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HomeMy WebLinkAbout(RFR Responses - Revised 8.9.23)Miracle Way Peer Review 1 2023-03-02March 2, 2023 (August 9, 2023 – Robert Reedy Responses in red text) 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: 1. Construction Protocol, dated December 27, 2022. 2. Draft Stormwater Pollution Prevention Plan (SWPPP), dated October 19, 2022 3. Stormwater Management Report And Operations & Maintenance Manual, dated January 18, 2023 4. 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; 1. 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. USGS Scientific Investigation Map I-2857 with site location pinned has been included with the revised submission package. 2. 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 primary source for groundwater elevation determination for stormwater leaching pits is the on-site soil investigations (test holes) which were conducted on two separate occasions as documented on C1. The test holes were conducted in the same area as the proposed stormwater leaching facilities. As shown in the test hole logs, perched groundwater was found during one of the soil investigations, but the proposed bottom of leaching is greater than one foot above the perched water elevation. The USGS Map is intended only as a secondary groundwater resource which illustrates that the actual groundwater elevation at the site is lower than the perched groundwater that was found during test holes. Regarding bedrock, underlying bedrock at the site is greater than 200 feet below sea level. Cape Cod was formed during the last glacial retreat approximately 14,000 years ago and it’s underlying soil is made up largely of glacial till. Notes added to C5 that depth to groundwater shall be confirmed by the engineer prior to installation of the M1 BMP’s. 3. 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. No specific restrictive material was found, though the soil was firmly in place during the subsequent test hole excavations which extended several feet below the elevation where perched water had previously been observed. The areas where perched water was observed were both existing natural low points and the initial test holes were during a wet period. It appears likely that groundwater elevation is typically much lower than what was observed during the initial test holes, however the perched groundwater elevation will be used for design purposes. (l) The existing and proposed vegetation and ground surfaces with runoff coefficient for each; 1. 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. Separate plans provided with revised submission. (m) A drainage area map showing pre- and post-construction watershed boundaries, drainage areas, and stormwater flow paths; 1. Two separate drainage area maps should be provided for clarity: one for pre-development conditions and a second for post-development conditions. Separate plans provided with revised submission. 2. The proposed contours should be plotted darker in the post-development map for legibility. Done. 3. 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. Done. Existing drainage areas separated on hydrocad report. Minor reductions in pre-development peak discharge rate as a result of the separation of catchments, revised calcs shown on Table 1. Peak Discharge Comparison in the Stormwater Management Report. 4. Pre-development time of concentration flow paths should be shown. Done. 5. Flow direction arrows should be added to all time of concentration flow paths for clarity. Done. (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. 1. Details of the proposed roof drain/Cultec system should be provided. 2 Details added to C5 for roof runoff. 2. 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. The addition of stone allowed reduction of chambers, each of the two leaching facilities per dwelling is now 10.5’ x 6.33’ x 3.5’ (one cultic unit with 1’ of stone around and underneath). HydroCAD wizard shows the length in two sections: 7.0’ for effective length and 1.5’ for overlap. With only 1 unit, the length is 8.5’. 3. 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. Stone added, see roof runoff details and notes on C5. Stone added to Hydrocad roof calculations as well. 4. The minimum required horizontal separation distances from each infiltration BMP to building foundations and soil absorption systems should be indicated on the plans. Title 5 and MA Stormwater Handbook setback notes added to C4 in top left corner. A couple roof leaching footprints were shifted so as to clearly show they meet the required setbacks. 5. Proposed design elevations should not be indicated as approximate (“±”). Revised on C4 and C5 plans. 6. 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 water quality swales are identified as such because they meet the design requirements of a dry water quality swale in the MA handbook, now that pre-treatment BMPs have been incorporated into each treatment train. The 2nd bullet point for Design – Dry Swales in the Water Quality Swale section of the MA handbook notes that 18” is the minimum soil bed depth. My understanding is that the detail given in the handbook which notes 30” is an example and not a required minimum. 7. 