HomeMy WebLinkAboutWPO202200020 Correspondence 2023-02-23 (3)SHIMP ENGINEERING, P.C.
Design Focused Engineering
February 21, 2023
Mr. John Anderson
Albemarle County
Department of Community Development
401 McIntire Road
Charlottesville, VA 22902
RE: Response Letter 2 for WPO202200020 South Pantops - Parcel 20R
Review Comments
Dear John Anderson,
Thank you for your review of the WPO South Pantops — Parcel 20R. We have done our best to satisfy all
comments dated 07 February 2023. Please see below for a detailed responses to each of your comments:
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. Registration Statement:
a. Sec.1 B. — Check box (Rev. 1) Addressed.
b. Sec.11 F. — Change to Albemarle County (Rev. 1) Addressed.
c. Sec.11 H. — Check box (Rev. 1) Addressed.
d. Sec. III — Complete prior to permit issuance
(Rev. 1) Persists. Information required prior to Grading Permit issuance.
RESPONSE: We analyzed the grading on the site and realize the cut soil in front of
the basement would be sufficient to generate filled soil behind the retaining wall. Not
outside as support needed.
e. Sec. V — Sign & date (Rev. 1) Addressed.
2. Complete Section 9. Signed Certification prior to plan approval. (Rev. 1) Addressed.
3. Complete other sections prior to pre -con. (Rev. 1) Persists. Sec. 6.E. /Sec. 8, require named
individual be listed: an individual responsible for PPP (6.E.) and for compliance inspections (Sec.
8) prior to state VAR10 permit registration, or Grading Permit issuance.
RESPONSE: Noted, these are not yet known, repeat comment expected.
4. (Rev. 1) New: Revise Sec. 4/5, once ESC and SWM plan sheets revised, reviewed, and
approved
RESPONSE: Noted, this will be updated as needed.
B. Pollution Prevention Plan (PPP)
The PPP content requirements can be found in County Code section 17-404.
1. Appears complete; revise as needed. (Rev. 1) May persist.
RESPONSE: No revision needed for this submittal.
912 E. High 5t. Charlottesville, VA 229021434.227.5140 1 shimp-engineering.com
C. Stormwater Management Plan (SWMP)
VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a SINMP.
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.
1. Show the Forest & Open Space easement(s) areas to be protected. According to VRRM
Engineering Review Comments Page 2 of 7 4.13ac F/OS protected. (Rev. 1) Partially
addressed. Pls. see item 4.g.i.3., below.
RESPONSE: Noted, See response below.
2. A swm narrative will need to be included on the plans. (Rev. 1) Partially addressed. As
follow up: Once calculations revised, revise narrative as needed.
RESPONSE: Narrative revised.
3. 1 would prefer that information on the type and location of stormwater discharges,
predevelopment and postdevelopment drainage areas be shown on the plans rather than
separately in calc. packet. (Rev. 1) Addressed.
4. Calculations packet:
A. Pg. 16 — Revise title of map to VOSTDEV....". (Rev. 1) Addressed.
B. Your pre- & post- development drainage area totals should match. (Rev. 1) Addressed. As
follow-up, see item 4.e.iii. below.
RESPONSE: For this project, the onsite area routed to the outfall has been increased,
so area different.
C. It's not clear from the maps how you are reducing ottsite impervious area (1S/8S)
postdevelopment. (Rev. 1) NA given need for design /calc revision. Ref. item 4.e.iii., below.
RESPONSE: No longer applicable.
D. Label/show offsite & onsite drainage areas. (Rev. 1) Addressed.
E. Pg. 24 —
i. Please cite the page numbers. (Rev. 1) Addressed.
ii. Clarify why you are subtracting Max Qdev from the Qpredev? Max Qdev determined
(0.15cfs) is less than the Achieved Reduction (0.89cfs). (Rev. 1) Withdrawn. Applicant
response, SE Letter d. 1/11/23: 'This analysis now uses the DEQ Offsite run-on EBE
analysis, so this comment is no longer applicable. Note: Limits of Disturbance area has
been added, because we got the new survey topo area. Also note, we have utilized
permeable paver driveways to help achieve compliance with the energy balance
equation.' As follow-up: See comments elsewhere concerning EB equation (4.e.iii, next
item), and permeable pavers (item 4.g.i.).
RESPONSE: No longer applicable
iii. Use the Energy Balance (offsite) equation. Consult the Plan reviewer SWM participant
guide. (Rev. 1) Not addressed. There is fundamental error in calculation /SWM design
strategy:
1. Pg. 22 indicates subcatchment 2S: P--` ^A " 11 'nn A- ` "RRM runoff=2.23cfs, with Tc
=5.0 min.
