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HomeMy WebLinkAboutMiracle Way Peer Review 1 2023-03-02 (2)March 2, 2023 Ms. Brittany DiRienzo, Conservation Administrator Town of Yarmouth Yarmouth Town Hall 1146 Route 28 South Yarmouth, MA 02664 Re: Miracle Way & Faith Road Cluster Subdivision Technical Stormwater Peer Review Route 6A Yarmouthport, MA 02675 TRC Project No. 532739 Dear Ms. DiRienzo, TRC has completed a technical stormwater peer review of the Stormwater Management Permit Application prepared by J.M. O’Reilly & Associates, Inc. (O’Reilly) submitted to the Commission on January 20, 2023 and provided to TRC on January 23, 2023. 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 O’Reilly were reviewed: Construction Protocol, dated December 27, 2022. Draft Stormwater Pollution Prevention Plan (SWPPP), dated October 19, 2022 Stormwater Management Report And Operations & Maintenance Manual, dated January 18, 2023 Site Plans, dated October 19, 2022. (Plans provided consist of drawings C1, C2, C4, C5, C7, C9, and an un-numbered “Erosion & Sediment Control Plan”. No drawings numbered C3, C6, or C8 were provided.) TRC’s observations are provided below, following the applicable standard in italics. TRC 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 (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; Page 3 of the stormwater report states “the USGS Cape Cod Groundwater Map places groundwater at approximately elevation 8”. A supporting figure illustrating the groundwater elevation and the site location should be provided. The bottom of the proposed leaching pits are over 20 feet below existing grade and over 10 feet below the bottom of the test holes. The designer should provide documentation supporting the accuracy and precision of the USGS groundwater map and the suitably of this source for design purposes. Similarly, documentation of the estimated depth to bedrock should be provided. The soil test logs provided on drawing C-1 indicate the presence of “perched” groundwater within sandy soils. The logs should include a description of the restrictive layer beneath the perched groundwater. (l) The existing and proposed vegetation and ground surfaces with runoff coefficient for each; The pre-development and post-development ground covers and associated runoff coefficients should be shown on the “Stormwater Contributory Area plan”. Two separate plans should be provided as described below. (m) A drainage area map showing pre- and post-construction watershed boundaries, drainage areas, and stormwater flow paths; Two separate drainage area maps should be provided for clarity: one for pre-development conditions and a second for post-development conditions. The proposed contours should be plotted darker in the post-development map for legibility. The pre-development delineation assumes a single “off-site” design point, which does not consider drainage divides or discharge rates to abutting properties. The pre-development drainage area map should delineate subcatchments based on existing drainage patterns and design points. Pre-development time of concentration flow paths should be shown. Flow direction arrows should be added to all time of concentration flow paths for clarity. (n) A description and drawings of all components of the proposed drainage system including: i. locations, cross sections, and profiles of all brooks, streams, drainage swales and their method of stabilization; ii. all measures for the detention, retention, and/or infiltration of stormwater; iii. all measures for the protection of water quality; iv. the structural details for all components of the proposed drainage systems and stormwater management facilities; v. notes on drawings specifying materials to be used, construction specifications, and typical details and cross-sections; and, vi. proposed hydrology with supporting calculations. Details of the proposed roof drain/Cultec system should be provided. Roof Run-Off note 2 on drawing C5 specifies that the Cultec units installed on each lot shall consist of two 15.5 ft x 4.33 ft leaching facilities. However, the HydroCAD model includes an 8.5 ft x 4.33 ft leaching facility. This discrepancy should be corrected or clarified. In addition, the HydroCAD chamber wizard details should be provided for review. Roof Run-Off note 3 on drawing C-5 specifies “no stone required”. The designer should explain in detail why stone is not required, as this is a typical design element. The minimum required horizontal separation distances from each infiltration BMP to building foundations and soil absorption systems should be indicated on the plans. Proposed design elevations should not be indicated as approximate (“±”). The designer should clarify why the “water quality swales” are described as “swales” as they do not appear to convey water. In addition, the “swale detail” provided on drawing C5 specifies 18” of permeable soil. This detail appears consistent with the typical organic filter provided in the Massachusetts Stormwater Handbook. Conversely, the Stormwater Handbook indicates 30” of permeable soil for a dry swale. The inverts of the M1 and M2 “water quality swale” outlet pipes