HomeMy WebLinkAboutWPO202100063 Other 2022-06-29�q oa nig 401 McIntire Road, North Wing
Charlottesville, VA 22902-4579
County of Albemarle Telephone: 434-296-5832
COMMUNITY DEVELOPMENT DEPARTMENT WWW.ALBEMARLE.ORG
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VSMP Permit Plan Review
Project title:
Dunlora Village (Farm) VSMP & SWM Plan
Project:
WPO2021-00063
Plan preparer:
Chris Mulligan, PE — Roudabush, Gale & Assoc., Inc.
999 Second Street, SE, Suite 201
Charlottesville, VA 22902 cmulligan(a),,roudabush.com
440 Premier Circle, Suite 200, Charlottesville, VA 22901
Owner /Applicant:
E. L. Phillips and Ann P. Phillips Liv. Trust, etal C/O Caroline Molina -Ray
6704 Menchaca Road, Unit 33, Austin TX, 78745
Applicant:
Southern Development Homes, 142 South Pantops Dr.
Charlottesville, VA 22911 charlesa(a),southem-development.com
Plan received date:
8 Nov 2021
(Rev. 1)
6 Dec 2021
(Rev. 2)
20 May 2022
Date of comments:
18 Nov 2021, QC -denied /email sent November 18, 2021 4:25 PM
(Rev. 1)
30 Dec 2021
(Rev. 2)
29 Jun 2022 —background /Additional review detail
Reviewer:
John Anderson, PE
County Code section 17-410 and Virginia Code §62.1-44.15:34 requires the VSMP authority to act on any
VSMP permit by issuing a project approval or denial. This project is denied for reasons listed below. The
VSMP application content requirements can be found in County Code section 17-401.
Note: SWM design for project presents fundamental challenge: proposed CDAs exceed VA DEQ
Stormwater Design Specification No. 12 (Filtering Practices) 5.0 Ac. threshold for 3 of 5 proposed surface
sand filters. Albemarle urges reconsideration of SWM strategy for Dunlora Village. Engineering regrets
length of additional review detail, but misgivings with initial concept carry through (and worsen) with May
submittal (additional SWM facility design detail). Recommend virtual (or in -person) meeting to discuss
SWM and ESC plan design. Further submittal based on surface sand filter concept is likely unworkable.
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 ESOP, (3) a SWMP, and (4) any TMDL measures necessary.
SWPPP
1. Please ensure SWPPP cover includes reference to WP02021-00063. (Rev. 2) Not Addressed, but county
approval stamp will identify project. No follow-up required. (SWPPP will be approved once all plan
revisions complete, and SWPPP I I" x 17" ESC/SWM plan sheet inserts are revised, as needed).
2. Submit SWPPP using county template located at: (Rev. 2) All other SWPPP review comments
addressed. https://www.albemarle.orp/home/showpublisheddocument/166/637202310327530000
a. Sec. 1: Registration Statement, please complete. Feel free to call if any questions. (Rev. 2)
Addressed, As follow- At Registration Statement, Sec. II.F., please list `Albemarle' as MS4.
b. Sec. 6.A.: PPP Exhibit: Please show initial location of:
i. Rain gauge.
ii. Portable sanitary facilities (porta-john), as required.
iii. Covered non -hazardous waste dumpster, if required.
Engineering Review Comments
Page 2 of 16
iv. Vehicle wash waters, draining to trapping measure (Not a sediment trap design, per se,
but shallow depression — 1-2 backhoe buckets. Avoid direct drainage to Ex. storm
system, or pond/s.)
v. Concrete wash -out.
vi. On -site fuel, if required.
vii. Paint, stucco, chemical storage, if required.
c. Sec. 6E: List named individual responsible for PPP measures.
d. Sec. 8: Although county personnel will inspect, Applicant is required to retain qualified
(contractor/third-party) E&S inspection personnel to perform ESC inspections and evaluate
compliance relative to VESCH, 3rd Edition, 1992.
e. Sec. 9: Ensure Signed Certification is signed and dated.
B. Pollution Prevention Plan (PPP) —see SWPPP item 2.b.- above (Rev. 2) Addressed.
The PPP content requirements can be found in County Code section 17-404.
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.
Note: Applicant (letter d. 5/19/22) responds to county email (March 9, 2022 4:48 PM, J. Anderson to Chris
Mulligan) rather than Engineering review comments d. 30-Dec 2021. Given this, Engineering evaluates VW plan d. 5/12/22 against
county email, first, and 12/30/21 WPO plan review comments, second. Review comments and guidance (3/9/22 email) reflect
Engineering position on Dunlom Village development design —both are relevant.
Partial text: County email
To the concept presented:
1. 9VAC25-870-69: As design is refined with more detailed spatial elements, including size of facilities,
Engineering expects design to hold to phosphorus reduction values approximated by VaRRM.xIs. (Rev. 2)
Persists. Applicant: `This [16.33 lb.] is the maximum offsite nutrient offset/load needed to support the
current design. In all likelihood, the final dedicated Open Space area will be greater than 14.5 Ac.' As
follow-up: FOS easement, nutrient purchase. a) Sheet 2 indicates 14.5 Ac. open space phased minimums,
if Applicant coordinates FOS easement with phased plats, WPO plan must indicate (via land use table for
each phase) that post -developed land cover within a phase (x.xx Ac. FOS) provides compliance with
stormwater water quality requirements. To this end, please provide a separate VaRRM.xIs for phased
development indicating (by phase lines, and calculations) that minimums shown on plans are sufficient.
Sheet 2 lists Ph I, I1, I11, IV (4.0, 4.0, 4.0, 2.5 Ac.). FOS easement effect is seen through post -developed
land use values (a sinpJe.yJs is insufficient for evaluating sheet 2 ref to minimum phased FOS easement).
Phased road or WPO plan bonds would require phase lines, with ESC-SWM /subdivision bond expense
estimated per phase.
a. Engineering accepts that statements concerning site constraints (email below) are reasonable
factual and accurate in context of practical limits to on -site SWM water quality control.
b. Compared with initial design, current concept presents an alternative concept that avoids steep
slopes, and aims to achieve maximum on -site phosphorous reduction.
c. Engineering views 16.33 lb. remaining TP load reduction required as a reasonable and practically -
attainable target:
i. Though not representing 75% on -site phosphorus reduction, proposed concept represents
63.55% on -site reduction.
ii. With subsequent submittal, provide narrative text (SWM Notes) that specifically address
items listed at 9AVC25-8700-69.B.3.(i)-(iv).
iii. Engineering review will reference /link narrative text SWM Notes as supportive of
accepting lesser on -site reduction as sufficient.
