HomeMy WebLinkAboutWPO201200062 Review Comments Erosion Control Plan 2012-10-31ALg�,��
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COUNTY OF ALBEMARLE
Department of Community Development
401 McIntire Road
Charlottesville, Virginia 22902 -4596
Phone (434) 296 -5832 Fax (434) 972 -4126
Project: WPO- 2012 - 00062, Old Trail Creekside Phase III Street Plans
Plan preparer: Mr. Scott Collins, PE; Collins Engineering
Owner or rep.: Jackie B. Shifflett
Date received: 18 July 2012
(rev]) 24 September 2012
Date of Comment: 14 August 2012
(rev]) 31 October 2012
Engineer: Michael Koslow
The 2nd submittal of E &SC, Mitigation, and SWM plans for Old Trail Creekside Phase III Street Plans,
received on 24 September 2012, has been reviewed. This review does not include comments on the road
plans (SUB2012- 00086). The plans cannot be approved as submitted and will require the following
changes /corrections prior to final approval.
A. Stormwater Management Plan Review Comments (WPO- 2012 - 00062)
1. This project's stormwater treatment requirement is to treat the first flush in lieu of providing
detention required by ordinance per comment #4 from 10/10/2005 review of Old Trail Creekside 2
preliminary plat (SUB20500259):
...A detention waiver has been received. The detention waiver can be granted with the condition
that first flush treatment is provided through BMP's, and adequancy of channels are adequately
addressed at drainage outfalls. Please note that some BMP's proposed in the buffers may be
reconsidered by Tamara Greene, Water Resources Manager. Also, mitigation for buffer
disturbance will be a separate issue.
The pro -rata share of Lickinghole Basin fee will be required and will be calculated after approval
of final plans.
(Rev]) As we discussed at our meetings, the approved plans and the biofilters involve more than
simply stormwater management and treatment versus detention. We understand these biofilters
were proposed as mitigation measures (as well as treatment in lieu of detention for channel
protection). As mitigation measures they were structural BMP trade -offs for buffer
disturbances. We must keep in mind that these disturbances have already been made, so
reducing the treatment on the approved plans is not an option unless other equivalent
mitigation measures are provided.
In our discussions we talked about planting buffer areas at a 2:1 ratio, as is traditionally done
for mitigation. It seems to be difficult to find sufficient area that has not already been sold or
otherwise spoken for. However, if suitable buffer area can be found to plant, it would have to
be compared to the overall original plan disturbances to establish the 2:1 ratio. If sufficient
additional mitigation cannot be provided in this way, it is not an acceptable solution to
substitute dry detention basins for the previously agreed -to biofilters.
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An appropriate demonstration of how previously agreed to mitigation and treatment measures
including alternate proposed mitigation areas and /or treatment measures is needed to approve
any change to this previously agreed -to treatment and mitigation scheme for Old Trail
Creekside.
2. A stormwater management maintenance agreement will be required for all proposed stormwater
management features.
(Rev]) Comment has been acknowledged.
3. Phases D & E as proposed rely on stormwater management infrastructure proposed south of Phase
G. Stormwater management infrastructure must be approved and bonded to treat the first phase of
proposed construction.
(Rev]) Comment has been acknowledged.
4. Please include a drainage area map showing the drainage area of bio- retention area #1.
(Rev]) Comment has been addressed.
5. Please include Albemarle County general stormwater notes.
(Revl) Comment has been addressed.
6. Please label 100 year flood plain on sheets E -1 and SWM -1. Please label 100' stream buffer and
the outer 50' of the stream buffer on sheet SWM -1.
(Rev]) Comment has been addressed.
7. Please relocate proposed bio - retention area #1 to ensure it is outside the 100 year flood plain and is
in the outer 50' of the 100' stream buffer.
(Rev]) Comment has been addressed.
8. Please include a forebay for proposed bio - retention area #1.
(Rev]) Please see comment #A] response above.
9. Please provide the derivation of 129,810 sft of impervious surface proposed to drain to bio -
retention area #1.
(Rev]) Please see comment #A] response above.
10. Please label 100 year water surface elevation in bio- retention area #1 section on sheet SWM -1.
(Rev]) Please see comment #A] response above.
11. Please label contour elevations on bio- retention area #1 plan view on sheet SWM -1.
(Rev]) Please see comment #A] response above.
12. Please include by note or detail an impervious core to the berm and a cutoff trench of minimum 4'
wide, 4' deep at 1:1 sloped sides on sheet SWM -1.
(Rev]) Comment has been addressed.
13. As shown on sheet SWM -1, there appear to be trees in conflict with outlet protection for bio -
retention area #1. Please update landscape plan.
(Rev]) Comment has been addressed.
14. Please modify "weir elevation. = 738.5" label on the biofilter #1 detail on sheet SWM -1 to point to
the bottom of the riprap and update to read "weir elev. = 737.5."
(Rev]) Please see comment #A] response above.
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15. Please label minimum floor dimensions for bio- retention area #1.
(Rev]) Comment has been addressed.
