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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 @embarqmail.com]
Woodbrier Associates L [P. O. Box 5548, Charlottesville VA 22905]
Plan received date: 11 April 2014
Date of comments: 22 May 2014
Reviewer: John Anderson
A. Stormwater Management Plan (WP0201400030)
1. 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').
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.
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)
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 44, 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,
above (or elsewhere). Furnish topographic map that shows project site's (Minus DA #4) 1% relationship to
watershed.
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.
Engineering Review Comments
Page 2 of 3
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.
7. 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.
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.
9. 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.
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.
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.
12. INV in/out for three -66" DIA pipes appear slightly off (0.03'); please check against 0.5% slope and revise
if necessary.
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
3613") and arrow are confusing.
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.
15. ConTechTM manufacturer's approval of the underground detention system design is required. ConTech plan
details are not legible.
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.
B. Erosion Control Plan (WPO201400030)
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).
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
Engineering Review Comments
Page 3 of 3
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.
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
the 2 -phase ESC plan under the Road plan WPO will be followed. DAs in particular should match.
4. With revision of DA contributing to the sediment trap, area and volume design requirements of the trap will
likely increase.
5. Furnish diversion dikes upslope of silt fence, adjacent to U.S. Rte. 29.
6. Show SAF on plans. Briarwood Drive is in close proximity to residential properties.
7. Identify any off -site borrow or waste sites.
File: WPO201400030 -UG storage – Briarwood - 052114