Engineering Review Comments
Page 3 of 16
iv. Remaining approximate 16.33 lb. to be met via combination of off -site compliance
options available and outlined via state code.
2. Engineering will evaluate filtering practices in strict comparison with VA DEQ Stormwater Design
Specification No. 12 (strict evaluation against):
a. Maximum contributing CDA of 5 Ac. for surface sand filters, and a maximum CDA of 2 Ac.
(recommended for perimeter or underground filters). (Rev. 2) Persists. Applicant response
(5/12/22 letter): `Plunge pools and demising walls have been introduced to reduce the maximum
drainage area to under 5 acres to each filtration bed.' As ollow-up: Plan sheet 12 shows five
Level 1 sand filters (A-E); DA listed on plans for these 5 filters are 8.96, 2.26, 8.91, 4.18, 9.92
Ac., respectively. 3 of 5 sand filters exceed maximum CDA, significantly. It is unclear how
plunge pools /demising walls reduce CDA; plunge pools and demising walls are features within
CDAs. 5 Ac. CDA is a design maximum for a surface sand filter. Please revise design.
b. Please note that "filters have been used on larger drainage areas in the past, but greater clogging
problems have typically resulted." VA DEQ Tech Spec No. 12,.44. (Rev. 2) Reminder.
c. Soil testing requirements: Ref. 6.2, Soil testing requirements,.77, BMP specification No. 12.
(Rev. 2) Persists. Applicant: `Soil testing will be provided as available.'
d. Overall sizing requirements, Ref. 6.1., including storage volume of at least 75% of Design
Treatment Volume (p. 6). (Rev. 2) Partially addressed. Applicant: `Sand Filter Area and Volume
sizing provided in plan -set.' As follow-up: Please notify where filter sizing design calculations are
found. For example (VA DEQ Stormwater Design Specification No. 12, 6.1 Overall .S'lzlnQ): `The
storage volume is the volume of ponding above the filter. For a given Treatment Volume,
Equation 12.1 is used to determine the required filter surface area:
Where:
Also:
Equation 12.1. Minimum Filter Surface Anna for Filtering Practices
Af = (TVXdf)/[(KXhf+dfXtf)]
Af = area of the filter surface (sq. ft.)
TV = Treatment Volume, volume of storage (cu. ft.)
d f = Filter media depth (thickness) = minimum 1 ft. (ft.)
K = Coefficient of permeability — partially clogged sand (ft./day) = 3.5 ft./day
h f = Average height of water above the filter bed (ft.), with a maximum of 5 ft./2
if = Allowable drawdown time = 1.67 days
A storage volume of a least 75% of the design Treatment Volume — including the volume over
the top of the filter media and the volume in the pretreatment chamber(s), as well as any
additional storage — is required in order to capture the volume from high -intensity storms prior to
filtration and avoid premature bypass. This reduced volume takes into account the varying
filtration rate of the water through the media, as a function of a gradually declining hydraulic
head.
Where:
As follow-up:
Equation 12.2. Required Treatment Volume Storage for Filtering Practices
K = 0.75(TV )
Vr = Volume of storage (cu. ft.)
TV = Treatment Volume (cu. ft.)
Provide Equation 12.1 for Level 1 Sand Filters A and C, with calculation packet.
Provide Equation 12.2 (Val for Level 1 Sand Filters A and C, with calculation
packet. Equations (include with calculation packet) should correlate with plan
linework which in turn should delineate each post -developed drainage area.
Note: Eq. 12.1 and 12.2 are shown on profiles for filters B, D, E, but need to be
included with calculations packet. Typically, labels describe graphic elements
on a plan profile, with calculations presented with SWM information on a plan
(not on a facility profile), and more typically included with a calculation packet.
If sand filter concept is viable (concept is likely infeasible): Provide a detailed
SWM profile for each surface sand filter, A-E. See VA DEQ Spec. No. 12, Fig.
12.1. for information, which would include gravel underdrain system INV OUT,
base elev. of 18" clean washed concrete sand, 3" topsoil with labels specifying
Engineering Review Comments
Page 4 of 16
geotextile, all sides, top, and bottom. Please note that inflow bypass /flow
splitter /high -flow bypass are system elements located exterior to a surface sand
filter, such that high flows do not pass through a sand filter but instead bypass it.
This is critical to limit velocity, sedimentation, or negative impacts that may
otherwise result (obscuring, blinding, diminishing sand filter efficiency).
SECTION 4: m2 Off.
12.1 sM IL2 povNe qpi xSmtic fm a vula¢ 1 MW m w w fi[W,
FpvefLf. 9tlwtl[da4vq[a 1L+0lMs
• Note: Plan lacks specificity. Please reference DEQ Spec. 14 Level 1 filtering
practice guidance and provide comprehensive plan /profile design data required
of surface sand filters.
• Sheet 12: Label existing contours (Additional comments possible.)
e. Depth to water table and bedrock, Ref. p. 4. (Rev. 2) Persists. Applicant: `Preliminary borings
indicate this is a minimal concern, but will verify with soils borings.'
f. 7.3, Steep Terrain, several key design criteria, including (p. 11): (Rev. 2) Persists. Please see
items 2.a. and 2.d. above, and Applicant responses, below:
i. Gradient of slopes contributing runoff to sand filters, Applicant: `Utilized plunge pools
and gravel diaphragms to alleviate concerns of sediment transport.'
ii. 2-cell, terraced design for steep terrain, energy dissipation, Applicant: `Multi -cell with
riser & barrel to minimize energy transfer; only largest storm events utilize above
ground/weir flow to progress downstream to outfall locations. In particular, SWM `A'
has a concrete spillway with rip -rap toe to enter into the lowest extended detention
facility.' Asfollow-up:
1. Energy dissipation prior to a detention facility downstream of a sand filter does
not provide energy dissipation of energy at the sand filter. Sedimentation issues
may persist given mixed pervious /impervious post-dev land cover of CDAs.
