Loading...
HomeMy WebLinkAbout225 Whites Path Peer Review Response Letter 6.16.2022June 16, 2022 Ms. Kathy Williams, PE, Town Planner Town of Yarmouth Conservation Commission 1146 Route 28 South Yarmouth, MA 02664 Re: Response to Technical Stormwater Peer Review Two Twenty Five White’s Path Units Two and Three, LLC 225 White’s Path, Yarmouth Dear Ms. Williams, On behalf of the Applicant, Two Twenty Five White’s Path Units Two and Three, LLC (Applicant), CHA has prepared this response letter to address the Technical Stormwater Peer Review prepared by Jason M. Gold, P.E., ESS Group, Inc. dated June 7, 2022. Included with this submittal package for your consideration are the following documents: Site Development Plans prepared by CHA, dated June 15, 2022; Stormwater Report prepared by CHA, dated June 2022; DRAFT SWPPP prepared by CHA, dated June, 2022. Mr. Gold’s original comments are presented below in regular text followed by CHA’s responses in bold- italic text. 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. CHA Response: An Existing Conditions plan is included in the resubmittal package. (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. CHA Response: Test pits were performed on the site on May 13, 2022. The test pit data from those test pits are included in Appendix A of the revised Stormwater Report. An adjustment to the estimated 225 White’s Path Yarmouth, MA CHA Project No. 72846 2 Response to Peer Review seasonal high groundwater of 1.9 feet (depth of 14.6 feet in Test Pit No.3) results by applying Technical Bulletin 92-001. Associated calculation worksheets are also included in Appendix A and test pit locations are presented on the Site Grading and Drainage Plan, Sheet C-103. 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. CHA Response: Test pits were performed on the site on May 13, 2022. The test pit data from those test pits are included in Appendix A of the revised Stormwater Report. The soils found in the test pits was classified as sand. An infiltration rate of 8.27 inches/hour was utilized based on Table 2.3.3 1982 Rawls Rate found in the MADEP Stormwater Handbook. The existing and proposed hydrology models have been updated utilizing the 8.27 inch/hour infiltration rate. The test pit locations are presented on the Site Grading and Drainage Plan, Sheet C-103. (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. CHA Response: The drainage are maps have been updated to include the times of concentration. Please see the updated maps in Section 3 of the revised Stormwater Report. The time of concentrations have been revised in both the existing and proposed hydrology models. Please see the revised calculations in Section 3 of the revised Stormwater Report. (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. 225 White’s Path Yarmouth, MA CHA Project No. 72846 3 Response to Peer Review 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. CHA Response: A detail of the washed stone strip and proposed trench drain are presented on the plans. Please see the details on DetailS-1, Sheet C-601 and the additional invert information on the Site Grading and Drainage Plan, Sheet C-103. (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 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. CHA Response: Section 1.4.3 of the Stormwater Report has been revised for consistency. A drainage area plan delineating subcatchments to each catch basin is provided in Section 4 of the revised Stormwater Report. Hydraulic pipe calculations using the Rational Method are included in Section 3 of the revised Stormwater Report. Note that all the proposed portions and most of the existing portions of the stormwater conveyance system convey the 25-year design storm, based on this Rational Method analysis. With the update to the appropriate infiltration rate of 8.27 inches/hour, the elevation within the proposed basin has been reduced to 36.89 in the 100-year storm. The 36.89 elevation is nearly equivalent to the condition in the existing basin for 4P which was found to be 37.31. At an elevation of 36.89, there may be some minor, temporary flooding in the paved parking areas as the stormwater runoff is recharged through the bottom of the basin. A HydroCAD analysis of the proposed drainage system components was also performed to evaluate the system with tailwater condition. With the update to the appropriate infiltration rate of 8.27 inches/hour, the water surface elevation within the proposed basin is 35.50 for the 25-year storm. The HydroCAD analysis demonstrates that the system is designed with adequate capacity to convey the 25-year design storm without the water surface exceeding the elevations of the rims of the proposed upgradient structures. Please see the referenced HydroCAD model in Section 3 of the revised Stormwater Report. 