RESPONSE: Noted, SWM strategy revised, no longer relevant.
912 E. High St. Charlottesville, VA 22902 1434.227.51401 chimp-engineering.com
2. Pg. 22 indicates subcatchment 1S: Post DA offsite (51.540 Ac.) runoff=29.47cfs, Tc =31.2 min.
RESPONSE: Noted, SWM strategy revised, no longer relevant.
3. This development cannot combine development discharge with bypass discharge, and model
merged flow as post -developed flow at the point development runoff exits concrete ditch
/flume. The bypass flow is independent of South Pantops Drive Parcel 20R development.
RESPONSE: Noted, SWM strategy revised.
4. 2/1/23 site photos, calc. packet, and plan indicate post -development discharge is to an
existing concrete flume.
RESPONSE: Noted, yes this is correct.
5. At point concrete flume discharges to natural stream, which is at plunge pool for 48" DIA
concrete culvert beneath S. Pantops Drive, 9VAC25-870-B(3) energy balance applies.
RESPONSE: Noted.
6. Calc. packet, p. 22, indicates post-dev Q peak 1-yr undetained,=2.23cfs.
RESPONSE: Noted. SWM strategy Revised as requested.
7. Apply Energy balance to 1.10 Ac./2.03 Ac. (Ditch DA), using Rv values (pre -post) and Q 1 pre,
site, = 0.87 cfs, which yields Q 1 post, site, = 0.285cfs.
RESPONSE: Noted. SWM strategy Revised as requested.
8. Revise design to meet Energy balance for the 1.10 Ac./2.03 Ac. DA development at point post-
dev. discharge at base of concrete flume discharges to natural stream, coincident with 48" DIA
concrete pipe.
RESPONSE: Noted. SWM strategy Revised as requested.
9. Remove reference to 51.540 Ac. off -site DA when calculating EB for this development) from
plan, narrative, and calc. packet.
RESPONSE: Noted. SWM strategy Revised as requested. Large offsite area, which is not
conveyed through the site, no longer analyzed.
10. 51.540 Ac. enters flood protection evaluation since at point bypass and development runoff
merge at 48" pipe outfall/natural stream plunge pool, site DA has not reached 1% of overall
watershed DA (110 Ac.), but is only 52.640 Ac. at that location. Flood protection evaluation
`inues to Rivanna River me -I floodplain.
RESPONSE: Correct, however 9VAC25-870-66-C(3)C applies discharge to (Mapped
Floodplain) so no 10yr reduction is required.
11. In this instance, strategy of meeting EB by offsetting hydrographs is impermissible. To
reiterate, provide channel protection evaluation at Point Al, top of concrete flume, per
9VAC25-870-66-B(1) /manmade channel, and per 9VAC25-870-66-B(3)/energy balance, at the
bottom, at the plunge pool. Ref. representative 2/1/23 photos, below:
RESPONSE: Noted, we agree in this project, because this =51.54 offsite runoff does not
pass through the site/ the limits of disturbance.
912 E. High St. Charlottesville, VA 22902 1434.227.51401 chimp -engineering (
12. Last: In this case, bypass flow is simply a 'receiving stream.' Development discharge must
meet channel protection at top (manmade) and bottom (EB) of the concrete ditch (natural
stream).
RESPONSE: Revised as requested only offsite runoff considered is runoff which flows
through the site (site being, limits of disturbance)
F. Please clarify: On pg. 24 you state you're meeting flood protection under 66-C(2)b, but on
pg. 3 for flood protection it looks like your trying to meet 66-C(1).
RESPONSES: Flood Protection 9VAC25-870-66-C(3)C.
You're not showing you meet 66-C(2)b, Qpost>Qpre. (Rev. 1) Partially addressed. As
follow-up: Pg. 29 Calc. packet indicates Q10 postdev /design=109.50 cfs while Q10pre
=Max Q10post=109.16. Qdesign > QMax. Revise design, review, or clarify if typo. Also,
provide cross reference to hydrograph, by calc. p. #. Note: Calculations require revision,
with flood /channel protection evaluation of site development runoff compliance at top
of concrete flume /ditch, and further channel/flood protection evaluation at bottom of
912 E. High St. Charlottesville, VA 22902 1434.227.51401 chimp-engineering.com
the flume. At bottom of flume, site development runoff must meet EB Eq., for channel
protection, while below point of plunge pool, where offsite 51.540 Ac. bypass runoff joins
1.10 Ac. site development runoff (2.03 Ac. ditch DA), natural channel must be evaluated
for flood protection (for areas prone to flooding, or not prone to flooding, to limit of
mapped floodplain).
RESPONSE: Reference updated, flood protection is per 9VAC25-870-66-C(3)C. This
outfall is within the Rivanna River regulatory Floodplain.