2. Provide small-scale plan/profile design insets for each filter: A, B, C, D, E.
a. Recommend scale 1" =20'.
b. Level of plan/profile detail at F =40' detail is insufficient to aid review
or construct surface sand filters to clearinghouse specification.
3. Label floor dimensions of each surface sand filter.
4. Label side -slopes at each sand filter.
5. Provide small-scale plan/profile detail for distilling pond (sheet 13).
iii. Drop in elevation between cells limited to 1 foot; Applicant: `Not really applicable here,
since drainage is being conveyed via storm pipes between facilities.' Asfollow-up: DEQ
guidance refers to drop in elevation between sections of sand filter rather than between a
sand filter and downstream facility (SWM `A', extended detention basin, for example).
iv. Slope armored with river stone or equivalent
g. 7.4, Cold climate and winterperformance (p. 11). (Rev. 2) Not addressed. Applicant: `Plunge
pools and optional by-pass can be utilized to alter, protect and /or maintain the facilities while
operational.' Review of bullet items at VA DEQ Stormwater Design Specification No. 12 cold
climate and winter performance section indicate all are design -related, not post -construction add-
ons /HOA-based response to freezing /temporary flooding. Given SWM plan reliance on surface
sand filters, design stage is place to consider cold climate and winter performance concerns.
h. 6.5, Filter media and surface cover, Impervious drainage area: The contributing drainage area
should be as close to 100% impervious as possible in order to reduce the risk that eroded
Engineering Review Comments
Page 5 of 16
sediments will clog the filter. (Rev. 2) Persists. Applicant: 'These facilities are maintained by the
HOA and should have regular inspections. In addition, plunge pools and/or gravel diaphragms
have been introduced prior to most of the facilities to maximize the longevity of the filters.' As
follow-up: Maintenance by HOA will not prevent sediment reaching sand filters that may occur if
design does not recognize "the contributing drainage area should be as close to 100% impervious
as possible in order to reduce the risk that eroded sediments will clog the filter." Proposed design
neglects DEQ guidance: each filter's CDA includes substantial pervious areas. Provide, for each
sand filter, portion of CDA that is pervious (- x.xx Ac.) with rationale for why this pervious
portion is eligible for treatment via surface sand filter, or why county should have confidence that
design outside DEQ design parameters would be expected to perform well, or as intended.
i. 6.4, Conveyance and overflow (p. 8): Engineering notes BMP practice states that `Most filtering
practices are designed as off-line systems so that all flows enter the filter storage chamber until it
reaches capacity, at which point larger flows are then diverted or bypassed around the filter to an
outlet chamber and are not treated. Runoff from larger storm events should be bypassed using an
overflow structure or a flow splitter. Claytor and Schueler (1996) and RAC (2001) provide design
guidance for flow splitters for filtering practices. Some underground filters will be designed and
constructed as on-line BMPS. In these cases, designers must indicate how the device will safely
pass the local design storm (e.g., 10 year event) without resuspending or flushing previously
trapped material.' (Rev. 2) Persists. Applicant: `The larger filters have been located offline, and
also have plunge pools to minimize filter sedimentation. There are adjacent risers located within
each to pass the larger storm events.' As ollow-ug: On balance, Engineering view is this treatment
practice is a mismatch for proposed CDAs (>5 Ac.), percentage of pervious areas that drain to
each surface filter, and conveyance and overflow design. Also, please see item 2.d.(ollow-
Ap)bullet 3, Fig. 12.1—inflow bypass/flow splitter/high-flow bypass are located exterior to a sand
filter. Risers within a sand filter do not meet DEQ bypass specification, design requires revision.
As example of obstacle/s that prevent approval of proposed design, see sheet 13 SB#1 conversion
to SWM A. Please compare plan /profile to each other, and to DEQ design specifications.
i. Plan:
SWM A - 2 FILTRATION FACILITIES wl DOWNSTREAM LEVEL 1 EXT'D DETENTION
1. Vehicular access to features 2., 3., 4. in treatment train is not provided, and
perhaps not possible given prevalence of surrounding steep slopes. SWM
facilities require vehicular access (10' wide, Max. grade 20%).
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Engineering Review Comments
Page 6 of 16
2. Design is unclear at MH structure at sand filters, profile does not resolve
confusion. What happens at MH? Does portion of flow split at this location?
Profile indicates point of entry to surface sand filters, where all flow appears to
enter, and a separate exit (while plan shows single entry/exit).
3. Without existing contour labels, it is not possible to evaluate proposed treatment
basin (1.,2.,3.,4.). For example, spillway (50cy Cl. I riprap) appears proposed in
cut section, but review requires assumptions. Provide existing contour labels in
any inset detail for all SWM facilities (A, B, C, D, E).
4. Note: While plan /profile suggest conversion from ESC (SB) to SWM facility,
amount of information and detail presented /required to be presented requires
smaller scale, with comprehensive elevation data for each treatment train
feature: distilling pond, sand filter, dry pool, Level I1 ED, etc.
5. Profile labels also appear in plan view to guide review and construction.
6. Function of 11.5' high wall between the two surface sand filter beds is unclear.
(TW =400', BW=388.5') Please clarify.
7. Unclear where /how SWM A plunge pool weir box functions (detail, Sheet 17).
ii. Profile
SB#7 CONVERSION TO SWM A
THE ADAPTER SHALL SO THE "NEX OURREAfT ABUSE ON TO ASO L"ICONNECTOR TO TH. MANHOLE ,..NATO TIGHT INIAL OF THE PIPE TO
CONSIST OF A RUBBER GAISKET WHICH PITS CLOSELY INTO THE CORRUCATION Of
,pW ,1H0 12. 1].W I..10� .6.m
SWM POND 1 PROFILE
HT=40; V:1'=10
1. 41.32% pipe slope; Max. =16%, else anchors (per VDOT drainage manual
standard 9.4.8.7, pg. 9-37). Proposed slope is well over twice max. slope that
requires anchors. Revise slope. Provide anchors.
2. Plan shows 4 distinct treatment features with embankments between each.
Profile, showingjust 3 embankments, is inconsistent with plan view.
3. Profile reverses left -to -right sequence of plan, is unconventional and confusing.
4. Profile pipe slope (41.32%) as well as sheet 17 OP1A Q10=37.95cfs is outside
limits of design for a surface sand filter. Design does not meet DEQ
specifications for conveyance /bypass.
5. Profile labels include `dry pool' and `distilling pond', unfamiliar terms. Cannot
evaluate dry pool and distilling pond based on design data provided.
Engineering Review Comments
Page 7 of 16
6. Two `top' elevations are shown for single riser, profile should clarify whether
373.25 or 375.00 is accurate, or both are accurate with one applying to sediment
basin #1, and the other to (modified) top elevation for Level II ED. Please
clarify.
7. Profile should include base detail for each riser.
8. Provide buoyancy calculations for ED.
9. Proposed grade, floor of ED, is discontinuous, show continuous floor.
10. ED includes 3 orifices: 12", 8", 6" DIA, with 6" orifice INV=364.00"
coinciding with WQ Vol.=15,762 C.F. @ NWSE=364.00'. Design intent
unclear. ED typically releases detained volume (no permanent pool), yet it
appears possible feature 4., profile, proposes a permanent pool.
11. MH ht. > 12' requires safety slab. Provide VDOT SL-1, label profile as needed.
12. Complete linework for pipe that conveys runoff between sand filter and ED.
13. Show and label each pipe end section (ES-1, for example).
14. Show /label 11.5' high wall.
15. Provide detailed geotechnical design for 11.5' high wall. Consider saturation,
proximity to sand filters, filter underdrains, and future maintenance.
16. SWM A plunge pool weir box (sheet 17) proposes 3 rows of (2) -l" holes.
a. Debris cage with openings <1" square is problematic.
b. If weir box 1" holes obstruct, nothing prevents volume of stormwater
bypassing plunge pool during certain events, with negative effect on
surface sand filters (if weir box upstream of filters).
c. Show /label SWM A plunge pool weir box in profile view.
d. Note: cannot correlate SWM A plunge pool weir box elevation details
(sheet 17) with sheet 13 profile (or plan) elevations.
iii. It is difficult to understand plan /profile details, sheet 14: SB#2 conversion to SWM B &
C filtration facilities. For example:
1. Design appears to route street runoff to SWM B, but without provision for high -
flow bypass. At `B', plan/profile indicates:
a. Top sand filter=401.00 (plan, but top of sand filter is not shown in
profile).
b. Filtration filter `B' Tv calculation shown on profile, include in calc.
booklet.
c. Inlet pipe to filter `B' not shown.
d. Filter B' underdmin not shown.
e. MH (splitter) design at filter `C' not shown (if overlooked, please
notify).
f. SWM Pond 2 profile (caption) ambiguous.
i. Floor elev. SWM Pond 2 unclear.
ii. NWSE SWM Pond 2 unclear.
iii. Bypass structure detail missing, unless overlooked.
iv. Unclear if Filter `C' plunge pool intended to receive plunge
pool weir box, since Filter `A' and `D' have either plunge pool
weir box, or plunge pool dewatering details (sheet 17).
Note: plunge pool `C' has depressed floor with only apparent
outlet to sand filter `C' (2, separated by demising wall) being
the elevation of the top of a gravel diaphragm (not shown in
profile /and no sheet 17 detail).
v. Emergency spillway discharge to undefined slopes (proposed
grade should define a channel downslope of each spillway).
vi. 8' high demising wall is not shown.
vii. Profile labels ST4 outfall and SB2 top cattail confuse.
Subsequent revision should prioritize clarity for contractor and
plan reviewer relative to trap, basin, SWM facility, sequence,
conversion, etc., and may require separate plan /profile details
for each facility at each phase, to include distinct elevations,
Engineering Review Comments
Page 8 of 16
with close correspondence between plan and profile, whether
S1392 CONVERSION TO SWM B & C FILTRATION FACILITIES
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3. It is unclear if filtering practices with associated upslope forebays and distilling ponds are capable of
meeting energy balance at release points to natural conveyances:
a. Each discharge to a natural channel or terrain feature is required to meet 9VAC25-870-66.B.3,
Energy balance. (Rev. 2) Partially addressed. Applicant: `Shown for each concentrated outfall
proposed. In addition, all capturable sheet -flow is to be directed to the SWM devices. Perimeter
Engineering Review Comments
Page 9 of 16
terrain offers some limitations to achieving this goal, and is depicted on the Outfall Summary
Sheet.' As follow-up:
i. A combined calculation packet with water quality /quantity tabulated QI_yr data for each
of the 8 outfalls depicted, with narrative (numeric /summary) information is required.
Initial submittal included a series of individual routings. Please combine individual
routings to create a single calculation packet with compliance narrative for each outfall.
ii. Provide /show (in calc. packet) energy balance equation for each outfall to a natural
receiving stream using equation listed at state administrative code 9VAC25-870-66.B.3.
a. In accordance with the following methodology:
O�i�d< LF.'(Q,aane,a 'RVme-tMrelapea)�Vorelopea
Under no rendition shall Qa , be greater than Q,ri� nor shall Q ,, be required to be less
than that calculated in the equation (Ocow` W,r ,)/RVoe,;where
I.F. (Improvement Factor) equals 0.8 for sites> 1 am or 0.9 for sites <1 acre.
Q� - The allowable peak Flow rate of runoff from the developed site.
RV, -The volume of mnofffrom the site in the developed condition.
0,a_i,, =The peak Flow rate of runoff from the site in the pre -developed condition.
RV, -The wlume of runoff from the site in pre -developed condition.
Qs_t=The peak flow rate of runoff from the site in a forested condition.
RV,,=The wlume of runoff from the site in a forested condition; or
iii. Provide calculations for flood protection for each outfall, ref. 9VAC25-870-66.C.
iv. Perform analysis to limits of analysis (9VAC25-870-66.B.4 /9VAC25-870-66.C.3.).
v. Sheet 12 depicts a series of Level Spreaders (ELS); for each ELS:
1. Provide discrete design: plan /profile (at a scale small enough to discern detail).
2. Reference ELS design criteria at VA DEQ Storrnwater Design Specification No.
2, Sheet Flow to Filter or Open Space, Table 2.2.
