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Home My WebLink AboutWPO202000032 Correspondence 2021-06-03SHIMP ENGINEERING, P.C. Design Focused Engineering May 20, 2021 David James County of Albemarle Department of Community Development 401 McIntire Road, North Wing Charlottesville, Virginia 22902 RE: Response Letter #4 for WP02020-321300 Richmond Road — Flow Automotive Dear David, ! 11/yy/o. 4 183 jl�Z� �ttr� ,rOUAL ti�G Thank you for your review of the V SMP permit plan for 1300 Richmond Road — Flow Automotive. This letter contains responses to County comments dated May 13, 2021. Our responses are as follows: D. Stormwater Management Plan (SWMP) 4. Detention basin: f. (Rev. 1/2) Provide seepage control: design anti -seep collars for barrel. (Rev. 3) Not currently addressed: Provide something from DEQ that will allow this exception based upon corrugations of the pipe used. Show the 4:1 paretic line from the riser weir. (Rev. 4) Clarify placement dimension of anti -seep device. RESPONSE: The profile has been updated to match the dimensions and a station of 10+20 is noted for the installation location. It. (Rev. 3) Embankment width should be 8' min. See comment 4.a. (Rev. 4) Provide an 8' embankment. Please submit a flood development permit (FDP) for minimal impact. Minimal disturbance to floodplain is expected. RESPONSE: We have reviewed the Zoning Ordinance and find that the fill in the floodplain is not permitted as a by -right use for the purpose of constructing a stormwater pond embankment. The purpose of the embankment width is to provide adequate structural stability given the depth of water that may pond behind the structure. In this instance, the actual amount of fill is approximately 2.6'. Given this minimal amount of fill, the width of embankment is more than adequate structurally in this situation. Further, Section A-3 of Appendix A of 2013 BMP Regulations states that the requirements are not mandatory for embankments which have a fill height of less than 3' as measured from the downstream toe of fill to top of dam which is the case here. Note that in this instance, the pond does not span a stream or watercourse, therefore, the measurement would not be from the pipe to the top of dam. Practically, the fill component of the embankment is only subject to pressure from ponding to a depth of 5.5" for the 100-year storm. It is possible that in my lifetime, the fill section of the embankment will never experience pressure from the basin filling during a rainfall event. With all of the above taken into account, I am certain that the fill portion of the structure will be more than adequate in width, and have certified these drawings accordingly. 8. Contact Ana Kilmer regarding the Nutrient Credit Agreement. PriortoDEQpeFmitissuatnee (Rev. 1/2/3/4) Comment still valid. Nutrient credit purchase is needed prior to plan approval. 912 E. High Sr. Charlottesville, VA 22902 1434.227.5140 1 shimp-engineering.com RESPONSE: We are ready to purchase the credits. Please confirm that Ana Kilmer has approved the nutrient credit purchase contract that was submitted on 04-14-2021. 11. (Rev. 3/4) Record the SWM facility easements with a deed & plat prior to approval. RESPONSE: The SWM maintenance agreement has been recorded. We are waiting on the County to provide us with the facility easement deed. 12. (Rev. 4) Sheet C10: Correct the profile detailidimensions. a. Add flit compaction note for embankment. RESPONSE: Note requiring 95% compaction has been added to the profile. b. Add note to key in fill material to existing slope. RESPONSE: As noted above, the embankment is very minimal in depth and keying in the fill with a cut-off trench or similar measure would be unnecessary in this circumstance. A note has been added to require the scarification of the existing surface prior to placement of fill. c. Str. A5a — Clarify what ex. standpipe? Show invert out connection. RESPONSE: The existing outlet control located in the manhole originally aligns with the 15" concrete pipe that is to be removed. With the proposed utilities, the existing outlet control needs to be rotated within the manhole to match the new outlet pipe alignment. The existing outlet pipe ran in the northwest direction, while the new outlet pipe runs in the northeast direction, so the standpipe will need to be rotated to match this new direction. 13. (Rev. 4) Provide more information/analysis of the channel to stream [ACDSM, p. 7]. a. Verify report pre -flows (i.e. 16.70cfs vs. 27.97 cfs don't match). RESPONSE: This table has been moved to sheet C15 to match the other stormwater conveyance calculations. We updated the table based on current flows and channel conditions. b. Provide you are meeting Channel protection under Part IIB — Provide the stone diameter lining required and what is provided for channel. RESPONSE: The rip -rap channel is existing and is in sufficient condition to convey stormwater per Part IIB regulations, specifically, by meeting the energy balance equation at the point of outfall at the top of the man-made rip -rap channel in accordance with 9VAC25-870-66-B. Lb. Since run-off complies with energy balance rates, there is no further required analysis for erosion at this manmade channel. However, we still confirmed that the existing channel still met channel protection by analyzed this downstream rip -rap channel in the Stormwater Conveyance Channel on C 15 to show that the channel does not have erosive velocities. The 7.4 fps velocity within the channel is not erosive for a rip -rap channel. c. Provide that you are meeting flood protection under Part IIB — Show the 10-yr is contained in the channel to the LOA, or floodplain. RESPONSE: We meet downstream flood protection by discharging through a stable channel into a mapped floodplain per 9VAC25-870-66-C-3.c. This known flooding is the mapped 100-year Zone AE floodplain. Since the pipe outfall is within a mapped floodplain, the outfall meets the limits of analysis for flood protection, and no further analysis is required. However, in good practice, we confirmed that the existing rip -rap 912 E. High St. Charlottesville, VA 22902 1434.227,5140 1 shimp-engineering.com channel into which the pond is discharged can convey the 10-yr flow without overtopping. As shown on the calculation table on C15, it is adequate for this depth of flow. E. Erosion and Sediment Control Plan (ESCP) 11. (Rev. 4) Add to note that the device will have to outlet to bottom of slope if there's any concentrated release. RESPONSE: The design of the silt sacks is to allow for seepage out of the entire bag at a slow rate, which would by design, not discharge in a concentrated form. We understand that over the length of the slope, water seeping from the sack may become shallow concentrated flow down the natural slope, however, that would match the current condition for any run-off over the slope, even without development occurring. The slope in question is to remain undisturbed with native vegetation, so we do not anticipate any erosion issues. Thus, no note was added. If the silt sack discharge was released at the bottom of the slope, it would have to be piped, which would concentrate the flow and cause more concern for runoff, and we cannot specify this in good practice. If you have any questions or concerns about these revisions, please feel free to contact me at Justin @ shimp-en ing eering corn or by phone at 434-227-5140. Regards, Justin Shimp Shimp Engineering, P.C. 912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom