HomeMy WebLinkAboutWPO201100037 Review Comments Stormwater Management Plan 2011-11-10ALg�,��
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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- 2011 - 00037, Glenmore -K2B ESC and SWM Plan
Plan preparer:
Mr. Scott Collins, PE; Collins Engineering
Owner or rep.:
Glenmore Associates L.P.
Date received:
14 June 2011
(Rev. 1) 8 August 2011
(Rev. 2) 26 September 2011
Date of Comment:
29 June 2011
(Rev. 1) 15 September 2011
(Rev. 2) 10 November 2011
Engineer:
Phil Custer
The ESC and SWM plans for Glenmore -K2B, received on 26 September 2011, have been reviewed. The
plans cannot be approved as submitted and will require the following changes.
1. The 30 acre active construction limit will be monitored and enforced by the on -site ESC inspector
in conjunction with the County Engineer. Because of this, agreements in lieu of ESC plans will
not be accepted for lot construction. The current plan will add 3.43acres towards that 30acre limit.
(Rev. 1) The current plan adds 15.03 acres to the 30 acre limit. The perimeter road plan
disturbed approximately 7 acres.
(Rev. 2) The current plan adds 15.58 acres to the 30 acre limit. The perimeter road plan
disturbed approximately 7 acres.
2. The existing site is mostly wooded, not open grass field as described in Existing Condition section
of the narrative.
(Rev. 1) Comment has been addressed.
3. The limits of disturbance does not match up with the construction of a portion of the sanitary
sewer line.
(Rev. 1) Comment has been addressed.
4. Silt fence must not be used across contours because it will act as a diversion. This standard is
broken adjacent to both the water and the sewer line.
(Rev. 1) Comment has been addressed.
5. Construction entrances must drain to a sediment trapping measure. [VESCH 3.02]
(Rev. 1) Comment has been addressed.
6. Please remove the silt fence from the stream.
(Rev. 1) Comment has been addressed.
7. Please provide a Utility Stream Crossing symbol at the sewer line's crossing of the intermittent
stream.
(Rev. 1) Please specify the diversion channel dimensions and lining as well as the culvert size.
(Rev. 2) Comment has been addressed.
Please detail how construction equipment will cross the intermittent stream. Please provide a
Stream Crossing symbol for this method.
(Rev. 1) Please see the previous comment.
9. Please provide Army Corps and DEQ approval of the stream work referenced in the previous two
comments.
(Rev. 1) Comment has been acknowledged by the applicant.
(Rev. 2) Comment remains unchanged.
10. The ESC bond will be computed after the plan has been approved and the applicant has submitted
a Bond Estimate Request Form to the County Engineer.
(Rev. 1) Comment remains unchanged.
(Rev. 2) Comment remains unchanged.
The ESC plan has been considerably changed and expanded since the first submittal. Therefore, additional
comments are necessary.
11. (Rev. 1) The diversions dikes shown on this plan must remain in place to direct runoff to the
stormwater facilities. These must be called out as diversions and designed as permanent channels
with 2 -year velocity and 10 -year capacity calculations.
(Rev. 2) Comment has been addressed.
12. (Rev. 1) Please accurately show the existing travelways west of lot 1. This set appears to show the
future alignment of Carroll Creek Drive which has not been approved or reviewed. Please also
show accurately where the existing waterline ends and where the construction of the new line
begins.
(Rev. 2) Comment has been addressed.
13. (Rev. 1) The current plan proposes fill within the floodplain. Please pull all fill out of the
floodplain or get a Special Use Permit.
(Rev. 2) Comment has not been addressed. The ESC Plan shows fill in the floodplain.
14. (Rev. 1) The plan does not appear to address MS -19 concerns below SB -1, and SB- 21SB -3.
Please refer to page 7 of 35 of the current Design Standards Manual for what is expected for a
MS -19 analysis.
(Rev. 2) Adequate channel concerns have been addressed for cross - sections 3 -9. From the
outlet of facility 3 to the headcut of the existing intermittent stream, it does not appear an
adequate channel exists. The increase infrequency of concentrated discharge will likely cause
erosion in this reach since the root systems and trunks along the cross - section will create
trouble spots. From the outlet of facility 3 to the headcut please construct an appropriately
sized rip- rapped channel, since grass will be difficult to establish in the forest. At cross - section
10, the channel disappears into the floodplain and appears to be stable in the current condition.
This will likely be okay even considering the increase in water this reach will see on an annual
basis.
MS -19 concerns have not been addressed for the outfall of SWM facility 1. Most of the design
storms will exit the BMP through the spillway (e.g. 80% for the 2-year storm), not the riser
structure which has a level spreader at the base of the slope. Please redesign the outlet to this
facility in light of the downstream erosion concerns. Also, an 11.5ft wide level spreader for
almost 10cfs will likely not operate as effectively as intended. A larger level spreader will be
needed if current discharge rates are maintained.
