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HomeMy WebLinkAboutWPO201500033 Correspondence 2015-07-24 r . err IMP SH' I• MPMANAGEMENT ENGINEERING LAND PLANNING ENGINEERING July 24, 2015 Justin Deel Regarding: WPO.2015.00033 Hillbrook VSMP Plans Dear Mr. Deel, I have attached four copies of the VSMP Plans for Hillbrook. Your comments for the first submittal and the changes we have made in response are below. Please forward two copies of the plans to VDOT per the request of Troy Austin. A. Stormwater Pollution Prevention Plan (SWPPP) 1. Please remove calculation packet. Include only the SWM, itself. Supporting documentation is not required here. The supporting documentation has been removed,and only the SWM is now included in the SWPPP. 2. Provide required signatures (Delegation of Authority). The Delegation of Authority signatures will be provided upon selection of a contractor for this project,which will occur after the SWM concept has been greenlit by the County. 3. Reflect below requested changes in resubmitted SWPPP. The changes requested below are now reflected in the SWPPP. B. Pollution Prevention Plan (PPP) 1. Provide PPP. Provided plan is E&SC Phase 1 plan. The plan has been updated to comply with Pollution Prevention Plan requirements. C. Stormwater Management Plan (SWMP) 1. "Stormwater that enters inlets 3B and 4 flows to the bioretention until the water surface elevation reaches 495.0, then it flows through the bypass structures... "; the consensus among the reviewers here is that this is impractical. Will all drainage up to 495' go to the biofilter? I think much of it will go down the overflow, untreated. The 175' pipe is basically flat. The overflow is 3.6" off the bottom, which will likely fill with sediment and debris. Please explore alternative options. The system has been re-designed to eliminate detention in the storm drain pipes.A vault now functions as the flow splitter,and there is 1.3' of elevation difference between the invert of the pipe flowing to the biofilter and the pipe flowing to the detention pipe. • NNW lore 2. The storm drain pipe slopes are too flat. The minimum should be 0.5%. This includes the 24" pipes, which are effectively drain pipes, not detention pipes. Please correct. Does the system rely on 1/100th of a percent precision(0.25%, 0.28%)? The storm drain pipes have been re-designed to have a minimum of 0.5%slope and to have self-cleaning velocities for the 2-yr storm. 3. You cannot claim sheet flow to a conservation area downstream of the bioretention facility, which requires a 10 year storm (495') to achieve. The sheet flow to conservation claim has been removed and the computations have been updated accordingly. 4. If your inlet 3 detail is to be used with the new design,please explain further. What size gravel will be used to protect the 3" PVC? The detail has been modified to specify clean#5 gravel for the inlet protection. 5. Please clean up storm drainage area maps, clearly showing all acreages used in VRRM. The storm drainage area maps have been changed to show acreages directly and to differentiate the BMP areas from the storm areas more clearly. 6. Please do not place detention pipes beneath structures (sidewalk). The sidewalk and street are private,and the HOA will be able to tear apart the sidewalk to access the detention structure without any special permissions. Because this portion of the sidewalk does not connect to any existing pedestrian walkways, it will be used very little, and thus create little inconvenience if it does have to be replaced during maintenance operations. 7. A permanent conservation easement is required for the forested open space claimed in VRRM. The forested open space claimed in the VRRM is protected by a tree preservation easement which belongs to the County. It is additionally protected by Zoning, as it is a required element for the bonus density, and the entire development will be in Zoning violation if the open space is developed. No disturbance of any kind may take place without County permission. 8. Please identify waste area(s), if needed. A note has been added to the Erosion and Sediment Control Narrative specifying that dirt export will be placed at Free State Run. 9. Provide retaining wall plans. Because the retaining wall is less than 3'tall, the contractor will be able to build it accurately.We have provided additional spots at the tallest point of the wall to show that the height will be minimized. D. Erosion and Sediment Control Plan (ESCP) 1. Please relocate and redesign Sediment Trap 1. The site will not be stabilized before the trap will have to be removed to construct the road, sidewalk, and storm sewer. A better location may be in lot 6? Unfortunately, it was not feasible to relocated Sediment Trap 1 to lot 6, as the pad's existing and final elevations are both well above the road entrance final elevation. To fix the problem of installing the road, ...r w.r sidewalk, and storm sewer, the silt trap is now designed to be removed after the pads and road bed are stabilized and silt fence is installed around the back of sidewalk.As the lots are very short, this will protect the road during construction. 2. Please clean up grading lines for sediment traps. Ensure that all proposed contours are solid lines and existing contours are dashed. Show grading for downstream dam embankment for ST 2 and include in LOD. Provide more proposed contour labels for sediment traps. Please provide scaled cross-sections showing existing and proposed contours. The grading lines for the sediment traps have been cleaned.All proposed contours are shown as solid lines. The grading for the downstream dam embankment for ST-2 has been clarified.Additional contour labels have been added.The sediment traps have been dimensioned carefully and graded in detail, and the detail from the Virginia Erosion and Sediment Control Handbook has been provided to clearly show the standard for construction. The contractor will be able to excavate the sediment traps according to plan and the field inspector will be able to check for adequacy. 3. Provide sediment trap labels. Sediment trap labels have been added. 4. Please label critical slopes hatching. Critical slopes were accidentally omitted in the previous submittal due to a glitch in the file structure, and are now provided. 5. Phase 1 drainage divide appears incorrectly drawn, please correct. The Phase 1 drainage divides previously accounted only for the existing condition of the site and have now been updated to reflect the construction of ST-1. 6. Prove more silt fencing at the site's perimeter (southern and northern corners?). Silt fencing has been added up to the limits of clearing and grading at the corners of the site to protect adjacent properties. If you have any questions, please do not hesitate to contact me at your earliest opportunity. I may be reached at: Justin(c�shimp-engineerinq.com or by phone at 434-953-6116. Best : -gard .40 J tin ' ., _. . chimp Engineering, P.C.