HomeMy WebLinkAboutWPO202000005 Correspondence 2020-04-20 (2)TIMMONS GROUP
April 20, 2020
John Anderson
County of Albemarle
Dept. of Community Dev.
401 McIntire Rd, Room 227
Charlottesville, VA 22902
608 Preston Avenue P 434.295.5624
Suite 200 F 434.295.1800
Charlottesville, VA 22903 www.timmons.com
RE: Boys and Girls Club, Northside —VSMP Permit Plan Review — WP0202000005 -Comment
Response Letter
Dear Mr. Anderson:
We have reviewed your comments from (Rev. 1) (Emailed Items from 04/11/2020 Email) April
11, 2020 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. Sec. 1, Registration Statement is incomplete. Complete Sec. 1 as condition of VSMP/WPO plan
approval.
a. Sec. IV, E., (MS4); Please leave blank.
Sec. IV, E has been left blank as requested.
b. Sec. VI, Certification: Please complete (print name, sign, date).
A signed certification will be provided once a contractor is chosen.
2. Sec. 4, 5: Update these sections with revised ESC and SWM plan sheets, once comments are
addressed.
Reduced ESC and SWM plan sheets have been provided.
3. Sec. 6A./Exhibit: Indicate location rain gauge.
A rain gauge location has been indicated on all PPPs.
4. Sec. 6E. — List named individual responsible for pollution prevention practices prior to VPDES
permit registration.
This will be provided once a contractor is chosen.
5. Sec. 8 — List named individual qualified to perform compliance inspections.
6. This will be provided once a contractor is chosen.
ENGINEERING I DESIGN I TECHNOLOGY
7. Sec. 9., Signed Certification: Individual who prepared SWPPP document must sign as condition
of VSMP/WPO plan approval. Also, Albemarle cannot register project with DEQ until SWPPP Sec.
9 is complete.
Acknowledged. A signed certification will be provided prior to VSMP/WPO plan approval.
8. Include 2019 Notice of Termination form. Link:
The 2019 Notice of Termination form has been provided in the SWPPP.
9. Revise SWPPP title to include ref. to VSMP/WP02020-00005.
SWPPP has been revised to include ref. to VSMP/WP02020-00005.
B. Pollution Prevention Plan (PPP) — See above.
The PPP content requirements can be found in County Code Section 17-404.
PPPs have been revised based on ESC changes.
C. Stormwater Management Plan (SWMP)
VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a
SWMP. This plan is disapproved for reasons listed below. The stormwater management plan
content requirements can be found in County Code Section 17-403.
1. Easement Plat is required for SWM Facilities (SWMI, SWM2), SWM Facility Access, and
Public drainage easements downslope of SWM facilities.
Easement plat will be provided in a future submission.
2. C6.2 (Channel protection) Energy Balance Equation should use pre -developed Analysis point
1 combined drainage areas 1+2 values (4.66 cfs, 37,810 CF), which represent technical
estimate of channel energy at Point of Analysis 1 based on pre -developed DA characteristics.
Engineering is unaware of rationale that allows portion of pre-dev runoff to be excluded
from the calculation, then added at the end. Doing this has effect of increasing Qpost-dev 1-
year peak Allowable. Engineering believes the correct approach is to calculate using
combined values, which yields: Q post 1-yr peak Allowable = I.F x Q pre (1-yr peak) x Rv
pre/Rv post. Or, Q1-yr Allowable = 0.8 (4.66cfs)(37,810cf) / 37,810cf = 3.73 cfs. It appears
design relies on calc. value (4.44cfs) = 19%> if using combined drainage areas 1+2, while
routings indicate Q 1-year peak post-dev = 4.21 cfs, a value = 13% higher than Q1-year peak
post-dev. Please review /revise Energy balance calculation, as needed.
Pre -developed analysis point 1 combines pre -developed drainage area 1, which is onsite
water, and pre -developed drainage area 2, which is offsite water. As a result, drainage
area 1(onsite) is used within the energy balance equation, and the reduction factor of
80% is applied to this portion of the water. Drainage area 2 (offsite) is then added to the
energy balance equation separately; this is so that the reduction factor is not applied to
the offsite drainage. The total of these two quantities then becomes the Qpre-developed for
the project. This rationale has been utilized and approved on various projects within
Albemarle County, including WP0201800030 and WP0201900010. We are happy to
discuss further if there are remaining questions about the stormwater scheme for this
project.
