HomeMy WebLinkAboutWPO201100071 Correspondence 2012-04-30 NP Engineering
April 30, 2012
County of Albemarle
Department of Community Development
401 McIntire Road
Charlottesville, VA 22901
Re: SWM Plan comments
Restore'n Station(WPO-2011-00071)
Mr. Phil Custer:
I am writing to address the Storm Water Management (SWM) Plan comments provided in your
letter dated March 2, 2012 for the Restore'n Station site plan. I have addressed each comment
individually and listed the comment and intended response for clarity.
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.
RESPONSE: The review letter from Filterra is attached.
2. Please provide an approval letter from ACF Environmental that the current layout is
acceptable for the use of the modular Raintank system.
RESPONSE: The SWM structure has been revised and will be provided by Contech. Their
review letter of the design is attached.
(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.
REV 1 RESPONSE: The review letter from Contech is attached.
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.
RESPONSE: The SWM maintenance agreement has been submitted under separate
cover.
(Rev. 1) This has been provided and is currently being processed by Ana Kilmer.
REV 1 RESPONSE: Ok.
680 IVY FARM DRIVE•CHARLOTfESVILLE,VIRGINIA 22901
•PHONE:(434)531-7387•
Restore'n Station-WPO C nents (WPO-2011-00071)
Page 2
April 30, 2012
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.
RESPONSE: The modified simple spreadsheet is attached.
(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 1 RESPONSE: The modified simple spreadsheet for the cistern (catching only the
water from the building roof and large canopy) is attached.
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.
RESPONSE: Removal credit is not desired for the cistern. For information, the design
calculations have been attached.
(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 runofffrom
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 1 RESPONSE: No credit is desired from the cistern. A roof drain filterra has been
added to provide water quality measures for the building roof and large canopy area.
6. Please provide pre-treatment and screening for the drainage system upstream of the
cistern per VA DCR Stormwater Design Specification No. 6.
RESPONSE: The pre-treatment and screening is incorporated into the design of the
system by Contech-please refer to Contech's review letter.
(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 1 RESPONSE: A roof drain filterra has been added to provide water quality measures
for the building roof and large canopy area.
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
680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901
•PHONE:(434)531-7387•
Restore'n Station-WPO C rents (WPO-2011-00071)
Page 3
April 30, 2012
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.
RESPONSE: Previously addressed in your email dated 1 1/16/1 1 from the Virginia
Department of Health.
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.
RESPONSE: No response required.
(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 in direction 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 modem all contech details to
show this stubout at the proper angle.
b. Please modem 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 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 1 RESPONSE: All items have been addressed in this plan revision;specifically:
a. Contech design was changed to add an MH structure;
b. SW-2 was modified to remove reference to raintank;
c. Grading is shown and approved by Contech.
d. Contech detail was revised to match the layout;
e. The line above the 6" orifice is reinforcing. A sharp crested weir has been
added at elevation 676.53 (see detail of system 2).
f. Note was added to provide anchor blocks per VDOT Special Design Drawin
A-73 and MA-73.
9. The control structures for the detention system must be accessible for inspection and
680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901
•PHONE:(434)531-7387•
.
Restore'n Station-WPO iinents (WPO-2011-00071)
Page 4
April 30, 2012
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.
RESPONSE: The storm water detention facility has been revised as provided by Contech.
Inspection and maintenance of this system will be less problematic than the previously
designed system.
(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 sped in all Contech details that VDOT standard ST-1, or equivalent, must
be provided in all risers into the facility.
REV 1 RESPONSE: SW3 was revised to show rim and invert elevations. A note that all
Contech facilities will have a VDOT Standard ST 1 is included.
10. Please use a minimum of 15" diameter pipes in the stormwater facility to decrease the
likelihood of clogging.
RESPONSE: The pipe size has been changed to 15" diameter.
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.
RESPONSE: MS-19 requirements have been addressed through the hydraulic calculations
of the storm water detention facility. It is impossible to gain access to the downstream
channel for analysis.
(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 1 RESPONSE: Detention calculations for the MS-19 requirements are attached
(rational and SCS methods). The calculations include using a 3"orifice and routing each
storm frequency through the SWM facility. It is our opinion that over-detention
requirements are met for the site and downstream channels improvements are not
necessary.
12. To receive a bond estimate, please provide a completed Bond Estimate Request Form to
the County Engineer after all comments have been addressed.
RESPONSE: The bond estimate request is attached as noted above.
(Rev. 1) WPO bond amounts will be calculated after plan approval.
680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901
•PHONE:(434)531-7387•
Restore'n Station-WPO Cui mnents (WPO-201 1-0007 1)
Page 5
April 30, 2012
REV 1 RESPONSE: OK.
I trust that the above comments are adequately addressed for final site plan and WPO
approval. Should you have questions regarding the revisions, please call me at (434) 531-7387.
