HomeMy WebLinkAboutWPO202000032 Correspondence 2021-04-15SHIMP ENGINEERING, P.C.
Design Focused Engineering
April 14, 2021
David James
County of Albemarle
Department of Community Development
401 McIntire Road, North Wing
Charlottesville, Virginia 22902
RE: Response Letter #1 for (WPO202000032) 1300 Richmond Road — Flow Automotive
Dear David,
Thank you for your review of the VSMP plan for 1300 Richmond Road. This letter contains responses to
County comments dated March 16, 2021. Our responses are as follows:
A. General
1. Comment Addressed.
2. Comment Addressed.
B. 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. Comment Addressed.
2. Comment Addressed
3. Comment Addressed
4. (Rev.3) Complete Section 2 & 11 prior to pre -con.
Section 11 (general permit copy) is included, we cannot have Section 2 (notice of general permit
coverage) until the permit is received. That will be added as soon as it is received.
5. (Rev.3) Submit revised Registration Statement; please:
a. Complete offsite (fill) export section.
Noted, there is no offsite export, site soil work is balanced. Section filled as required.
b. Ensure construction dates are accurate.
Our best estimate for construction added.
c. Ensure form is signed and dated.
Form is signed and dated.
d. Ensure MS4 data is accurate
MS4 listed as Albemarle County, this is accurate.
6. (Rev.3) Sign/date cover seal.
Signed & dated seal provided for SWPPP.
912 E. High Sr. Charlottesville, VA 22902 1434.227.5140 1 shimp-engineering.com
C. Pollution Prevention Plan (PPP)
The PPP content requirements can be found in County Code section 17-404.
1. Comment Addressed.
2. Comment Addressed.
3. Comment Addressed.
D. Stormwater Management Plan (SWMP)
VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a SWMP.
This plan is disapproved, and the reasons are provided in the comments below. The stormwater
management plan content requirements can be found in County Code section 17-403.
1. Comment Addressed.
2. Comment Addressed.
3. Sheet 10:
a. Comment Addressed.
b. Comment Addressed.
c. Comment Addressed.
d. Comment Addressed.
e. Comment Addressed.
f. Comment Addressed.
4. Detention basin:
a. Rev.3) Not addressed. Align channel centerline to be more perpendicular with the
contour of basin or Provide a state channel design standard & liner that meets what your
proposing. Embankment height is currently 346-340=6' high (see C 10 profile); not the
height of fill. Therefore, embankment standards apply.
Channel has been aligned to be more perpendicular. The current alignment represents a
balance between ditch slope and angle of alignment — a more perpendicular alignment
would have a steeper slope. The channel is existing rip -rap, but the new portion where it
outlets into the pond has now been designed and included with the other outlet protection
design on Sheet C11, as well as on the plan sheet C4.
b. Comment Addressed.
c. Comment Addressed.
d. Comment Addressed.
e. (Rev.1/2) Provide at least 2' freeboard from the 100-yr WSE to top of dam or l' with a
vegetative spillway if design allows.
1' freeboard with vegetated (grassed) spillway provided.
f. (Rev.1/2) Provide seepage control: design anti -seep collars for barrel. (Rev.3) Not
currently addressed: Provide something from DEQ that will allow this exception based
upon the corrugations of the pipe used. Show the 4:1 paretic line from the riser weir.
For seepage control, we have specified a concrete cradle (refer to plans/profile). US Army
Corps of Engineers no longer recommends anti -seep collars. See sheet from current
embankment spec. which is attached to this comment letter. Please note, the anti -seep
design we have provided is a conservative measure, as water in the pond will draw down
quite quickly due to its size and the 8" orifice at the bottom of the standpipe. As such, the
creation of a phreatic surface which could cause seepage is unlikely. If any seepage were to
occur, it would happen near the principle spillway pipe. The concrete cradle around this
pipe will provide adequate seepage control. Finally, we have specified the extra -robust
912 E. High St. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom
ADS HP -Storm for the principle spillway pipe. HP -Storm has superior performance to the
typical N-12 HDPE pipe, and better performing pipe joints. We are confident that this
principle spillway design protects against seepage per best engineering practice.
g. (Rev.1/2) Provide backslope no steeper than 3:1 on dam. (Rev.3) Not addressed. See
comment 4.a.
3:1 backslope provided.
h. (Rev.3) Embankment width should be 8' min. See comment 4.a.