3. Please note maximum slope/minimum width requirements downslope of an ELS
if ELS converts concentrated runoff to sheet -flow. Without max. slope /min.
width, sheet flow reconcentrates, and must then meet energy balance.
4. Furnish figure similar to Fig. 2.3 in VA DEQ Spec. No. 2 for each ELS depicted
on plan. Please furnish information required by specification sufficient to
review, construct and inspect each ELS. Fig. 2.6 on sheet 17 is generic, this
generic detail includes this text Aabel: `dimension based on design discharge.'
Provide discrete ELS design for each ELS.
5. Consider items required of Construction Record Drawings (As -built) for VSW
(plan sheet 2), as this information is needed for each ELS and surface sand filter.
b. Can RGA provide detail or at a conceptual outline of how proposed SWM concept meets energy
balance at each discharge point? (Rev. 2) Persists. Applicant: `Runoff Reduction Calculations
including pre/post mapping provided with this submittal.' Unless overlooked, calculations (single
document) not attached to May submittal. Please see item 3.a. for information requested.
c. Is it via forebays, distilling ponds, and sand filters alone?
d. Do preliminary routings indicate this to be the case?
4. Given BMP design criteria, Engineering evaluates concept plan (as presented): (Rev. 2) Persists.
Applicant: `... hope submittal has addressed the majority of your concems.' As follow-up: Please see
comments elsewhere.
a. Likely requires adjustment /concept revision to ensure compliance with:
i. Max. CDA
ii. Impervious CDA
iii. Steep terrain design criteria
iv. SWM quality /quantity requirements (likely not met with limited number of filtering
devices proposed)
v. Note: Numerous, smaller, strategically placed filtering practices may be required.
vi. Engineering cannot approve, per ordinance (17408.C.2.), BMP design that does not
attain BMP Clearinghouse Website design criteria.
Design that deviates from BMP Clearinghouse design criteria is an unapproved BMP
(design).
Engineering Review Comments
Page 10 of 16
vii. It is not possible to provide a more detailed review of the concept, without more detail.
5. From email, below: ' Open space area of 14.77 acres shall be dedicated for restriction across the entire
80.77 acre parcel, to include future recreation fields, and access from Dunlom to a parking lot a boat ramp,
alongside."
I may be misreading this, but recreation fields or access from Dunlora to a parking lot and boat ramp are
not eligible for consideration as forest /open space from a SWM perspective.
Recreation fields are mowed. Access to parking /boat ramp, same. Eligible areas may not be developed, or
include impervious surface, or be bush -hogged more than 4 times per year. (Rev. 2) Persists. Applicant:
`Final Open Space needed is 14.5 ac. to meet WPO requirements.' As ollow-up: Engineering anticipates
FOS easement will be proposed with phased final subdivision plats. Also, related comments, elsewhere.
6. Rooftop runoff not captured /conveyed to SWM facilities: SWM /WPO plan design (and later, independent
homebuilders) may not propose release of concentrated downspout runoff to fill or cut slopes (Ref. 18-
4.3.3.C.4). (Rev. 2) Persists. Applicant: `Noted in plans. Physically capturable rear yard drainage is being
directed via rear yard swales toward stormwater inlets of facilities.' As follow-up: Please direct reviewer to
plan sheet or /linework that show rear yard drainage directed to stormwater inlets of facilities. Sheet 13
note `See Phase 1 Dunlora Village Road & Utility Plans' is helpful, to the extent Phase 1 road plan shows
rooftop runoff directed to SWM, please also show on the WPO plan, and for the entire development.
Proposed grading (unless overlooked) does not clearly define swale conveyance to stormwater inlets of
facilities. Sheet 17 includes detail titled sheet flaw measure-uncaptured, non-SWMareas with caption that
reads `Note: Utilize measure as necessary or required to prevent re -concentration of runoff from adjacent
impervious surfaces. Rear yards shall be directed to an approved SWM facility when possible.
Concentrated runoff leaving the rear yard toward steep slopes without sheet -flow management or storm -
water management controls is prohibited.' Sheet 17 note that reflects ordinance requirement/s, prohibition,
measures as needed, while helpful, lacks detail required to prevent rooftop runoff (concentrated storm
runoff) release without capture /treatment.
7. Please exercise caution when representing uncollected runoff as sheet flow, in post -developed condition,
for example: rooftop runoff requires transition to sheet flow, if not conveyed to a dedicated SWM facility.
(Rev. 2) Persists. Applicant: `Since the uncollected runoff areas were not included /treated as a portion of
the necessary stormwater compliance strategy, areas of concentration are being directed to Level Spreaders,
so as to achieve sheet -flow. Smaller, more remote and uncapturable areas —due to terrain constraints —shall
have graded yards to initiate the sheet -flow conditions.' As follow-up: Please see comments related to ELS,
elsewhere.
8. Items above relate to compliant filtering practice design ( Engineering evaluates against BMP
specifications), compliance with 9VAC25-870-66.B.3, and 9VAC205-870-69.
9. Additional comment may be possible with additional detail.
best, J. Anderson
John E. Anderson, PE I Civil Engineer II
Department of Community Development I County of Albemarle, Virginia
401 McIntire Road I Charlottesville, VA 22902
434 296 5832 ext. 3069
1. General (Rev. 2) Items below persist.
a. Provide access easement to each permanent SWM facility.
b. Provide easement for each SWM facility.
c. Provide forest -open space easement.
Engineering Review Comments
Page 11 of 16
d. Provide public drainage easement downstream of SWM facilities to the point discharge reaches a
natural receiving stream, or to the property line [see 14428].
e. Note: Easements La, b, c, d may be recorded with a final subdivision plat. Off -site easement (or
written agreement) needed to construct /maintain temporary ESC measures, sediment basins or
outfalls, for example, must be recorded (or copy of the written agreement provided to county)
prior to WPO plan approval.
f. Nutrient credit purchase is required prior to receiving a grading permit, but is not required for
WPO plan approval.