15. (Rev. 1) Please provide dewatering orifice calculations. It looks as though the orifices may be
larger than should be permitted.
(Rev. 2) Dewatering orifices have been modified.
16. (Rev. 1) Please show an adequate channel between lots 8 and 9 to sediment basinlbiofilter 2.
(Rev. 2) Please show a culvert at the entrance to the stormwater management access path.
Please also provide culvert calculations on the 12 "pipe and make sure a proper sump is called
out at the inlet to make sure water does not continue downhill, but is directed to the stormwater
facility.
17. (Rev. 1) Please show an adequate channel between lots 12 and 13 to sediment basin /biofilter 3.
(Rev. 2) Comm —fit has been addressed.
18. (Rev. 1) On sheet 11, please provide a note stating that all existing biofilters must remain in place
for a long as possible and that their removal may only be authorized by a county inspector.
(Rev. 2) Comment has not been addressed. Between submittals, the sheet set was reconfigured.
The sheet that the note is now needed on is Sheet 13.
The first resubmittal of this application included the SWM plan associated with final plats SUB -2011-
00083 and SUB - 2011 - 00056. The following comments must be addressed prior to plan approval.
1. (Rev. 1) On sheet 13, please clearly show the channels behind all lots directing water to the new
stormwater facilities. These channels are imperative to this stormwater plan.
(Rev. 2) Comment has been addressed.
2. (Rev. 1) Please provide the county's modified simple spreadsheet for each stormwater.facility.
(Rev. 2) Comment has been addressed.
3. (Rev. 1) Stormwater detention does not appear to be addressed with this plan. Please provide
detention calculations with the next submittal for each of the stormwater facilities. [17 -314]
(Rev. 2) Detention calculations have been provided. However, the following issues were
discovered:
a. Pre - Development target rates must be based off a curve number of 70, not 74. The site
is composed of C (or D) soils and is almost entirely forested. Also, brush has a curve
number of 65, not 77, in C soil.
b. The top of the riser for basin 3 in the detail does not match the routing.
c. The emergency spillway of basin I does not match between the routing and the detail.
4. (Rev. 1) During the 100 -year storm, one foot of freeboard must be provided on all of the
stormwater facility embankments. [VSMH 3.01]
(Rev. 2) All routings show the proper freeboard. Please maintain this freeboard with the update
to the calculations required by the previous comment and ESC Comment 14.
5. (Rev. 1) Sheet 13 indicates that lot 6 is directed to stormwater facility 2, but it appears that the
back half of this lot, if not all of it, will not be captured because there is no diversion ditch.
(Rev. 2) Comment has been addressed.
6. (Rev. 1) Please see ESC comment 13.
(Rev. 2) No fill is shown in the SWM plans.
7. (Rev. 1) Please provide a stormwater access path to biofilter 3. For all stormwater access paths
steeper than 10 01c, the path must be graveled or equivalent.
(Rev. 2) Comment has been addressed.
8. (Rev. 1) The risers for each facility should be designed in such a way it works for both conditions
(ESC and SWM). Right now, the plan requires the risers to be raised by 2.03ft, 2.02ft, and 2.14ft
for basins 1, 2, and 3 which seems impractical.
(Rev. 2) Comment has been addressed.
9. (Rev. 1) On sheet 13, a note references a concrete spillway but all stormwater facilities do not
appear to have any spillway at all.
(Rev. 2) Spillway widths do not match between plan view and the details for facilities 2 and 3.
Though, please see the following comment.
10. (Rev. 2) The applicant has changed all proposed stormwater BMP from biofilters to extended
detention facilities. Each extended detention facility was designed with a I" orifice because of
the calculations. The Virginia Stormwater Management Handbook states that 3" should be the
smallest orifice used because of potential of clogging. The county will grant a variance for the
use of 3" orifices if safety improvements are incorporated into the design of each facility.
Before submitting the complete plan, please forward proposed safety improvement to county
engineering for conceptual signoff.
Also, the earthern forebays with the wide concrete spillways appear burdensome and will likely
not be effective. County Engineering recommends the use of gabion baskets with smaller stone
on the uphill side to allow more time for the settling of the incoming water.
Lastly, all details of the dewatering orifices are for enhanced, extended - detention facilities
where a permanent pool exists. Please update all details to remove the inverted pipe and
provide more detail on the proposed trashracks (size of pipe, size of perforations, frequency of
perforations, solid endcap, etc.).
11. (Rev. 2) Please move the emergency spillway for Stormwater Facility #2 to existing grade. The
spillway cannot be placed over the center of the embankment.
File: E3 esc swm PBC WPO -201 1 -00037 Glemore K2B.do