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HomeMy WebLinkAboutWPO201400028 Correspondence 2014-06-02 SHIMP PROJECT MANAGEMENT CIVIL ENGINEERING LAND PLANNING ENGINEERING June 2, 2014 Mr. John Anderson Regarding: Pantops Corner Early Grading Plan (WPO201400021) Dear Mr. Anderson, I have attached four copies of the plans addressing your comments dated May 30, 2014. The specific changes are detailed below. 1. Comment addressed,yet computational or detail discrepancies remain. Part(a) of this comment addresses the dewatering orifice. The reason for this inconsistency is that the calculation in the design data sheet is based on an algebraic formula provided in the VESCH, III-108. 5-inch tubing is hard to acquire for the contractor, so we have specified 6-inch instead, which will provide adequate capacity and is readily available. Part(h), regarding the Sediment Trap Design Data, has been addressed by revising the table to be consistent with VESCH Plate 3.13-1. The rest of the comment provides calculations for the sediment basin using a principal spillway capacity that exactly equals the 2-year storm. From VESCH Plate 3.14-8, the best choice for riser diameter is 30". 27"CMP is hard to acquire, and the maximum capacity of the 24" riser is 13.6 cfs, which doesn't leave much of a margin of error for a calculated 2-yr flow of 13.1 cfs. Moreover, a 30" riser allows for less head, which allows for less cut in a design which already requires a lot of excavation. The 30"riser's maximum capacity is 23.1 cfs, which exceeds the calculated 25-year flow of 20.1 cfs. This allows the 25-year storm to flow through the principal spillway with 0.9'of head, so the design data sheet was assembled using the "no emergency spillway"scenario. However, VESCH Plate 3.14- 2 requires additional freeboard for sediment basins without an emergency spillway, so an emergency spillway is specified. It will remove water in storm events greater than the 25-yr flood. 4. Please furnish a plunge pool to receive diverted runoff flowing west. Design should redirect this westward flow, and release it to the north to be conveyed along diversion dike to SB-1. The idea is to redirect runoff flowing W to the N, to prevent and avoid runoff release at the SW corner of the site. As discussed, a sediment trap has been designed and added at the location in question. 5& 6. Provide more silt fence(SF). Silt fence is now specified at all requested locations. IWO, VOW 11. Show OP for 90.9'-18"CMP outfall below outfall of sediment basin (SB-1). The existing rip-rap is now shown at that location, and a note has been added to reinforce it as necessary. 12. Show sediment trap floor dimension (ST-1). Sediment traps floor dimensions have been added for ST-1 and for the new ST-2. 13. Provide 1'depth of cover(MIN) for 18"CMP culvert for construction access;furnish CIP/OP for temporary CMP. If you have any questions please do not hesitate to contact me at your earliest opportunity. I may be reached at: Justinna.shimp-engineering.com or by phone at 434-953-6116. Best Re• _ ustin Shimp, P.E. Shimp Engineering, P.C. ..r iu.rf SHIMP PROJECT MANAGEMENT CIVIL ENGINEERING LAND PLANNING ENGINEERING May 05, 2014 Mr. John Anderson Regarding: Pantops Corner Early Grading Plan (WPO201400021) Dear Mr. Anderson, I have attached two copies of the plans addressing your comments dated April 10, 2014. The specific changes are detailed below. 1. Move and redesign the sediment basin to the low point on the site's perimeter without encroaching on preserved steep slopes. (We also noticed discrepancies between computations and plan details, which should be cleaned up with the redesign.) There is not enough room to locate the sediment basin to the low point on the site's perimeter without requiring massive amounts of cut. Instead, the sediment basin has been re-located north of the fiber- optic cable line, to control the majority of the site runoff. 2. Update the topography to reflect actual site conditions, demolished structure, actual trees and paved areas, etc. The topography has been updated to reflect current site conditions. 3. Ensure the storm conveyance channel or diversions do not impede construction access or grading. A construction access road is now graded in the plan, and a culvert is specified to carry water from the diversion underneath. 4. Provide a sediment trap and diversions at the SW corner on Rt. 250. The diversion dike along the southern property line has been adjusted to transport all runoff to the sediment basin. 5. Provide silt fence (SF)between LOD and off-site stream below the new basin location, E side of site. Silt fence is now specified at this location. 6. Provide SF along N side of site(approx same locations as safety fence) and W side. Runoff along this portion of the site will be captured by a diversion dike. 7. SAF(safety fence)is needed on the south side of site to protect the pedestrian walkway adjacent to US 250. SAF or better perimeter fencing should be provided at all sidewalks and areas where the public may be at risk. Safety fence is now specified on the south side next to the sidewalk. Nor.' Niere 8. Limits of disturbance(LOD) must include the construction entrance and all land disturbance. The limits of disturbance have been adjusted to include all E&S measures and the area of disturbance has been updated in the narrative. 9. It appears an off-site waste area is required. Please amend sheet 2 Notes, and include on the plan the destination of waste dirt, which must be an approved off-site waste area. A note has been added to sheet C2 stating that dirt export will be hauled to the Route 250/McIntire Interchange Project. 10. Provide stockpile locations. A stockpile location has been added south of the sediment basin. If you have any questions please do not hesitate to contact me at your earliest opportunity. I may be reached at: Justinna shimp-engineering.com or by phone at 434-953-6116. Best Regards, 41,3P Justi himp, P.E. Shimp Engineering, P.C.