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Home My WebLink AboutWPO202100007 Correspondence 2021-08-03® COLLINS ENGINEERING June 30, 2021 Emily Cox, PE Department of Community Development 401 McIntire Road Charlottesville, VA 22902 RE: Brookhill Blocks 16-17 VSMP Plan (WPO 20210007) 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902 ��434.293.3719 PH 434.293.2813 FX www.coll ns-eng ineeri ng.com Thank you for your comments dated July 26, 2021 on the above referenced project. Below are our detailed responses to your comments. Should you have any questions during your review and approval of this project, feel free to contact Graham Murray at ¢raham(acollins-eneineerine.com or at (434) 566-3011. A. Stormwater Pollution Prevention Plan (SWPPP) 1. Attached is the updated DEQ coverage registration statement with the modified land disturbance (increase in the land disturbance permitting). 2. Attached is the updated overall coverage mapping for Brookhill B. Pollution Prevention Plan (PPP) 1. No objection. C. Stormwater Management Plan (SWMP) 1. This comment is acknowledged and the client is currently working on the SWM agreement. We will reach out to Ana to get the SWM agreement executed. 2. This comment is acknowledged. A FDP application has been submitted and the applicant is awaiting approval on it. 6. The required reinforcement length for the retaining walls have been added to the grading plan sheets (Sheets 11-13). 8. Sheet 17 has been revised, in the bottom -left corner of the page, to include 'Splash Block Notes'. These 'Splash Block Notes' now more clearly state which homes are required to have splash blocks installed. These notes also mandate the contractor stabilize the areas downstream from the splash blocks prior to the installation of the downspouts and splash blocks. These notes supplement the information provided on sheet 15 in the bottom portion of the'Stormwater Management Narrative'. 13. This comment is acknowledged. A waiver request dated June 911 was submitted with the plans previous submittal. Please note, in order to meet minimum channel protection requirements via the energy balance computations, a low -flow orifice less than 3" in diameter is required. The SWM report submitted shows this. However to reduce the risk of clogging, the applicant is proposing low -flow orifices with a minimum diameter of 3". The minor increase in the low -flow orifices' diameters increase the 1-year SCS outflows slightly, and is now above the energy balance's thresholds. As a result, the request was submitted to the County requesting this minor overage be accepted. In an earlier May 13t^ phone conversation this was deemed an acceptable resolution to addressing this comment. 16. The previous plans proposed three (3) outfall pipes associated with the stormwater management facilities that had steep slopes generating velocities in excess of 15 ft/s during the 10-year design storm. The slopes for these three (3) pipes have now been revised and are now gentler, resulting in velocities less than 15 ft/s during the 10-year design storm. Please see the SWM calculations packet, under the'Storm Sewer Analysis of the SWM Facilities' Outfalls' section, for evidence of this revision. 18. Sheet 20 has been updated with the most current Brookhill Stormwater Quality Summary Table. Please note, following County Engineering's last review of this table, this project's post -development subarea A has been revised and now receives water quality treatment from a StormTech isolator row (40% RR) AND the previously provided discharge to conserved open spaces in a sheet flow manner. The result was a minor increase in phosphorous removal provided onsite. Please also note, the SWM report has been updated to reflect the changes associated with underground detention system 'A' change. And lastly, 'Stormwater Quality Summary Notes' have been added to sheet 20 in the middle -right portion of the page that more clearly states the minimum 75%threshold requirement for onsite treatment is being met. 19. The 'Stormwater Management Narrative' shown on sheet 15 has been revised to state this project will now require the purchase of 5.58 lbs/yr. This will take advantage of all the remaining phosphorous removal credits available at this time within the Brookhill Development. 20. The retaining walls located in the vicinity of lot 123 have been revised in efforts to address the concerns outlined in this comment. The retaining walls are mostly outside the limits of the storm sewer easements now, the storm sewer easement is 38' wide in this location, the storm sewer easement connects into the SWM easement where additional room is available for maintenance should it be required, the wall is only 2-3' in height near structure 56 and the wall itself is located upland of the storm sewer (meaning a geogrid would not conflict with the storm sewer's footprint should one of these structures need to be maintained). In regards to the retaining wall located near lot 7, unfortunately there is less area available for adjustments since the wall is located between a lot's buildable area and sanitary sewer (both existing and proposed to limit impacts to the floodplain). The plans have however been revised to alleviate some of the concerns noted in this comment. The plans now provide an expanded width for the storm sewer easement, the easement is connected to the SWM easement for additional maintenance room, and the retaining wall will be accompanied by a maintenance agreement in the event modifications are required. Also, structure 28B represents the closest point to the retaining wall (11' away) and the geogrid associated with the retaining wall will not encroach into this structure's area. The pipes angle away from structure 28B providing additional separation from the retaining walls. And lastly, Collins Engineering consulted with a contractor about these two areas and the challenges associated with them if maintenance would be required. The Contractor noted both areas would be largely accessible since they are near the SWM facilities' accesses, and the easement widths would afford them the ability to install trenches with shoring walls (pictured below) minimizing their impact on the surrounding areas. In summary, the Contractor felt maintenance could be preformed on the walls and the storm pipes if it were needed. D. Erosion and Sediment Control Plan (ESCP) 1. Roudabush Gale and Associates could not find any evidence of a recorded Greenway plat for the single-family residential lots at Brookhill. This was per a correspondence dated July Wh with Collins Engineering. 2. This comment is acknowledged and the applicant will be pursuing a WPO plan, or WPO plan amendment, showing the installation details and sequence for the pedestrian tunnel and culverts. In a July 29`h phone conversation between County Engineering and Collins Engineering, it was determined this 'comment' was for informational purposes only and would not delay the approval of this plan since notes are shown within this plan set mandating the construction of the Polo Grounds Road Phase II Improvements prior to the issuance of certificates of occupancies at blocks 16 and 17 (sheet 18). 10. The erosion and sediment control 'Sequence of Construction'shown on sheet 3 has been revised, under note #2, and mandates the preserved slopes and greenway be flagged or marked before land disturbance can start. 11. All of the diversion dikes have been reviewed and have been confirmed to have drainages areas less than 5 acres. The dikes' subareas are now shown and labeled on sheets 4-12. Please note, the previously submitted plans had two diversions with drainage areas in excess of 5 acres during ESC phase I. This has been corrected with the revised plans. 12. The applicant is currently working with Vincent Pero on his approval/permission for the installation of the riprap ouffalls. Sincerely, Graham Murray, PE