HomeMy WebLinkAboutMiracle Way Peer Review 2 2023-08-25
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August 25, 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. 532739.0000
Dear Ms. DiRienzo,
TRC has completed a technical stormwater peer review of the response to comments prepared by J.M.
O’Reilly & Associates, Inc. (O’Reilly). 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/revised PDF documents prepared by O’Reilly were reviewed:
1. Responses to March 2, 2023, TRC comments dated August 9, 2023
2. Draft Stormwater Pollution Prevention Plan (SWPPP), dated May 4, 2023
3. Stormwater Management Report dated May 2, 2023, including
a. HydroCAD Report dated August 9, 2023
b. HydroCAD Report, 100-year storm only, dated August 9, 2023
4. Operations & Maintenance Manual, dated May 2, 2023
5. Total Suspended Solids (TSS) Calculations
6. United States Geological Survey (USGS) Soil Map
7. Site Plans dated August 9, 2023. (Plans provided consist of drawings C4, C5, C7.1, C7.2, C9,
and an un-numbered “Erosion & Sediment Control Plan”. No revised drawings numbered C1,
C2, C3, C6, C8 or C9 were provided.)
The March 2, 2023, 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
(l) The existing and proposed vegetation and ground surfaces with runoff coefficient for each.
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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.
O’Reilly Response: “Separate plans provided with revised submission.”
TRC Comment: Separate Pre- and Post- Development Watershed Plans have been provided.
However, the ground cover types and boundaries of each (woods, woods/grass, paved parking,
grass cover, roof area) should also be shown, along with their associated curve number (CN)
values.
(m) A drainage area map showing pre- and post-construction watershed boundaries, drainage areas,
and stormwater flow paths.
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.
O’Reilly response: “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.”
TRC Comment: The pre-development HydroCAD models 6 subareas to six design points,
totaling 511,730 square feet (sf) of area. The post-development models 6 subareas to 6 design
point, totaling 495,015.84 sf of area. Computations for SR2 are not included, although these
may be the “ROOF” subarea. HydroCAD and associated tables in the Stormwater Management
Report should be revised to account for the 16,714.16 sf discrepancy.
(r) Calculations supporting the design of the stormwater management system and its compliance with
the performance standards established in these regulations.
3. 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.
O’Reilly response: “Existing design points evaluated individually to confirm post-development
reduction, see HydroCAD reports and Table 1 of Stormwater Report.”
TRC Comment: See response to m(3), repeated here. The pre-development HydroCAD models
6 subareas to six design points, totaling 511,730 sf of area. The post-development models 6
subareas to 6 design point, totaling 495,015.84 sf of area. Computations for SR2 are not
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included, although these may be the “ROOF” subarea. HydroCAD and associated tables in the
stormwater management report should be revised to account for the 16,714.16 sf discrepancy.
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.
O’Reilly response: “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.”
TRC Comment: We feel that the waiver request is reasonable but defer to the Commission.
2.05 Stormwater Management Performance Standards
(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.
O’Reilly Response: “See response to previous comments.”
TRC Comment: See response to m(3), repeated here. The pre-development HydroCAD models
6 subareas to six design points, totaling 511,730 sf of area. The post-development models 6
subareas to 6 design point, totaling 495, 015.84 sf of area. Computations for SR2 are not
included, although these may be the “ROOF” subarea. HydroCAD and associated tables in the
stormwater management report should be revised to account for the 16,714.16 sf discrepancy.
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Findings
Based on these observations, TRC 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,
TRC
Sharon Burke, PE
Senior Technical Director/Civil Engineering