The URL can be used to link to this page

Home My WebLink AboutWPO202000032 Correspondence 2021-02-05SHIMP ENGINEERING, P.C. Design Focused Engineering February 5, 2021 David James County of Albemarle Department of Community Development 401 McIntire Road, North Wing Charlottesville, Virginia 22902 RE: Response Letter #1 for (WPO202000032) 1300 Richmond Road — Flow Automotive Dear David, Thank you for your review of the VSMP plan for 1300 Richmond Road. This letter contains responses to County comments dated November 30, 2020. Our responses are as follows: A. General 1. Comment Addressed. 2. Comment Addressed. B. 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. Comment Addressed. 2. Section 9 included 3. Section 10 included. 4. Section 11 is completed, section 2 cannot be completed until the land disturbance permit is issued at the precon meeting. C. Pollution Prevention Plan (PPP) The PPP content requirements can be found in County Code section 17-404. 1. Comment Addressed. 2. Comment Addressed. 3. Comment Addressed. D. Stormwater Management Plan (SWMP) VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disapprove a SWMP. This plan is disapproved, and the reasons are provided in the comments below. The stormwater management plan content requirements can be found in County Code section 17-403. 1. Comment Addressed. 2. Comment Addressed. 3. Sheet 10: a. Partially addressed. Show the calculations table (LD-347), HGL elevation at each inlet. LD-347 added to plan sheet C15. 912 E. High Sr. Charlottesville, VA 22902 1434.227.5140 1 shimp-engineering.com b. Comment Addressed. c. Provide 2' minimum cover height over pipes. Note added to these profile locations, ADS detail added to C10. d. Comment Addressed. e. (Rev.1/2) Acknowledged. Show end section for HDPE pipe (detail is for RCP). ES-2 is now specified since this is a corrugated plastic pipe. Detail has been replaced as well. f. Comment Addressed. 4. Detention basin: a. Provide a sediment forebay. Follow the erosion & flood control guidelines from VADEQ, Appx. D. (Rev.2) Not addressed. Design will also short-circuit runoff to outlet. Provide design to maintain 2:1 length to width ratio. Additional Rip -rap added to remove possibility for erosion, and to catch debris in that area. Short-circuiting is not a concern because this BMP provides no real treatment or sediment removal. The pond is not designed to remove neither nutrient loads nor the (very minimal anticipated amount) sediment which enters it. Short circuiting is only a nutrient/sediment removal issue, the peak flow rate out of the pond is unaffected by where the runoff enters the pond nor how fast this runoff gets to the orifice. However, in the spirit of your request, we have achieved a forebay style area by extending rip -rap past the outlet riser to prohibit scour within the basin, to further reduce runoff velocity in the area, and to promote any debris to be caught in this area for increased ease of maintenance. b. Design a low -flow, smaller diameter orifice(s) in riser structure before considering larger diameter. (Rev.1/2) Install a trash rack over orifice Trash rack now specified in BMP profile. c. Show detail of trash rack. (Rev.1/2) Partially addressed. Show for 8" orifice. Detail included on C 10. d. (Rev.1/2) Provide riser/outlet structure flotation analysis (factor of safety = 1.25 min.). Analysis included on C15. Please note the structure is now concrete. This is a more robust riser, and due to the weight (see calcs) does not require an additional base. e. (Rev.1/2) Provide at least 2' freeboard from the 100-yr WSE to top of dam or 1' with a vegetative spillway if design allows. This requirement comes from VA MS 3.01 Embankment requirements, which are not required for small embankments under 3' tall. Since the embankment is 2.9' this requirement is not applicable to the pond. Reference attached image of page from VA SWM Handbook Volume 1. Please note, this simple detention pond which provides only water quantity reduction by lowering the peak flow rate, and does not have an additional corresponding 2013 BMP specification which adds additional requirements. Per that document, the main requirement for review of this such dam is "a stable outfall for the 10- yr storm." This has been achieved (along with the other current quantity requirements) at the outfall, and state requirements do not require a freeboard for the 100 year storm. f. (Rev.1/2) Provide seepage control: design anti -seep collars for barrel. Modem double -wall exterior corrugated HDPE has been shown to provide similar anti - seepage protection due to greatly increased exterior surface area, which provides a longer theoretical flow path for seepage due to the exterior corrugations. We have also observed that anti -seep collars can be counterproductive because they can interfere with compaction and can hinder uniform compaction along the pipe. Thus, we tend to only specify these for 912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom Reinforced Concrete Pipe, which has a smooth exterior surface. Finally, these collars are not required in this application because the pond dam is below the threshold for the embankment requirements, as mentioned above. g.