Sincer ly,
•
Nat Perkins, P.E.
680 IVY FARM DRIVE•CHARLOTTESVILLE,VIRGINIA 22901
•PHONE:(434)531-7387•
Nat Perkins
NP Engineering ,
Ivy Farm Drive
Charlottesville, VA. 22901 fi
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April 23, 2012 A Griming Idea in Stomtwater Filtration.
Plan Review of Filterra®
Re-Store'n Station, Albemarle County, VA
Dear Sirs
Thank you for submitting the revised plans on 23 April 2012 for our review of the Re-
Store'n Station project.
Filterra®structures Fl (8x4),F2(8x4),F3 (6x4),F4 (6x4),F5 (6x4), F6 (6x4), F7 (8x4)
and F8 (8x4)F9(6X4 FTRD)were studied for;
•Planned Filterra®box size
•Filterra®contributing drainage area meeting project's regional sizing specification
•Filterra®invert elevations are higher than effluent invert elevations
• The bypass is lower than the Filterra®elevation(spot elevations)
• The grading pattern encourages cross linear flow and not head-on flow
• The Filterra®outlet drain pipe is sized correctly and exits perpendicular to the wall
•For any conflicting structures such as storm drain pipes below Filterra®
•For most efficient placement of Filterra®units.
•Filterra roof drain inverts meet published requirements
The plan review concluded that the Filterra®structures listed above is sited and sized
appropriately to treat stormwater to our published specifications.
Operational consistency with these specifications is contingent upon the stormwater unit
being installed correctly and according to the plans,as well as regular maintenance being
performed. Installation Help documents will be forwarded to the Buyer at time of order.
The Filterra®Installation,Operation and Maintenance Manual will be made available
upon request.
Yours sincerely
(0)v a:""
Duane Vincent
Engineer Support
Filterra®Bioretention Systems
Manufactured by Americast T:(804)798-6068 A M E R I CAS T
11352 Virginia Precast Road F:(804)798-8400 not just concrete.concrete solutions.
Ashland,VA 23005 E:design@filterra.com www.filterra.com
605 Global Way,Suite 113
Ok. 1/ — � LinthicumMO 21090
CONSTRUCTION PRODUCTS INC. www.contech-cpi.com
Nat Perkins, P.E. April 23rd, 2012
NP Engineering
680 Ivy Farm Drive
Charlottesville,VA 22901
RE: Review of stormwater management design for Re-Store'N Station,Albemarle County, Virginia
The purpose of this letter is to document for Albemarle County and VA DCR our review of the plans and the
proposed application of the water quality treatment structures at this site as you have requested.
CONTECH has reviewed stormwater management designs for both the rainwater harvesting system and the
underground detention pond.
Rainwater Harvesting System
Based on the catchment area of 7,300 sf (gas station canopy) and irrigation requirements, CONTECH
recommends a 72" diameter SPRE cistern with a total length of 741f. This tank will provide a storage volume
of approximately 15,000 gallons and a runoff reduction of 78%. The system is designed to irrigate 18,000 sf
from April to October with an average depth of 0.785"per week(varies by month).
Underground Detention System
Perforated 72" diameter CMP was selected to minimize the overall footprint. The dimensions of the system
are 44'x86' including a 1' stone perimeter. The system has a total of (4) 36" access risers including one
downstream of the outlet control weir plate.
In summary, both the rainwater harvesting system and underground detention system are expected to
operate in accordance with CONTECH Construction Products'design intent.
Please feel free to contact me if you have any questions or concerns.
Sincerely,LL�i
Av
Aaron M. Lowell
Design Engineer—Stormwater Products
CONTECH Construction Products Inc.
