HomeMy WebLinkAboutWPO201100071 Review Comments Stormwater Management Plan 2012-05-21ALg�,��
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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- 20 1 1 - 0007 1, RE- Store'n Station SWM Plan
Plan preparer:
Mr. Nat Perkins, PE; NP Engineering
Owner:
Jefferies II, LLC
Owner's Rep.:
Mrs. Jo Higgins; Project Development Limited
Date received:
12 September 2011
(Rev. 1) 14 February 2012
(Rev. 2) 30 April 2012
Date of Comment:
8 November 2011
(Rev. 1) 2 March 2012
(Rev. 2) 21 May 2012
Engineer:
Phil Custer
The second revision to the SWM plan for the RE- Store'n Station (WPO- 20 1 1 - 0007 1), received on 30
April 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 rantank 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.
(Rev. 2) Comment has been addressed. Though, please provide an updated letter after
modifications to the system design, detailed below, are completed.
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 S 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.
(Rev. 2) Comment has been addressed.
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).
(Rev. 2) Comment has been addressed. The county will consider only Filterra 9 as a
storm waterfacility. The cistern is not a storm waterfacility covered by this plan.
5. If removal credit is desired for the cistern, the four pages of the state's Cistern Design Spreadsheet
91
7
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.
(Rev. 2) Filterra 9, provided upstream of the cistern, is providing the water quality treatment for
this watershed. The cistern is not a stormwater facility part of this stormwater management
plan. Comment has been addressed
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.
(Rev. 2) Filterra 9, provided upstream of the cistern, is providing the water quality treatment for
this watershed. The cistern is not a stormwater facility part of this stormwater management
plan. Comment has been addressed.
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.
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 —25 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.
(Rev. 2) The design of the outlet was modified but the outlet profile was not altered
accordingly. Please update the profile of the pipe out of the facility. Please make sure
the pipe between the facility and the manhole has a 0.5% minimum slope on it. Please
also make sure the invert out of the manhole is at least O.Ift below the invert in.
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.
(Rev. 2) Comment has been addressed.
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 aft 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.
(Rev. 2) The grades drawn by the applicant do not match the note requiring at least two
feet of fill over the facility. The pipe stubout with the weir wall and the top portion of
the southern edge of the facility will be exposed. The outlet pipe through the
downstream property is proposed too high in the ground, it cannot be constructed with
Ift of cover that close to the new channel. The grading plan also demonstrates that the
soil between the southern end of the facility and the downstream face of the
embankment does not come close to meeting the standards of VSMH 3.01 especially
considering the design of the swale at the base of the embankment. Please modify the
grading for the swale and the embankment to show the fill required to provide the
necessary cover for the facility while also providing an adequate slope (3:1) on the
downhill side of the embankment. It seems the pipe detention system must shift at least
15ft to the north and/or be lowered to achieve these requirements.
d. The contech plan view detail does not match the layout of the facility on Sheet SW -3.
Please correct.
(Rev. 2) The length of the stubout that contains the weir plate is inconsistent between
the detail and SW -3.
e. Is the line above the 6" orifice in the Orifice Plate Detail a spillway? Ifso, please specify
its length and elevation. If not, please provide more clarification in the detail as to what
the line is.
(Rev. 2) Comment has been addressed.
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 VDOT 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.
(Rev. 2) Comment has been addressed.
g. (Rev. 2) For many of the routing reports, the inputs for the design of the stormwater
facility do not match what is shown in the plan. Please make sure all stormwater
calculations are consistent with the final design of the stormwater facility.
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 speck in all Contech details that VDOT standard ST
or equivalent, must be provided in all risers inter thv f"cility.
(Rev. 2) Comment has not been addressed. The rim elevations on the detail also do not work
with the grades proposed on SW -3.
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.
(Rev. 2) In a meeting with the applicant, I agreed to consider MS -19 addressed if the 2-year
SCS storm was completely routed through the 3" orifice (smallest achievable) because of the
size of the contributing drainage area and size of the existing channel. However, this
requirement was not fulfilled. The applicant's calculation clearly indicates that the 6" orifice is
activated during two year SCS storm. It seems that if the 6" orifice was eliminated, the system
would be approvable.
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.
(Rev. 2) Please provide me with the supplier's price for the underground detention system, so
that a bond can be calculated after the few remaining comments are addressed. A price for the
cistern should be excluded.
File: E2 swm PBC WPO- 2011 -00071 RE- Store'n Station SWM Plan.doc