HomeMy WebLinkAboutWPO201000057 Review Comments Stormwater Management Plan 2011-01-11� OF AL
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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: Avinity SWM Plan [WP0201000057]
Plan preparer: Scott Collins [scott @collins- engineering.com]
Owner or rep.: Avon Properties, LLC
Plan received date: 9 September 2010
(Rev. 1) 29 November 2010
Date of comments: 26 October 2010
(Rev. 1) 11 January 2011
Reviewer: John P. Diez — Current Development - Engineering Technician
(Rev. 1) Phil Custer
Engineering has reviewed the first revision to the SWM plan associated with the Avinity site plan (WPO-
2010- 00057), received 29 November 2010. The review of the site, road, and esc plans will be provided in
a separate comment letter. The swm plan can be approved after the following comments are addressed:
B. Stormwater Management Plan (WP0201000057).
1. Please provide a completed copy of the standard stormwater maintenance agreement and fee for
recordation for any stormwater management facilities.
(Rev. 1) Comment has not been addressed.
2. SWM sheets show details for proposed Filterras. However, no Filterras are shown on the plans.
Please revise.
(Rev. 1) Filterras have been removed from the plan.
3. Please provide a trash rack on 15" HDPE pipe entering the stormfilter vault
(Rev. 1) Comment has been addressed.
4. Please provide plans and details for the proposed Retention Basin Type III. Please be advised that
Type III basins require an aquatic bench. Please refer to the Virginia Stormwater Management
Handbook.
(Rev. 1) Please remove all references to a Retention Pond from all plan sets. If the project is
delayed long enough that the county is considering calling the SWM bond to construct the
underground facilities, it will be up to the county engineer's discretion to allow a new plan to be
submitted showing a wet pond.
There seems to be some issues in the stormwater routing calculations:
A.) The plan and details for the Contech system shows five (5) 90' long barrels and
two (2) 62' long barrels. However the routing seems to use only 4 of the 5 -90'
long barrels. Please revise.
(Rev. 1) If the applicant wants to include the 62' long headers /manifold in the
routing, use a length of 75ft for the 5 pipes. Currently, the program is double
counting the majority of the volume within the header /manifold. Also, the
inverts of each pipe should vary based on the 0.5% slope of the system. When
the above corrections are made to the facility, there is about a 20 -25%
reduction in the volume of the facility, which will require adjustments to the
routing. This volume reduction will also necessitate the raising of the first
orifice in the weir plate because a portion of the water quality volume will
Engineering Review Comments
Page 2 of 3
escape out the llinch orifice.
B.) The length of the weir (emergency spillway) is incorrect. At the 521.00 elevation,
the weir length must be 8.39', not 10'. In addition the weir discharge must be
changed to "in riser ".
(Rev. I) Comment has been addressed.
C.) The invert elevation for the barrel (Outlet Structure #2) seems to be incorrect.
Please revise.
(Rev. 1) The invert should be 512.75.
D.) The 10" orifice for the weir (emergency spillway) seems to be missing in the
routing calculations.
(Rev. 1) The routing appears to have been modified in a way that this comment
is no longer necessary.
E.) Please provide calculations from Contech showing diameter or depth being used
in the routing calculations.
(Rev. 1) Comment has been addressed.
F.) Please provide a signed manufacturer's approval letter from Contech for the
facilities.
(Rev. 1) Comment has been addressed. However, a new letter will be needed
each time any of the Contech facilities are modified.
G.) If the area used for the time of concentration from post - development drainage area
#IA is excluded will a greater discharge be the result? If yes, please exclude this
area from the analysis.
(Rev. 1) The flow path for the post- development scenario begins in a grass
channel sloped at >IO %, not sheet flow. It appears the minimum time of
concentration of 6min should be used.
H.) (Rev. 1) The stormfilters should be not be listed as "out of riser'; but "dam ".
The way the routing is currently set up, the model assumes the stormfilters are
behind the weir wall.
6. (Rev.]) 3100sf for patios and porches for the IB watershed appears to bean underestimate
after a measuring a few on the plan. Please recalculate this number.
7. (Rev. 1) The pre- development curve number and, therefore, the pre- development discharges are
too high. All pre- development calculations must assume the existing site is in "good"
hydrologic condition. The calculations provided by the applicant assume "poor" condition. By
using the applicant's areas from his calculation table and the CN's for good conditioned brush
and woods, I found a CN of 51.9 for drainage area 1B. This results in 2 and 10 year pre -
development rates of - y1.4cfs and -- 8.5cfs. Below are excerpts from the Virginia's SWM
Regulations (4VAC- 50- 60 -50) and the Virginia Stormwater Management Handbook that speak
to this requirement.
C. For ptuposes of computing nuioff, all pervious lands in the site shall be assumed prior
to development to be iu good condition (if the lands are pastor -es, lawns, or parks), with good
corer (if the lands are - woods), or iAith conservation treatment (if the lands are cultivated);
regardless of conditions existing at the time of computation.
Engineering Review Comments
Page 3 of 3
N'47ien a watersh d is being analyzed to determine the impact of proposed development,
Virginia's stormwater management regulations require the designer to consider all
existing or undeveloped Land to be in Iivdrological4v good condition. This results in lower
existing condition peak nmoff rates - which, in nun, results in greater post-development peak
control_ In most cases, undeveloped laud is Mi good hydrologic condition sunless it has been
altered in some way— Since the goal of most shorts water programs is to reduce the peak flows
8. (Rev.]) According to the pipe calculations on DP -7, the maximum 10 year flow experienced by
the CDS structure is 24cfs, not 15cfs as indicated in the table on SWM -3. Please correct and
provide an updated letter from Contech.
The water quality flow rate for the structure is noticeably low at 3.4cfs (14% of the 10 year
flow). It seems a considerable percentage of the annual rainfall for Albemarle County will
bypass this system. Is there any way this facility can be moved just upstream of the stormfilter
vault or can the structure be increased in diameter?
9. (Rev. 1) There cannot be a sump at the outlet of the sediment basin /swm facility. This will
create a mosquito breeding ground that negatively affects the public's health, safety, and
wellness. Please remedy this condition with this plan.
10. (Rev. 1) Please modify the grading plan so that the watershed shown on SWM -2 at the front of
the site can be achieved.
11. (Rev. 1) To receive an SWM bond estimate, please submit the Bond Estimate Request Form
and a preliminary cost estimate by Contech to the County Engineer after the SWM plan is
approved.
CDE2- swm_PB C_Avinity. doc