As we agreed in email correspondence, we will provide all other requirements for the
typical embankment spec except for the 8' minimum width. This is because the
embankment height is less than 3', and because increasing the width would cause fill in
the floodplain, which is not allowed, and because the additional 2' width would not
provide a noticeable improvement to design due to the small amount of fill placement.
5. Comment Addressed.
6. Comment Addressed.
7. Comment Addressed.
8. Contact Ana Kilmer regarding the Nutrient Credit Agreement. Prior to DEQ permit issuance.
(Rev.1/2/3) Comment still valid. Nutrient credit purchase is needed prior to plan approval.
Purchase has been initialized, unsigned agreement and affidavit is included for reference. We will
send the executed documents via email as soon as received.
9. Record the SWM maintenance agreement. - Prior to approval (Rev.1/2/3) Comment still valid.
Acknowledged. Unsigned maintenance agreement attached for final review.
10. Comment Addressed.
11. (Rev.3) Record the SWM facility easements with a deed & plat prior to approval.
Easement plat with deed have been submitted (SUB 202100053). We understand based on the
review timeline, this will be the last item required for approval.
E. Erosion and Sediment Control Plan (ESCP)
Virginia Code Sec. 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 plan content
requirements can be found in County Code section 17-402.
1.
Comment Addressed.
2.
Comment Addressed.
3.
Comment Addressed.
4.
Comment Addressed.
5.
Comment Addressed.
6.
Comment Addressed.
7.
Comment Addressed.
8.
Comment Addressed.
9.
Comment Addressed.
10.
Comment Addressed.
912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom
If you have any questions or concerns about these revisions, please feel free to contact me at
keane@shimp-en ing eering corn or by phone at 434-299-9843.
Regards,
Keane Rucker, EIT
Shimp Engineering, P.C.
912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom
VA DEQ STOFMNWATER DESIGN SPECIFICATION INTRODUCTION- APPENDIX B- PRINCIPAL SPILLWAY
for pipes 24 inches or less in diameter. Larger pipes require 23-inch wade bands with 24inch
wide flat gaskets and four "rod and li type connectors. Flanged pipe with gaskets is also
permitted. Refer to the Construction Specifications in this standard for more information -
All pipe gaskets should be properly lubricated with the material provided by the pipe
manufacturer. Use of an incorrect hibricant may cause deterioration of gasket material.
Conduit Piping and Seepage Control — Seepage or piping along a pipe conduit. which extends
through an embankment. should be controlled by use of one of the following: (1) anti -seep
collars. as shown in Figure B-2. or (2) filter or drainage diaphragms as shown in Figure B-3.
Concrete cradles. as discussed in item 3 below. may also be used.
Seepage control will not be required on pipes less than 6 inches in diameter.
1. Ann -Seep Coffars: Tbese collars lengthen the percolation path along the conduit.
subsequently reducing the exit gradient. which helps to reduce the potential for piping. While
this works well in theory, the required quality of compaction around the collars is very difficuh
to achieve in the field.
The Bureau of Reclamation. the U.S. Army Caps of Engineers. and the USDA -Natural
Resource Conservation Service no longer recommend the use of anti -seep collars. The U.S.
Department of the Interior -Bureau of Reclamation issued Technical lfemorundum Mo. 9 in 1987
that states:
Wren a conduit is selected for a waterway through an earth or roddill
embankment cutoff collars will not be selected as the seepage control measure."
Alternative measures have been developed and used in the designs of maior structures. These
measures include graded filters or filter diaphragms, and drainage blankets. These devices are
not only less complicated and more cost-effective to construct than the cutoff collars, but also
allow for easier placement of the embankment fill.
Designers and engineers. however. continue to use anti -seep collars as the sole method of
seepage control for small dams. This may be due to the complexity of the design procedure for
graded filters. It may also be due to the designei s concern that little engineering supervision
and or inspection will occur during construction- whuch is generally necessary for the successful
installation of graded filters.
Anti -seep collars. when used. should be installed around all conduits through earth fills
according to the following criteria:
a Enough collars should be placed to inctease the seepage length along the conduit by a
nun u ium of 15%. This percentage is based on the length of pipe in the saturation zone.
b. The assumed normal saturation zone should be determined by projecting a line through the
embankment. with a 411:IV slope. from the point where the aortal water elevation meets the
upstream slope to a point where it intersects the invert of the conduit. This line, referred to as
Introdu on Appendix B: Principal Spillways 6 d 26 Version 1.0, March 1. 2011
912 E. High St. Charlottesville, VA 22902 1434.227.51401 shimp-engineenng.rom
912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom