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HomeMy WebLinkAboutWPO201000032 Review Comments Erosion Control Plan 2010-06-17� OF AL ,. vIRGI1`IZP COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Miller School New Entrance Road (WPO- 2010 - 00032) Plan preparer: Mr. Brian Smith, PE Owner or rep.: The Miller School of Albemarle Plan received date: 17 May 2010 Date of comments: 17 June 2010 Reviewer: Phil Custer The engineering review of the ESC and mitigation plans for the new entrance drive at the Miller School, received on 17 May 2010, has been completed. The plans can be approved after the following comments are addressed. A. General Review 1. Before these plans can receive approval, a site plan must be approved. At the time of the site plan submittal, the applicant should request a waiver to disturb critical slopes to construct the proposed access road and any other waiver of zoning standards the applicant wishes. 2. The disturbance to stream buffer shown in this plan cannot be authorized by the program authority with the current ordinance language. Please provide a request for exemption from the Water Protection Ordinance as outlined in 17 -308. Once received, we will schedule the request for the next available Board of Supervisor's meeting. The letter should request exemption from all county stream buffer regulations to construct the proposed plan and, per 17- 321.13, permission for two additional stream crossing. Another review of this ESC and mitigation plan will not be performed until this request is received. Ideally, the site plan should be submitted at the same time too so VDOT has the opportunity to evaluate the entrance location. County engineering has not performed this evaluation and will likely do so when preparing the staff report to the Board of Supervisors. 3. A Stormwater Management Plan is required unless an equivalent amount of impervious area is removed from one of the other access roads for the school. If impervious areas in other parts of the site are not to be removed, swm facilities must be placed in locations where as much of the new impervious area can be captured per 17- 315.A. I recommend sloping the main travelway at 2% to the north and providing a biofilter around the drop inlet at the entrance by laying back the slope adjacent to the state road. The bed area of the biofilter should be no less than 4% (2.5% if the modified simple spreadsheet shows a removal rate of around 50 %) of the impervious area draining to it. The road embankment at drainage structure 7 should be used as a swm facility as well. The travelway south and east of this facility should be sloped to the north to capture and as much runoff as possible. Alternatively, the travelway can be moved over the existing dam and the existing pond converted to a stormwater facility (biofilter or extended detention depending on the results from the modified simple spreadsheet). 4. Please provide the approval letter from the Army Corps of Engineers for the proposed disturbance to the streams. 5. The stream buffer along Mechum's River is 100ft from the banks of the waterway or the limits of the 100 -year floodplain, whatever is greater. Engineering Review Comments Page 2 of 2 B. Erosion and Sediment Control Review 1. All proposed culverts (including pipe 7) must be computed as culverts and not by Manning's Equation. There should be at least Ift of freeboard on the 10 -year storm for Pipe 1 (VDOT requirements may be stricter). The existing box culvert section should be checked for capacity using the Mannings Equation however. 2. Remove the CIP symbol from the inlet end of Pipes 1 and 7. This measure is limited to drainage areas under 3 acres. Please replace with USC in both locations which is more appropriate for the placing of pipes in streams. 3. The erosion control concept for the construction of the main travelway from Sta. 10 +00 to Sta. 19 +00 is a major concern. The drainage area to the inlet protection is much larger than the maximum allowable specified in the VESCH and the silt fence below the roadway will act more like a diversion because it is placed on a slope. A sediment trapping measure will be needed around the inlet at the base of the entrance road. Please grade a 2:1 slope down from the 640 contour to provide an area where a modified sediment basin can be located. Since a basin of the necessary volume can likely not be provided, engineering review will grant a variance of volume requirements given the site constraints as long as the area is maximized. After the stream crossing, inlet, and modified sediment basin are constructed, the grading of the roadway should be performed in a manner that a diversion directs as much runoff as possible to the basin. The entrance road must have a 2% superelevation to the roadside ditch. 4. The construction sequence must be more detailed. Access to the site through the construction entrance cannot be provided until the installation of the pipe in the stream occurs. The first step in the construction sequence must be to install the stream crossing. Borrow soil to construct this crossing will need to be identified. The sequence noted in the previous comment should follow the stream crossing construction step. 5. The ESC portion of the WPO bond will provided at the time of plan approval. C. Mitigation Review 1. Calculations will need to be updated if necessary based on changes necessitated during the review of the WPO plan or conditions given by the Board of Supervisors. 2. The perennial crossing requires a bridge, arch culvert with earthern bottom, or a box culvert. [17- 320.D.1.a] 3. Please specify the type of seedlings to be planted. 4. The Mitigation portion of the WPO bond will provided at the time of plan approval.