(Rev.1/2) Provide backslope no steeper than 3:1 on dam. As noted in responses above, since this 2.9' tall pond dam does not qualify as an embankment per DEQ definitions, this is not a requirement for this pond dam. This backslope revision would also provide minimal difference, and in our opinion, provide no additional benefit to public health or safety. 5. Comment Addressed. 6. Comment Addressed. 7. Show letter of availability from the nutrient credit bank or provider on the plans. (Rev.1/2) Provide this letter on the plans. Currently, 2.59 credit purchase needed. Added to C15. 8. Contact Ana Kilmer regarding the Nutrient Credit Agreement. Prior to DEQ permit issuance. (Rev.1/2) Comment still valid. Acknowledged. 9. Record the SWM maintenance agreement. -Prior to approval (Rev.1/2) Comment still valid. Acknowledged. We have contacted Ana Kilmer to begin this process. 10. (Rev.1/2) Show SWM facility easement over ex. 48" detention pipes. Easement added to these pipes. E. Erosion and Sediment Control Plan (ESCP) Virginia Code Sec. 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 plan content requirements can be found in County Code section 17-402. 1. Comment Addressed. 2. Comment Addressed. 3. Comment Addressed. 4. Comment Addressed. 5. Comment Addressed. 6. Sheet 8/10: a. Provide a detail of what this IP looks like and refer to that detail (see comments for picture). (Rev.1/2) Partially addressed. Ensure that ponding will not be excessive here. Provide some sort of adequate overflow containment for excess or de -watering device. Noted, dewatering instructions and linework clarification added for this area during Ph. 3. b. Comment Addressed. 7. Comment Addressed. 8. (Rev.1/2) It is recommended to trench out and install these pipes/str. at the same time you construct STl (Phase 1). Noted, this is a good suggestion, note added to sequence of construction. 9. (Rev.1/2) Sheet 7/8: BM & MU needed for the slope; add symbols. Symbols & details added. 10. (Rev.1/2) Sheet 8: I think you need to keep the STl in place until the detention pond is completed. Please mention this in your construction sequence. Otherwise, show Sediment Basin design for phase 3 & 4. This is now noted in Phase 3 Sequence note 3. 912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom If you have any questions or concerns about these revisions, please feel free to contact me at keane@shimp-en ing eering com or by phone at 434-299-9843. Regards, Keane Rucker, EIT Shimp Engineering, P.C. 912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom 912 E. High St.. Charlottesville, VA 22902 1434.227.51401 shlmp-engineenrig.rom \fl.NL R-11 ST.A\DARD 3.01 CELAPTER 3 NM'BfVM STANDARD 3.01 EARTHEN EMBA\KNIE\T Definition An earthen embankment is a raised impounding structure made from compacted soil. Purpose The purpose of an earthen embankment is to impound stormwater runoff. Conditions Where Practice Applies An earthen embankment is appropriate for use with infiltration. detention. extended -detention or retention facilities. The design procedures presented in this section may not apply to small embankments or to storm drainage outfall structures with less than 3 feet of embankment height. The review and approval of such structures should be based on sound engineering practices and supporting calculations that verify a stable outfall for the 10-year storm. at a minimum Similarly. this section does not apply to embankments with a height of 25 feet or more and a maximum storage capacity of 50 acre-feet or more, as measured from the top of the embankment. Such structures may be regulated under the Virginia Dam Safety Act and the Virginia Dam Safety Regulations (VR 625-01-00). The height ofan earthen emban anent is the vertical distance from the natural bed of the stream or watercourse, measured at the downstream toe of the embankment, to the top of the embankment. If the embankment does not span a stream or watercourse. the height is the vertical distance between the lowest elevation, measured at the outside limit of the emmbamknent, and the top of the embankment. 3.01 - 1 912E. High St Char l ottesvi l l o, VA 22902 434227 S140 I shirnp-engineering mm