HomeMy WebLinkAboutWPO202000051 Correspondence 2021-02-05f 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
February 4, 2021
John Anderson
County of Albemarle
401 McIntire Rd, North Wing
Charlottesville, VA 22902
RE: Boys & Girls Club Driver's Ed — VSMP Permit Plan Review — WP0202000051- Comment
Response Letter
Dear Mr. Anderson:
We have reviewed your comments from December 21, 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.
Please ensure that SWPPP, once submitted:
1. Sec. 1, Registration Statement is complete. Complete Sec. 1 as condition of VSMP / WPO plan
approval.
Section 1 has been completed to the extent possible at this time.
a. Sec. IV, E., (MS4); Please leave blank.
Section IV, E has been left blank.
b. Sec. VI, Certification: Please complete (print name, sign, date).
A signed certification will be provided with a future submission.
c. Please submit revised Registration Statement, Eff. 1/1/21 [Also, email sent January 29,
2021 11:20 AM]
The revised Registration Statement has been provided with this submission.
2. Sec. 4, 5: Include revised ESC and SWM plan sheets, once comments addressed.
Revised ESC, SWM plan sheets have been provided.
3. Sec. 6.A. / Exhibit: Indicate location rain gauge / other required PPP measures. As follow-up:
Please revise Phase 1 Exhibit to show vehicle wash waters draining to downslope trapping
measure; not a VESCH sediment trap per se, but a 1-2 backhoe scoop depression, or to concrete
washout.
Rain gauge location has been provided within PPPs. Concrete washouts have been provided
ENGINEERING I DESIGN I TECHNOLOGY
with proposed construction entrances in each phase.
4. Sec. 6.E. — List named individual responsible for pollution prevention practices prior to project
preconstruction meeting.
Named individual responsible will be provided with a future submission.
5. Sec. 8. — List named individual qualified to perform compliance inspections prior to pre -
construction meeting (prerequisite to receive Grading Permit).
Named individual responsible will be provided with a future submission.
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.
A signed certification will be provided with a future submission.
7. Include 2019 Notice of Termination form.
The 2019 Notice of Termination form has been included.
8. SWPPP title includes ref. to VSMP / WP02020-00051.
SWPPP title references WP02020-00051.
B. Pollution Prevention Plan (PPP) —See above.
The PPP content requirements can be found in County Code Section 17-404.
PPPs have been included with this submission.
C. Stormwater Management Plan (SWMP)
VSMP Regulations 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. Revise WPO plan per Engineering Initial Site Plan Review comments (10-Dec 2020) items 2-4, 9-
11, 12 (b, c, g, h). Ensure ISP review revisions carry over to WPO plan resubmittal.
Plans have been coordinated to address all comments received from the County.
2. C1.3: Provide VDOT ST-1, IS-1, PB-1 details on the plans (steps, inlet shaping, pipe bedding).
VDOT details have been provided on Sheet C1.2.
3. C5.0:
a. Clarify whether pipe 301 discharge is to surface or manmade subgrade structure. If
discharge is to surface, ensure discharge is to a stormwater conveyance channel, as
existing topography at pipe 301 discharge area evenly spaced contours without a
defined channel. This slope will erode if concentrated flow is released with no proposed
change in grade, to define a channel.
Pipe 301 discharges below grade, tying into the existing manhole structure. The profile
has been updated to remove end section label and clarify intent at this location.
Additionally, a label has been added requiring contractor to field verify condition and
invert of structure prior to construction.
b. Align pipe 301 outfall with pipe. An outfall structure should not deflect direction of
discharge. Provide MH structure at pipe 301 discharge to deflect discharge, unless
design relies on ex. MH.
The end section outfall was showing due to a printing error that has since been
resolved. See response to 3a above for further clarification.
4. C3.4: Label Ex. SWM facility (Mfr. Wetland).
The existing SWM facility has been labeled on all sheets.
5. C5.1:
a. Recommend a vault -style cover with locking feature for underground detention access
located paved surface between loop and west wing of building. This point of access
should be limited to authorized personnel only with no possibility of trespass by youth
patrons of the facility.
Details for the underground detentions systems have been provided on Sheets C6.4 —
C6.5, which includes access.
b. With next submittal, provide tiered retaining wall TW/BW elevations.
Detailed grading, including all relevant spot shots, has been provided with this
submission. See Sheets C5.2 — C5.3.
6. C5.0, C5.1:
a. Show and label SWM facility easements for underground detention systems. Label
SWM1, SWM2. Ensure easement widths meet ACSM easement diagram guidance; p. 15.
SWM facility easements have been provided for both underground detention systems
on Sheet C5.0 — C5.1. Systems have been labeled to differentiate between the two.
b. Clarify via inset details the precise location / structure type/s that house SWM1, SWM2
weir plates. Precise detail is needed.
