HomeMy WebLinkAboutWPO201800077 Correspondence 2022-06-15•����01
0 608 Preston Avenue
P 434.295.5624
Suite 200 F 434.295.1800
T I M M O N S GROUP
Charlottesville, VA 22903 www.timmons.com
June 14, 2022
Emily Cox
County of Albemarle
Community Development
401 McIntire Rd, North Wing
Charlottesville, VA 22902
RE: Old Trail Block 32 — VSMP Permit Plan Review — WP0201800077 Amendment 3 -
Comment Response Letter
Dear Ms. Cox:
We have reviewed all of your comments from May 16, 2022 and made the necessary revisions.
Please find our responses to the comments below in bold lettering.
A. Stormwater Pollution Prevention Plan (SWPPP)
The SWPPP content requirements can be found in County Code section 17-405. A SWPPP
must contain (1) a PPP, (2) an ESCP, (3) a SWMP, and (4) any TMDL measures necessary.
1. Please insert amendment into SWPPP book once it is approved.
Acknowledged.
B. Pollution Prevention Plan (PPP)
The PPP content requirements can be found in County Code section 17-404.
1. No changes to PPP.
Acknowledged.
C. Stormwater Management Plan (SWMP)
VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a
SWMP. This plan is denied, and the reasons are provided in the comments below. The
stormwater management plan content requirements can be found in County Code section 17-
403.
ENGINEERING I DESIGN I TECHNOLOGY
1. Ensure all criteria for the grass channel from table 3.2 are met, such as: The maximum
flow velocity of the channel must be less than 1 foot per second during a 1-inch storm
event.
The one-year runoff into the proposed grass channel is 5.41 cfs, which produces a
velocity of 0.97 ft/s. The calculation has been added adjacent to the SWIM A — Grass
channel detail.
2. For the image below: 1) It must outfall to a channel and 2) please specify the rip -rap
design.
After review, it makes more sense to outfall and daylight the 12" pvc into the
extended detention pond. Outlet design has been included to accommodate the 4.55
cfs inflow to the pond with 6" diameter cobble at 1' deep as designed by Stantec for
biofilter inflows in the swm master plan.
3. Ensure filter media mix specified on the plan matches the approved DEQ design from
2011 specifications: 6.6 General Filter Media Composition. The recommended
bioretention soil mixture is generally classified as a loamy sand on the USDA Texture
Triangle, with the following composition: o 85%to 88% sand; o 8%to 12% soil fines; and
o 3% to 5% organic matter.
Filter media composition information from the latest 2013 DEQ specifications has
been added on sheet C2.0 under the bioretention media specifications.
4. Both the bioretention and grass channel specifications require knowing the water table.
Do you have that information? It should be shown or noted on the plan/design.
The soil located in the biofilter and grass channel location is classified as 4E Ashe loam
and has a depth to the water table of more than 80 inches. The most restrictive design
of both swm measures would be the excavated material to the bottom of filter media.
The recommended separation distance between the water table and media is 2'.
Given the bioretention media will be a depth of 4' maximum, this leaves more than 2'
of separation to the water table and should pose no issues with intersecting the filter
bed and causing potential groundwater contamination within the facility. The grass
channel has a depth of 1.5' and is significantly higher than the water table. A note has
been added to sheet C2.0 adjacent to the suggested annual maintenance for
bioretention.
5. Provide a specific landscaping planting plan for the bioretention.
The original plantings from the approved landscape plan have been shown on sheet
C2.1 with updated quantities due to the reduced footprint.
6. Provide this checklist on the plan:
httos://www.aIbema rle.org/home/showpublisheddocument/212/63720232750537000
0.
Checklist has been provided on the cover sheet of the plans.
7. Please show the calculations used for Lremoved. What efficiency was used?
Calculations are shown on sheet C2.1. The efficiencies used remain identical to the
original approved plan with the level II biofilter at 90% and the extended detention at
25%. The addition to this plan only includes the grass channel with an efficiency of
32% (A/B soils) as found in VA DEQ spec no. 3 snippet below.
5FA:11UiN Z: 1'!':KP11KrdA1 UJL
Table 3.1. Summary of Stormwater Functions Provided by Grass Channels 1
HSG Soils A and B
HSG Soils C and D
Stormwater Function
No CA 2
With CA
No CA
With CA
Annual Runoff Volume Reduction (RR)
20%
NA 3
10%
20%
Total Phosphorus (TP) EMC
Reduction4 by BMP Treatment
15%
15%
Process
Total Phosphorus (TP) Mass Load
32%
24% (no CA) to
Removal
32% with CA
Total Nitrogen (TN) EMC Reduction4
by BMP Treatment Process
20%
20%
Total Nitrogen (TN) Mass Load
36%
28% (no CA) to
Removal
36% with CA
Partial.
• Use VRRM Compliance spreadsheet to calculate a
Curve Number (CN) adjustments, OR
Channel & Flood Protection
• Design extra storage in the stone underdrain layer and
peak rate control structure (optional, as needed) to
accommodate detention of larger storm volumes.
C1h'P and CSN (2008) and CWP (2007)_
8. Please update the master OLD Trail tracking spreadsheet. Since drainage area has not
changed, it may not be affected. If so, please provide a statement explaining the change
or no change.
This is correct. Due to the net zero change in drainage area to the swm facilities, the
tracking spreadsheet will remain the same for the approval of this facility. The basis of
the master plan is dependent on the captured drainage and this facility ultimately
captures the same amount of drainage as the original approved plan.
D. Erosion and Sediment Control Plan (ESCP)
Virginia Code §62.1-44.15:55 requires the VESCP authority to approve or disapprove an ESCP.
This plan is disapproved, and the reasons are provided in the comments below. The erosion
control plan content requirements can be found in County Code section 17-402.
1. Please include any changes necessary to the US Plan such as: sequence of construction,
protecting the SWM facilities, outlet protection, etc.
An updated sequence of construction and notes for the proposed amendment have
been added to the cover sheet.
We have included PDF copies of the plans and calculations for your review. If you have any
questions or comments, please feel free to give me a call at 434.295.5624 or email at
aeremy.fox@timmons.com .
Sincerely,
Jeremy Fox, PE
Project Engineer