HomeMy WebLinkAboutWPO201300044 Correspondence 2013-09-03 COLLINS 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902
434.293.3719 PH 434.293.2813 FX
www.collins-engineering.com
Glenn Brooks
Community Development
County of Albemarle
401 McIntire Road, Room 227
Charlottesville,VA 22902
RE:Avinity-ACSA&WPO Amendment Plan (WPO 201000057)
Thank you for your comments on the project referenced above. Please let this letter supplement the
revised plans in response to your comments dated August 1s`, 2013, as well as the feedback provided to
Mr. Murray of Collins Engineering during your August 8th e-mail correspondence to him and the meeting
held on August 15th with County Engineering.
Erosion &Sediment Control Comments (Comment letter dated 8/1/13)
The limits of disturbance has been revised on the erosion and sediment control plan to only include the
area of construction for the sediment basin and utility relocations, more specifically, the area west of
the residence. Sheet DP-1 has also been revised to include text mandating the sanitary sewer pump
around be performed in dry weather, please see the sequence of construction.
Storm water Management(Comment letter dated 8/1/13)
The plans have been revised to include an aquatic bench for the retention basin.The facility now
provides a removal efficiency of 65%and has been enlarged.
Additional Comments (8/8/13 e-mail correspondence and 8/15/13 meeting with Engineering)
1. The facility has been revised to have an aquatic bench for the entire perimeter of the basin,with
the exception of where the forebay is proposed.The aquatic bench is not proposed in this
location because plantings would likely be damaged during maintenance operations and this is a
typical design standard. During the August 15th meeting the aquatic bench configuration was
deemed an acceptable resolution to this comment.
2. The plans have been revised to be safer for pedestrians. In addition to having an expanded
aquatic/safety bench in the upper 12" of the pond, other revisions have been made to the plan
to make it more pedestrian friendly. On the southern and eastern portions of the facility a fence
is proposed on top of the retaining wall to reduce risks for the pedestrians.The northern portion
of the facility has a 10' wide top of dam and access to reduce pedestrian risks and the western
portion of the pond now has a 3' level safety bench. During the August 15th meeting these
measures were deemed acceptable revisions to the plans to address this comment. Also note
the sediment basin grading has been revised to be pedestrian friendly.The sediment basin now
has a 3' level safety bench or guardrail around its entire perimeter in locations where a wider
10'safety bench is not located. An orange safety fence with warning signs is also proposed
around the sediment basin. These revisions provide adequate safety for pedestrians during all
phases of construction.
3. The plans were revised to have a smaller forebay.The forebay shown is in general accordance
with what was deemed acceptable during the August 15th meeting.
4. During the August 15th meeting various locations for the proposed spillway were discussed.
Upon further evaluation, and at the request of Engineering, the principal spillway was
redesigned to pass more flow and therefore reduce the width of the emergency spillway. The
reduction in the spillway width, coupled with a revised grading plan, allowed for the revised
spillway location to be placed in cut. This no longer required the facility to be armored with
concrete. Though in accordance with the August 15th meeting the spillway is now armored with
a riprap lining. These revisions are in accordance with Engineering's recommendations during
the August 15th meeting.
5. The plans were revised to relocate the outfall location of the storm sewer and to only have one
point of inflow into the facility.This revision allowed the forebay to be smaller in accordance
with comment 3 referenced above, as well as allowed the SWM access to terminate at the
outlet of structure 4C. During the August 15th meeting this configuration was deemed an
acceptable resolution to providing adequate access to the forebay and the storm sewer's outfall
location.
6. During the meeting with Engineering it was determined the storm sewer connecting the
stream's inlet, structures 65C and 65D, to structure 65A would adequately address this
comment. As a result of this the plans were revised to include a DI-7 to redirect drainage to
structure 65A and the pond. It is intended that this DI and pipe also be used for the outfall of the
future development into the pond.
If you have any questions regarding the proposed plans please do not hesitate to contact me.