Details for the underground detention systems have been provided on Sheets C6.4 —
C6.5.
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a. Revise SWM1 weir plate detail top of pipe elevation to 548'.
Top of pipe elevation has been corrected on Sheet C6.3.
b. Provide small-scale plan views for SWM1, SWM2 weir plate structures that show access
ladders, pipe penetrations, weir plates and manway clearances (access to each side of
each weir plate) to check for potential conflicts. Also, 8.d., immediately below.
Details for the underground detention systems have been provided on Sheets C6.4 —
C6.5, which includes information about the weir plates.
c. Provide debris cage details for SWM1, SWM2 weir plate orifice openings, unless
overlooked.
A debris cage detail has been provided on Sheet C6.3.
d. Provide additional weir plate details; steel gauge, manner of connection to housing
(prefabricated, welded, etc.).
Details for the underground detention systems have been provided on Sheets C6.4 —
C6.5, which includes information regarding the weir plates.
e. Provide UG detention system profile details (bedding; inv elevations; system slope, if
any; min. depth of cover, etc.).
Details for the underground detention systems have been provided on Sheets C6.4 —
C6.5.
8. C6.2:
a. Consider and offset any effect of diminished treatment of existing stormwater facility.
C3.4 depicts post -developed condition with no Drainage Area 1 concentrated runoff
reaching Ex. SWM. C2.4 shows 24" RCP outfall removed with project. Please update
design, VRRM.xIs re -development worksheet, and / or calcs if current / existing
condition DA 1 runoff reaches wetlands, which perform water quantity / quality
functions, which may diminish in the post-dev condition.
No runoff from DA 1 reaches the existing manufactured wetland in either the existing
or proposed conditions. See response to comment 8.b below.
b. Pre -developed drainage area map may be inaccurate in that it may underestimate
volume of runoff or areas currently draining to ex. Stormwater facility. Instead, the pre-
dev map implies DA 1 does not route to Ex. SWM facility / wetlands, when linework
shows concrete flume which appears to route a portion of runoff to this facility via 24"
RCP outfall (removed with project). Ex. Contours / riprap appear to indicate a portion at
least of pre-dev DA 1 runoff reaches Ex. SWM (blue circle, image, below). If existing Mfr.
Wetland is overgrown or poorly maintained, pre-dev runoff analysis must assume a pre -
developed condition with functional existing SWIM facility.
Please see screen clip below, which is an as -built plan of the existing SWM facility
provided by the County to illustrate the existing drainage patterns. The area circled
represents an emergency spillway from the existing SWM facility to the existing
concrete flume. All drainage that gets into the concrete flume via the northern outfall
bypasses the existing SWM facility. See note that states "TRANSITION EXISTING
SWALE INTO CONCRETE DITCH". It appears that the existing manufactured wetland
was designed to serve the Ivy Creek School building and associated parking, not the
driver's education course.
c. Label analysis point 2 on post-dev DA map. Additional comments possible.
Per response to 30.g, we have not modified the analysis points with this submission.
See below for reasoning.
9. Include construction installation (Mfr.) notes and periodic maintenance requirements for
proposed underground detention, on the plans.
Mfr. recommended installation notes have been provided with details on Sheets C6.4—C6.5.
Periodic maintenance requirements for underground detention have been provided on Sheet
C1.0.
10. Calculation booklet:
a. Revise cover to include WPO202000051.
Cover references WPO202000051 as requested.
b. Provide LD-204, 1=6 in. for inlet capacity analysis.
LD-204 has been provided for both 1=4 in. and 1=6 in. as requested within the
calculation booklet.
c. Revise inlet length, Str. 202, to limit excessive spread (=12.093').
Inlet structure 202 has been revised to ensure spread is less than 8'.
d. Recommend increase inlet lengths, Str. 206 and 208, from 6' and 4', respectively, to
allow drivers to exit vehicles to dry pavement (recommend avoid 9.599' and 9.164',
spread, respectively).
Inlet structures 206 and 208 have been modified to ensure spreads are less than 8'.
e. Revise 6' inlet length, Str. 402, to ensure spread (9.012') is <1/2 travel lane width + 2'
gutter pan, or, <8'. Drive aisle = 24', two-way travel; this means % travel lane width = 6'.
Structure 402 has been modified to ensure spread is less than 8'.
f. Revise narrative, p.3, to include discussion of Ex. County SWM facility (mfr. Wetland).
Provide comprehensive description of quality / quantity benefits provided by existing
on -site SWM, and in the proposed post-dev condition. Project narrative / SWM does not
mention ex. SWM facility.