2. Provide Calculation packet: (Rev. 2) May persist. 5/20/22 submittal does not appear to include a
combined routing document. Please provide single document that combines routings, with index and
narrative summary correlating routings with summary findings.
a. Combine multiple routing documents submitted 12/6/21 into a single document. Additional
comments are possible. (Rev. 2) Persists.
b. Provide stormwater quantity and quality narrative that indicates compliance for overall
development, and at each discharge point. (Rev. 2) Persists.
c. Reference WP0202100063 on cover. (Rev. 2) Addressed.
d. Include VaRRM.xIs. (Rev. 2) Partially addressed. As ollow-up: Comments elsewhere request
VaRRM.xls to show post -developed forested condition for phased development, with minimum
post -development land cover (forest) values that correspond with intent to develop Dunlora
Village in phases. These min. values are likely to be 4,4,4,2.5 Ac. for Phases I, I1, lII, and IV.
e. Discuss runoff where plan indicates post -developed condition is sheet flow. Please note
requirements for sheet flow analysis at 9VAC25-870-66.D. (Rev. 2) Persists. Also, seefollow-up
at (email) review items 3.a.v., and 6., above.
D. Increased volumes of sheet flow resulting from pervious or disconnected impervious areas, or from physical
spreading of concentrated flow through level spreaders, must be identified and evaluated for potential impacts
on down -gradient properties or resources. Increased volumes of sheet flow that will cause or contribute to
erosion, sedimentation, or flooding of down gradient properties or resources shall be diverted to a stormwater
management facility or a stormwater conveyance system that conveys the runoff without causing down -
gradient erosion, sedimentation, or flooding. If all runoff from the site is sheet flow and the conditions of this
subsection are met, no further water quantity controls are required.
f. Transfer Sheet 16 routing data to the Calculation packet. (Rev. 2) Persists.
g. It appears from Open Space Note that WPO plan intends to rely on all non -lot areas (35.77 Ac.
Open Space) to serve as forest -open space (FOS), for SWM purposes. Please provide discrete
areas that may be placed in forest -open space easement, since not all open space managed by the
HOA will qualify as forest -open space for SWM purposes. For example, areas will be maintained
/manicured as lawn or kept clear for aesthetic purposes, will not be allowed to revert to a natural
condition, and will not meet DEQ concept of post -developed SWM forest -open space
(unmaintained, max. bush -hogging 4 x per year, no plant removal, etc.). Far less than 35.77 Ac. is
likely available to list as forest -open space in VaRRM.xls new development spreadsheet. (Rev. 2)
Withdrawn. Engineering recognizes portions of residential lots not cleared and disturbed during
construction are eligible to be placed in forest -open space (FOS) easement. Ref email sent to
RGA, June 24, 2022 8:25 AM.
h. Revise VaRRM.xIs to reflect post -developed forest -open space land cover available to be placed
in permanent deeded easement, that meets DEQ concept of FOS. (Rev. 2) Persists. Please see
comment request for individual VaRRM.xIs tied to each phased plat, since plan indicates a
minimum area to be platted as FOS easement, with each of (the 4) final subdivision plats.
3. VAC25-870-69.B.: Revise design to achieve at least 75% phosphorus reduction on -site. Sheet 17 indicates
TP load reduction required is 16.5 lb. if 35.77 Ac. is placed in (deed -protected) forest -open space. At least
12.375 lb. of the 16.5 lb. reduction must occur on -site. (Rev. 2) Addressed; also, see email section, above.
4. Please confirm that preserved steep slopes shown with this WPO plan are consistent with revised preserved
steep slopes shown on approved Steep Slopes Exhibit by RGA, approved 3/23/21 (with preliminary plat),
Dunlora Farm, SUB202000223. This appears to be the case, that the WPO plan is consistent with the
Engineering Review Comments
Page 12 of 16
Exhibit since preserved slope limits reflect saw -like points /abrupt change in limits. If inconsistent, please
revise WPO plan to ensure consistency. (Rev. 2) Withdrawn —Engineering accepts design is consistent
with Steep Slopes Exhibit.
5. Sheet 10
a. Shading of preserved steep slopes is ambiguous. It is unclear whether structures are proposed on
preserved steep slopes. Please clarify that no building sites are proposed to occur on preserved
steep slopes, that no structures are proposed to be built on preserved steep slopes. (Rev. 2)
Withdrawn. See item 2.g., previous page.
b. Please label lots. (Rev. 2) Persists.
c. Show roof leader line runoff discharge to stormwater conveyance system. Sheet flow from lots is
untenable unless roof runoff is routed to storm collection and conveyance. Contractors and home
owners routinely direct downspout runoff to graded slopes, which is impermissible. Ref. 18-
4.3.3.C.4.: "Surface water diversions. Surface water shall be diverted from the face of all cut or
fill slopes or both, using diversions, ditches, and swales, or conveyed downslope by using a
designed structure. The face of the slope shall not be subject to any concentrated flows of
surface water such as from natural drainage ways, graded swales, downspouts, or similar
conveyances." (Rev. 2) Persists. Please ref. email review item 7. above, for follow-up
comments.
d. Label SWM facilities. (Rev. 2) Partially addressed. As follow-up: Please review plan sheets that
display SWM facility design, and ensure facilities are labeled in plan view: SWM `A', `B', etc.
e. At each SWM facility outfall, indicate discharge is to an adequate receiving channel. SWM
facility discharge must be to an adequate receiving channel. (Rev. 2) Persists. Please ref. email
review item 3.a.iii-iv. above, for follow-up comments.
f. All SWM facilities appear to discharge to natural terrain. Provide energy balance equations for
each SWM facility discharge to natural terrain. Also, please see Calculation packet comment,
below. (Rev. 2) Persists. Please ref. email review item 3.a./3.a.ii., above, for follow-up
comments.
g. Delineate Forest /Open Space Easement for SWM purposes. (Rev. 2) Partially addressed. Please
ref. item 2.d. previous page, for follow-up comments.
It. Include Letter of Nutrient Credit availability, since nutrient credit purchase is proposed. (Rev. 2)
Addressed.
6. Sheets 12-15 (Rev. 2) Partially addressed. Please see email item 2_i., above, for follow-up comments.
a. Label primary spillway outfall elevation on profile.
b. Label primary spillway pipe type, length, slope, DIV IN/OUT on profile
7. Include VA DEQ Stonnwater Design Specification construction, maintenance, and periodic inspection
guidelines for extended detention facilities on the plan. (Rev. 2) Addressed.
