HomeMy WebLinkAboutWPO201000073 Review Comments Stormwater Management Plan 2010-12-21ALg�,��
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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- 2010 - 00073, Great Escapes Movie Theater at Hollymead Town Center
Plan preparer: Mr. Scott Collins, PE; Collins Engineering
Owner or rep.: Route 29 LLC
Date received: 25 October 2010
Date of Comment: 21 December 2010
Engineer: Phil Custer
The SWM plan for Great Escapes Movie Theater at Hollymead Town Center, received on 25 October
2010, has been reviewed. Comments from the review of the site and esc plans have been provided in
separate comment letters. The SWM plan can be approved after the following corrections are made:
Proffer 9D requires the applicant to provide a removal rate 20% greater than what would be
required per county policy, up to a removal rate of 80 %. The modified simple spreadsheet
provided by the applicant shows that a removal rate of 74% is normally required. Therefore, the
applicant must provide stormwater management for this project to achieve a 80% removal rate.
Since no single facility exists that achieves such an efficiency, facilities in series, or a "treatment
train ", will be required.
Currently, the applicant's water quality concept proposes a 65% biofilter sized for Areas A and B
as well filterras treating the remainder of the development (Area Q. However, the biofilter must
be designed based on the entire upstream watershed, not simply the area that doesn't get any
pretreatment from the filterras. I recommend sizing the biofilter for 50% removal rate (2.5% of the
impervious area of the total watershed, assuming lft of ponding) and supplementing the biofilter
with upstream filterras and an underground SWM WQ system (downstream of the contech
detention system). In this case, the bed area for the biofilter will need to be approximately 8300sf,
which would require minimal adjustment of the grading plan. (Providing a biofilter with a 65%
removal rate does not appear to be feasible given the layout approved with the latest rezoning
plan.)
Please also note it is impossible to provide more than 100% removal for a watershed. When
performing the treatment calculation, the percentage not treated by the first facility is multiplied by
the Removal Rate of the downstream facility. For instance, two 50% RR facilities have a
combined removal rate of 75 %.
2. In the modified simple spreadsheet provided by the applicant on sheet 23, the value entered in the
post - development pasture /grass cell is greater than the total drainage area. Please correct.
3. This stormwater plan must be designed to capture as much of Meeting Street as practicable. [17-
315.A] Please update all calculations and maps accordingly.
4. The approved application plan for Hollymead Area A2 (ZMA- 2007 - 00001) possesses a note that
states this stormwater facility will provide detention for Block B3. Please show this within the
calculations or process a variation through the Planning Department to allow onsite detention in
Block B3.
The pre - development curve number seems much too high. What year was the aerial photograph
provided by the applicant taken? The 2002 orthophotograph, which I have provided below, shows
the majority of the drainage area as wooded (with —15% as pasture with some other impervious
areas for the house and gravel driveway) in B soils. The aerial photograph provided by the
applicant appears to have captured the site in the initial stages of land preparation for the
development. A curve number close to 57 is more appropriate. Please update the calculations
6. The ESC details for the biofilter riser indicate it is a CMP of 24" diameter. However, the
applicant proposes a hanson concrete anti -vortex top in a detail on sheet 27. Will the CMP riser be
able to handle this load safely? Also, how will the concrete trashrack be fixed to the riser? If a
metal anti - vortex /trashrack is used, please update the calculations accordingly. Engineering
review recommends using concrete risers and pipes within permanent stormwater facilities because
they tend to last longer than CMP.
7. The bioretention basin detail on sheet 27 specifies the bed elevation of the facility as 497.40,
though it is 496.40 on the grading plan. Please correct. Please also be sure to label the top of the
riser structure to match the computations.
8. As designed, the forebay appears to pass water too quickly into the main cell of the biofilter
without allowing enough time for suspended solids to settle. Please provide a gravel diaphragm in
the spillway or similar element to provide a greater chance that settling will occur within the
forebay before passing to the main cell of the biofilter.
9. The forebay must be accessible to vehicles. Please make the emergency spillway traversable for a
typical truck.
10. The landscape plan for the biofilter must meet the Virginia Stormwater Management Handbook
standards with regard to the amount of shrubs and trees: 10 plantings required for every 1000sf of
biofilter bed, 2:1 -3:1 shrub to tree ratio, and at least three species each of shrubs and trees. Please
also provide a biofilter-only planting schedule on Sheet 27.
11. The note within the contech system schematic states that the weir plate outfalls into a 24" pipe, but
the detail shows an 18" pipe. Please clarify.
12. On the weir plate detail, please show the 10" low -flow orifice at the bottom of the plate if its invert
does match the invert of the weir plate.
13. Please label the pipes on the contech detail to match the titles within the routing.
14. In the routing, the pipe inverts are all shown at 527.00. Please use the actual inverts based on the
slope of the system.
15. The angle of the pipes in the routing should be in degrees, not slope. For 0.5% pipe slopes, use
0.2865.
16. The post - development curve number calculation for drainage area 5B assumes that 37% of the
watershed is grassed. This appears to be an overestimation of the pervious area. Also, any water
that falls on the stormwater facility stays within the facility and it must be considered impervious
for the routing calculations.
17. Because the emergency spillway is used more frequently than the 100 -year storm, provide an
adequate channel from the spillway to the stream. Use culverts to cross the greenway trail.
18. What is the existing barrel pipe composed of? If it is a CMP, use 0.025 as the manning's
coefficient in the routing. Also, the invert in appears to be 492, rather than 488 as stated in the
routing. Also, 492- 488/60ft is equal to a slope of 6.67 %, not 3.3 %. Please clarify.
19. Using your same inputs, I am encountering different outputs from the biofilter routing. Please
provide the stage- discharge table of the anti -vortex device with your next submittal.
20. Please provide approval letters from each manufacturer of the stormwater systems being proposed
within this plan. It looks like there could easily be less than Ift of space between the top of the
contech system and the surface elevation on the south side of the facility. This aspect should be
specifically referenced in the contech approval letter.
21. A stormwater facility maintenance agreement will need to be recorded for each parcel prior to
plan approval.
22. After all technical comments have been addressed, the applicant must request a SWM bond be
computed. To request a bond, please complete the bond estimate request form.
File: El_swm_PBC _ wpo- 2010 -00073 Great Escapes Movie Theater.doc