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HomeMy WebLinkAboutPeer Review 6.07.2022A TRC Company TRCcompanies.com June 7, 2022 Ms. Kathy Williams, PE, Town Planner Town of Yarmouth Conservation Commission 1146 Route 28 South Yarmouth, MA 02664 Re: Two Twenty Five White’s Path Units Two and Three, LLC Technical Stormwater Peer Review 225 White’s Path, Yarmouth ESS Project No. 495497 Dear Ms. Williams, ESS has completed a technical stormwater peer review of the Stormwater Management Permit Application prepared by CHA Consulting, Inc. submitted to the Commission on May 5, 2022 and provided to ESS on May 17, 2022. The goal of the review was to determine if the application is technically complete and to evaluate general compliance with the Yarmouth Stormwater Management Regulations, effective July 1, 2021. The following PDF documents prepared by CHA Consulting, Inc. were reviewed: 1. Site plans titled “Site Development Plans; 225 White's Path, Yarmouth, MA”, revised May 5, 2022 2. Draft NPDES SWPPP, dated May 2022 3. Stormwater Management Permit Application, dated May 2022 4. Stormwater Report, dated May 5, 2022 ESS’s observations are provided below, following the applicable standard in italics. ESS has determined that the application is neither complete nor in compliance with the Stormwater Management Regulations as summarized below. Revised documents and a detailed response to the following comments should be provided for further review. Yarmouth Conservation Commission Stormwater Management Regulations 2.04(1) Stormwater Management Site Plan (f) The location of existing and proposed utilities; (g) The site's existing and proposed topography with contours at 1-foot intervals; An existing conditions plan should be included with the Site Development plans. (k) Estimated seasonal high groundwater elevation using the Cape Cod Commission adjustment method (Cape Cod Commission Technical Bulletin 92- 001, as amended) in areas to be used for stormwater retention, detention, or infiltration; The stormwater report table of contents lists “Test pit data” in Appendix A; however, no test pit data has been provided. A TRC Company TRCcompanies.com Ms. Kathy Williams, June 7, 2022 2 495497 Whites Path SW Peer Review Section 1.4.1 of the Stormwater Report states that four test pits will be excavated in the area of the proposed retention/recharge basin to confirm the seasonal high groundwater elevation. If the Conservation Commission chooses to issue a permit without the test pit data, ESS recommends the following: 1. The designer should clarify that the design infiltration rate will be confirmed in addition to the seasonal high groundwater elevation. 2. The designer should clarify that construction will not begin until the findings of the test pit observations along with the test pit logs and any resulting design changes have been provided to, and approved by, the Conservation Commission. 3. The site plans should be revised to show the proposed test pit locations as well as appropriate construction notes indicating the previous two recommendations. (m) A drainage area map showing pre- and post-construction watershed boundaries, drainage areas, and stormwater flow paths; 1. Stormwater flow paths used to calculate the time of concentration are not provided. The drainage area maps should be revised accordingly. 2. The listed times of concentrations for existing drainage areas 1, 3, 4, 5, and 7 and proposed drainage area 3 conflict with times provided in the HydroCAD calculations. The plans should be revised for consistency. (n) A description and drawings of all components of the proposed drainage system… 1. A detail of the “washed stone strip” specifying gradation, thickness, and filter fabric should be provided. 2. A detail of the proposed trench drain should be provided. Inverts of the pipe connecting the trench drain to the existing catch basin/proposed manhole should be provided. (r) Calculations supporting the design of the stormwater management system and its compliance with the performance standards established in these regulations; 1. Section 1.4.3 of the stormwater report states that the size of subcatchment 5S is reduced under proposed conditions. The drainage area maps and HydroCAD calculations indicate the size is equal under proposed and existing conditions. The report should be corrected for consistency. 2. The pipe hydraulic calculations are missing from Section 3.2 of the Stormwater Report. These calculations should be provided for review. o A drainage area plan delineating subcatchments to each catch basin should be provided to support the calculations. o The hydraulic calculations should demonstrate that the drainage system is designed to convey at least the 25-year design storm. Tailwater conditions should be evaluated since A TRC Company TRCcompanies.com Ms. Kathy Williams, June 7, 2022 3 495497 Whites Path SW Peer Review the HydroCAD calculations indicate the 25-year water surface elevation in the basin (37.29) is higher than the inverts and rims of many of the upgradient structures. 