8. Include Construction Record Drawings (As -built) for VSMP on the plans, link:
httos://www.albemarle.org/home/showpublisheddocument/3381/637327510536700000 (Rev. 2)
Addressed.
9. Display permanent outfall protection, graphically. If OP is proposed off -site, obtain permanent public
drainage easement on adjacent parcel. (Rev. 2) Addressed. Ref. sheet 17.
10. Label SWM facility outfall protection L x W x D. (Rev. 2) Addressed.
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.
1. Provide Notes, sheets 6 and 7:
a. Preserved steep slopes (>25%) to be marked in the field and reviewed by Albemarle County ESC
inspector prior to land disturbance. (Rev. 2) Partially addressed. As follow-up: Please revise
Note 3, sheet 6, 7, to read: "Preserved steep slopes (>25%) to be marked in field by contractor and
Engineering Review Comments
Page 13 of 16
reviewed by Albemarle County ESC inspector prior to land disturbance." That is: Albemarle does
not mark slopes, but reviews contractor /engineer flagging.
b. Field markers delineating preserved steep slopes are to be maintained throughout the course of the
project, are to be replaced if damaged. (Rev. 2) Addressed.
c. No permanent or temporary disturbance of preserved steep slopes is permissible. (Rev. 2)
Partially addressed. As follow-up: Please revise Note 5, sheet 6, 7, to read: "No permanent or
temporary disturbance of preserved steep slopes is permissible unless related to storm or utility
line installation, and only if. No alternative exists, impacts are minimized, slopes are immediately
stabilized, and pipe location across steep slopes is shown on an approved WPO or site plan." That
is: extremely limited temporary disturbance is permissible for utility line or storm pipe installation
only, where no other utility corridor exists, or is impracticable.
2. Sheets 6, 7
a. Label limits of disturbance, x.xx Ac. (Rev. 2) May persist. Please direct reviewer to label (x.xx
Ac.) on plan sheet 6 or 7, if overlooked.
b. Show and label ST weirs. (Rev. 2) Addressed.
c. For sediment basins, show accurate contours at entrance to each emergency spillway. (Rev. 2)
Addressed. Note: SB#3 emergency spillway removed.
d. If WPO plan is consistent with SUB202000223 Steep Slopes Exhibit, please revise to remove all
grading across preserved steep slopes. No temporary grading, ESC measures, permanent SWM
facilities, or proposed permanent grade across preserved steep slopes (limits revised per approved
SUB202000223 Steep Slopes Exhibit) is permissible. (Rev. 2) Addressed.
e. Remove SB# 1 from preserved slopes. Eliminate grading of preserved steep slopes downslope of
SB41. Additional comments possible with revised plan. (Rev. 2) Addressed.
f. Remove SB#2 from preserved slopes. Eliminate grading of preserved steep slopes downslope of
SB#2. Additional comments possible. (Rev. 2) Addressed.
g. Remove ST# 1 from preserved slopes. Additional comments possible. (Rev. 2) Addressed.
h. Remove ST#4 from preserved slopes. Additional comments possible. (Rev. 2) Addressed.
i. Eliminate grading of preserved steep slopes downslope of ST# 10. Additional comments possible.
(Rev. 2) Addressed.
j. Sediment basin and sediment trap outfalls are problematic:
i. Provide off -site permanent public drainage easement for SB#4 outfall (OP4 is off -site).
(Rev. 2) Persists. Space for future maintenance problematic: limited or non-existent
where detail shows SB#4 emergency spillway touching property line.
ii. Revise SB #2 emergency spillway, which is shown parallel with contours rather than
perpendicular to contours. Avoid emergency spillway in fill section. Ref. DEQ
Clearinghouse document, Vegetated Emergency Spillway,
htti)s: Hswbmpvwrrc.wp. prod. es. cloud. vt. edu/wp-
content/uploads/2017/11/lntroduction_App-C_Vegetated-Emergene-
Spillways 03012011.pdf (Rev. 2) Addressed.
From p. 2 of DEQ document:
SC I'lli I:A: EMVIIIUry�'IALA DU'UMMUrNffYI.UNSIDEH ilUNJ
The adjacent topogmphy (steepness ofthe abutments), the existing or proposed land use, and
other factors (such as a roadway over the embankment) influence the design and construction of
a vegetated emergency spillway.
Vegetated emergmry spillways must he built I. existing ground or `cut. Ere. though an
emergency spillway helps to extend the life expectancy of an impoundment and lowers the
associated downstream hard coalitions, it should not he located on Ray portion of the
embankment fill. Therefore, additional land disturbance beside the embankment most he
accounted for during the planning stages ofa project. Sometimes, as emergency spillway may
not be practical due to this or other considerations.
If site topography or other constraints preclude the use ofa vegetated emergency spillway in
"cm," the principal spillway rear he oversized to pass the additional flows or an armored
emergency spillway may be pmvided. A cost analysis may be helpful to aid in the selection of
the spillway type. If rumoring is chosen, then ripmp, concrete or any other permanent,
roncrodible surface may he used. Note, however, that an armored emergency spillway aver
the top of an embankment should be doeigned by a qualified prafessinnal.
Engineering Review Comments
Page 14 of 16
iii. Revise SB#3 Emergency Spillway: (Rev.2) Persists. Emergency spillway eliminated
with revision. Plan now shows sediment trap spillway at property line, which does not
address concern expressed with initial review comment.
1. Eliminate curved exit. Design using straight exit channel, per DEQ ref. doc.
From p. 3, DEQ Emergency Spillway document:
The topography most be carefully considered when constructing an emergency spillway. The
alignment of the exit channel must be straight to a point far enough below the embankment to
insure that any flow escaping the exit channel cannot damage the embankment. This may result
in additional clearing and/or grading requirements beside the abutments, property line, etc.
2. Current design overlaps emergency spillway and fill slope at base of fill slope.
There is a relatively high likelihood that flow may escape the exist channel and
erode the embankment. Revise design.
3. Insufficient space: Emergency spillway is coincident with development property
line. There must be adequate space to construct /maintain the spillway on the
development parcel. Alternatively, off -site easement is required prior to WPO
plan approval. Easements in connection with ESC measures may not be
deferred until final subdivision plat, but any easement (or written agreements)
required to implement VESCP measures must be in place prior to WPO plan
approval.
iv. SB#4 Emergency spillway: (Rev. 2) Not addressed.
