HomeMy WebLinkAboutWPO202000005 Correspondence 2020-05-08s t # t t
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TIMMONS GROUP
May 5, 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. 2) dated April 23, 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.
(Rev. 2) Persists. Applicant: 'TBD'
a. Sec. IV, E., (MS4); Please leave blank.
Sec. IV, E has been left blank as requested.
(Rev. 2) Addressed.
b. Sec. VI, Certification: Please complete (print name, sign, date).
A signed certification will be provided once a contractor is chosen.
(Rev. 2) Persists.
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.
(Rev. 2) Persists.
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.
(Rev. 2) Addressed.
ENGINEERING I DESIGN I TECHNOLOGY
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.
(Rev. 2) Persists.
This will be provided once a contractor is chosen.
S. Sec. 8 — List named individual qualified to perform compliance inspections.
This will be provided once a contractor is chosen.
(Rev. 2) Persists.
This will be provided once a contractor is chosen.
6. 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.
(Rev. 2) Persists.
Acknowledged. A signed certification will be provided prior to VSMP/WPO plan approval.
7. Include 2019 Notice of Termination form. Link:
The 2019 Notice of Termination form has been provided in the SWPPP.
(Rev. 2) Addressed.
8. Revise SWPPP title to include ref. to VSMP/WPO2020-00005.
SWPPP has been revised to include ref. to VSMP/WPO2020-00005.
(Rev. 2) Addressed.
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.
(Rev. 2) Persists. See A.4. (SWPPP Sec. 6.E.). above.
Person responsible for pollution prevention practices will be provided once a contractor is chosen.
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.
(Rev. 2) Persists.
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.
(Rev. 2) Withdrawn: review error but please see related follow-up, below. As follow-up:
Revised grade downslope of post -developed drainage Area 213 is sheet flow, with revised
grading. This flow does not concentrate until flat slopes undisturbed by project grading give
way to, and intersect, a defined drainage feature (see blue arrows, below). This sheet flow
does not reach Ditch C or SWM2 (C5.0). DA Area 213 is excluded from stormwater
considerations by virtue of proposed grade. It appears Qpre-post (DA Area 213) has no place
in pre- or post -development calculations. Timmons may find that proposed design of SWM2
is more conservative than anticipated.
Engineering requests:
a. If SWM2 design proves adequate/more conservative, provide Notes only on C6.2
explaining that DA Area 213:
i. Persists as relic of initial design; shown to minimize revision effort with
county approval.
ii. Is not required to be shown and including Area 213 has conservative effect
on SWM2 design.
iii. Is included in SWM2 detention/capacity calculations, though not required to
be included.
iv. Runoff is routed, though runoff from DA Area 2B will not actually reach
SWM2.
b. If SWM2 design proves inadequate once DA Area 213 is excluded (from routing,
energy balance, quantity, or SWM2 detention capacity calculations), then:
Revise calculations/routings.
Revise design of SWM2 (ensure adequate).
iii. Revise Analysis point 1 Q pre -/post calculations.
iv. Revise Energy balance equation, which includes DA Area 2B.
V. Revise calculations booklet.
Note: Engineering cannot predict effect of excluding DA Area 213, but notes
that: pre- = post -developed condition (Area 213); Qpre-/post values appear
identical; Area 213 land cover and topography are unaffected by project; and
DA Area 213 runoff (once concentrated by existing defined drainage beyond
limits of project) discharges below Analysis Point 1.
Area 2B has been removed from the stormwater analysis in both the pre- and post -
developed conditions. The calculations have been rerun and updated on Sheet C6.2 and
within the calculation book.
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.
(Rev. 2) Withdrawn, but see SWMP item 2, above, since aspects of calculation -related
comment at item 2 may be relevant/persist.
Acknowledged. See response to 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.
(Rev. 2) Addressed. Also, SWMP item 2, above.
Acknowledged. See response to 2 above.
S. Provide profile views of each 96" DIA detention system; include elevations for:
(Rev. 2) Partially addressed. As follow-up: Snippets of SWM1, SWM2 visible in storm profiles
are partial, and insufficient.
