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Home My WebLink About Second Review 2022-11-17 1 November 17, 2022 Ms. Brittany DiRienzo, Conservation Administrator Town of Yarmouth Yarmouth Town Hall 1146 Route 28 South Yarmouth, MA 02664 Re: JDB 212 Mid-Tech Drive, LLC Technical Stormwater Peer Review 186 & 212 Mid-Tech Drive West Yarmouth, MA 02673 TRC Project No. 510575 Dear Ms. DiRienzo, TRC has completed a technical stormwater peer review of the response to comments prepared by Baxter Nye Engineering and Surveying (BNES). 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 additional PDF documents prepared by Baxter Nye Engineering and Surveying were reviewed: 1. Comment response letter by Baxter Nye Engineering and Surveying dated October 6, 2022. 2. Site plans titled “Proposed Site Redevelopment, Contractor Bays, 186 & 212 Mid-Tech Drive, West Yarmouth, Massachusetts 02673” dated October 6, 2022. 3. Stormwater Management Report, dated October 6, 2022. The September 28, 2022 review comments are provided in italics below, followed by a review of the response. Previous comments that have been adequately addressed with no further comment are excluded for brevity. Yarmouth Conservation Commission Stormwater Management Regulations 2.04(1) Stormwater Management Site Plan (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; An analysis of the estimated seasonal high groundwater table has not been provided. Test pits have not been excavated in the vicinity of the proposed infiltration areas. Grading and Drainage Note 10 on Sheet C4.0 of the plan set specifies that the contractor shall verify the assumed 8.27 in/hr infiltration rate and groundwater elevation prior to construction. The BNES response letter states “At time of construction, the contractor will dig down to four feet below the bottom of the drainage systems to verify that no groundwater is encountered.” It should be noted that the absence of groundwater at the time of excavation is not necessarily indicative of seasonal high groundwater. If the Commission chooses to waive test pits, it is recommended that the Commission consider a condition of approval requiring documentation that the infiltration rate and 4-foot separation to seasonal high groundwater and restrictive layer have been confirmed prior to installation. (l) The existing and proposed vegetation and ground surfaces with runoff coefficient for each; The ground covers and associated runoff coefficients should be shown on the Drainage Area Plans. The curve number areas listed in the Pre-Devt Drainage Areas Plan (Drawing C4.2) for Drainage Area D-2 are inconsistent with the values provided in the HydroCAD report. While this discrepancy is not likely critical to the stormwater design, it should be corrected for the record. If the Commission chooses to proceed with a conditional approval, it is recommended that the submission of a revised report be required. (r) Calculations supporting the design of the stormwater management system and its compliance with the performance standards established in these regulations; 1. The proposed and existing “bio-swales” and associated Cultec Chambers are modeled as a single unit. This approach does not demonstrate the inlet capacity of the beehive grate or 12 inch Cultec inlet pipe. Nor does it demonstrate that sufficient head is provided between the grate invert and overflow weir to prevent the “bio-swales” from over toping before the chambers fill. Additional documentation and supplemental calculations should be provided to support this approach or the model should be revised. The additional calculations and documentation provided do not fully address this comment. Additional calculations demonstrating the capacity of the behave grate were provided. However, the “orifice check” model does not account for tailwater conditions from the Cultec chambers. Additionally, while the surface area of the forebay is appropriately excluded from the exfiltration calculations in the “Beehive and Cultec” model (presumably to account for sedimentation of the forebay), it is not excluded in the “orifice check” calculations. Furthermore, the “Beehive and Cultec” model implies that the Cultech chambers fill before the forebay. This is not an accurate representation of the design conditions. While it is not unreasonable to assume that the system may perform generally as designed based on the information provided, the calculations and documentation are not sufficient to fully support that conclusion. If the Commission chooses to proceed with a conditional approval, it is recommended that adequate supporting calculations and narrative are required to be submitted for peer review prior to issuance of a building permit. 2. Groundwater recharge and Water Quality Volume calculations provided are based on utilizing the “bioswales” as infiltration practices. However, the “bioswales” function as sediment forebays and provide pre-treatment for the Cultec chambers. Therefore, the Cultec chambers are the infiltration practice. The recharge and water quality volume calculations should be revised accordingly. The revised calculations base the recharge volume on the Cultec chambers. However, the water quality calculations are still based on the forebay volumes. The forebays function as the pretreatment BMP rather than the Water Quality BMP. Therefore, the water quality calculations should also be based on the Cultec volumes. The recharge volume calculations indicate that the infiltration volumes provided in the Cultec system exceed the required water quality volumes listed in the water quality volume calculations. Therefore, this error does not appear to impact the final design. If the Commission chooses to proceed with a conditional approval, it is recommended that the water quality volume calculations be corrected for the record. 3. Since the “bioswales” function as sediment forebays, sediment forebay design calculations should be provided. It is recommended that the “bioswales” be relabeled “sediment forebays” in the plans and documentation for clarity and consistency with the Mass Stormwater Handbook. While sediment forebay design calculations have not been provided, the water quality volume calculations indicate the forebay is adequately sized. 2.06 Erosion and Sediment Control Plan (4) Erosion and Sediment Control Plan Content… 1. An Erosion and Sediment Control Plan is not included in the plan set. BNES revised drawing C4.0 to include erosion and sediment control measures. The requirements listed below are not included in the revised drawing. If the Commission chooses to proceed with a conditional approval, it is recommended that a revised erosion and sediment control plan be submitted for peer review prior to issuance of a building permit. (4)(c)(ii) Existing vegetation including tree lines, canopy layer, shrub layer, and ground cover, and trees with a caliper twelve (12) inches diameter breast height or larger, noting specimen trees and forest communities; (4)(e) Volume and nature of existing and proposed soil materials; (4)(g) …number of square feet of the land area to be disturbed; (m) Location and description of, and implementation schedule for, temporary and permanent seeding, vegetative controls, and other stabilization measures; (4)(o) A description of where and how construction vehicles and equipment will be cleaned within the site or at designated entry/egress stations at the site boundary; 2. A copy of the NPDES Construction General Permit Stormwater Pollution Prevention Plan (SWPPP) has not been provided. BNES states “The SWPPP information provided meets the NPDES General Permit requirements”. Several elements of the NPDES Construction General Permit Stormwater Pollution Prevention Plan (SWPPP) requirements have not been provided including, but not limited to, a list and description of all pollutant- generating activities, endangered species protection, etc. If the Commission chooses to proceed with a conditional approval, it is recommended that the Commission consider a condition of approval requiring the submission of a SWPPP using the EPA Construction Stormwater Pollution Prevention Plan (SWPPP) Template prior to the issuance of a building permit. Sincerely, TRC Jason M. Gold, P.E. Manager, Civil/Site Engineering Services