HomeMy WebLinkAbout2023/03/11 - Response to 1st Stormwater Peer Review - Minor Modifications - - JN i
r 1
March 11, 2023
Ref: 11625.34
Ms.Jean Enright, Planning Director
North Andover Planning Department
120 Main Street
North Andover, MA 01845
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. The following comments relate to the Site Preparation and Erosion Control Plan (Sheet C2.0)
a. The existing concrete path that cuts diagonally across the site is not called out for removal.
HW recommends that the Applicant confirm whether the path is to remain, and revise the
plans to clarify that the path is to be removed if not.
RESPONSE: Note to remove existing path has been added to the Site Preparation
and Erosion Control Plan Sheet C2.0.
b. The catch basin adjacent to Existing Tree 4 (along Medina Drive) is not called out for
removal, but does not appear to be active in the proposed Grading and Drainage Plan
(Sheet C4.0). HW recommends that the Applicant clarify if this existing catch basin is to
remain, or revise the plans as needed.
RESPONSE: Note to remove existing catch basin adjacent to Existing Tree 4 has
been added to the Site Preparation and Erosion Control Plan Sheet C2.0.
c. A callout stating "Remove existing wooden post and rail fence"is shown on the north side of
the site, but does not point to anything specific. HW recommends that the Applicant revise
or remove this callout.
Ref: 11625.34 N�,
March 11, 2023
ry
Page 2
RESPONSE:This note location has been revised on the Site Preparation and Erosion
Control Plan Sheet C2.0 to point to the fence north of the existing walkway on the
corner.
2. The Applicant notes on Sheet C 1.0 that earth materials shall be "exposed for a minimum of time
before they are covered, seeded, or otherwise stabilized to prevent erosion"(Erosion Control Note 4).
HW recommends that the Applicant specify a maximum time of 14 days of exposure before earth
material stockpiles are seeded and stabilized.
RESPONSE: Erosion Control note 4 on the Legend and General Notes Sheet C1.0 has
been revised to include that earth material stockpiles must be seeded or otherwise
stabilized to prevent erosion within 7 days of exposure given the Project is
discharging to a Tier 2 water as noted under the SWPPP and NPDES CGP.
3. The Applicant has included a Dewatering Filter Bag detail on Sheet C6.1, but does not make
reference to dewatering in the notes on Sheet C7.0. HW recommends that the Applicant clarify
whether dewatering is anticipated, and revise the plans to include a note on dewatering if so.
RESPONSE:A note about dewatering in accordance with the Environmental
Protection Agency (EPA) Dewatering and Remediation General Permit (DRGP) has
been added to the Site Preparation and Erosion Control Plan Sheet C2.0 and the
Grading and Drainage Plan Sheet C4.0.
4. The following comments relate to the grading and drainage design depicted on the Grading and
Drainage Plan (Sheet C4.0):
a. A spot grade is covered by the trench drain at the inner corner of Proposed Building 2. HW
recommends the Applicant revise the plans to make the spot grade visible. Relatedly, the
adjacent trench drain does not appear to be at a low point, and it seems possible that water
will pond along the edge of the building. HW recommends that the Applicant revise the
grading in this area to prevent ponding.
RESPONSE:The conflicting spot shot has been removed on the Grading and
Drainage Plan Sheet C4.0.An additional spot shot has been added between the
landscape retaining wall and proposed Building 2 to clarify the intent for this
landscape area to act as overland relief in the situation where the trench drain
and/or area drain C115 are blocked.The trench drain rim elevation has been revised
to a low spot of 219.9.
b. At structures C110 and D 101,proposed outlet inverts are higher than proposed inlet inverts,
which appears likely to cause backing up of the drainage system. HW recommends that the
Applicant revise the drainage design to allow constant flow.
RESPONSE:The outlet inverts of C110 and D101 have been lowered to allow
constant flow.The inverts downstream of C110 leading to Subsurface Infiltration
Ref: 11625.34 N�,
March 11, 2023
ry
Page 3
System C have been revised accordingly.The inverts of structures D102 and D103,
downstream of D101, have also been revised accordingly.
c. The rim elevation of structure C709 appears to be 1 foot above the proposed grade. HW
recommends the Applicant confirm whether this design is intentional and revise the plans if
not.