3. Alternatively, offsite runoff that transits site may rely on piped conveyance that bypasses
the site entirely and does not compromise compliant calculation (Energy balance equation)
at Analysis Point 1.
See response to comment 2 above.
4. Related: Post-dev (Analysis Point 1) Rv is given as 37,810 cf, which is identical with pre -
developed Rv. As forest is generally converted to post-dev site with 2.83 Ac. Impervious
cover, if Rv pre = Rv post = 37,810cf (i.e., values accurate) please explain. Else recalculate
Q1-yr peak Allowable, post-dev, using revised Rv-post. Revise detention system / SWM
facility design, if needed.
Post -developed volumes have been revised. See Sheet C6.2.
5. Provide profile views of each 96" DIA detention system; include elevations for:
a. Base elevation of bedding stone.
b. Base of 96" pipe [incl. 0.15% slope, consistent with Hydrocad routings],
c. Top of 96" pipe,
d. Depth of select fill above pipe,
e. Proposed grade above each system (in profile view/s, several spot elevations along L
of pipe).
Profile views have been provided for all stormwater pipes on Sheets C7.0 — C7.1.
Additionally, details have been provided for both underground detention systems on
Sheets C6.3 — C6.4.
6. If underground detention is a Mfr. System (ADS, Contech, etc.), please include Mfr.
Recommended installation and construction inspection guidelines on the plan. Please also
include periodic maintenance inspection guidelines on the plan (Mfr., or otherwise).
Details are provided for the proposed underground detention systems on Sheets C6.3 —
C6.4. Additionally, periodic maintenance and inspection notes are provided on Sheet C1.0.
7. Include Albemarle county guidance document found at: www.albemarle.org titled
Construction Record Drawings (As -Built) for VSMP on the VSMP/WPO plan.
Albemarle County guidance document has been provided on Sheet C3.0.
8. Revise plan set title to include ref. to VSMP/WPO2020-00005.
Title has been revised as requested.
9. Easement plat must be recorded prior to VSMP/WPO2020-00005 approval.
Acknowledged.
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 for reasons listed, below. The erosion control plan content
requirements can be found in County Code Section 17-402.
C3.0 — (Rev. 1) C3.0 comments Addressed.
1. As condition of Engineering support of 0.45 Ac. Critical slope impact (waiver request; email
to Timmons/Developer, 02/28/2020 8:59 AM), and Engineering support of a revised
estimate of =0.60 Ac. Impact to critical slopes (incl. storm line crossing critical slopes;
03/05/2020 11:09 AM), please note that comments, below, or requested to revise the ESC
plan may not be limited to initial review or follow-up review comments but may also include
ESC inspector directive (a circumstance already assured by VPDES permit) to deploy
additional control measures that may be needed or evaluated necessary to limit impacts to
critical slopes.
2. Revise sequence to accommodate additional requested (Phase II-b) ESC Plan measures; see
below.
3. ST3 with bottom trap dimensions = 4' x 178' are so narrow that, although wet/dry storage
volumes are provided, they are not available. The trap is equivalent to a 178' flat channel, 4'
wide. Sediment -laden runoff will reach ST3, lose velocity and particles will settle in portions
of the trap nearest the diversion trap inlets at either end. ST3 will short-circuit; this design
cannot be approved. Revise design to activate a broader -width sediment trap; consider, for
example, a series of smaller traps moving downslope, each more conventionally shaped and
receiving reduced LOD runoff, removed once no longer needed. ST3 is a linear ST proposed
in Phase I yet is at the very outer limit of eventual site disturbance, but it appears that a trap
at this location could be delayed if smaller, successive areas are cleared via phased grading.
(Note: it is rare for Engineering to recommend phasing or specific locations of ESC measures,
but given Critical Slopes Waiver, a conservative least -impact design approach to minimize
critical slope impacts is crucial.
