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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road, Room 227
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project:
Briarwood Underground Detention
Plan preparer:
Scott Collins; Collins Engineering [200 Garrett St., Suite K, Charlottesville, VA
22902, scott(a)collins -en ing eering com]
Owner or rep.:
Wendell Wood and Nena Harrell [ulcwww @embargmail.com]
Woodbrier Associates L [P. O. Box 5548, Charlottesville VA 22905]
Plan received date:
11 April 2014
Rev. 1
10 June 2014
Date of comments:
22 May 2014
Rev. 1
16 July 2014
Reviewer:
John Anderson
A. Stormwater Management Plan (WP0201400030)
Revise routing calculations to eliminate outlet structure 3. Change outlet structure 1 type to orifice, or
revise the design so that weir flow is more likely. Please revise SCS routing calculations. Storage pipe
length should be identical in plan section view (sheet 5) and as model input (pipe 2 given as 181'). (Rev. 1)
Comment addressed.
2. Update (or replace) critical slopes (sheet 2) with managed slopes (County GIS -web) so that review can
compare development with steep slope overlay district, and preservation or management requirements.
Steep slopes exist within project LOD. (Rev. 1) Comment addressed.
3. Drainage areas are confusing. For example, DA #lA is problematic, and does not define an actual drainage
area. This DA does not exist in prior submissions (see pre - development DA, WP0201300072, 11/25/13,
sheet 2; structure 24 DA, SUB201400066, sheet 8 of road plan). Please show drainage divides' upper
boundaries coincident with topographical high points. Topographic lines or labels require attention and
revision at several locations: Post - development DA (sheet 3) should bear close resemblance to post -
development DA, sheet 8, SUB201400066, which shows contours for the same development (portion of
new roadway); eliminate post - development contours associated with roadway from pre - development view
(Master Stormwater Development Plan —sheet 3); DA #lA is called out as 1.02 Ac. in plan view, 0.70 Ac.
in Watershed Summary table (sheet 4); and several other plan view -table mismatches (DA #1C /post; totals)
(Rev. 1) Comment addressed.
4. Engineering comment (19 Dec 2013): "Please provide MS -19 with all WPO applications." With this
application, information pertaining to MS -19 and proposed development outlined in Briarwood Channel
Adequacy Report (Nov -4, 2013) has changed. The development areas and drainage areas have changed.
With loss of DA #4, required points of analysis for receiving channels now include outfalls from SWM
facilities to pipes on the west side of U.S. Rte. 29 (existing 24" CMP and 18" RCP associated with MS -19
cross - sections 1 and 3). With elimination of DA #4, 1% analysis continues beneath U.S. Rte. 29 to Herring
Branch. Please include MS -19 for downstream 1% analysis points along Herring Branch. Analysis should
reflect routing calculations which will change given control structure and DA revisions noted in items #1, 3,
Engineering Review Comments
Page 2 of 4
above (or elsewhere). Furnish topographic map that shows project site's (Minus DA #4) 1% relationship to
watershed. (Rev. 1) Comment partially addressed: See ESC comment #1, WPO201400059, 7- 14 -14,
and ESC comment #11, WPO201300072, 7- 15 -14: MS -19: photos indicate erosion down slope of pipe
outfalls beneath U.S. Rte. 29, an indication that receiving channels are inadequate. The computations
should reflect this, and appropriate measures should be specified to correct the problem. The computations
(roughness and velocities) appear to indicate conditions of adequacy that do not exist.
5. When submitting revised MS -19 data or report, please ensure that channel cross - sections of ditches match
existing ditches, that photographs and captions correspond with analysis points. (Rev. 1) Comment
partially addressed. In response to letter request received with revised MS -19 report (rev: 9- Jun -14),
channel cross section locations, Channel Adequacy Exhibit (watershed areas; d. 6/4/14), and photographs
present suitable MS -19 analysis points. Only cross - section locations are provided. Channel geometry is
not included with MS -19 report, but would help when evaluating statement of adequacy for cross sections
#1 and #3 against existing conditions (slope ditches) at outfalls of 24" DIA and 18" DIA storm drain pipes
beneath U.S. 29.