G. VRRM — Please use New development spreadsheet. The parcel is u,,,,. v, ,,,,,, ,.. (Rev 1)
Addressed. As follow-up:
Engineering notes permeable pavers and forest /open space (FOS) postdeveloped land
cover are used to comply with storm water quality requirements. FOS easement must be
platted prior to WPO plan approval.
1. Please confirm whether development is to be subdivided; if so, FOS easement may be
platted with subdivision plat, after WPO plan approval.
RESPONSE: Yes, this is correct
2. If no subdivision, please submit easement plat, via separate plat application process for
review/approval prior to WPO plan approval.
RESPONSE: Final subdivision plat is being prepared. Preliminary already
submitted.
3. oiiuvv linework /boundary of 3.26 Ac. FOS easement oi, plan sheet C4 (shown on C9,
but please show portions visible on C4, as well).
RESPONSE: Noted. Shown on Sheet C4.
4. Provide public SWM facility easement for permeable pavers:
a. Permeable pavers constitute material surface type of each of 13 townhouse
driveways.
RESPONSE: Pavers Removed.
b. Acknowledge that individual owners of each townhouse will in effect be responsible
for SWM facility maintenance of permeable pavers, in perpetuity.
RESPONSE: Pavers Removed.
c. This means, pavers are the only permissible driveway surface material, and may not
be removed unless water quality phosphorus reduction provided by pavers is offset
via alternative SWM BMP, or nutrient purchase that provides equivalent phosphorus
reduction.
RESPONSE: Pavers Removed.
d. Were credits purchased instead of driveway permeable pavers, Engineering estimates
expense to purchase 0.32 lb. credits would be approximately $4,000.
RESPONSE: Pavers Removed.
e. While Engineering cannot prohibit this design strategy, i.e., perpetual maintenance of
permeable paver driveways by 13 individual townhouse owners, we wish to point out
the strained logic of developer avoidance of relatively modest $4000 expense,
912 E. High St. Charlottesville, VA 22902 1434.227.51401 chimp-engineering.com
transferring maintenance of SWM facilities located on every townhouse (lot?) to 13
individual townhouse owners that will likely take exception to this design, perpetual
maintenance responsibility, and maintenance expense.
RESPONSE: Pavers Removed.
f. Engineering recommends developer instead purchase 0.27 lb. credits since this
practice along with 3.26 Ac. FOS easement net a 0.05 lb. credit, meaning, 0.27 lb.
purchase (with FOS) would meet site phosphorus reduction requirement.
RESPONSE: Pavers Removed.
g. Engineering would not object to permeable pavers placed within private access if CCR
assigns SWM facility maintenance expense to HOA, but finds permeable driveways
problematic in the extreme.
RESPONSE: Pavers Removed.
h. Note: Under proposed design, each driveway requires independent SWM facility
easement (metes /bounds) since driveways are discontinuous, with sanitary laterals
between.
RESPONSE: Pavers Removed.
i. Each driveway /SWM facility will be owned and maintained separately, by a private
individual /townhouse owner.
RESPONSE: Pavers Removed.
j. C4 shows permeable paver underdrains; west rear corner of each townhouse spills to
permeable pavement.
i. C3-C4 show permeable paver common UD connecting with individual UDs beneath
pavers at each driveway. The common UD collects individual UD runoff, once surface
runoff passes through permeable pavers.
RESPONSE: Pavers Removed.
The two common UDs serve 5 lots, and 8 lots. A common UD that spans multiple
privately -owned SWM facilities (permeable paver driveways) poses obstacle; which,
for example, of thirteen townhouse owners is responsible for investigating or
repairing an issue affecting common underdrains?
RESPONSE: Pavers Removed.
Remove common underdrains from driveways.
RESPONSE: Pavers Removed.
k. Advantages of 0.27 lb. nutrient purchase to meet SW quality requirements are
practical, and multiple, for future owners, and current developer.
Expense of SWM facility easement plat and underdrain redesign may exceed expense
of 0.27 Ib. nutrient purchase, while individual townhouse owners would likely far
prefer piped collection /conveyance of rear roof runoff to an UG detention system
placed beneath rear driveway private access.
RESPONSE: RESPONSE: Pavers Removed.
912 E. High St Charlottesville, VA 22902 1434.227.51401 chimp-engineering.com
ii. It is important to note that a detention system appears unavoidable to meet energy
balance for 2.03 Ac. ditch DA at bottom of the concrete ditch /flume.
RESPONSE: yes, you are correct, 200 LF underground detention system is
required, see updated plans.
iii. It seems expedient to route rear roof runoff to such a dedicated underground
detention system as opposed to 13 individual SWM facilities that are, at the same
time, private' I driveways.