4. Please confirm ST 1 dry vol. vs. elevation values. It appears that wet volume (depth=3') and
dry volume (depth=1.5') with 2:1 side -slopes are not equal, though reported equal. Dry
volume (Vol. above base of weir), if calculated, appears = %2 wet volume (1.5' v. 3') + (6' x 6'
x 1.5' + 1/3 (1.5' x 6' x 6')) (2"d term = Vol. pyramid/corners; 15Y term = additional distance
dry volume shifts outward /2:1 side -slopes), or % wet + 2.66 cy; or (1/2 wet Vol.) = 72.35 cy
+ 2.66 cy. There is some discrepancy between listed dry vol. = 144.7 cy and value derived
using elevations and 2:1 wet/dry volume side slopes. Please ensure reported design ST1
wet -dry Vol. is sufficient and consistent with proposed grading and ST typical detail
elevations.
5. C3.1: Provide paved construction entrance detail.
C3.2:
6. Let SAF be discontinuous across CE, unless gated. If gated, please include SAF gate label.
(Rev. 1) Addressed.
7. Label floor dimensions, L x W, each sediment trap (ST).
(Rev. 1) Addressed.
8. Include L x W x D dimensions for all weirs, and riprap outfalls. Applies to other sheets, as
well.
L x W x D dimensions have been provided for all weirs and riprap outfalls. See Sheets C3.3
— C3.5 and C5.2 for dimensions.
(Rev.1) Addressed. As follow-up, please see email sent 4/11/2020 7:55PM, email item 1.
Email Item 1: ST1, ST2, ST3, ST4, (C3.2; details) are listed as 13', 9', 105, 6.5' which don't
match weir labels on C3.3 (this is follow-up, ESCP review item 8.)
Weir labels have been revised to match details on Sheet C3.2.
9. C5.0, C5.1, C5.2, C5.3: Label proposed 2:1 fill slopes. Specify Landscape Plan that includes
partial reforestation of slopes, and stabilization of slopes steeper than 3:1 with vegetation
hardier than grass. Species and plant qty. require bond.
(Rev.1) Comment persists. Also, 04/11/2020 7:55PM email, item 4.
Email Item 4: Comment 9 persists. Landscape plan required. C3.0 Permanent Stabilization
Note (slopes>3:1) is insufficient. VESCH Table 3.32-D lists 3 grass species (Kentucky 31 Tall
Fescue, Common bermudagrass, Red Top Grass), while a 4t" species, seasonal nurse crop,
includes rye/millet grass. ACDSM, Sec. 8.A.2 (p. 22), stabilization on constructed slopes>3:1
requires permanent vegetation hardier than grass. Please provide landscape plan for
slopes>3:1 that does not rely on grass species. Include canopy tree species, if part of
permanent stabilization plan (reforestation).
Fill slopes have been labeled on Sheets C5.0 — C5.5. Slopes steeper than 3:1 have been
identified, and stabilization has been provided on Sheets L1.0 — L2.0.
10. Show and label critical slopes across all plan sheets that show LOD, ESC measures, or
improvements.
(Rev. 1) Addressed.
11. C3.3, C3.4, C5.2 (ST1): (Rev. 1) Addressed.
Silt fence along is insufficient given proposed extent of 2:1 fill slopes, and vertical interval.
At a minimum, provide reverse slope benches (permanent feature) with corresponding
contour lines labeled to clearly convey design intent. At base of proposed 2:1 slopes,
provide intermediate sediment trap, ESC Plan Phase II-b (once ST1 is removed) to intercept
sediment -laden runoff from proposed 2:1 slopes at base of slope. Blue -circles indicate
vulnerable 2:1 slopes that may degrade quickly without reverse slope benches. Even with
reverse benches, there is potential for sediment -laden runoff that merits trap as an
intermediate perimeter measure to help limit impact to critical slopes SW of basketball
court. Trap should be sized for DA and removed with county ESC inspector approval once all
upslope areas (2:1 fill slopes) are stabilized. (C3.3, C3.4, C5.2, below)
C3.3, C3.4, C5.0 (ST2): Provide intermediate sediment trap, ESC Plan, Phase II-b, once ST2
removed and Ditch C installed to intercept sediment -laden runoff prior to Str. 704 (grate); IP
alone is insufficient inlet protection given disturbance required to construct the play field
2:1 embankments, and to remove ST2 — images, below; blue -circle area (3rd image) is a
possible small trap location. Once Ditch C and play field embankments are stabilized, this
small trap should be removed, with county approval; (C3.3, C3.4, C5.0, below)
12. C3.3, C3.4, C5.2 (ST3):
Similar to previous items, provide intermediate ESC measure/s between C3.3 and C3.4 until
proposed 2:1 slopes on critical slopes are stabilized. Consider 2-layer protection (dash lines).