6. Given that post - development DA #1C is slightly smaller than pre - development DA #1C, and with increased
impervious area, the post - development CN value should increase rather than decrease. (Rev. 1) Comment
addressed.
Final build -out (SCS routing, sheet 4) chart references inclusion of 28,OOOsf, post - development chart
references exclusion of 34,OOOsf. Correct discrepancy or clarify. (Rev. 1) Comment addressed; per
discussion/e -mail [June 26, 2014 10:47 AM], anticipate post - development chart will reference 29,500 SF.
8. Water quality: BMP Computations for Worksheets 2 -6 list %RR = 58 %. Provide required treatment for
improvements shown on this plan. The grass swale does not appear sufficient. Water quality treatment
requirements attach to proposed development; requirements that apply to proposed development may not be
met by referencing treatment elements associated with projects which may be delayed, or never built. (Rev.
1) Comment addressed if project eligible for technical review criteria found at 9VAC25- 870- 93/ -99
(technical criteria in effect before Jul- 1 -14). Design provides 4 Filterra units on Road `A' [ref. sheet 5, str.
#28, 30, 36, 38; letter, Filterra to Collins Engineering, June 9, 2014 affirms design adequate for contributing
drainage area]. VDOT restrictions on use of Filterra units within public RW set practical limits on extent of
water quality treatment.
Elm Tree Court cul -de -sac is not included in any post - development drainage area. Please include Elm Tree
Court turnaround (increase in impervious area) in hydrologic models, DAs, etc. It appears runoff will reach
the cul -de -sac and release to slopes. Provide treatment for this area. (Rev. 1) Comment addressed;
structure 46, DI.
10. Show 24" CMP and 3' X 3' box culvert continuing beneath U.S. Rte. 29 SBL/NBL; sheet 2 is confusing in
this respect. (Rev. 1) Comment addressed.
11. Do not show the proposed three line SWM underground detention system as pre - development;
WPO201300018 approved a two -line system which was never constructed. Pre - development condition is a
sediment basin that serves as SWM control for a portion of Briarwood Drive. (Rev. 1) Comment
addressed.
12. INV in/out for three -66" DIA pipes appear slightly off (0.03'); please check against 0.5% slope and revise
if necessary. (Rev. 1) Comment withdrawn, per Collins' response, but revisions were actually made to
Engineering Review Comments
Page 3 of 4
the underground detention system approved Aug -20, 2013 (WPO201300018; 2 -line system). Current
design proposes a 3 -line underground detention system.
13. Show STM 36A in detail inset (sheet 5); show 24" CMP between STM 36A and 36B as section of pipe to
be removed. Note ( "Contractor shall plug and remove the existing riser & barrel upstream of structure
36B ") and arrow are confusing. (Rev. 1) Comment addressed.
14. It is unclear how the proposed 1' tall weir plates located inside each 66" DIA storage pipe will divert 1 x
WQV to the grass swale BMP given that top elev. of the lowest weir is 403.84'while INV IN of the 15"
bypass overflow is 408.5'. Unless water elevation at top of 1'weir plates is > 408.5', it would appear that
water in the storage system will not reach the grass swale via the 15" bypass overflow. Please clarify.
Provide a profile with these elements. (Rev. 1) Comment addressed.
15. ConTechTM manufacturer's approval of the underground detention system design is required. ConTech plan
details are not legible. (Rev. 1) Comment partially addressed; please confirm that Jul -8, 2013 ConTech
letter that affirms adequacy of design applies to 3 -line system under consideration, and not the 2 -line system
approved under WPO201300018. The debris cage detail is faint; please furnish legible detail.
16. Show pipe DIA and L for existing and proposed pipes beneath Briarwood Drive that carry ditch flow south
to north, across Briarwood Drive, near Int with U.S. Rte. 29. Existing pipe is approx 125 -ft, while
proposed pipe is approx 140 -ft in length. (Rev. 1) Comment addressed.
B. Erosion Control Plan (WPO201400030)
Text, J. Anderson email, 6/2/2014 3:54 PM: "It would be fine to transfer ESC plan for the
underground detention system into the Road Plan WPO since Road Plan WPO already integrates
UG storage construction into a 2 -phase ESC plan. It would be clearer if all aspects of ESC required
to install the UG detention system are located on the Road Plan WPO."