3. The Stormwater Report states that the runoff generated by the 100-year design storm will be retained on-site. However, according to the HydroCAD calculations, the 100-year water surface elevation in the basin (38.98) is almost 2 feet higher than the parking areas along the western and southern sides of the building and approximately 3.5 feet higher than the low point at the northeastern property line. The designer should demonstrate how runoff from the 100-year design storm will reach the infiltration basin without flowing off-site or causing on-site flooding. 4. Calculations and/or figures supporting the water quality unit subcatchment areas should be provided for review. 5. The drawdown time calculations provided in Section 4.4 of the stormwater report are based only on the recharge volume rather than the volume of the 100-yr design storm for which the basin is designed. The entire volume of runoff generated by the 100-yr design storm should infiltrate within 72-hours, per the MA Stormwater Handbook (Volume 2, Chapter 2, page 89). Revised drawdown calculations should be provided. 6. The time span of the HydroCAD calculations is only 5 to 20 hours, which may not provide the full runoff volume from the 24-hour design storm. The post development model should span at least 0 to 36 hours or longer as needed to demonstrate that the drainage system and infiltration basin can accommodate the full runoff volume from the 100-year design storm. 7. The ground cover for subcatchment 2S/4S does not account for precipitation falling directly on the infiltration basin. A water surface ground cover area should be included as appropriate. 2.05. Stormwater Management Performance Standards (2) (d) Restrictions on the application of fertilizers… The 7 items of this subsection should be included in the fertilizer section of the long-term Operation and Maintenance plan for future reference by O&M personnel. 2.06. Erosion and Sediment Control Plan (2) If a project requires a Stormwater Pollution Prevention Plan (SWPPP) per the NPDES General Permit for Storm Water Discharges From Construction Activities (and as amended), then the permittee is required to submit a complete copy of the SWPPP. If the SWPPP meets the requirements of the General Permit, it will be considered equivalent to the Erosion and Sediment Control Plan described in this section. 1. The Construction Period Pollution Prevention And Erosion And Sedimentation Control Plan and Section 2.3.7 of the Draft NPDES SWPPP references temporary sediment basins and designated stockpile areas. Drawing C-003 should be revised to indicate the location of these measures. 2. Drawing C-003 should be revised to show filter sock along the upper edge of the proposed infiltration basin. A TRC Company TRCcompanies.com Ms. Kathy Williams, June 7, 2022 4 495497 Whites Path SW Peer Review 3. The inspection frequency specified in Site Demolition, Erosion and Sedimentation Control Note 11 on Drawing C-002 is inconsistent with that specified in Section 6.1.2 of the Draft NPDES SWPPP and the inspection and maintenance schedule provided in the Construction Period Pollution Prevention And Erosion And Sedimentation Control Plan. The drawing note specifies inspections every 7 days and within 24 hours of a storm event of 0.5 inches or greater. The SWPPP specifies inspections be conducted either every 7 days or every 14 days and within 24 hours of a storm event of 0.25 inches or greater. The Construction Period Pollution Prevention And Erosion And Sedimentation Control Plan is similar to the SWPPP, with the exception of the precipitation amount listed as 0.5 inches or greater. While the inspection frequency specified in the draft SWPPP appears consistent with the 2022 NPDES Construction General Permit, the Yarmouth Stormwater Management Regulations are more stringent. Section 2.08(3) of the Stormwater Management Regulations requires inspections “no less than weekly or as specified in the Permit, and prior to and following anticipated storm events”. The inspection frequency in all documents should be revised for consistency and for compliance with both the Construction General Permit and the Yarmouth Stormwater Management Regulations. 2.07. Operation and Maintenance Plan (post-construction) (1) (g) The signature(s) of the owner(s); The copy provided has not been signed. Checklist for Stormwater Report B. Stormwater Checklist and Certification Registered Professional Engineer’s Certification The checklist should be stamped and signed by the Professional Engineer. Findings Based on these observations, ESS concludes that the application is not in compliance with the Stormwater Management Regulations. Additional calculations, drawings, and documentation should be provided to demonstrate compliance as described above. A detailed response to the review comments should be provided along with a reference to the associated revision or supporting material. Sincerely, ESS GROUP, INC. Jason M. Gold, P.E. Manager, Civil/Site Engineering