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. These
views have been expanded to show greater lengths of the underground detention pipes,
as well as top/bottom elevations and depth of cover. An additional profile has been added
to Sheet C7.1 to demonstrate the 0.15% slope across SWM1. Lastly, details have been
provided for both underground detention systems on Sheets C6.3 — C6.4; these details
show minimum bedding requirements below and fill requirements above detention pipes.
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.
(Rev. 2) Partially addressed. As follow-up: Partial system are shown on C6.3, C6.4 (Lane,
SWM 1, 2) and 6.2. Technical installation notes provided. Please furnish periodic inspection
and Lane mfr.- recommended maintenance procedures on the plans.
Periodic inspection notes have been provided on Sheet C1.0. Lane mfr.-recommended
maintenance procedures have been provided on Sheets C6.3 — C6.4.
Also:
a. Provide trash rack details and mounting details for 10", 8", and 4" DIA weir plate
orifices.
Orifice trash rack details have been provided on Sheets C6.3 — C6.4.
b. Revise SWM1 10" orifice inv. Elevation, which appears to be 3' -7" above 8' DIA pipe
invert, or at 527.58'.
SWM1 orifice has been revised to match on Sheets C6.2 and C6.3. The 10" orifice
invert elevation is 527.40'.
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.
(Rev. 2) Addressed.
8. Revise plan set title to include ref. to VSMP/WP02020-00005.
Title has been revised as requested.
(Rev. 2) Addressed.
9. Easement plat must be recorded prior to VSMP/WP02020-00005 approval.
Acknowledged.
(Rev. 2) Persists.
An easement plat will be provided in a future submission and recorded prior to
VSMP/WPO approval.
New (Rev. 2):
10. WP0202000005 proposes 4.87 lb. nutrient credit purchase to meet stormwater quality
compliance requirements. Please coordinate nutrient credit purchase documents with Ana
Kilmer prior to purchase to avoid duplicative effort, or delay (Ana Kilmer, Analyst,
434.296.5832-x3246, akilmer@albemarle.org).
A letter of credit availability has been provided on Sheet C6.2. Purchase of nutrient credits
will be coordinated with Ana Kilmer.
11. Please provide identical notes on at least one ESC, at least one grading, and at least one
spot -grade plan sheet. (Notes need not appear on the FSP.) Notes should state:
a. Contractor will flag limits of critical slopes to be disturbed with survey ribbon or
stakes prior to disturbance of critical slopes.
b. Contractor will flag limits of critical slopes to be preserved with survey ribbon or
stakes prior to disturbance of critical slopes.
c. Contractor is to notify Albemarle County inspector/provide opportunity to review
limits of critical slopes prior to disturbance.
d. Contractor must maintain ribbons or stakes that delineate critical slopes not to be
disturbed, for the duration of the project.
e. Pre -construction staked/flagged limits of critical slopes to be disturbed need not be
maintained, once slopes are disturbed.
Notes have been provided as requested on Sheets C3.0 and C5.0 — C5.5.
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 approved once revised in response to single follow-up, item 12, below.
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 = % wet volume (1.5' v. 3') + (6' x 6'
x 1.5' + 1/3 (1.5' x 6' x 6')) (2nd term = Vol. pyramid/corners; 15t 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', 10.5', 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.
(Rev. 2) Addressed. Note: Timmons' (Rev. 2) 04/20/2020 comment response letter
addresses all four (4) 04/11/2020 email items.
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 41" 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.
(Rev. 2) Addressed.
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) [Images removed with
Rev. 2 comments.]
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) [images
removed with Rev. 2 comments.]
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.
(Rev. 2) Partially addressed. As follow-up: C3.5 reference to reverse bench slope detail
obscured by Note to contractor. Please shift this Note to contractor. [Images removed with
Rev. 2 comments].
The note has been shifted to eliminate text conflicts. See Sheet C3.5.
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.
(Rev. 2) Note: Ensure Site Plan, WPO Plan, and Critical Slopes Waiver request are consistent.
Site Plan, WPO Plan, and Critical Slopes Waiver are now consistent.
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.
(Rev. 2) Addressed.
We have included PDF copies of the plans and calculations for your review. If you have any
questions or comments, please feel free to email me at kim.melIon@timmons.com.
Sincerely,
Kimberly Mellon
Project Engineer