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COMPREHENSIVE
ENVIRONMENTAL
INCORPORATED
April 11, 2023
Ms. Brittany DiRienzo
Conservation Administrator
Town of Yarmouth
1146 Route 28
South Yarmouth, MA 02664
RE: Eversource Energy Staging Lot, 484 Willow Street, Yarmouth, MA
Peer Review of Stormwater Management Permit Application
Dear Ms. DiRienzo,
As requested by the Yarmouth Conservation Commission, Comprehensive Environmental Inc.
(CEI) has provided a technical review of the Stormwater Management Permit Application
submitted by Eversource Energy (the Applicant) for 484 Willow Street (the Site). Proposed
construction at the Site includes clearing of the existing forested lot, paving a vehicle storage lot,
and installation of an infiltration basin, storm sewer, site lighting, landscaping, fencing, and a fire
hydrant. This letter includes the findings of our review for the stormwater design components for
the Site.
CEI’s review is based on the application materials listed below:
1. 484 Will Street Stormwater Management Permit Application, dated February 2023,
prepared by Shive-Hattery
2. 484 Will Street Stormwater Management Permit Site Plans, dated January 19, 2023,
prepared by Shive-Hattery
3. Stormwater Management Report Narrative, dated January 19, 2023, prepared by Shive-
Hattery
4. Eversource ERP Staging Yard Stormwater Pollution Prevention Plan, dated January 2023,
prepared by Tighe & Bond
CEI’s review findings and comments are provided below.
I. Compliance with Stormwater Management Standards
CEI believes the project design addresses the Massachusetts Stormwater Standards and good
engineering practice as follows:
Yarmouth Conservation Commission
April 11, 2023
Page 2 of 5
Standard 1: No New Untreated Stormwater Discharges
The stormwater management plan for this project is centered around this implementation of
three infiltration basins. The infiltration basins are proposed to be constructed along the
western edge, the southwest corner, and the east side of the property.
Based on reasons explained for Standard 4 below, it is unclear whether Standard 1 is met.
Standard 2: Peak Rate Control
Due to the lack of test pits completed in the vicinities of basins 2 and 3, the soil type and
therefore the conductivity cannot be determined. For this reason, it is unclear whether
Standard 2 is met.
Standard 3: Groundwater Recharge
The Stormwater Management Report provided with the Stormwater Management
Application uses an infiltration rate of 2.41 in/hr for the Hydrologic Soil Group (HSG) A
soils found at the Site. Table 2.3.3 in the MA Stormwater Handbook specifies that this rapid
infiltration rate is for loamy sand observed and described in the provided narrative and test
pit logs evaluated by JC Engineering, Inc. CEI notes that modeling stormwater drainage with
the assumed infiltration rate of 2.41 in/hr in loamy sand is appropriate based on the soil
evaluations at the test pits in the location of basin 1. However, the same soil type and
infiltration rate was assumed for basins 2 and 3 although no soil borings or test pits were
completed in these areas.
The Representative indicates that at the time of the completion of the Stormwater
Management Report Narrative, a completed geotechnical report had been requested to
confirm soil conditions, however, the report was not provided.
CEI is unable to confirm that Standard 3 is met until these soil types are confirmed or
corrected and the recharge volume calculations for basins 2 and 3 are rectified.
Standard 4: Water Quality
a. Water Quality Volume Calculations
The water quality volume (WQV) for each of the three drainage areas were calculated as
follows:
= ()∗ ( 43,560
!"#$ ), &’(ℎ = 1+,ℎ,
Basin 1: = 25,272 /.
Yarmouth Conservation Commission
April 11, 2023
Page 3 of 5
Basin 2: = 1,643 /.
Basin 3: = 274 /.
It appears that the Stormwater Management Report Narrative did not include WQV for the
entirety of the site since the three identified drainage areas only account for 3.86 acres of the
total 4.39-acre site. The three driveways onto the site are not accounted for in any of the
drainage areas.
Additionally, the WQV provided for basin 3 has differing values in the equation (274 CF)
versus the value identified in Table 23 (1,945 CF).
b. TSS Removal
According to the provided TSS removal calculation worksheets, pretreatment removal rates
are based on the use of both a deep sump catch basin and a hydrodynamic separator which is
not always the case on this site. In basin 1, ST FEW E-1 discharges water with no
pretreatment structures. This drainage network receives runoff via two rectangular intake
catch basins with no connection to a hydrodynamic separator and no forebay at the outfall to
basin 1.
