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HomeMy WebLinkAboutWPO201400023 Correspondence 2014-03-20 ......., ,,..„ A iaENGINEERS SURVEYORS PLANNERS ASSOCIATES May 8,2014 Glenn Brooks, P.E. County of Albemarle Department of Community Development 401 McIntire Road, Room 227 Charlottesville, VA 22902-4596 Re: B.F. Yancey Elementary School Wastewater Treatment Improvements Erosion and Sediment Control Plan—WPO 201400023 WWA Project No. 213003.04 Dear Mr. Brooks: Enclosed please find two (2) copies of the revised plans for the B.F. Yancey Elementary School Wastewater Treatment Improvements Erosion and Sediment Control Plan. I have prepared the following summary in response to your comments and concerns: A. Erosion Control Plan (WPO 201400023) 1. Comment: Provide limits of clearing and grading. Response: Limits of clearing and grading are shown on the plans. Where trees are being cleared, the limits of clearing and grading are indicated by the new tree line as noted on drawing number EC-4. 2. Comment: Use diversion dikes to direct drainage to the trap(s). Response: A diversion dike has been added to direct drainage to the trap. Please see drawing number EC-4. 3. Comment: Off-site water through the ditch is being directed through the disturbed area, and released without protection. Unless there is a way to avoid disturbed area, this is considered sediment laden water. Response: The new culvert has been extended to a point upstream beyond the disturbance to prevent runoff from the disturbed area from mixing with the clean off-site water that is being bypassed around the site. Outlet protection is being provided at the outlet of the culvert and the culvert discharges into a level spreader to convert the flow into sheet flow. The culvert will be installed before the site is 3040 Avemore Square Place •Charlottesville,VA 22911 Telephone(434) 984-2700 •Fax(434) 978-1444 Charlottesville •Lynchburg cleared to bypass the off-site water around the site. Please see drawing number EC- 4. 4. Comment: The drip field clearing is too large to be protected by silt fence. Response: The size of the drip field clearing has been reduced. Also, the drip field area is not to be cleared and grubbed. Trees will be cut flush with the ground and their stumps will be ground in place at noted on drawing number EC-4, thus minimizing the disturbance. Wire supported silt fence is provided downslope of the clearing to provide further protection. 5. Comment: Silt fence should not be used across contours, as this will channelize rather than filter. Response: The location of the silt fence has been revised to not cross contours. 6. Comment: The initial drainage area for the trap was not found. It appears to be the same as the proposed permanent stormwater facility in the calculation. This misses parts of the disturbed area, and does not appear to be the worst case. Response: The drainage area for the sediment trap has been added to the plan. Please see drawing number EC-4. 7. Comment: MS-19 must be considered from the discharge points of the project to the receiving stream. Response: There are no existing channels at the discharge points of the project leading to the stream. The existing ditch fans out and the flow spreads out into sheet flow prior to entering the stream. The discharges from the project are directed into level spreaders to convert the flows back into sheet flow to mimic the current natural drainage pattern at the site. B. Stormwater Management 1. Comment: The Yancey school site already has two BMPs on record. Both are on this side of the site. Locations can be viewed at the county web site GIS application. These must be reflected on the plans. Response: The locations of the two BMPs present on the Yancey school site as indicated on the GIS have been noted on the plans. Please see drawing number EC- 3. 2. Comment: This plan appears to use the rational method for routings. A 24hr rainfall distribution must be used to demonstrate detention compliance. The 3040 Avemore Square Place •Charlottesville,VA 22911 Telephone(434) 984-2700 • Fax(434)978-1444 Charlottesville •Lynchburg Page 2 of 3 analysis should be considering the school development area, or the area of improvement only. Response: The routings have been revised to use the TR-55 method with a 24 hour rainfall distribution as requested. Please see drawing number EC-10 for the revised routings. 3. Comment: The water quality calculation has used a 4.5 acre area. This is incorrect. The project area in the erosion control narrative is 2 acres. The drainage area to the facility is 1.28 acres. Ideally, removal rates should be based on an analysis of the entire school site (not the parcel area, but the school development area), with an overall approach to stormwater management for the entire development. If removal rates are to be computed based on the proposed improvements, which would be roughly the captured area to the facility, this gives about 12%impervious, and a removal rate of about 68% in the rural areas. Response: Per the meeting with the county on May 1, 2014, we understand that an overall approach to stormwater management would be preferred. For the purposes of this project, stormwater quality will be analyzed for the 4.5 acre parcel on which the proposed improvements are located. We respectfully request approval of this approach. We trust that the above responses and revised plans properly address the outstanding items. If you have any questions,please contact me. Sincerely, WW Associates, Inc. Herbert F. White III, P.E. President 3040 Avemore Square Place • Charlottesville,VA 22911 Telephone(434) 984-2700 •Fax(434) 978-1444 Charlottesville •Lynchburg Page 3 of 3