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COMPREHENSIVE
ENVIRONMENTAL
INCORPORATED
May 12, 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, Revised dated May 2, 2023;
3. Stormwater Management Report Narrative, dated January 19, 2023, prepared by Shive-
Hattery, revised dated May 2, 2023;
4. Eversource ERP Staging Yard Stormwater Pollution Prevention Plan, dated January 2023,
prepared by Tighe & Bond;
5. Long-term Pollution Prevention & Operation and Maintenance Plan, dated May 1, 2023
prepared by Shive-Hattery.
The Applicant’s responses to CEI’s review letter are provided in red, followed by CEI’s
responses in blue.
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
May 12, 2023
Page 2 of 10
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.
SH Response: See response to Standard 4 below. Now that Standard 4 is met, Standard 1 is
also satisfied.
CEI Response: Comments addressed. Standard 1 is now 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.
SH Response: See response to Standard 3 below. Now that Standard 3 is met, Standard 2 is
also satisfied.
CEI Response: For the reasons explained for Standard 3, 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.
Yarmouth Conservation Commission
May 12, 2023
Page 3 of 10
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.
SH Response: Per conversation with CEI on 04/18/2023, test pits are not required due to the
proximity of the basins. Included in Appendix J of the Stormwater Report is a document from
Terracon stating the high likely-hood that the in-situ soils are sandy throughout the site.
Additionally, Terracon has previous soil borings logs taken on the Eversource property north
of this project area that also showed sandy soils.
The existing trees obstruct the ability to perform soil borings in this project area, but the trees
cannot be cleared until a Stormwater Management Permit is approved. Therefore, a
geotechnical report will not be available until after permit approval.
CEI Response: After reviewing the document from Terracon, CEI concurs that the soils are
anticipated to be homogenous sandy soils throughout the proposed project site. Standard 3 is
met.
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 /.
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.
SH Response: Per conversation with CEI on 04/18/2023, WQV is required for all new
impervious area. The design has been updated to capture all of the proposed impervious area.
The remaining difference between disturbed area and drainage area is general pervious areas,
such as the backside of the basin berm. See below response for further explanation.
Yarmouth Conservation Commission
May 12, 2023
Page 4 of 10
The site has been adjusted to treat runoff from the west and north driveways. Runoff from
those two driveways pass through a hydrodynamic separator and are recharged in the
proposed basins.
Due to site constraints, the east driveway is unable to drain to a basin. Instead, a
hydrodynamic separator and Basin 1 have been sized to treat an area of existing untreated
pavement equivalent to the proposed east driveway. This existing untreated pavement runoff
enters ST E-3, flows through the proposed hydrodynamic separator ST E-4, and discharges to
Basin 1. Therefore, there is no additional untreated impervious area than what is existing
today.
CEI Response: CEI has reviewed the updated proposed site plan and agrees that due to site
restraints, the new drainage areas collect adequate runoff for treatment via best management
practices to the extent possible which includes treatment of additional off-site to on-site
runoff.
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).
SH Response: Typo has been corrected.
CEI Response: All WQVs for each basin has been adjust to account for the additional runoff
from the proposed driveways and off-site runoff where applicable.
The WQV calculations portion of Standard 4 is met.
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.
SH Response: TSS removal calculation worksheets have been provided for each treatment
train. Hydrodynamic separators have been added to Drainage Areas 1 and 3 to meet TSS
removal requirements. In Drainage Area 2, all of the impervious area drains to a
hydrodynamic separator at a low point. The separators are only sized to treat impervious area
added by this project.
CEI Response: Comment addressed.
Yarmouth Conservation Commission
May 12, 2023
Page 5 of 10
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.
SH Response: Per conversation with CEI on 04/18/2023, overflow from Basins 2 and 3 do
not need to be treated again since runoff from those drainage areas are already being
adequately treated by other hydrodynamic separators. The storm sewer connections are
intended to provide a relief path during events larger than the 100-year storm. Basins 2 and 3
are designed to detain and infiltrate the 100-year storm for their respective drainage areas.
CEI Response: Comment addressed.
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.
SH Response:
1. An updated TSS removal calculation worksheet has been provided for Basin 2.
2. Hydrodynamic separators have been added upstream of Basin 3 to capture and
treat impervious runoff for Basin 3’s drainage area.
CEI Response: Comment addressed.
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.
SH Response: TSS removal calculations worksheets have been provided for each
basin/drainage area. WQV calculations have been updated to include all proposed impervious
area.
CEI Response: TSS removal calculations comments have been addressed.
Standard 4 is now met for both TSS removal and WQV.
Standard 5: Land Uses with Higher Potential Pollutant Loads (LUHPPL)
The Site is site not a LUHPPL, therefore Standard 5 does not apply.
SH Response: Noted
CEI Response: No response required.