RESPONSE:The rim elevation of structure C109 has been revised to match the
proposed grade on the Grading and Drainage Plan Sheet C4.0.The inverts at
structure C104 downstream of C109 have been adjusted accordingly.
d. The proposed Subsurface Infiltration System A is not connected to an outlet control
structure (OCS), and the outlet pipe is set to an elevation that promotes limited
storage/infiltration (i.e., the outlet is only 0.1 feet above the bottom of the infiltration
system). The Applicant may consider raising the outlet or proposing an outlet control
structure to maximize the storage and infiltration capacity of System A.
RESPONSE: The outlet elevation of pipe leaving Subsurface Infiltration System A
has an invert of 224.4 which is 0.3 feet above the bottom of the system (224.1).The
outlet pipe is intended as the outlet control due to the shallow nature of structure
A106.With this outlet pipe, System A contributes to meeting the required water
quality treatment on site via infiltration.
5. On the Site Details (Sheet C6.3), the OCS with Weir detail shows two orifices in the weir in the
section view and only one orifice in the plan view. In the HydroCAD model, only one orifice is
modeled. HW recommends that the Applicant clarify the number of orifices proposed for each OCS
and revise the plans/HydroCAD model as necessary for consistency.
RESPONSE: The OCS with Weir detail on Site Details Sheet C6.3 has been updated
to show only one orifice in section view to be consistent with the HydroCAD model
and plan view.
6. The proposed Subsurface Infiltration Systems are within 2 feet of the estimated seasonal high
groundwater(ESGHW) table and contribute to attenuating peak flows during the 10 year storm.
HW recommends that the Applicant provide a mounding analysis to verify that the proposed basins
will not experience mounding. Further, HW recommends that the Applicant modify the design of
System B to provide a minimum of 2 feet of separation between the bottom of stone (currently
217.0) and the ESGHW elevation (216.0).
RESPONSE: Based on subsurface explorations, the shallowest observed ESHGW
table was noted at 2 feet below existing grade. Subsurface Infiltration System B has
been shifted so that the highest existing grade is 217.0 and therefore the ESHGW
table is conservatively at 215.0.This revision maintains the minimum 2 feet of
separation between the bottom of stone (217.0) and the ESGHW elevation (215.0).
For additional confirmation boring GEO 107W within the footprint of the proposed
Subsurface Infiltration System B was completed in January 2024 by GeoEngineers.
Ref: 11625.34
March 11, 2023 ' 00
Page 4
11bl.
This boring indicated groundwater at approximately 4 feet below existing grade.
Please see attached excerpt from the Soil Pre-Characterization Memorandum
prepared by GeoEngineers.
7. As shown on the Utility Plan (Sheet C5.0), the inlet to SMH A202 from Proposed Building 1 is 1.8 feet
above the outlet of the SMH. To reduce the potential of scour in the manhole, HW recommends that
the Applicant consider lowering the inlet invert to 0.7 feet above the outlet invert.
RESPONSE:A drop less than 2 feet within a sewer manhole is standard practice. If
the drop exceeds 2 feet an exterior drop SMH would be used to avoid the potential
of scour in the manhole.The current 1.8 feet drop can be reduced to 0.2 feet by
lowering the invert from Building 2 and raising the outlet. Decreasing the drop to
0.1 feet would exceed a 5% slope for the incoming pipe and we want to be mindful
of high velocities in the system.The revised inverts are shown on the Utility Plan
Sheet C5.0.
8. The Operation and Maintenance (O&M) Plan specifies an inspection rate for the subsurface
infiltration systems of at least once per year. HW recommends that the Applicant revise this rate to
twice per year,per MSH Volume 2, Chapter 2.
RESPONSE:The recommended inspection rate for the subsurface infiltration
systems has been revised to twice per year in the O&M Plan per the MSH Volume 2,
Chapter 2 recommendation.
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: 11625.34
March 11, 2023
Page 5
ATTACHMENTS:
• Site Plans—Local Approvals, revised 03/11/2024
• Operations and Maintenance Plan, revised 03/11/2024
• Excerpt from Soil Pre-Characterization Memorandum by Geoengineers, 03/05/2024