed.
9. Please reduce the landscaping plan to the minimum necessary for approval so future county
inspection does not require all of these plantings to be maintained. Only 4 plants in each facility is
needed: 1 tree and 3 shrubs. Each shrub should be of a different species. Please refer to VSMH
3.11 for additional details. A different planting plan may be issued to the contractor showing
landscaping above and beyond county requirements.
10. Please specify an adequate sump downstream of the DI -5 structure to ensure that no bypass of the
biofilter occurs.
11. A SWM bond will need to be held by the county in order for the county to approve a plat to
modify the stream buffer lines. To request a bond amount, please submit a Bond Estimate Request
Form to the County after the SWM plan has been approved.
File: El_swm_PBC_WPO- 2012 -00057 Kings Grant SWM Amendment.doc