Standard 6: Critical Areas
Yarmouth Conservation Commission
May 12, 2023
Page 6 of 10
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.
SH Response: See above responses for Standard 3 and 4. Since Standard 3 and 4 are now
met, Standard 6 should also be satisfied.
CEI Response: Comments addressed. Standard 6 is now met.
Standard 7: Redevelopment
As the project is not a redevelopment, Standard 7 does not apply.
SH Response: Noted
CEI Response: No response required.
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.
SH Response: Noted
CEI Response: No response required.
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;
o SH Response: Added in Section 1 the requirement that future
responsible parties must be notified of their responsibility to operate
and maintain the system in perpetuity.
o CEI Response: Comment addressed.
Yarmouth Conservation Commission
May 12, 2023
Page 7 of 10
· A plan that is drawn to scale and shows the location of all stormwater BMPs in each
treatment train along with the discharge point;
o SH Response: An exhibit has been included in the O&M Manual that
labels the basins, hydrodynamic separators, and the deep sump catch
basins. It also shows the storm sewer system and flow arrows.
o CEI Response: Comment addressed.
· A description and delineation of public safety features; and
o SH Response: Per conversation with CEI on 04/18/2023, a fence along
the basins is not necessary due to the surrounding area being industrial
in nature, a lack of immediate adjacent neighbors, and that the basins
will not have permanent standing water. During 100-year storm events,
Basin 1 will only have a maximum 1.75’ of standing water, Basin 2 will
have 0.54’, and Basin 3 will have 0.40’ of standing water. All of the
basins are designed to drain out within 72 hours.
o CEI Response: Comment addressed.
· An estimated operations and maintenance budget.
o SH Response: an estimated operations and maintenance budget has
been included in Section 11 of the O&M Manual.
o CEI Response: Comment addressed.
For these reasons, Standard 9 is not met.
SH Response: See above responses. With those updates, Standard 9 is now met.
CEI Response: Comments addressed. Standard 9 is now met.
Standard 10: Prohibition of Illicit Discharges
Standard 10 does not apply.
SH Response: Noted
CEI Response: No response required.
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
Yarmouth Conservation Commission
May 12, 2023
Page 8 of 10
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.
SH Response: The design and calculations have been updated to include the three driveways
and all proposed impervious area. Per conversation with CEI on 04/18/2023, WQV is
required for all new impervious area. The remaining difference between disturbed area and
drainage area is general pervious areas, such as the backside of the basin berm. See responses
to Standard 4 for further explanation.
CEI Response: Comment addressed.
2. Please provide a grading map with the drainage system and infiltration basins overlayed
with all applicable elevations.
SH Response: An exhibit has been added to Appendix D of the stormwater report. It includes
contours, flow arrows, and the storm sewer system.
CEI Response: Comment addressed.
3. According to the Yarmouth Stormwater Regulations Section 2.07, the Operation and
Maintenance Plan must include the following:
a. Address of the person(s) responsible for operation and maintenance,
SH Response: Added to the cover of the O&M.
CEI Response: Comment addressed.
b. The signature(s) of the owner(s), and
SH Response: Added to the cover of the O&M.
CEI Response: Comment addressed.
c. An estimate for operation and maintenance budget.
SH Response: an estimated operations and maintenance budget has been
included in Section 11 of the O&M Manual.
CEI Response: Comment addressed.
Yarmouth Conservation Commission
May 12, 2023
Page 9 of 10
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.
SH Response: Section 9 of the O&M Manual has been updated to include the provisions
from Section 2.07(1)(i).
CEI Response: Comment addressed.
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.
SH Response: Sections 3.h and 3.i have been added to the Stormwater Report covering
Regulation Sections 2.08 and 2.09, respectively.
CEI Response: Comment addressed.
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.
SH Response: Per conversations with CEI on 04/18/2023, 04/19/2023, and 04/20/2023,
hydrodynamic separators are acceptable upstream of an infiltration basin in lieu of a
sediment forebay. Eversource Energy is already accustomed to maintaining separators
and would prefer to continue that treatment practice as opposed to introducing a new one
such as sediment forebays. Therefore, hydrodynamic separators will be used to meet
stormwater requirements instead of a sediment forebay.
CEI Response: Comment addressed.
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.
SH Response: Overland impervious flow for Basin 2 enters a hydrodynamic separator at
a low point before it discharges to Basin 2. Therefore, pretreatment is met for Basin 2.
Off-site runoff is not required to be treated.
Yarmouth Conservation Commission
May 12, 2023
Page 10 of 10
Hydrodynamic separators were added at low points upstream of Basin 3 to collect the
proposed impervious runoff. Pretreatment is now met for Basin 3. Off-site runoff is not
required to be treated.
CEI Response: Comment addressed.
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