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HomeMy WebLinkAboutWPO202200033 Correspondence 2023-02-08 (2)SHIMP ENGINEERING, P.C. Design Focused Engineering January 06, 2023 Mr. David James diames2(c�r�albemarle.org Albemarle County Building 401 McIntire Rd Charlottesville, VA 22911 Regarding: WP0202200033 —LOT A-13 BUN DORAN FARM Comments & Responses Dear Mr. James, Thank you for your review the VSMP Plan for Lot A-13 Bundoran Farm. We have revised the plans per your most recent comments. Please see below for a detailed response to each of your comments. A. Stormwater Pollution Prevention Plan (SWPPP) The SWPPP content requirements can be found in County Code section 17-405. A SWPPP must contain (1) a PPP, (2) an ESCP, (3) a SWMP, and (4) any TMDL measures necessary. 1. 1. You are only required to complete the SF SWPPP. View template here. https://www. abemarle. org/home/showpublisheddocument/2662/6378655379265000 The Single Family SWPPP is now provided B. Pollution Prevention Plan (PPP) The PPP content requirements can be found in County Code section 17-404. PPP plan and measures have been updated and are now shown. C. Stormwater Management Plan (SWMP) VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a SWMP. This plan is denied, and the reasons are provided in the comments below. The stormwater management plan content requirements can be found in County Code section 17-403. WP02022.00033 Has been added on the Cover sheet. 2. Level spreaders: Ensure design meets water quantity standards per BMP Spec. #2, Sec. 6.3. Including but not limited to: a. A bypass/overflow to pipe/channel will be necessary for design flows greater than the 1-in/hr storm or consider an alternate (LS) design. Upon review, to satisfy section D of the stormwater quantity regulations the level spreaders have been revised to sheet flow the 10yr 24-hour storm. Please refer to the submitted HydroCAD calculations for information on the calculated flows. Sheet C4 now includes a level spreader sizing table showing compliance with the latest DEQ guidance. b. Provide a minimum 6 el width from c/l to lip. Channel widths have been revised. 3. Lever spreader C should be relocated outside of the stream buffer. Provide justification (Of-- --F.1........ A t- 1-,-.1 ;r +111. ,.N-.-.1 The Level spreader has been relocated outside of the stream buffer. 4. Sheet C3- etc Ensure fill disturbance will he no more than 30 LF of stream. (17-604 C(3` The stream crossing has an existing 48" pipe, our secondary pipe will add capacity but not be physically placed in the stream bed. 5. Cales: Sh. 26, Revise: I noticed th_ The calculation table has been corrected. Note that the table uses the flattest channel slope for the 10yr storm and the steepest channel slope for the 2yr storm. This may lead to some velocities that don't seem right between the 2 and 10yr storms however that is a function of using separate slopes as we are checking the 10yr storm for capacity and the 2yr storm for peak velocity. 6. Provide analysis of ditches specifically 4 &5: It is preferable to use more current and conservative rainfall values for ditch design and wondering how you arrived at those The ditch calculations have been revised. Some errors were present in the LD-268 which has now been revised. Note that we use the lowest grade slope in the ditch to evaluate the 10yr capacity and the highest grade slope to evaluate the 2yr velocity. In some situations this results in a 10yr velocity that is less than the 2yr, that is simply a function of using a lower slope in the manning's equation. The prior spreadsheet had errors for that calculation that have been corrected now. The LD-268 table shows that ACC x 12 = Q2 (Design Storm) The calculation obtained for 2YR velocity is using Q2 and the maximum slope. The calculation obtained for 10YR depth is using Q10 and the minimum slope. D. Erosion and Sediment Control Plan (ESCP) Virginia Code §62.1-44.15:55 requires the VESCP authority to approve or disapprove an ESCP. This plan is disapproved, and the reasons are provided in the comments below. The erosion control plan content requirements can be found in County Code section 17-402. Provide county paved construction ci,.,ance detail per ACDSM, p. Construction entrance detail has been added on Sheet C9 I D.-,...;,J- -11.- +;..L.♦I-- 1--i-, iI N-1"I'll R. I -+.;I rX UQC UD 3.08] Silt fence culvert Inlet protection detail has been added on Sheet C8 3. Provide utility stream crossing (IJSC)- VSS- SSS for culvert installation and bar Utility stream crossing detail is not provided because the stream is not going to be disturbed, the proposed pipe will be installed next to the existing pipe. stabilization & deta. ..B, 3.22, 3.23, 3.25] As noted above, no stream disturbance will occur. Revise: Sed. Trap - The weir width should be 6 x D.A. (12.42-ft). A & C elevations should Thank you for pointing this out. I have updated the calculation. I added a new column in the table to show the provided weir length. _)posed drainfield and downhill of sed. trap. Silt fence has been provided. F. Mitigation Plan (MP) revised to address all comments. No changes to mitigation plan were made. If you have any questions, please do not hesitate to contact me at odria@shimp-engineering.com, or you may contact Justin Shimp at Justin@shimp-engineering.com or by phone at (434)-227-5140. Best Regards, Odri Barefoot -Diaz Shimp Engineering, P.C.