Sincerely,
Graham Murray, PE
U.S.Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06J04
TR 55 Worksheet 2:Runoff Curve Number and Runoff
Project: Avinity(Approved Layout&Potential Future Development) Designed By: FGM,PE Date: 9/3/2013
Location: Located off Avon Street Extended,South of Cale ES Checked By: FGM,PE Date: 9/3/2013
Check One: Present X Developed X
1.Runoff curve Number(CN)
Soil name and Cover description
Drainage Area (Cover CN(weighted)_
hydrologic group type,treatment,and hydrologic condition; CN Area Product of total product/ Calculated
Description percent impervious;unconnected/connected
(Appendix A) p (Acres) CN x Area total area S'Value
impervious area ratio)
D A#lA Impervious Areas 98 0.20 20.0
B Woods in Good Condition 55 8.58 471.7 56.4 7.73
(Present)
Lawns in Good Condition(75%+Groundcover) 61 0.77 46.9
D.A#18 Impervious Areas 98 0.00 0.0
(Present)
B Woods in Good Condition 55 26.26 1444.3 55.0 8.18
Lawns in Good Condition(Approx.15%) 61 0.00 0.0
D.A#SC Impervious Areas 98 0.00 0.0
(Present)
B Woods in Good Condition(Approx.25%) 55 2.09 114.7 59.5 6.81
Lawns in Good Condition (Approx.75%) 61 6.26 381.6
D.A.#1A
(Developed)
B Approved DA#1A Watershed 81.6 9.11 743.4 81.6 2.25
D.A.18 Impervious Areas 98 . 0.00 I 0.0
(Not Modified) I 8 Woods in Good Condition 55 26.26 1444.3 55.0 8.18
Lawns in Good Condition(Approx.15%) 61 0.00 0.0
D.A.1C Impervious Areas(incl.previously bypassed areas) 98 4.63 453.8
B Woods in Good Condition(Approx.25%) 55 0.00 0.0 80.5 2.42
(Developed) Lawns in Good Condition (Approx.75%) ' 61 4.15 253.1
2.Runoff
2-Year Storm 10-Year Storm Drainage Area Description
Frequency-years 2 10 n/a
Rainfall,P(24 hour)-inches 3.7 5.6 n/a
Runoff,Q-inches 0.47 1.39 D.A#1A(Present)
Runoff,Q-inches 0.42 1.29 D.A#1B(Present)
Runoff,Q-inches 0.60 1.63 D.A#1C(Present)
Runoff,Q-inches 1.92 3.58 D.A.#1A(Developed)
Runoff,Q-inches 0.42 1.29 D.A.1B(Not Modified)
Runoff,Q-inches 1.83 _ 3.47 -D.A.1C(Developed)
U.S.Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06/04
TR 55 Worksheet 3.lime of Concentration(T )or Travel Time IT,)
Project: Avinity(Approved Layout&Potential Future Development) Designed By FGM,PE Date: 9/3/2013
Location: Located off Avon Street Extended,South of Cale ES Checked By FGM,PE Date: 9/3/2013
Check One Present X Developed X
Check One T„ X T, Through subarea n/a
D.A NIA D.A N1B ' D.AR1C I D.A.#1A D.A.16 D.A.1C
Segment ID: (Present) (Present) (Present) (Developed) (Not Modified) (Developed) [
Sheet Flow:(Applicable to T ,only)
Dense
1 Surface description(Table 3-1) Gras;
2 Manning's roughness coeff.,n(Table 3-1 0.24
3 Flow length,L(total L c 100)(ft) 100
4 Two-year 24-hour rainfall,P ,(in.) 3/
5 Land slope,s(ft/ft) 0.04
6 Compute T,_(0.007(n•L('5)/P.,as s°' f .1 I 0.17 ) I I 1 I I
Shallow Concentrated Flow:
7 Surface description(paved or unpaved) Unpaved
8 Flow Length,L(ft) 700
9 Watercourse slope,s(ft/ft) 007
30 Average velocity,V(Figure 3-1)(ft/s) 4.2
11 T,=L/3600`V ' ! 005 i I 1 1 ' I
Channel Flow:
12 Cross sectional flow area,a(ft `)
13 Wetted perimeter,P,(ft)
14 Hydraulic radius,r=a/P w(ft)
15 Channel Slope,s(ft/ft)
16 Manning's Roughness Coeff,n
17 V=[1.49r�7's0-')/n
18 Flow length,L(ft)
19 T,=L/3600`V I I I 1 I I J
20 Watershed or subarea T or 7, Approved Approved Approved Approved Future
IAdd T,in steps 6,11 and 19) ` 0.25 I 0.42 I 0.22 1 0.22 1 0.42 I 0.17 I I '
(Formerly DARS) (Formerly DA#3) (Formerly D703)
U.S.Department of Agriculture
FL-ENG-21C
Natural Resources Conservation Service 06/04
FR 55 Worksheet 4.Graphical Peak Discharge Method
Project Avinity(Approved Layout&Potential Future Development) Designed By. FGM,PE Date: 9/3/2013
Location: Located off Avon Street Extended,South of Cale ES Checked By FGM,PE Date: 9/3/2013
Check One Present X Developed X
Drainage Drainage Area Drainage Area Drainage Area Drainage Area Drainage Area Drainage Area
1.Data Area Description Description Description Description Description Description
D.A NIA D.A R1B D.A atC DA a1A U.A.18 D.A.1C
(Present) (Present) (Present) (Developed) Not Modified) (Developed) ,
Drainage Area(Am)in miles '= 00149 00410 0.0130 0.0142 0.0410 00137
Runoff curve number CN= 564 55.0 59.5 81.6 55.0 80.5
Time of concentration(To)- 0.25 042 0.22 0.22 0.42 0.17
Rainfall distribution types II ii Ii li II u
Pond and swamp areas spread 0 0
throughout the watersheds I i (.• 0 0
2.Frequency-years 2 10 2 10 2 10 2 10 2 10 2 10
3.Rainfall,P(24 hour)-inches 3.7 5.6 3.7 5.6 3.7 5.6 3.7 5.6 3.7 5.6 3.7 5.6
4.Initial Abstraction,la-inches 1.546 1.546 1.636 1.636 1.361 1.361 0451 0.451 1.636 1.636 0.484 0.484
5.Compute la/P 0.42 0.28 0.44 0.29 0.37 0.24 0.12 0.08 0.44 0.29 0,13 0.09
6.Unit peak discharge,Qu-csm/in 475 650 350 500 575 700 750 775 350 500 725 825
7.Runoff,Q from Worksheet 2-inches 0.47 1.39 0.42 1.29 0.60 1.63 1.92 3.58 042 1-29 1.83 3.47
8.Pond and Swamp adjustment factor,Fp 1 1 1 1 1 1 1 ; 1 1 1 1
9.Peak Discharge,Qp-cfs Routed through Routed through Routed through
where Qp=Qu Am Q Fp 3.33 13.53 5.97 26.54 448 14.84 SWM Facility SWM Facility SWM Facility
•
NOTE:For SWM Facility routing,the accumulation of postdev.DA 41A,1B&1C is 44.15 ac.w/Tc=O 42 hrs&CN=65.6