16. Please locate the emergency spillway for bio- retention area #1 in cut to ensure soil stability in an
inundation event.
(Rev]) Comment has been addressed.
17. Embankment for Sediment basin #1 and bio - retention area #1 should match in elevation and
layout for ease of constructability.
(Rev]) Comment has been addressed.
B. Stormwater Management Computations Review Comments (WPO- 2012 - 00062)
1. Please provide stormwater management computations including:
a. Routing model and results for bio - retention area #1
b. Hyrology for bio - retention area #1 used to generate inflow hydrographs for a 100 year
storm.
c. Contour areas and elevations used for storage, matching the plan
d. The hydraulic dimension and coefficients for each control structure used in the routing
model, matching the plan
(Rev1) Please see comment #AI response above.
C. Mitigation Plan Review Comments (WPO- 2012 - 00062)
1. Please revise mitigation planting calculations: required mitigation planting area is 2x impacted
area (it appears the calculations include 3x impacted area). Also note impacted areas for sanitary
sewer and stormwater management facilities are exempt from stream mitigation requirements.
(Rev]) Impact and mitigation areas appear to be correct. Please see response C2 below.
2. Mitigation plantings are required to be proposed in stream buffer. Please revise proposed
mitigation planting areas.
(Rev]) Proposed mitigation areas 2 & 3 do not appear to be in the stream buffer.
3. Please include location, plan, and recorded easements for additional offsite mitigation if needed.
(Rev]) Because the easement is designed to allow for county inspection of mitigation plantings,
easements for mitigation areas are still required in existing open space areas.
4. Please include typical plant spacing requirements on mitigation plan.
(Rev]) Comment has been addressed.
D. Erosion and Sediment Control Plan Review Comments (WPO- 2012 - 00062)
r,ca�,c luvl�,c `Existing Site Conditions' and `Adja,,r-,1L . -.1F-a�1 LU UF- muir- ,�t,ecific regarding
streams in and around the project. It appears the description "an unnamed tributary to Lickinghole
Creek" and "Lickinghole Creek" would be more appropriate.
(Rev]) Comment has been addressed.
2. Please label existing soils and divides on sheets ESC -2 and ESC -3.
(Rev]) Comment has been addressed.
3. Please indicate all proposed staging, stockpiling, and parking areas on sheets ESC -2 and ESC -3.
(Rev]) Comment has been addressed.
4. Please include right of way diversions across Birchwood Hill Road at limits of existing
construction to ensure no sediment laden runoff drains to existing storm drains.
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(Rev]) Comment has been addressed.
5. Silt fence capacity is only 0.25 Ac / 1001ft. Please include sediment traps to catch sediment from
0.99 Ac and 0.94 Ac drainage areas proposed to drain to silt fence.
(Rev]) Please ensure flow length: width ratio for all proposed sediment traps is 2:1 [VESCH p.
111 -721.
6. Please include a sediment trap or basin at each of the 3 un- captured corners of culvert #1 crossing.
(Rev]) Comment remains.
Please include a utility stream crossing and diversion for apparent stream during construction of
culvert #1.
(Rev]) Comment has been addressed.
8. Please relocate proposed sediment basin #1 to ensure it is outside the 100 year flood plain and in
the outer 50' of the 100' stream buffer.
(Rev]) Comment has been addressed.
9. Please provide a north arrow for sediment basin 1 detail on sheet ESC -4.
(Rev]) Comment has been addressed.
10. Please provide a details and a capacity analysis for each the two existing sediment basins similar to
details provided for sediment basin #1 to ensure there is capacity for 3.93 Ac and 4.37 Ac of
runoff proposed to drain to each respectively. Please include description of existing riser including
existing invert elevations for each basin.
(Rev]) Comment has been addressed.
11. Please include safety fence and "danger, quick sand, do not enter" signs around sediment basin #1.
(Rev]) Comment has been addressed.
12. The baffle shown for sediment basin #1 is not needed.
(Rev]) Comment has been addressed.
13. Please provide an MS -19 adequate channel analysis to ensure the unnamed tributary to
Lickinghole Creek is adequate for proposed discharge from this project.
(Rev]) Comment has been addressed.
14. Please locate the emergency spillway for sediment basin #1 in cut to ensure soil stability in an
inundation event.
(Rev]) Comment has been addressed.
Once the comments have been addressed, please submit 2 copies of the revised plans, calculations, and
narratives to Community Development Engineering along with the required review fee and transmittal
form.
Community Development Engineering is available from 2:30 -4 PM on Thursdays to discuss these review
comments. Please contact Michael Koslow at 434 - 296 -5832 ext. 3297 or email mkoslow @albemarle.ora
to schedule an appointment.
[17- 204.f] An application for an erosion and sediment control plan that requires modifications, terms, or conditions to be
included in order for it to be approved shall be deemed to be withdrawn if the owner fails to submit a revised plan addressing the
omitted modifications, terms or conditions within six (6) months after the owner is informed of the omitted information as
provided under paragraph (B).
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