Calculations for pretreatment of ST FES A-1 should take into account the two inlet pipes to
the hydrodynamic separator which are connected to separate deep sump catch basins and
originate from overflow of basins 2 and 3, respectively. Similarly, the inlet pipe along ST
FES B-1 also originates from basin 2 and the calculations should reflect this.
Basin 2 only has one inlet pipe which receives runoff from a hydrodynamic separator. There
is no deep sump catch basin associated with this pipe and the WQV should be calculated as
such. Basin 3 does not have any identified pretreatment solutions.
Since TSS removal calculations have not been completed for each basin nor have WQV
calculations been completed for the entirety of the site, Standard 4 is not met.
Standard 5: Land Uses with Higher Potential Pollutant Loads (LUHPPL)
The Site is site not a LUHPPL, therefore Standard 5 does not apply.
Standard 6: Critical Areas
The site is located with Zone II and has a proposed impervious area greater than 2,500 square
feet, therefore, is subject to the requirements of Standard 6. Based on previously mentioned
concerns for Standard 4 regarding WQV and TSS removal calculations, CEI is unable to
determine if Standard 6 is met. Since there is no pretreatment design for basin 3, Standard 6
is not met.
Yarmouth Conservation Commission
April 11, 2023
Page 4 of 5
Standard 7: Redevelopment
As the project is not a redevelopment, Standard 7 does not apply.
Standard 8: Construction Phase Erosion and Sediment Controls
A Stormwater Pollution Prevention Plan has been provided and is utilized as the Construction
Period Erosion, Sedimentation, and Pollution Prevent Plan for this project. It appears to meet
the requirements of Standard 8.
Standard 9: Operation and Maintenance
A Long-term Pollution Prevention & Operations and Maintenance (O&M) Plan is provided
in the Stormwater Management Report Narrative. The O&M does not include the following
requirements of Standard 9:
· Information for how future property owners will be notified of the presence of the
stormwater management system and the requirement for proper operation and
maintenance;
· A plan that is drawn to scale and shows the location of all stormwater BMPs in each
treatment train along with the discharge point;
· A description and delineation of public safety features; and
· An estimated operations and maintenance budget.
For these reasons, Standard 9 is not met.
Standard 10: Prohibition of Illicit Discharges
Standard 10 does not apply.
II. Stormwater Management Design
1. The entirety of the site area is not included in the three defined drainage areas. These
three drainage areas account for 3.86 acres of the total 4.39 acres (4.12 acres of disturbed
area). Where does the rest of the site drain? In particular, the proposed paved driveways
on the north, west, and east sides of the property are not incorporated in any of the three
drainage areas. Please indicate to where these impervious surfaces will drain and provide
new calculations as needed.
2. Please provide a grading map with the drainage system and infiltration basins overlayed
with all applicable elevations.
3. According to the Yarmouth Stormwater Regulations Section 2.07, the Operation and
Maintenance Plan must include the following:
Yarmouth Conservation Commission
April 11, 2023
Page 5 of 5
a. Address of the person(s) responsible for operation and maintenance,
b. The signature(s) of the owner(s), and
c. An estimate for operation and maintenance budget.
Additionally, requirements in regards to the maintenance log and submittal of the annual
certification as identified in Section 2.07(1)(i) of the Yarmouth Stormwater Regulations
should be explicitly stated in the Operation and Maintenance Plan.
4. The Stormwater Management Report Narrative should include provisions for the
Yarmouth Stormwater Regulations Section 2.08, Inspection and Site Supervision, and
Section 2.09, Final Report.
5. In order to meet the pretreatment requirements for Basin 1, CEI may suggest
implementing a sediment forebay along the west edge of the basin where all four inlets
are located. With the use of a sediment forebay, hydrodynamic separators may not be
required.
6. Based on grading plans provided, it appears that there may be overland flow to basins 2
and 3. Since these basins are designed for infiltration, pretreatment is required. Consider
regrading edge of parking lot away from basin 2 so water may first enter the
hydrodynamic separator for pretreatment. If pretreatment for basin 3 is intended to be in
the form of a grassy swale, provide designs and the applicable calculations.
If you have any questions or comments regarding this review letter, please contact Matt Lundsted
or Sara Nelson at 603-424-8444.
Sincerely,
COMPREHENSIVE ENVIRONMENTAL, INC.
Matthew Lundsted, PE Sara Nelson
Principal Review Engineer