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 225 White’s Path Yarmouth, MA CHA Project No. 72846 4 Response to Peer Review 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. CHA Response: The hydrology model has been revised to include the infiltration rate of 8.27 inches/hour for the basin exfiltration rate. This rate significantly reduces the water surface elevation in the basin down to 36.89 which is at the rim elevations of the lowest catch basins. This water level in the 100-year storm in the proposed condition is slightly lower than in the existing condition at 37.31. With this reduction from the existing case and the slight high point behind the existing catch basin (EXCB9 as shown on the Catch Basin Area Plan, Sheet DA-003) of 37.7 (existing DMH), there is storage on the site to detain water in the 100-year storm. Please see the revised HydroCAD models in Section 3 of the revised Stormwater Report. 4.Calculations and/or figures supporting the water quality unit subcatchment areas should be provided for review. CHA Response: The tributary areas to the water quality unit are presented on the drainage area plans indicating the subcatchments associated with the existing and proposed catch basins. Please see the drainage area plan in Section 3.3 of the revised Stormwater Report. The area was found by adding the impervious areas of the catch basins tributary to the water quality unit which is 1.87 acres of impervious area. The area was used to determine the water quality discharge rate per MADEP’s Standard Method to Convert Water Quality Volume to a Discharge Rate for Sizing Flow Based Manufactured Proprietary Stormwater Treatment Devices. Please see Section 4.1 of the revised Stormwater Report. 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. CHA Response: The drawdown calculations have been revised utilizing the entire volume resulting from the 100-year design storm. Please see the revised calculations in Section 4.3 of the revised Stormwater Report. 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. CHA Response: The time span of the hydrology model has been adjusted to 0 to 48 hours to account for the full runoff volume. Please see the revised calculations in Section 3 of the revised Stormwater Report. 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. 225 White’s Path Yarmouth, MA CHA Project No. 72846 5 Response to Peer Review CHA Response: The hydrology models were updated to include a water surface ground cover in the existing and proposed cases. Please see the revised calculations in Section 3 of the revised Stormwater Report. 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. CHA Response: The Long-term Operation and Maintenance Plan has been updated to include the seven items of the subsection. Please see the O&M plan in Section 2 of the revised Stormwater Report. 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. CHA Response: Please see the updated Demolition and Erosion Control Plan, Sheet C-003 for locations of the temporary stockpile areas and sediment basins. 2.Drawing C-003 should be revised to show filter sock along the upper edge of the proposed infiltration basin. CHA Response: The filter sock is shown on the upper edge of the proposed infiltration basin. Please see the updated Demolition and Erosion Control Plan, Sheet C-003. 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 225 White’s Path Yarmouth, MA CHA Project No. 72846 6 Response to Peer Review “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. CHA Response: The Site Demolition, Erosion and Sedimentation Control Note 11 on Sheet C-002, the Construction Period Pollution and Erosion and Sedimentation Control Plan, and the draft SWPPP have been revised to indicate the requirement of weekly inspections per Section 2.08(3) of the Stormwater Management Regulations. Please see the revised documents. 2.07. Operation and Maintenance Plan (post-construction) (1) (g) The signature(s) of the owner(s); The copy provided has not been signed. CHA Response: The document has been signed by the Owner. Please see the updated document in Section 2 of the revised Stormwater Report. 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. CHA Response: The checklist is stamped and signed by a Professional Engineer. Please see the checklist included in the Stormwater Report. We trust that the responses provided above, along with the enclosed plans and documents, satisfactorily address the peer review comments. We greatly appreciate your time and consideration and should you have any questions or require any additional information, please don’t hesitate to contact me at sarnold@chacompanies.com or (781) 982-5400. Sincerely, CHA Consulting, Inc. Scott Arnold, P.E. Senior Engineer