RESPONSE: Agreed
H. Provide inlet drainage area map and assumed values to match calc. table. (Rev. 1) May
persist: C9 shows ditch and inlet drainage map. As noted elsewhere, calculations must be
revisited.
RESPONSE: Noted.
I. Provide channel adequacy/protection is met from the Al discharge point to outfall natural
channel (i.e. portion of ex. manmade channel). 66-B(1)a (Rev. 1) Persists. Applicant
response, letter d. 1/11/23: TD-268 table has been used to demonstrate that 2-yr velocity
and 10-yr depth at the concrete channel is adequate up to the outfall point. See table on
sheet C9.' As follow-up: 66-B(1)a, channel protection for manmade system requires Q
peak 2-yr post dev not cause erosion of manmade channel from Al to outfall to natural
channel. Modified LD-268, p. 34, Calc. packet, lists Allowable velocity=10.Ofps and 2-yr
velocity=17.25fps, 72% higher than Vmax. Clarify; reconcile 2-yr design velocity that
appears to exceed Vmax; ensure ditch velocity (Al to outfall), V2-yr, is non -erosive, and
will also be non -erosive to plunge pool at point of discharge to natural receiving stream.
RESPONSE: We updated allowable velocity--20 Up. This velocity is from VDOT road
& Bridge design guidelines for concrete channel
j. Upstream diater.t'e neGessarybased upon impervieus area post _ development
Dr..r.. sed .Jeyelelarnent . ltiFnately diSChare..r to a na*,Hr-.I rL.-.r npl and Energy galance
meqt hP Met there
5. New: Provide Construction Record Drawing (As -built) for VSMP on the plan.
RESPONSE: Added to C9.
6. New: Provide construction, installation, inspection, and periodic maintenance for
permeable pavers on the plan; ref. BMP Clearinghouse VA DEQ Stormwater Design
Specification No. 7.
RESPONSE: No longer applicable.
D. Erosion and Sediment Control Plan (ESOP)
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. Cover— (Rev. 1) Addressed.
a. Provide date(s) of LIDAR data.
b. Provide date of field verification of topography & existing conditions.
2. Please show LOD on the existing conditions sheet. (Rev. 1) Addressed.
912 E. High St. Charlottesville, VA 22902 1434.227.51401 chimp-engineering.com
3. Show demolition operations. (Rev. 1) Addressed.
4. Show existing treeline. (Rev. 1) Addressed.
5. Show accurate horizontal depth (batter) of retaining wall. (Rev. 1) Addressed.
6. Show geogrid extent of retaining wall. (Rev. 1) Addressed.
7. Label TW/BW elevations at each bend and wall ends. (Rev. 1) Addressed.
8. Tie contour lines along new road to the retaining wall. (Rev. 1) Addressed. As follow-up:
Tie east end of retaining wall to existing contours. (TW =417; BW=412.50, but no
proposed contours are shown.)
RESPONSE: The bottom of wall contour have been corrected ( in thus case, changed
to 416.00)
9. Note: Please submit retaining wall design & computations with the site plan. Separate
building permit required. (Rev. 1) Persists. Applicant: 'Comment noted. We have this in
progress we will submit it separately.'
RESPONSE: We are still waiting on Circeo Geotech to complete wall Design.
10. Show all proposed contours at 2' intervals. (Rev. 1) Addressed.
11. Reduce grade to 2:1 or less adjacent to lot 1. (Rev. 1) Addressed.
12. Provide B/M (VAESC HB, 3.36) between lots 5 & 6, and adjacent to lot 1. (Rev. 1)
Addressed.
13. Where grade over 3:1 provide low -maintenance (not grass) groundcover. (Rev. 1)
Addressed.
14. Show a staging and stockpile area. Show SF around stockpile. (Rev. 1) Addressed.
15. Show temporary construction easement for offsite improvements. (Rev. 1) Addressed.
16. Provide letter of permission from VDOT for work in the ROW prior to disturbance. (Rev. 1)
Addressed. (Not required for WPO plan approval; required for any activity within VDOT
RW. Slight correction: VDOT Approval will take form of a Land Use Permit.) Applicant:
'Letter should be provided from the contractor.'
RESPONSE: Slight correction on our end, contractor will apply for land use permit.
17. Provide volume estimate of cut & fill balances. (Rev. 1) Addressed.
If you have any questions, please do not hesitate to contact me at odria@shimp-engineering.com, or you
may contact keane Rucker at keane@shimp-engineering.com or by phone at (434)-227-5140.
Best Regards,
Odri Barefoot -Diaz
Shimp Engineering, P.C.
912 E. High St Charlottesville, VA 22902 1434 227 5140 1 shimp-engineering.com