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. Done. (r) Calculations supporting the design of the stormwater management system and its compliance with the performance standards established in these regulations; 1. The HydroCAD chamber wizard details should be provided for review as noted above. See response to #2 in above section. 2. The M1 and M2 “water quality swale” outlet pipes should be included in the HydroCAD model. Done. 3. The pre-development subcatchment should be divided into multiple subcatchments as determined by drainage divides, flow patterns, and key design points. Done, see previous comment responses. Each design point should be described in the stormwater report. Pre-development and post-development conditions for each design point should be evaluated individually. Existing design points evaluated individually to confirm post-development reduction, see HydroCAD reports and Table 1 of Stormwater Report. 4. 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. Changed hydrocad settings to use “horizontal area” instead. 5. TSS removal calculations should be provided for the “water quality swales” located in the residential lots and associated pre-treatment. TSS Calculations added for each treatment train. See TSS Calc Sheets and Tables 2 and 3 of stormwater report. 6. 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. In order to address this, proprietary pretreatment units have been provided for each of the two main stormwater systems (M1 and M2). TSS calc sheets have been updated accordingly. 7. 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. TSS calcs added for each treatment train, including overland flow. Tables 2 and 3 added to stormwater report. Our office would like to request a waiver from the requirement that all impervious surfaces must have adequate pre-treatment prior to entry into stormwater BMP’s: • Of the 5,997 SF of total impervious area that flows into the S1 BMP, 1,988 SF has less than the required minimum 25’ vegetated strip for pre-treatment. Non-treated impervious areas consist of portions of residential driveways that are too close to the proposed BMP to allow for feasible pre- treatment devices. • The 791 SF of total impervious area that flows into the S4 BMP has less than the required minimum 25’ vegetated strip for pre-treatment. Non-treated impervious areas consist of portions of residential driveways that are too close to the proposed BMP to allow for feasible pre-treatment devices. • All other site BMP’s provide pre-treatment. As shown in the TSS calcs and Tables 2 and 3 of the stormwater report, the proposed project will provide a cumulative pre-treatment removal efficiency of 75.8% sitewide, as well as a 91.7% sitewide cumulative treatment removal efficiency. • Project requests that the Commission allow 52,887 SF of the site’s 55,666 total impervious (non-roof) area to meet the pre-treatment requirement, with a waiver requested for the remaining 2,779 SF of non-pre-treatment impervious area. 8. 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. See HydroCAD report, all system components included and no backups or submergences reported for the 100-year storm. Outfalls M1 and M2 are set 0.5’ above rim elevation for the catch basin in each swale, preventing submergence. See Hydraulic sizing calculations on page 5 of the stormwater management report. M2 main conveyance system pipe size increased from 12” to 18”, as noted on M2 facility detail. 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; 1. This requirement should be included in the Operation and Maintenance Manual. Added to Long-term Lawn care section of O&M. (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… 1. Each pre-development design point should be evaluated individually as described in previous comments above. See response to previous comments. (3)(b) Structural pretreatment is required for all proposed infiltration devices to remove 44% TSS from runoff before it enters the infiltration device… 1. 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. CDS Pre-treatment units added, see response to previous comments. 2.06(4) Erosion and Sediment Control Plan (4) Erosion and Sediment Control Plan Content… 1. 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. Corrected in SWPPP section 4.2. 2. 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. Apron, apron detail, and stockpile locations shown on erosion control plan. Erosion control notes 4-6 added on plan. 3. Proposed silt sacks should be labeled on the erosion and sediment control drawing. Labels added. 4. Erosion controls should be provided to protect the infiltration BMPs during construction and until final site stabilization is achieved. See erosion control note 5 on Erosion Control Plan. 5. 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. SWPPP revised. 6. 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. SWPPP revised. 7. Section 4.4 of the SWPPP references the “Town of Chatham Zoning Regulations”. This should be corrected to reference Yarmouth. SWPPP revised. 8. 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. SWPPP revised. 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… 1. 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. Done. (f) A list of easements with the purpose and location of each; 1. The drainage easements should be listed and described in the O&M plan. Added to property info section of O&M. (g) The signature(s) of the owner(s); 1. The copy provided has not been signed. Signed copy provided.