1. Revise to avoid undercutting fill slope SB embankment. Provide straight exit.
v. Provide armored emergency spillway design for any SB emergency spillway proposed to
be in fill rather than cut. Consider velocity, channel dimensions, riprap classification, etc.
Provide calculations for armored emergency spillways. (Rev. 2) May persist. Please see
item 2.i.iii, above.
vi. SB#1 Emergency spillway: (Rev. 1) Not addressed.
1. Revise, since armored spillway terminates at top of fill slope, extend armor to
base of fill slope. Adhere to DEQ Vegetated Emergency Spilhvay design
criteria, and guidance
3. Sheet 8
a. Revise sediment trap design tables: (Rev. 2) Addressed.
i. Revise bottom trap dimensions to L x W, for each ST.
it. Provide weir length, each ST.
4. Sheets 12-15
a. Since each SB is destined for permanent service as a SWM facility, include relevant geotechnical
design, construction, inspection, and maintenance notes on the plan. Ref. VA DEQ Stormwater
Design Specification, Appendix A, Earthen Embankment. Without these notes, there is risk or
likelihood of inadequate geotechnical material testing, inspection, etc. during construction of
embankments that transition from VESCP measure to permanent SWM facility. (Rev. 2) Not
addressed. Note: Vertical interval top of dam SB#1 to 24" primary spillway pipe INV OUT>20'.
Geotechnical considerations are critical for earthen embankments.
b. Provide sediment basin profiles. Do not combine SB and SWM detention as a single profile. SBs
are temporary ESC measures, bonded, built, inspected, and maintained independent of future use
as a SWM facility. Segregate ESC profiles from SWM profiles. (Rev. 2) Partially addressed. As
follow-up:
i. SB#1. sheet 13 (comments (may) also apply to SB#2, SB#3, SB#4):
1. SB #1 profile indicates 1,380.2 CY storage required, 749.0 CY storage provided.
SB#1 appears undersized. If so, revise SB#1 to provide sufficient stored volume.
2. Provide profile consistent with VESCH Plate 3.14-2, to include these elevations
(in addition to top of dam and riser crest top elevations provided):
a. design high water
b. top of dam
c. dewatering device
Engineering Review Comments
Page 15 of 16
d. sediment clean -out point
e. bottom of base of riser structure
3. Show continuous proposed grade across floor of sediment basin #L
4. Show clay core, cutoff trench, collars, as needed. Sheet 17 typical provides
inadequate detail.
5. If collars required, show in profile, and label collar dimensions.
6. Show /label 24" pipe end section (VDOT ES-1, for example).
7. Permanent riser ht. >12' requires safety slab (VDOT SL-I).
8. Label SB#1 floor dimensions (L x W).
9. Provide baffles, as needed.
10. Label 24" pipe INV OUT.
ii. SB#2, sheet 14
1. Label ST#14 in plan view.
2. Apply SB#1 comments to SB#2, as needed.
3. WQV (storage) required /provided not listed on profile.
iii. SB#3 & Outfall to SB#4 (sheet 15)
1. Recommend compaction note for MH /pipe nearly entirely in fill section.
2. Comments for SB#1 may above also apply.
3. Label is confusing: `Sediment basin 3 shall be a sediment trap until the storm
sewer outfall into sediment basin 4 is constructed (limit contributing drainage
area to less than 3 Acres). Note: Divert all runoff to SB#4 until basin & SWM
`D' outfall is operational'. Clarify proposed sequence /interdependence of SB#3
(basin/trap), SB#4, SWM `D'.
iv. SB#4, sheetl6: Apply SB#1 comments, as needed.
c. Provide Sediment basin design (tables), per VESCH, 3.14 (size, baffles, collars, spillways, etc.).
(Rev. 2) Not addressed.
Process:
After approval, plans will have to be bonded. The bonding process is begun by submitting a bond estimate request
form and fee to the Department of Community Development. One of the plan reviewers will prepare estimates and
check parcel and easement information based on the approved plans. The County's Management Analyst will
prepare bond agreement forms, which will have to be completed by the owner and submitted along with cash,
certificates or sureties for the amounts specified. The agreements will have to be approved and signed by the County
Attorney and County Engineer. This may take 24 weeks to obtain all the correct signatures and forms.
Stormwater Management Facilities Maintenance agreements will also have to be completed and are recorded. The
County's Management Analyst or other staff will prepare the forms and check for ownership and signature
information. The completed forms will have to be submitted along with court recording fees.
This project appears to propose nutrient credit purchase, may require off -site written agreements or easement to
support ESC measures, and requires forest -open space easement (for SWM purposes), SWM facility and SWM
facility access easement, and public drainage easement. (Off -site easement or written agreement with adjacent
landowners required to construct or maintain temporary ESC measures is required for WPO plan approval.)
After bonding and agreements are complete, county staff will enter project information in a DEQ database for state
application processing. DEQ will review the application information based on local VSMP authority approval. At
this time, the DEQ portion of the application fees will have to be paid directly to the state. For fastest processing,
this is done electronically with the emails provided on the application. DEQ should notify applicants with
Engineering Review Comments
Page 16 of 16
instructions on how to pay fees. When DEQ approves the application, they will issue a permit coverage letter. This
should be copied to the county.
After DEQ coverage is issued, via the coverage letter, the County can hold a pre -construction conference.
Applicants must complete the request for a pre -construction conference form, and pay the remainder of the
application fee. The form identifies the contractor and responsible land disturber, and the fee remaining to be paid.
This will be checked by county staff, and upon approval, a pre -construction conference will be scheduled with the
County inspector. At the pre -construction conference, should everything proceed satisfactorily, a joint VSW and
grading permit will be issued by the County so that work may begin.
County forms can be found on the county website forms center under Engineering Applications:
httos: //www. albemarle. ore/government/community-develonment/aooly-for/engineering-anol is ations
Thank you.
Please call if any questions — tel. 434.296-5832-x3069, or email 4anderson2(&a1bemarle.org .
WPO202100063—Dunlora Farm-Village-062922rev2