HomeMy WebLinkAbout2023/09/26 - Response to 1st Stormwater Peer Review - - !'fin
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September 26, 2023
Ref: 1162 5.34
Ms.Jean Enright, Planning Director
Associate Principal
Horsley Witten Group, Inc.
112 Water Street
Boston, MA 02109
Re: Initial Stormwater Peer Review
Corner Development Project at Merrimack College
North Andover, Massachusetts
Applicant: Merrimack College
315 Turnpike St
North Andover, MA 01845
Dear Ms. Enright:
On behalf of the Owner and Applicant, VHB has received comments issued by the Horsley Witten Group
regarding the review of the Stormwater Management Report and Site Plan for the above-referenced
project.We appreciate the effort associated with this review and offer the following responses. Horsley
Witten Group comments are presented below in italics, followed by VHB's responses in bold type.
1. Standard 1:No new stormwater conveyances (e.g. outfalls) may discharge untreated stormwater
directly to or cause erosion in wetlands or water of the Commonwealth.
a. The applicant has analyzed the pre- and post-development stormwater runoff to one design
point(DP-7). DP-1 is the closed drainage system across Andover/Elm Street on the
northwest side of the Project Site. It is not clear on the plan set where the closed drainage
system eventually discharges though it appears to be piped into a resource area. HW
recommends that the Applicant clarify the outfall. If the outfall is a resource area, HW
recommends that the Applicant confirm that erosion is not currently occurring.
RESPONSE: Please see revised drainage figures that identify the closed drainage
system outfall into an existing intermittent stream (bordering vegetated wetland
and bank), which was flagged and located by VHB in March 2022.The existing
outfall is vegetated at a concrete headwall as shown in the photos below, VHB had
not proposed disturbance to avoid additional impact to the existing wetland
resource area, and due to the existing flows into this pipe from the Site being
reduced by approximately 4 CFS in the 25-year design storm as a result of the
proposed stormwater management improvements.We should also note that the
existing closed drainage system at DP-1 and down to the headwall is owned by
Ref: 1162 5.34n
September 26, 2023
Page 2
MassDOT, and upon receipt of Local Approvals, the Project will also be required to
obtain a State Highway Access Permit for the proposed site and drainage
improvements.
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Ref: 1162 5.34n
September 26, 2023
Page 3
2. Standard 2:Stormwater management systems shall be designed so that post-development peak
discharge rates do not exceed pre-development peak discharge rates.
a. In accordance with §250-22 B. (6) a summary of pre- and post-development peak rates and
volumes of stormwater demonstrating no adverse impacts should be provided as part of the
narrative. The Applicant has provided the peak flows and the peak volumes for the 2-year,
10 year, 25 year, and 100-year storm events. HW recommends that the Applicant provide a
revised table if needed based on the comments below.
RESPONSE: Per responses below, revisions have been made to the Project Site Plans
and Details to align with the HydroCAD model as initially submitted.
b. HW has reviewed the Existing and Proposed Drainage Conditions figures and the
HydroCAD model. The subcatchment areas, surface materials, times of concentrations, and
flow patterns appear reasonable. No further action required.
RESPONSE: N/A
c. The Applicant has included three subsurface infiltration chamber systems. HW notes that
the Outlet Control Detail(OCS)for Subsurface Infiltration System B indicates that the
structure is 4.0 feet diameter and illustrates the two 10-inch orifices in line vertically. The
HydroCAD model includes Device 2 as a 6-foot weir with two 10-inch orifices in line
vertically. The HydroCAD model includes Device 2 as a 6-foot weir with two 10-inch orifices
set at the same horizontal elevation. HW recommends that the Applicant revise the
HydroCAD model using a 4-foot weir or adjust the OCS detail. HW further recommends
that the Applicant adjust the OCS detail to show the orifices besides each other to avoid
confusion.
RESPONSE:The Outlet Control Structure with Weir (OCS) detail on Sheet C6.3 has
been updated to match the HydroCAD model.The 6-foot weir has two 10-inch
orifices in line horizontally.
d. The HydroCAD model for Pond 3, Subsurface Infiltration System B, utilizes an outlet device
set at elevation 215.8. The Grading and Drainage Plan includes a callout for an outlet
elevation at 216.2. HW recommends that the Applicant revisit this outlet and confirm the
system has been modeled correctly.