lowella@contech-cpi.com
www. contechstormwater. com
commit C'DS The StamweterManagemem YVortechs�
CMP DETENTION s StormFilter CONS SPAN'
DETENTION SYSTEMS
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Short Version BMP Computations For Worksheets 2 -6
Albemarle County Water Protection Ordinance: Modified Simple Method
Plan: Re-Store'n Station Water Resources Area: Rural Area
Preparer. NP Date: 20-Mar-12
Project Drainage Area Designation DA A
L storm pollutant export in pounds, L=[P(Pj)Rv/12][C(A)2.72]
Rv mean runoff coefficient, Rv=0.05+0.009(1)
Pj small storm correction factor,0.9
I percent imperviousness
P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= _1 0.14 I
C pollutant concentration,mg/I or ppm target phosphorus
f factor applied to RR
✓ required treatment volume in cy,0.5"over imperv. area= A(1)43560(0.5/12)/27
RR required removal, L(post)-f x L(pre)
%RR removal efficiency, RR100/L(post)
Impervious Cover Computation(values in feet&square feet) -_
Item
r •1
pre-develoLment _-- Area post-development
(Roads Length Width subtotal Length Width subtotal
y 01 0' 0 0 0 0!{ --- 01 0 0'
0 0 0 `
I
0 , 0
_ 0 0 _ 1 0 p l
0 p1
Driveways v_. Leith width no. subtotal Length Width no. subtotal 1 M^ i
and walks r - 0, 0 01 0 01 0 01 01 i
_
4� _ 0 - 01 0 0 - 0� 0 0F Oj 1
I 0 0 1 01 I 01
_. �
Parking Lots 1 2 3 4 1 2 3 4
I 0' 0
iiGravel areas ---Area Any 1
I 0ix0.70= 0, 0 x0.70= 1 0i
1Structures Area no. subtotal Area no. subtotal I
I 01 0 0 I 2775 1 27751 1
1 -�- 0 0 0 3495.41 1 3495.411
1 0 1 0,
0, 0 0 0 6270.41 i 6270.41
lActively-grazed pasture& Area Area
yards and cultivated turf 6270.36 x 0.08= 501.6288 [ 0 x 0.08= 0
Active crop land Area Area ( I
i _- { ix 025= 0 x 0.25= I 01
Other Impervious Areas
0 0
Impervious Cover 8% 100%
KPre) OW)
'Rv(Post) - V
0.95 9.7 ;
New Development(For Development Areas,existing impervious cover 20%) ,
C f I(pre)* Rv(pre) ' L(pre) L(post) RR %RR j Area Type
0.70 1.00 20% 0.23 I 0.20 0.84 0.64 76% Development Area
0.35 1.00 0% 0.12 1 0.05 0.42 0.37 , 87% Drinking Water Watersheds
0.40 1.00 1% 0.12 i 0.06 0.48 0.42 87%Other Rural Land
min.values
Redevelopment(For Development Areas,existing impervious cover>20%)
C f____1„,,I(pre)* Rv ,re I L(pre) L(post) RR %RR i Area Type
:, ,�a.00 1 1 -�� ., O.Z3 0 20 0.84 0.66 _ 78%' Development Area t
E 3 u" a 0.12 0.05 0.42 0.37 89% Drinking Water Watersheds 1
o' k 1, 0 Y ,J`. y 0.12 i 0.06 I 0.48 0.43 i 89% Other Rural Land
Interim Manual, Page 70 rev.16 Feb 1998 GEB
,
PLEASE PRINT AND FAX OR EMAIL THIS FORM 1-10 v01 FTRD
Filterra® Roofdrain System Project Information Form o
Complete&send to Americast by email,fax or mail.
Address: 11352 Virginia Precast Road Toll Free: (866)349-3458 Fax: (804)798-8400 fi Iterra)
Ashland,VA 23005 Email: design@filterra.com
A Growing Idea to Simmwater Filtration.
Project Information Engineering Contact Information
Project Name/Number: a Engineering Finn: Current Date:
Es zse .'Q s-r›-np,__, IJ'P F ,> e--- .\ 4(z307...-
Regulatory Municipality and State(City,County,etc.): Contact: Phone:
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Target Treatment Rate: Email: Fax:
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Filterra®Details(Email,mail or fax plans to Americast-Acceptable formats are AutoCAD or pdf)
Plans sheets should include(1)cover,(2)grading,(3)drainage areas,(4)stormwater schedules or profile,(5)landscaping&(6)Filterra®details.
Filterra Roofdrain Structure# (^
Filterra Filter Surface Area(ID ft) FTRD 6 X it_Y
Filterra Contributing DrainageArea Acres p,1 It
Runoff Coefficient C Factor 0 q
Maximum Flow Expected cfs Oa\-L(40 _
Influent Pipe Diameter inches 4ofr
Influent Pipe Direction* A,B,C or D ri'
FilterraSpot Elevation TOP of SLAB 7o 7 p 2�
INV IN �jf Q 0--e.0
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Modified Structure Y/N? A-.1 .. ) ) .�/y,, /�
Grate or Top Options Y/N? Al / /�J� LA10✓f PI. 1 hiik OF 0 —7//w a.! e-J /,4 i`JJC 4 e j " m
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MULCH PROVIDED BY AMERICAST PVC PIPE
PERFORATED UNDERDRAIN SYSTEM BY AMERICAST (BY CONTRACTOR)
UNDERDRAIN STONE PROVIDED BY AMERICAST
SECTION A-A
MODIFICATIONS OF DRAWINGS ARE ONLY PERMITTED DRAWING AVAILABLE IN TIF FILE FORMAT.
BY WRITTEN AUTHORIZATION FROM FlLTERRA
DATE: 10-15-10 DWG: FTRD 6x4 6
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PRECAST FILTERRA®
A gJE U L -1 LI® ROOF,DRAIN CONFIGURATION ifi Vtrra
WITH 6" PVC PIPED IN US PAT 6.277.274
Copyright 0 2007 by Amaicact AND 6.569,321