HomeMy WebLinkAboutWPO200800038 Review Comments Stormwater Management Plan 2008-09-24ALg�,��
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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 - 00038, Westhall V ESC and SWM Plan
Plan preparer: Mr. Doug March, PE; W & W Associates fax 434.978.1444
Owner or rep.: Shiflett Farm, LLC fax 434.975.3542
Date received: 15 April 2008
(Rev. 2) 21 August 2008
Date of Comment: 30 March 2008
(Rev. 2) 24 September 2008
Engineer: Phil Custer
The SWM and ESC plans for Westhall V, received on 21August 2008, have been reviewed. This letter
comments on the third submittal made to the County for the WPO and road plans. The second submittal
was reviewed under an alternative review procedure with just a checklist. All comments that were
addressed with the second submittal are stated as so. The plans can be approved after the following
remaining comments have been addressed.
A. General Review Comments
1. The work proffered in item 1.3 of ZMA- 2006 -00001 should be shown in this set.
(Rev. 2) This work does not have to be included in this set but must be completed to the
satisfaction of Albemarle County before the first building permit is applied for.
2. Please provide the county with all state and federal permits.
(Rev. 2) Comment has not been addressed.
3. Before this plan can be approved, all necessary easements on adjacent TMP 56 -53 must be
recorded. These easements include, but are not necessarily limited to, the SWM facility, drainage -
ways across all channels from the Westhall property, and the access path to the facility.
(Rev. 2) Comment has not been addressed.
4. This WPO plan is currently being reviewed independent of any subdivision plat because, at the
time of the WPO submittal, all plats for this area were either withdrawn or not yet submitted.
Additional WPO comments may be given in the reviews to follow based on required changes or
review of the plat and road plans.
(Rev. 2) The final plat is under review.
B. ESC Plan Review Comments
Please perform adequate channel analyses for the channels downstream of the forebay and pond
outlets. The single calculation for the channel immediately after the pond outlet is not extensive
enough.
(Rev. 2) Comment has been addressed.
2. Please provide a Phase 1 ESC sheet with the initial clearing of land and the installation of the
perimeter control measures. This sheet should show the existing drainage areas to the traps and
basins with the current site topography before grading operations begin. It is hard to review the
adequacy of the current plan and future comments may be forthcoming based on the new sheet.
Comment was addressed with the second submittal.
3. Please provide drainage area lines on ESC -2 for all sediment trapping measures. Sediment
trapping measures should be sized for the largest drainage area that could possibly be draining to
it. This could be in the initial disturbance phase, final site grading phase, or an intermediate phase
between the two. Again, please be sure that drainage area lines are provided for both existing and
proposed grading.
Comment was addressed with the second submittal.
4. Please provide letters of intent for all off -site work. From my initial review of the plans, letters are
needed from the owners of lots 57 -61, lot 7, TMP 56 -48, and TMP 56 -53.
(Rev. 2) Comment has not been addressed. The applicant has indicated that an additional
sewer easement is needed on TMP 56 -95. An easement will be needed on Lot 7. The grades in
this area should be flattened to 3:1. If slopes are steeper than 3:1 (but no steeper than 2:1), low
maintenance, non - grassed groundcover needs to be proposed.
5. Grade lines go over and through sediment traps. Please tie site grading into the sediment traps.
The final as -built grade lines should not appear on the ESC plans.
Comment was addressed with the second submittal.
6. Provide diversion dikes north and south of ST3 rather than silt fence. Please be sure to direct the
area of disturbance required to construct the extension of Westhall Drive into the sediment trap.
Comment was addressed with the second submittal.
7. The construction entrance cannot be placed on fill. I recommend moving the construction
entrance off of the temporary turnaround area on Brookwood Road. Moving the CE would reduce
the limits of construction for the Phase I operation to the area needed to just construct the
perimeter controls. To place the construction entrance in its current proposed location, the volume
of the existing basin would be reduced by significant amounts of fill and the ESC plan for Phase
III would become inadequate.
Comment was addressed with the second submittal.
8. The embankment for the sediment basin is not shown to be wide enough. [VESCH]
Comment was addressed with the second submittal.
9. Please provide a safety fence around all sediment traps and basins with signs stating: "danger,
quick sand, do not enter ".