should be provided on the details, in plan view, and in the HydroCAD model. (r) Calculations supporting the design of the stormwater management system and its compliance with the performance standards established in these regulations; The HydroCAD chamber wizard details should be provided for review as noted above. The M1 and M2 “water quality swale” outlet pipes should be included in the HydroCAD model. The pre-development subcatchment should be divided into multiple subcatchments as determined by drainage divides, flow patterns, and key design points. Each design point should be described in the stormwater report. Pre-development and post-development conditions for each design point should be evaluated individually. The HydroCAD calculations model exfiltration over the “wetted area” of infiltration practices, which includes the sidewalls. The Stormwater Handbook requires that exfiltration calculations include only the bottom surface area, excluding the sidewalls. The HydroCAD calculations should be revised accordingly. TSS removal calculations should be provided for the “water quality swales” located in the residential lots and associated pre-treatment. The TSS removal calculations include the “water quality swale” as pre-treatment. However, as this BMP is functioning as an infiltration device, it should be excluded from the pre-treatment calculation. Alternatively, the designer could consider designing “water quality swales” M1 and M2 as sediment forebays which would function as pre-treatment BMPs if appropriate. The TSS removal calculations and the stormwater report should address how pre-treatment is provided for overland flow that is not treated by the deep sump catch basins. Hydraulic sizing calculations should be provided for the stormwater conveyance system. The calculations should consider tailwater conditions if the outfalls into M1 or M2 will be submerged. If the drainage network is not sized to convey the 100-year design storm, an analysis of how stormwater runoff in larger storms will reach the infiltration BMPs should be provided. 2.05 Stormwater Management Performance Standards (2)(d) vii. Grass clippings, leaves, or any other vegetative debris shall not be deposited into or within 50 feet of water bodies, retention and detention areas, drainage ditches or stormwater drains, or onto impervious surfaces, such as, but not limited to, roadways and sidewalks, except during scheduled clean-up programs; This requirement should be included in the Operation and Maintenance Manual. (3)(a) Post-development peak discharge rates do not exceed pre-development peak discharge rates for the 2, 10, 25, 50 and 100-year 24-hour storms… Each pre-development design point should be evaluated individually as described in previous comments above. (3)(b) Structural pretreatment is required for all proposed infiltration devices to remove 44% TSS from runoff before it enters the infiltration device… As described in previous comments above, the M1 and M2 “water quality swales” can not be designed as both pre-treatment and an infiltration device. 2.06(4) Erosion and Sediment Control Plan (4) Erosion and Sediment Control Plan Content… Section 4.2 of the draft SWPPP specifies silt fence as a perimeter control, however, the site plans specify straw wattles. This discrepancy should be corrected. The proposed “aggregate apron” (construction entrance) and stockpile locations should be shown on the erosion and sediment control drawing. A detail of the proposed “aggregate apron” should be provided in the drawings. Proposed silt sacks should be labeled on the erosion and sediment control drawing. Erosion controls should be provided to protect the infiltration BMPs during construction and until final site stabilization is achieved. Access Road note 1 on drawing C5 specifies a 20’ wide stone apron at the entrance, however, the SWPPP specifies a 10’ wide “aggregate apron” in section 4.3. The SWPPP should be revised to specify a 20’ wide apron, to coincide with the proposed road width. Section 4.3 of the SWPPP specifies a 10’ long aggregate apron comprised of a 4” thick layer of ¾” – 1 ½” stone. This should be revised to be consistent with the Massachusetts Soil Erosion and Sediment Control guidelines which specify a 50' long construction entrance comprised of a 6" thick layer of 1” to 3" stone on fabric. Section 4.4 of the SWPPP references the “Town of Chatham Zoning Regulations”. This should be corrected to reference Yarmouth. Section 4.14 of the SWPPP references “three (3) residential homes”. This should be corrected to read “fourteen (14)” for consistency with the proposed project. 2.07 Operation and Maintenance Plan (post-construction) (1) A stand-alone Operation and Maintenance plan (O&M Plan) is required at the time of application for all projects… The O&M plan should be removed from the stormwater management report and written to function as a stand-alone document for use by maintenance personnel post-construction. (f) A list of easements with the purpose and location of each; The drainage easements should be listed and described in the O&M plan. (g) The signature(s) of the owner(s); The copy provided has not been signed.