HomeMy WebLinkAboutWPO200800086 Review Comments Stormwater Management Plan 2008-10-06ALg�,��
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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- 2008 - 00086; Hollymead Al
Plan preparer: Mr. Todd Philipp, PE; Dominion Development Resources
Owner or rep.: HM Acquisition Group, LLC.
Date received: 20 August 2008
Date of Comment: 6 October 2008
Engineer: Phil Custer
The ESC and SWM plans for the Hollymead Al have been reviewed. The following comments are
provided.
A. WPO- 2008 -00086 General Review Comments
1. A SWM facility maintenance agreement for all parcels will need to be recorded before the site plan
can be approved. Please submit this document with fee directly to Pam Shifflett after consulting
the guidelines available on the county website.
2. In both ESC and SWM sets, please label the existing stream buffer.
3. Any and all changes in the site plan must be reflected in the WPO plans.
4. There appear to be some pipes missing west of the Starbucks. Please clarify.
B. WPO- 2008 -00086 Stormwater Management Plan Comments
1. Please include in your resubmittal the design of the existing basin and all calculations confirming
that the design of the existing basin is adequate considering the proposed development and the
existing impervious areas draining to it. Required calculations include all items listed in the
Design Manual (detention compliance calculations including pre and post development analysis
with reference maps, calculations showing that the biofilter captures the required water quality
volume of the watershed, forebay sizing calculations, as -built information on riser and barrel, etc.).
Please note that after looking at the topographical data on this submittal and a visit to the site, it
appears that the embankment does not meet the 3:1 downstream slope requirement of the VSMH.
In this plan, please correct to make sure the embankment meets the design standards. Please call
to discuss your proposed remedy to this situation before resubmitting. Any disturbance to the
existing stream buffer will require the approval of the County Engineer. In addition, there appears
to be many trees and shrubs growing on the embankment that must be removed.
2. Please provide a 100 -year routing of BF -5 to confirm that there is adequate freeboard on the
embankment.
3. Please show SWM easements over all facilities.
4. There appears to be some discrepancies in the calculation of the drainage area to BF -4 between
sheets 4 and 5.
5. Vehicular access to the sediment forebay and riser structure in BF -5 is necessary. Access
requirements are listed in the design manual.
6. The Stormfilter details provided in the set are too general and must be site specific. Please provide
elevations on all of the Contech Stormfilter details. Please also specify the correct pipe diameters
and orientation in details as well. It appears the bypass structure is not shown correctly in the
detail regarding the number and direction of pipes entering it.
7. Please submit a letter from Contech stating that the facility as designed meets the state standard of
a 50% removal rate.
8. All stormwater treatment facilities require sediment forebays, sized for 0.1 "- 0.25 ", at each point
concentrated stormwater enters the facility. The forebay volume can be included in the facility's
water quality volume. Access to these forebays must also be provided.
9. For biofilters 1 -4, please provide a cross - section showing that each facility can pass the 10 -year
storm with lft of head. Because biofilters 3 and 4 must pass the 100 -year storm with lft of head
on the embankment.
10. Please increase the ponding depth in all of the biofilters to lft. As designed, the biofilters do not
appear to capture the required water quality volume.
11. The biofilter must be planted with at least 3 species of trees and at least 3 species of shrubs. [DM]
12. The permanent outlet structure for BF -5 must match the outlet structure for SB -1. [DM]
13. A SWM bond will be computed once the WPO plan is ready for approval.
C. WPO- 2008 -00086 Erosion and Sediment Control Plan Comments
1. Proffer 9 has not been satisfied with this plan regarding Erosion and sediment control.
Engineering review does not feel that the applicant has proposed adequate ESC measures to
achieve the "80% sediment removal" as required in the proffer. A few of the proposed additional
measures are already requirements (no disturbance inside of the buffer and dewatering of sediment
basins). Please provide information regarding Flexterra.
To meet the ESC proffer, engineering review recommends providing a phase of the ESC plan that
upgrades "Sediment Basins B and C" to VESCH standards (treatment train applied to ESC) and
keeps those facilities in place until most of the site is stabilized with grass or gravel, cleaning out
the existing basin's wet storage as the first step in the construction sequence, and limiting
disturbance to a certain percentage of the site at one time with a more detailed phasing plan.
2. Please provide calculations showing that the existing sediment basin meets all minimum standards
of the VESCH. For this calculation, please use as -built information on the riser and the recently
flown topography.
3. Please provide a note on sheet 2 and sheet 3 stating that any disturbed area must be stabilized
within 9 months after initial land disturbance.
4. How will the construction of Meeting Street be coordinated with this ESC plan?
5. Please provide a drainage area map showing the "post- development" drainage areas to all sediment
basins (including the existing basin).
6. The outlet protection and channel from the existing basin needs to be redesigned. The system is
failing.
7. The narrative suggests that no offsite waste or borrow areas are needed. Considering all of the
rock and unsuitable soil on -site, it is unlikely that offsite disturbance is not needed. Please specify
in the narrative that all borrow or waste areas must have an approved Erosion and Sediment
control plan. The borrow or waste area cannot be another block of Hollymead Towncenter without
the specific approval of the Program Authority.
8. Please provide a soil stockpile on the plan.
9. ESC measures for the installation of the sanitary sewer line and the paved pedestrian trail need to
be shown. It appears that grading will be needed for the pedestrian trail that is not shown on this
plan.
10. The construction of the waterline in Meeting Street is well outside of the limits of disturbance.
Please amend.
11. I do not understand the calculation provided sizing the dewatering orifice. By my calculation, it
appears the dewatering orifice should be 6 ".
12. The detail on page 4 shows that the 25 -year storm only passes through the dewatering orifice not
the riser's top. This does not appear to be accurate. The VESCH's method of calculating
freeboard for the 25 -year storm does not even consider the dewatering orifice.
13. Please clearly label the contours in the proposed SB -1. It appears the bottom of the pond is
mislabeled on sheet 3.
14. SB -1 must be designed for 10.26 acres not 9.18.
15. The existing pole must be removed from SB -1. Grading operations in the vicinity necessitate
removing this pole.
16. Please show safety fencing around all sediment basins with signs stating "Danger, Quicksand. Do
Not Enter."
17. Please show a ROW diversion at the entrance onto Route 29 directing water into structure 91. A
diversion east of the travelway should also be provided to direct sediment laden water from the
area of the trail construction into that structure.
18. Please dimension outlet protection on the plan. A few of the riprap blankets appear to be shorter
than the characteristics specified in the table.
19. The ESC plan should show the removal of the pole in the existing basin.
20. An ESC bond will be computed once the WPO plan is ready for approval.
File: E1_swm esc_PBC_wpo200800086 Hollymead Al.doc