1. Inlets on Briarwood Drive (STM -38, STM -37) outfall to the existing sediment basin that serves as storm
management via a 15" RCP (see sheet 2, 8/6/13/ WPO201300018 plan), but note on Sheet 7 does not
account for this runoff. It appears that the sediment trap should be designed to handle runoff from DA #113
( =2.30 Ac). (Rev. 1) Comment addressed.
2. The sediment trap outfall (stone weir) must continue to the roadside ditch (sheet 6, 8/6/13 plans). Furnish
scale plan/profile drawings of revised sediment trap, not just typical from VESCH, 1992 Edit. Show wet
storage side slopes of 2:1 or flatter. Provide 2:1 (min) length:width ratio for flow path. Proposed trap
dimensions (98'W x 14'L flow path) yield 0.14 ratio for overland flow, but ratio of 7.0 if overland flow is
diverted (dike) to the south end of the sediment trap to join 15" RCP runoff at that point. Diversion dikes.
not SF, must direct runoff to the sediment trap. Trap is centered on an existing drop inlet. Adapt trap
design to preserve inlet function and capacity suggest plate 3.08 -2, VESCH. (Rev. 1) Comment may
apply if sediment trap restored as ESC measure for underground detention system installation. [see
WP0201300072, Road Plan WPO, ESC plan review comment #6, 7/15/14]
3. The Road plan WPO (WPO201300072) indicates a 2 -phase ESC plan will be followed to make partial
roadway improvements ( Briarwood Drive /Elm Tree Court), and, in phase -II, to construct Road `A' and
Road `B'. Between Phase I and II, the underground SWM facility proposed with this application will be
built under its own ESC plan. Please ensure that ESC considerations tied to drainage areas contributing to
the existing sediment basin at the end of phase I road building are evaluated under this application. This
application bridges ESC Phase I and II of the road plan WPO. Phase I runoff, layout of piping, and
contributing drainage areas should match this application. Please revise ESC plan under this application Y
Engineering Review Comments
Page 4 of 4
the 2 -phase ESC plan under the Road plan WPO will be followed. DAs in particular should match. (Rev.
1) Revised comment (since sediment trap for underground storage system installation has been removed):
There is no apparent provision for ESC during installation of the underground detention system north of
Briarwood Drive. Please provide ESC for this phase of work. [Ref. 7/15/14 WPO201300072 ESC plan
comment #6]
4. With revision of DA contributing to the sediment trap, area and volume design requirements of the trap will
likely increase. (Rev. 1) Comment irrelevant, unless sediment trap restored to limit off -site sediment
transport during underground storage system installation. ( #2, above)
5. Furnish diversion dikes upslope of silt fence, adjacent to U.S. Rte. 29. (Rev. 1) Comment not addressed;
comment is relevant. ESC between underground storage system installation and U.S. 29 is required.
6. Show SAF on plans. Briarwood Drive is in close proximity to residential properties. (Rev. 1) Comment
partially addressed; furnish SAF as necessary to discourage trespass from neighboring residential areas.
Excavation is required to install the underground detention system. Show ESC features required to install
underground detention system north of Briarwood on WPO201300072, ESC sheets. [Ref. 1. -5., above.]
7. Identify any off -site borrow or waste sites. (Rev. 1) Uncertain if addressed. Please identify any off -site
borrow or waste sites.
8. (New) Sheet 5 — Typical Filterra Sidewalk Configuration: If geometry of typical detail is to be used, show
sidewalk width (plan view) at each location Filterra units will alter sidewalk width. Also, please correspond
or speak with Chris Perez since he has explained he has not seen and is not familiar with design intent to
follow or adopt an alternative to uniform sidewalk width. Filterra /sidewalk configuration has planning
implications; it may not meet minimum sidewalk width requirements.
9. (New) Per e-mail (June 26, 2014 10:47 AM), design will be revised to lower weir plate elevation to 407.55'
(sheet 4). SCS routing calculations will reflect increased segment area, modeled as an orifice. [SCS
routing, sheet 3, outlet structure 3: Equiv. DIA = 8.4 "]
File: WPO201400030 -UG storage— Briarwood - 071614Rev -1