HomeMy WebLinkAboutWPO201100071 Review Comments Stormwater Management Plan 2012-03-02ALg�,��
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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 - 00071, RE- Store'n Station SWM Plan
Plan preparer:
Mr. Nat Perkins, PE; NP Engineering
Owner:
Jefferies IL LLC
Owner's Rep.:
Mrs. Jo Higgins; Project Development Limited
Date received:
12 September 2011
(Rev. 1) 14 February 2012
Date of Comment:
8 November 2011
(Rev. 1) 2 March 2012
Engineer:
Phil Custer
The first revision to the SWM plan for the RE- Store'n Station (WPO- 2011 - 00071), received on 14
February 2012, has been reviewed. The plan can be approved after the following comments have been
addressed:
1. Please provide an approval letter from Filterra that the current layout and grading plan is
acceptable to them. I have concerns with the grading plan not allowing any water to some of the
structures. For instance, the contours are pitched away from the curb that F4 is placed on.
(Rev. 1) Comment has been addressed.
2. Please provide an approval letter from ACF Environmental that the current layout is acceptable for
the use of the modular Raintank system.
(Rev. 1) The raintank system has been replaced in this submittal with a Contech detention
facility and the applicant has provided an approval letter from this supplier. However, changes
to the system are required and an updated letter will be necessary.
3. This application will require that a Stormwater Management Facility Maintenance Agreement be
recorded. Please complete this form and submit it to Ana Kilmer with a $17 recordation fee after
reading the instructions online. Please include all parcels contained in this site plan on this
agreement.
(Rev. 1) This has been provided and is currently being processed by Ana Kilmer.
4. The modified simple spreadsheet must analyze the watersheds for each stormwater quality facility
proposed on site and should not analyze a watershed including offsite water. By doing so, you are
obligating yourself to treating that offsite water to that removal rate, which the current plan is not
doing and is also not desirable. Please update the modified simple spreadsheet for only the
drainage areas to the detention facility and to the cistern.
(Rev. 1) A modified simple spreadsheet has been provided for the areas draining to the filterras.
However, no modified simple spreadsheet has been provided for the cistern (please see the
following comment).
5. If removal credit is desired for the cistern, the four pages of the state's Cistern Design Spreadsheet
in Appendix C should be included in the Stormwater report in full so it can be evaluated. Page 4
is impossible to read and the first three pages are very difficult to read. Please indicate in the
stormwater report what the removal rate of the cistern is and provide justification in the
calculations. If the removal rate of the cistern does not meet the required rate after the modified
simple spreadsheet is amended per comment 4, the cistern must become larger, toilets in the gas
station must use water from the cistern, or another BMP facility (upstream or downstream) must be
provided.
(Rev. 1) Rather than addressing this comment, the applicant has chosen to state that the cistern
has not been provided as a stormwater facility, but no other water quality measure is provided
for the runoff from the canopy and store roof as mentioned in the initial comment. Please
either design this cistern as a water quality measure (in which case you will likely need to
provide year round usage) and provide the calculations referred to in the comments from the
first submittal or provide an appropriate water quality measure above or below this system.
6. Please provide pre- treatment and screening for the drainage system upstream of the cistern per VA
DCR Stormwater Design Specification No. 6.
(Rev. 1) A vortechs unit is referred to in the approval letter, but no details or alignment is
provided in the resubmittal. Details and placement of the system is required if the cistern is
designed as a stormwater quality unit. (Please see the previous comment.) If no other water
quality unit is provided up or downstream of the cistern, the cistern must be designed to meet
the required removal rate.
7. The proximity of the Raintank 2 and one of the drainfields is a concern of the county. I'm not
certain that the two systems are far enough apart not to affect each other and I have contacted the
Virginia Health Department regarding this issue. An impervious liner may be required on this
system to make sure septic effluent does not find its way into the detention system and that the
detention system does not affect the efficiency of the drainfield.
(Rev. 1) The Virginia Department of Health has concluded that the two underground systems
should not affect each other's performance.
