HomeMy WebLinkAboutWPO200800041 Review Comments Erosion Control Plan 2008-05-28ALg�,��
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road, Room 227
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: SDP - 2007 - 00105, Brownsville Elementary School, Major Amendment
Plan preparer: Scott Collins, Collins Engineering [email: scott @collins -en ing eering com]
Owner or rep.: Joseph Werres, County of Albemarle [email: jwerres @albemarle.orc]
Date received: 09 Oct 2007
Revl: 09 Jan 2008
Rev2: 11 Apr 2008
Rev3: 13 May 2008
Date of Comment: 10 Oct 2007
Revl: 08 Feb 2008
Rev2: 25 Apr 2008
Rev3: 28 May 2008
Engineer: Allan Schuck
Revl: Max Greene
Rev2, 3: Jonathan Sharp
A. Final site plan comments:
All comments have been addressed.
B. Stormwater Management Plans
1. The 48" manhole (Structure 22) should be sized larger because of the 42" pipe running to it.
This may have an affect on the outlet structure weir dimensions.
Rev]: The 12" diameter pipe must be upsized to a 15" minimum diameter. Also, on sheet 4,
one of the raintank details still specifies a 42 "pipe.
2. Please label the elevations of the orifice and weir, and label the weir length and location in the
manhole (is it centered in the manhole ?).
Rev]: comments addressed.
3. The low flow orifice should specify a trash rack.
Rev]: Please remove the trash rack from the invert in of the 12 "pipe. There would be no
way to access the trash rack for maintenance.
4. Please specify the critical durations used for the 2, 10, and 100 year routings.
Rev]: It appears that the critical durations are always set to the time of concentration. It
appears that the actual critical durations for the 2, 10, and 100 year storm occur before 12
minutes. My calculations came to 3 min, 2 min, and I min respectively. Please specify the
critical duration (duration of peak) in the calculation package.
5. The raintank detail specifies a cover of 2 feet required over the facility, when only 1 foot is
provided at the north -east corner of the facility.
Rev1: comments addressed.
6. The control structure information in the calculation package does not match the plans.
Rev]: Control structure information has not been provided in the latest calculation
package. Please provide the hydraulic dimensions and coefficients for each weir, orifice,
culvert or other control structure used in the routing model, matching plans. Is the facility
sized for future development? It appears that the facility could be reduced by 113 or more
and still be able to provide adequate detention and freeboard. I would double check your
calculations for the WSE of the 100 year storm; it appears to be overly high. Also, the 100
year storm does not need to pass through the facility, because detaining the 100 year storm
is not required.
A SWM bond is not computed for County projects.
C. Erosion and Sediment Control Plans
8. Please provide the Albemarle County Erosion Control Notes, verbatim (provided following
comments).
Rev]: comments addressed.
9. Please show the outlet protection symbol at the outlet of the SWM facility.
Rev]: comments addressed.
10. The drainage area map going to the facility appears to be wrong in some areas. Drainage may
exceed 3 acres; a basin may be necessary.
Rev]: The following comments are in regards to the Sediment Basin:
a. There is not enough freeboard for the principle spillway. Without an emergency
spillway, there should be 3 feet between the principle spillway and top of dam.
b. The head provided for the principle spillway is incorrect. The head should be measured
from the principle spillway to the design highwater elevation. The head is 0.48 feet, not
the 2.5 feet specified. As a result, the principle spillway is not adequate.
c. Baffle calculations are incorrect. It appears that baffles will be required.
d. Orifice calculations are incorrect. It appears that the orifice should be sized at 3" for
adequate drawdown time.
11. The wet storage for the trap cannot exceed 4 feet in depth.
Rev]: comments addressed.
12. Please provide an adequate channel analysis for the swale downstream of the facility running
to the buffered stream.
Rev]: comments not addressed. I can find nothing included with the submittal that shows
that MS -19 has been met.
13. An ESC bond is not con,, ated for County projects.