HomeMy WebLinkAboutWPO201300044 Review Comments Erosion Control Plan 2014-03-10� OF AL
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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: Avinity WPO amendment
Plan preparer: Collins Engineering, [Graham Murray, graham @collins - engineering.com]
Owner or rep.: Avon Properties LLC [Alan Taylor, alan @riverbenddev.com]
Plan received date: 25 Feb 2014
Date of comments: 10 Mar 2014
Reviewer: Glenn Brooks
A. Erosion Control Plan (WP0201300044)
1. The basin size appears to be too small for the drainage area. The sediment basin computation on the
erosion control plan says 17.48 acres, which is fine, but the drainage area map for the stormwater
management plans says 43.74 acres. This is confusing, and it appears I missed it in the original
approval, which has caused me to take a more thorough look at these plans. It does not work for the
sediment basin, which requires 67cy per acre wet and dry storage. If this is accurate, it will rescind the
previous approval.
2. Provide a copy of the state or federal permits for the project.
3. Show non -grass ground cover on slopes 3:1 or steeper for permanent stabilization.
4. A wash rack and sediment capture is need for the construction entrance.
5. The sediment basin is showing a core and key above the barrel, unlike the stormwater basin. This does
not appear correct.
6. The sediment basin is showing steel collars and base on RCP, which is not correct.
7. The basin shape appears to require baffles to achieve the 2:1 flow path.
B. Stormwater Management Plan (WP0201300044)
1. The hydrology is not documented and appears low. I compute composite CN values pre and post of
about 60 and 67, and this does not work for satisfying detention requirements in either phase.
2. Regarding the emergency spillway:
a. The emergency overflow must not be active in the 10yr storm. Using this as a control device is not
practical, as it is rip -rap, and cannot be constructed to the accuracy of a computer program.
b. With the removal of the residence in phase 2, there is room to place the emergency spillway so that
it does not run over a fill area.
c. Provide a detail to scale of the emergency spillway control section on the dam.
3. If the retaining wall is to be approved with this plan it must be engineered and shown accurately, as it
is to support roads and infrastructure. If it is shown this way, retaining walls plans and computations
are required.
4. Show all permanent drainage and stormwater facility easements.
5. Please draw all the cross - sections and details to scale.
6. Provide the Albemarle County general construction notes for stormwater.
7. The flow path should be about 2:1.
8. regarding the riser;
a. Provide a screen or trash rack detail that works for the proposed orifices in the riser. The one
provided is for a culvert inlet.
b. Change the riser so that openings at any one level do not cut away more than 25% of the structure.
Engineering Review Comments
Page 2 of 2
c. Remove the metal plate base option for the concerete riser.
d. It appears the riser is not accessible. Move the riser into the dam such that it is accessible.
9. Regarding the forebays;
a. demonstrate appropriate sizing.
b. show the gabion basket wall at the weir on the plan, and provide a detail.
c. provide a detail for the gabion wall foundation and tie -in to the sides.
10. Regarding the computations;
a. Provide a drainage area map for existing conditions.
b. On each drainage area on each map label CN, Tc and A values.
c. Removal rate values were not provided. I compute about 70% for developed areas, and resulting
water quality volumes are about 13800cf for phase 1 and 25400cf for phase 2. The facility should
actually have more storage than this to account for the runoff from the large "untreated" watershed
which passes through, potentially displacing the water which is held for treatment.
11. Sign and date professional seals.
12. A stormwater management facilities maintenance agreement is required.
File: E1_ ecp, swm _GEB_Avinity_wpo20l300044.doc