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HomeMy WebLinkAboutWPO201100089 Review Comments Stormwater Management Plan 2011-10-28ALg�,�� �'IRGINZ�` COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Charlottesville Self Storage SWM Plan Amendment Plan preparer: Mr. Jim Taggart, PE; TCS Engineering Company, LLC Owner: Virginia Self Storage Partners I, LLC Owner's Rep.: Ms. Jo Higgins Plan received date: 2 September 2011 Date of comments: 28 October 2011 Reviewer: Phil Custer A. Letter of Revision Review (SDP- 2006 - 00037) 1. The proposed changes are not in conflict with any ordinance requirements of Chapter 18. 2. This area is already covered by an active Erosion and Sediment Control Plan. For this reason, this plan amendment will not need to include an ESC Plan. The contractor must work out all necessary ESC measures with the ESC inspector on site. It is likely that well over 10,000sf of earth will be disturbed when you take into account the disturbance for cut -waste and access. B. Stormwater Management Plan Review (WPO Unknown) 1. The applicant must provide the county a WPO application and $180 fee ( #20) for this stormwater review as soon as possible. Upon resubmittal, another $180 fee will be needed. 2. The proposed facility does not meet the minimum requirements for an extended detention facility. The treatment method of an extended detention facility is the release of the 2xWQv over 30 hours. According to the Modified Simple Spreadsheet approved with the 2006 plan, the water quality volume for this watershed is 12,857cf. Therefore, 25,714cf would need to be released over 30hours. First of all, the total volume of the facility (below the spillway at 353.50) is less than this amount. Secondly, the orifice is not located at the bottom of the facility, so there will be close to a foot of permanently ponded water. Lastly, the proposed primary outlet does not restrict flows nearly as much as it must to release the required volume in 30 hours. Assuming a height of 4ft, I found that an orifice of 2" diameter at 350 is needed to release 25,714cf in 30 hours using the procedure outlined in Chapter 5 -6.2 of the VSMH. Because 3" orifices should not be used due to likelihood of clogging, the facility footprint could be expanded further so the height of the ponding can be shorter, which results in wider diameters. 3. Please provide a low flow channel from the pipe outlets to the riser structure. Riprap will likely be needed on this channel until the bottom of the facility is reached. 4. A sediment forebay is required. All stormwater quality facilities specified in the VSMH require forebays. 5. In Appendix 2A, please show the access route to the facility. The route on the last page is Albemarle County Community Development Engineering Review Comments Page 2 of 2 not clear. The route looks steeper than 10% in some places and may need gravel. In some areas the access route has a severe cross slope and may need to be graded. Please show this grading on the plan. Access to the forebay is necessary. Since this is a dry facility, access can be provided through the bottom of the basin if access cannot be provided any other way. 6. Please label all proposed contours. 7. Please provide this plan using a larger scale on 11 "x17" paper. The plan is difficult to read as drawn on a 8.5 "x11" sheet. 8. Grading is needed to daylight the barrel as currently proposed. Please show this grading. 9. What does "4in PVC (Vertical)" on Appendix 2A refer to? 10. Engineering will be focusing our review on the erosion potential downstream of this pond in the floodplain. For this reason, we will be focusing on the two year storm routings and making sure erosion will not be an issue between the facility and Moore's Creek. The following comments are provided on the applicant's routing: a. Use the minimum allowed time of concentration, 5 min., for the pond drainage area, not 2 min. b. The critical storm must be routed. c. The spillway length appears to be 200ft, not 271ft as specified in the routing. d. An exiiltration rate of 0.5in/hr is fairly high, especially in an area used as sediment traps for several years. Please provide permeability tests confirming this or remove this assumption from the routing.