8. Detention requirements have been met: the 2 and 10 year discharges for the 1.49 acre post -
development watershed has been reduced to the pre - development rates. For all other projects
submitted to this county, the updated IDF curves found in the Design Manual must be used. For
this project, you do not need to use the updated IDF curves because the routings are acceptable.
(Rev. 1) A new detention system has been provided in this submittal. An independent analysis
of the system has confirmed that detention rates have still been met. Please be aware that
future submittals to the county will require the applicant to model the critical storm duration
for all design storms. Please make the following corrections to the plan which are necessitated
because of the modified design:
a. All changes indirection and slope for pipes require a manhole. Please provide at least
one manhole in the outlet pipe from the detention system. The Contech detention
system shows a 15" stub out at the bulkhead. If this stubout cannot be constructed at
the required -y25 angle, a second manhole will be required. Please either provide this
manhole or modify all contech details to show this stubout at the proper angle.
b. Please modify all Stormwater management notes on sheet SW-2 since they refer to the
Raintank system, which was removed between the first and second submittals. Please
make sure these revised notes include the 3 county general notes for Stormwater
Management plans found in the latest edition of the design manual.
c. In the plan and profile of the system, show the grading required to provide the
minimum cover stated in the notes. It seems that the system could be dropped 3ft rather
and the need for fill would be eliminated. This would also place the outlet of the pipe
closer to the bottom of the stream which would help prevent future erosion at the
headcut. Please also show the proposed grading of the area above the facility, since
existing grades will be destroyed with the sediment basin construction.
d. The contech plan view detail does not match the layout of the facility on Sheet SW-3.
Please correct.
e. Is the line above the 6" orifice in the Orifice Plate Detail a spillway? If so, please
specify its length and elevation. If not, please provide more clarification in the detail as
to what the line is.
f. The slope of the pipe into the detention system has been increased from 15.99% to
18.52 %. The increase in slope above 16% requires that anchor blocks are required on
every other pipe joint. Please refer to VOOT Special Design Drawing A -73 and MA -73
for Anchor Details for Concrete Pipe. Alternatively, the plan can redesign the system
so no slope greater than 16% is proposed.
9. The control structures for the detention system must be accessible for inspection and maintenance.
The current design of the facility has the potential for clogging at the inlet end of the pipe inside
the facility where maintenance would be difficult. The control structure must have a trashrack
placed overtop of it that meets state minimum standards. I also recommend placing some sort of
screening or filter upstream of the detention system to prolong the life of the facility. This last
suggestion is not a requirement.
(Rev. 1) On sheet SW -3, please show all access points to the facility and provide a top elevation
for the rim of each structure. Please also specify in all Contech details that VDOT standard
ST -1, or equivalent, must be provided in all risers into the facility.
10. Please use a minimum of 15" diameter pipes in the stormwater facility to decrease the likelihood
of clogging.
(Rev. 1) Comment has been addressed.
11. An analysis of the downstream channel per Minimum Standard 19 was not provided with the
SWM or ESC plan. Please refer to page 7 (of 35) of the County's Design Manual and the Virginia
Erosion and Sediment Control Handbook for what the county expects for an analysis of
downstream channels. Per state law, this analysis must be performed using the 24 -hour storm.
According to DCR's Technical Bulletin #1, detention does not constitute compliance with MS -19
because total volume of runoff is increased and poses a potential threat to downstream properties.
(Rev. 1) Comment has not been addressed. A downstream channel analysis is required in any
situation where the velocity, peak rate, or volume of runoff is increased in a development.
Normal detention does not eliminate the requirement for the applicant to check downstream
channels because volume is increased unless a significant amount of infiltration is provided. If
an analysis cannot be performed, the applicant must proceed as if an inadequate channel is
present and provide over - detention meeting State Code Section 10.1 -561.
12. To receive a bond estimate, please provide a completed Bond Estimate Request Form to the
County Engineer after all comments have been addressed.
(Rev. 1) WPO bond amounts will be calculated after plan approval.
File: E2 swm PBC WPO- 2011 -00071 RE- Store'n Station SWM Plan.doc