Comment was addressed with the second submittal.
10. A baffle is required in the sediment basin.
(Rev. 2) Comment has been addressed.
11. I do not see where Culvert Inlet Protection is used on the plan though a detail was provided in the
set.
Comment was addressed with the second submittal.
12. Please correct the sediment basin table to show that only 810.7 cy are provided as opposed to the
total volume provided at this elevation. Please note that the 12.1 drainage has not been confirmed
by engineering review because it has not been shown on the plan. (Please also see ESC comment
#3) If it appears that the drainage area is in fact larger than 12.1 acres in the next review, the
sediment basin will be undersized.
(Rev. 2) Comment has been addressed.
13. Please modify the C coefficient used for the routing of the sediment basin. The entire drainage
area is not bare earth and contains some impervious area from existing developed phases.
Comment was addressed with the second submittal.
14. Considering the time of concentration calculation on sheet SWM -2, the sediment basin time of
concentration should be around 10 -11 minutes.
Comment was addressed with the second submittal.
15. Please divert SCC D into ST2.
Comment was addressed with the second submittal.
16. Please provide blowup details for the review of the sediment traps. It is hard to check the traps for
compliance with the state's design standards (regarding embankment heights, embankment widths,
volumes, etc.) with the plan at 60 scale.
Comment was addressed with the second submittal.
17. Please use diversion dikes on the south end of the property to direct water into sediment traps 1
and 2.
Comment was addressed with the second submittal.
18. Please provide a calculation for the dewatering orifice of the sediment basin. My calculations
indicate the orifice should be 5 ".
Comment has been withdrawn. A 6" dewatering orifice is correct.
19. The ESC portion of the WPO bond will be computed once all comments have been addressed.
(Rev. 2) Comment remains unchanged.
C. SWM Plan Review Comments
1. A Lickinghole basin pro -rata share fee will be calculated when the plans are closer to approval.
(Rev. 2) The Lickinghole basin fee has not yet been calculated.
2. Signed Stormwater Management Facilities maintenance agreements and recordation fees will need
to be submitted for both parcels. Please contact Pam Shifflett (x 3246) for questions regarding
this procedure.
(Rev. 2) Comment has not been addressed.
3. Engineering review staff will not approve this SWM plan for proposed development of the TMP
56 -53 parcel. This review is for the development of the Westhall V subdivision SWM plan only.
At the time of the development of TMP 56 -53, the existing SWM facility will need to be checked
for adequacy by the applicant.
(Rev. 2) Comment has been noted by the applicant.
4. Please improve the "existing unimproved road" to the standards specified in the design manual for
maintenance paths to SWM facilities.
(Rev. 2) Comment has been addressed.
5. In addition to the Lickinghole basin pro -rata share, SWM facility C needs to provide stormwater
quality treatment to be consistent with the approved rezoning plan. Please provide these
calculations. The sediment forebay sizing calculations should also be provided when the pond
calculations are submitted.
(Rev. 2) Comment has not been addressed. The applicant should provide a calculation for the
entire watershed to SWM facility C. Find the 3 x WQv of the entire drainage area considering
all existing impervious areas in previous phases of Westhall and the proposed impervious area
of Westhall V development. That volume should be compared to a calculation of the volume
between elevations 604 and 610.
6. Please show a trash rack on the riser weir.
Comment was addressed with the second submittal.
7. The maximum slope allowed around the sediment forebay facility will be 3:1, including the
downstream slope on the embankment.
Comment was addressed with the second submittal.
8. The downstream slope of the SWM pond must be 3:1 or shallower.
Comment was addressed with the second submittal.
9. The minimum pipe diameter permissible in SWM facilities is 15 ". For the forebay facility, please
make the pipe 15" and provide an orifice plate at the inlet to the pipe.
Comment was addressed with the second submittal.
10. Please specify on Sheet SWM -3 the elevations for the crests for both riser structures.
Comment was addressed with the second submittal.
11. On sheet SWM -5, please provide information for the sediment forebay drainage area so that
routing calculations can be confirmed.
Comment was addressed with the second submittal.
12. The SWM portion of the WPO bond will be computed once all comments have been addressed.
(Rev. 2) Comment remains unchanged.
File: E3 esc swm PBC 08 -038 Westhall V ESC and SWM.doc