RESPONSE: The outlet elevation of the Grading and Drainage Plan has been
updated to match the outlet shown in the HydroCAD model.
e. The Applicant has used precipitation values equal to or greater than the values provided by
National Oceanic and Atmospheric Administration (NOAA)Atlas 14 for the 24-hour storm
events, as outlined in §250-23 E. (79). No further action required.
RESPONSE: N/A
Ref: 1162 5.34n
September 26, 2023
Page 4
f. Per,§250-23 E. (25) Stormwater basins shall be sized to accommodate the 100 year storm
event with a minimum of one foot of freeboard. The Applicant has documented that the
proposed subsurface chamber systems can accommodate a 100-year storm event. The
proposed chamber systems do not provide one foot of freeboard, HW recommends that the
Planning Board provide direction to the Applicant on whether the subsurface chambers are
required to provide a foot of freeboard.
RESPONSE: The proposed stormwater management systems were not modeled
with a foot of freeboard for the 100-year storm, as they are subsurface systems
receiving flows from the closed drainage system, which is sized for the 25-year
storm event, and not at-grade detention basins. Regardless, the subsurface ponds
were still evaluated for the 100-year event and peak elevations relative to the
proposed site grading to understand the minimal ponding that would occur at low
points of the system during the peak of the 100-year design storm. Please refer to
Exhibit B3— 100-Year Storm: Ponding Elevations, which highlights the anticipated
ponding elevations of the closed drainage systems based on the 100-year storm
peak pond elevations.The systems were additionally modeled to confirm that peak
rates of runoff and runoff volumes were decreased from existing to proposed in the
100-year storm event.
g. Per,§250-23 E. (30)All drainpipes are to be reinforced concrete pipes and have a minimum
diameter of 12 inches. The Applicant has proposed HDPE drainpipes with some being less
than 12 inches. HW recommends that the Applicant review these criteria and revise the
plans or providejusti fication to the Planning Board to allow the use of the different pipe
material and sizes.
RESPONSE: Please refer to revised Sheets C4.0 and C5.0, and revised Hydraulic Pipe
Analysis included in Appendix A of the Stormwater Report.The proposed drainage
being picked up from existing area drains has been upsized to 12". Unless otherwise
required by the Town, we would propose the flexibility of utilizing both HDPE and
RCP pipe depending on the collection structures, HDPE in particular if being
connected from proposed HDPE landscape drains, and RCP in particularly if
connected from proposed pre-cast basin structures.
3. Standard 3 requires that the annual recharge from post-development shall approximate annual
recharge from pre-development conditions.
a. The Applicant has provided a Stormwater Evaluation Memorandum prepared by
GeoEngineers, dated July 27, 2023. The geotechnical report recommended using an
exfiltration rate of 0.52 inches per house(iph). The Applicant has used 0.52 iph in the
HydroCAD model and has provided the required 2 feet of separation to groundwater. No
further action required.
RESPONSE: N/A
Ref: 1162 5.34n
September 26, 2023
Page 5
b. HW concurs that the Applicant has provided the required recharge. However, HW was not
able to confirm the values listed in Table 5 of the Stormwater Management Report. For
documentation purposes HW recommends that the Applicant review the values listen and
confirm the depth proposed between the lowest outlet and the bottom of the system. It may
be beneficial for the Applicant to provide the stage storage area print out from the
HydroCAD model. If applicable, HW recommends that the Applicant update Table 5.
RESPONSE:The lowest outlet and bottom of system elevations have been added to
the details on Site Details Sheet C6.3. Please see attached Exhibit C4— HydroCAD
Staged Storage Areas which includes the storage volumes for each of the three
subsurface systems.Table 5 in the Stormwater Management Report has been
updated to include the bottom of system area, the bottom of system stone
elevation, and the lowest system outlet invert.
4. Standard 4 requires that the stormwater system be designed to remove 80% Total Suspended Solids
(TSS) and to treat 7-inch of volume from the impervious area for water quality.
a. The Applicant has provided a TSS worksheet that lists the water quality units and the
subsurface infiltration chambers to provide the 90% TSS removal per the Town's
requirements. No further action required.
RESPONSE: N/A
b. Per,§250-23 B. (7) the Applicant is required to remove 60% of the average annual load of
Total Phosphorus. The Applicant has provided the Phosphorus Removal worksheets that
indicate that the Applicant is removing 79% of the Phosphorus load via the subsurface
infiltration chambers. The calculations appear reasonable. No further action required.