Consider wire -backed SF. Permanent reverse slope benches are required. Consider small
traps at strategic points, and trap removal. It is imperative to limit impacts to or across
critical slopes. It is insufficient to remove ST3 without intermediate ESC measure/s in place.
Further, Engineering encourages a plan view of development, intermediate to partial site
grading shown in Phase II and final build out depicted in ESC Plan Phase III. For example, it
may be possible to retain ST2 and ST3 with diversion dikes while parking areas are graded.
Engineering encourages design to retain ST2 and ST3 as long as possible, prior to removal.
(C3.0 Phase II Sequence appears to suggest building pad elevation can be constructed prior
to ST2, ST3 removal.) Also, please note need to disperse concentrated ditch runoff via level
spreader (or similar) for channels, formed by proposed and existing slopes ('V' feature, blue
arrow, below, at approximate location TSD, circled). (Rev.1) Partially addressed. A follow-up,
please see email sent 04/11/2020 7:55PM, email item 2.
Email Item 2: Smooth contours downslope of ST1 (C3.5) do not channelize runoff, which
should help alleviate post -development issues across constructed slopes. Please confirm if
these contours were meant to carry over to phase IV, C3.6. If not, contours on C3.6, below
basketball court, include a channel that will concentrate runoff, which will require energy
dissipation at toe of slope (follow-up: item 11, 12, blue arrow).
Contours have been smoothed downslope of ST1. These contours were meant to carry to
phase IV, and this has been corrected for this submission. See Sheet C3.6.
13. C3.4: Remove construction entrance (CE), this view.
(Rev. 1) Addressed.
14. C3.4: Check dams are not a permanent feature. Replace any check dams proposed or
discussed in narrative as permanent with: rock -lined channel, concrete flume, EC -II or EC -III,
or other appropriate permanent stabilization structure. Check dams do not persist, but
weather and scatter, offering unreliable permanent protection against erosion. CDs pose
risk to equipment and personnel required to maintain landscape areas.
(Rev. 1) Addressed.
15. Ref. Engineering site plan review comments, d. 3/3/20. Address VSMP/WPO plan -related ISP
comments.
(Rev. 1) Site plan ESCP-related comments Addressed; site plan SWMP-related comments
may persist.
Engineering site plan review comments have been addressed and resubmitted to the
County on March 23.
16. Additional comments possible, once ESC Plan revised.
(Rev. 1) Addressed.
17. Recommend replace ST1, ST2, ST3 typical details with scaled (accurate) specific profile
depictions.
(Rev. 1) Withdrawn.
18. Revise alignment of CE across plan sheets to align with movement of construction traffic
approaching from the NE. A skew is preferable to 90-deg angle, given limited width of Lambs
Lane.
(Rev. 1) Addressed.
19. Include note on plans requiring each underground detention system to be free of sediment,
trash, and construction debris at two points during construction: once all upslope grading is
complete and denuded areas are seeded per VESCH standards; and once construction is
complete, prior to issuance of CO. This note does not replace Construction Record Drawing
(As -built) for VSMP requirements, but highlights both difficulty of preventing sediment
entering these underground systems during construction, and imperative to leave both
systems free and clear of debris, sediment, etc. before contractor is released, or
demobilizes.
(Rev. 1) Addressed.
20. New: (Email Item 3) C3.1: Recommend FD and TSD details.
Details have been provided as requested. See Sheet C3.1 for FD detail and Sheet C3.2 for
TSD detail.
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.
Sincerely,
Kimberly Mellon
Project Engineer