Narrative within calculation booklet has been modified to include mention of the
existing drainage patterns and existing SWM facility.
g. Narrative, p.3, explains that 'Two underground detention systems will be installed to
meet stormwater quantity requirements. SWM1 is 50 linear feet of 84" CMP while
SWM2 is 220 linear feet of 96" CMP. Together, these pipes reduce the 1-year, 24-hour
storm to meet the requirements of the energy balance equation.' Energy balance
applies at each discharge point. Revise narrative or design to ensure energy balance is
achieved at each post -developed discharge to natural conveyance.
The outfall to SWM1 has been fixed to clarify that it is discharging into an existing
structure and through a series of existing pipes. Due to a printing error, it appeared in
our previous submission that the proposal was to discharge via an end section above
grade; but this is not the case. All downstream pipes have been checked for capacity.
However, due to the fact that SWM1 does not discharge to a natural conveyance, it is
our understanding that this comment no longer applies.
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:
1. Develop a more detailed, additional -phase ESC plan that:
a. Shows proposed mass grading required to construct improvements (pavement, building,
walls, etc.).
b. See detailed ESC Plan developed for WPC202000005, Boys and Girls Club Northside, for
examples of interim mass -grading, ESC measures, etc.
An interim phase has been added to the ESC plans to further detail the sequence of
construction for this project. See Sheet C3.3.
c. C3.3:
i. Phase 1 shows no proposed grading, other than grading for ST1, ST2, ST3.
Phase 1 does not show proposed grading. However, we have added an interim
phase to further detail the sequence of construction for this project. See Sheet
C3.3.
ii. Label sediment trap floor dimensions.
Sediment trap floor dimensions have been labeled on applicable plan sheets,
as well as in the details on Sheet C3.1.
iii. Shift ST2 slightly downhill so DD inflow to ST2 travels a farther distance, with
increased residence time. DID runoff is introduced too close to ST weir outfall.
Diversion ditches have been modified to increase residence time as requested.
iv. Label ST2, ST3 weir riprap dimensions (L x W x D).
Weir riprap dimensions have been provided on all applicable sheets.
v. Revise diversion to ST2 to avoid 90-deg turns. Concentrated runoff will likely
break through bends. Recommend replace ST2 with separate STs, one at each
90-deg bend, with area in between these two smaller STs directed to drain to
one, or the other.
Diversion ditches have been modified to eliminate any sharp turns without
having to relocate or replace ST2.
vi. Coordinate location, size of all traps with mass grade plan, sequence of
construction, and final improvements. STs must function for a period of time,
must accommodate construction of walls, building, paved surfaces, etc.
Sediment traps have been coordinated with grading and final improvements.
See Sheets C3.2 — 3.4 as well as sequence of construction on Sheet C3.0.
vii. The proposed 2-phase ESC plan gives no clear understanding of how the site, or
ESC measures, will progress from existing condition thru final stabilization / site
buildout.
An interim phase has been added to the ESC plan to further detail the
sequence of construction for this project. See Sheet C3.3.
d. C3.0: Revise sequence of construction to provide additional specific ESC detail from
existing condition thru final improvements (building, walls, athletic field, pavement /
sidewalks).
Sequence of construction has been updated to match detailed ESC sheets. See Sheet
C3.0.
e. C3.4:
i. Consider additional ESC measures for 10' (vertical interval) steeply -graded
slopes south of building.
The site layout has been revised with this submission; the steeply -graded
slopes south of the building are no longer proposed. Steeply -graded slopes
south of the proposed basketball courts are protected by a sediment trap in
the interim phases and a conveyance ditch in the final condition to protect
downstream areas.
ii. Label steepest portion of graded slope west of stairs from elevation 561 to 544
(2.5:1, 3:1, etc.).
The steepest portion of graded slopes have been labeled on Sheets C5.2 —
05.3, both adjacent to the stairs and south of the proposed basketball court.
iii. Provide / specify permanent groundcover (vegetative cover) hardier than grass
for any proposed graded slope steeper than 3:1.
Slopes steeper than 3:1 are only proposed south of the basketball court. Hardy
vegetation is proposed in this area. See Sheet 1-1.2.
iv. Label proposed slopes steeper than 3:1, provide / reference plant schedule
(specify species other than grass).
Slopes are labeled on Sheets C5.2 — C5.3. Hardy groundcover is proposed in
steep areas. See Sheet L1.2.
2. C5.1: Provide ditch detail /calc. velocity in ditch created by new/proposed contours south of
building, west of tiered retaining walls downslope of building (blue arrow, image, below).
Channel sections and ditch calculations have been provided on Sheet C5.0 for all proposed
drainage ditches.
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
kim.roeser(@timmons.com .
Sincerely,
ck Hwy 40wk
Kim Roeser
Project Engineer