RESPONSE: N/A
c. The Applicant has noted it its narrative that the proposed stormwater system has been
designed to treat one-half inch of water quality volume. Per.§250-23 B. (7)(b) new
developments are required to retain the volume of runoff equivalent to one inch multiplied
by the total post-construction impervious surface or meet a combination of retention and
treatment that achieves the above standards listed under.§250-23 B. (a). It appears that the
criteria have been met. No further action required.
RESPONSE: N/A
5. Standard 5 is related to projects with a Land of Use of Higher Potential Pollutant Loads (LUHPPL).
a. The use of the site as dormitories within a college campus is not considered a LUHPPL.
Standard 5 is not applicable.
Ref: 1162 5.34n
September 26, 2023
Page 6
RESPONSE: Noted.
6. Standard 6 is related to projects with stormwater discharging into a critical area, a Zone 11, or an
Interim Wellhead Protection Area of a public water supply.
a. The proposed development is not charging near or into a critical area,Zone 11 or an IWPA
area. Therefore, Standard 6 is not applicable.
RESPONSE: Noted.
7. Standard 7 is related to projects considered Redevelopment.
a. The project is considered new development. The Applicant will increase the impervious
surface by 60,770 sf of root top and pavement area within a watershed area of 377,520 sf,
an increase of approximately 50%. Standard 7 is not applicable.
RESPONSE: Noted.
8. Standard 8 requires a plan to control construction related impacts including erosion, sedimentation,
or other pollutant sources.
a. The Applicant has provided a Site Preparation and Erosion Control Plan, sheet C2.0 with
details on Sheet C6.7. The Applicant has also provided a narrative for Recommended
Construction Period Pollution Prevention and Erosion and Sedimentation Controls in
Appendix E of the Stormwater Management Report.
RESPONSE: N/A
b. HW recommends that the Applicant add a note stating,prior to any land disturbance
activities commencing on the site, the developer shall physically mark limits of no land
disturbance with tape, signs, or orange construction fence, so that workers can see the areas
to be protected. The physical markers shall be inspected daily.
RESPONSE: Note has been added to the Site Preparation and Erosion Control Plan
Sheet C2.0.
c. The Applicant is proposing to disturb more than one acre of land. In accordance with the
EPA Construction General Permit, the Applicant is required to file a Notice of Intent with the
EPA and prepare a Stormwater Pollution Prevention Plan (SWPPP). HW recommends that
the Applicant provide the Town of North Andover with a copy of the SWPPP a minimum of
74 days prior to land disturbance. The Planning Board may choose to require receipt of the
final SWPPP signed by the contractor as a condition of approval.
RESPONSE:Acknowledged, the Applicant proposes to submit a copy of the SWPPP
to the Town of North Andover prior to filing the Notice of Intent with the EPA 14
days prior to land disturbance.
9. Standard 9 requires a Long-Term Operation and Maintenance(O&M) Plan to be provided.
Ref: 1162 5.34n
September 26, 2023
t
Page 7
a. The Applicant has provided an Operation and Maintenance Plan in Appendix D of the
Stormwater Management Report. The O&M Plan can be removed from the Stormwater
Management Report and be a standalone document for use by the college. HW notes that
the town is listed incorrectly on the cover page.
RESPONSE:The O&M plan has been removed from the Stormwater Management
Report and will be included as a standalone document. Stormwater Management
Report appendices have been updated accordingly.The cover page has been
revised.
b. In the Long-Term Maintenance/Evaluation Checklist, Section E.4 of Appendix D, HW
recommends that the Applicant add the water quality units.
RESPONSE:Water quality units have been added to Section E.4 of Appendix D of
the O&M Plan.
c. HW recommends that the Planning Board reference this document and require a signed
O&M Plan as a condition of approval.
RESPONSE: Please see the attached revised O&M Plan that has been signed by the
College.
70. Standard 70 Requires an Illicit Discharge Compliance Statement be provided.
a. The Applicant has provided an Illicit Discharge Compliance Statement signed by the
property owner. No further action required.
RESPONSE: N/A
Thank you again for your time and consideration.We look forward to ongoing coordination of the
proposed project with you as noted. If there are any additional questions/comments in the meantime,
please feel free to contact me atjkoetteritz@vhb.com or 617-607-0094.
Sincerely,
VHB
Jeffrey Koetteritz, P.E.
Ref: 1162 5.34
September 26, 2023
Page 8
TT
• Site Plans—Local Approvals, revised 09/15/2023
• Stormwater Report, revised 09/15/2023
• Operations and Maintenance Plan, revised 09/15/2023