HomeMy WebLinkAbout08/18/2022 - VHB Response 10
August 18, 2022
Ref: 11625.25
Janet Carter Bernardo, P.E.
Associate Principal
Horsley Witten Group, Inc.
294 Washington St
Boston, MA 02108
Re: Initial Stormwater Peer Review
Merrimack MAC Storage Building
North Andover, Massachusetts
Applicant: Merrimack College
315 Turnpike St
North Andover, MA 01845
Dear Ms. Bernardo:
On behalf of the Applicant,VHB has received comments issued by the Horsley Witten Group regarding the second
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.
Comment 2a. The Applicant has provided new Existing and Proposed Drainage Conditions figures which show a
portion of the site also flowing to WF2.
HW has the following additional comments about the Existing Drainage Conditions Figure:
i. The Time of Concentration (Tc)flow path for the roadway flows towards WF1. However, it appears
that existing curbing separates the roadway from the grass area that drains to WF1. HW recommends
that the Applicant explain how the runoff will reach WF1 or add a drainage boundary at the curb that
separates the roadway from the grass area.
RESPONSE. Per field confirmation,there is no curbing east of the gate post and water
flows over the edge of pavement to WF1.The existing drainage figure and associated
Hydrocad model have been revised to include a third design point(DP-3)to reflect the
small portion of the area that does not flow to WF1.
ii. Unless there is a break in the existing curb, it appears that the runoff from a large portion of the
existing pavement area flows to the catch basins located in the southwestern corners of the adjacent
Ref: 11625.25 jNl
August 18, 2022
10
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parking lot. HW suggests that the Applicant consider adding a third design point for this area which
flows to the catch basins and clarify where the catch basins discharge to or justi fy the catchment
areas provided.
RESPONSE:A third design point (DP-3) has been added to the existing drainage
figure and Hydrocad model to reflect the area that does not flow to WF1.The water
captured at DP-3 enters the existing Merrimack College campus closed drainage
system.
iii. It appears that the impervious portion of Drainage Area 2 flows in the easterly direction over the
pavement. HW recommends that the Applicant verify that this portion of the pavement does flow to
WF2.
RESPONSE:Confirmed, the impervious portion of Drainage Area 2 flows to WF2.
HW has the following comments about the Proposed Drainage Conditions Figure:
i. It appears that the roadway portion west of the trench drain flows to the catch basins located in the
southwestern corners of the adjacent parking lot unless there is a break in the curbing as noted above.
HW recommends adding a third design point for this area which flows to the catch basins. HW also
recommends that the Applicant add a drainage boundary at the existing curb that separates the
roadway from the grass area.
RESPONSE:A third design point (DP-3) and Drainage Area 31 have been added to the
proposed drainage figure. Drainage Area 31 is delineated on one side by the existing
curb that separates the roadway from the grass area.
ii. It appears that the impervious portion of Drainage Area 13 flows in the easterly direction over the
pavement similar to existing conditions. HW recommends that the Applicant verify that this portion of
the roadway does flow to WF2. The existing and proposed conditions are very similar, so the final
design point is not critical to the design.
RESPONSE:Confirmed, the impervious portion of Drainage Area 21 (formerly named
Drainage Area 13) flows to WF2.
Comment 8b. The Applicant has provided an inlet protection detail to Sheet C-5.02 of the Plan
Set. HW recommends that the Applicant include inlet protection for the trench
drain during construction once it is installed.
RESPONSE:Sheet C3.00 has been updated to include a note about inlet protection for the proposed
trench drain during construction.
Comment 8c. The Applicant has delineated the project's limit of work on the plan set. HW recommends that the
Applicant include the construction entrance and laydown area within the limit of work. Furthermore, HW recommends
that the Applicant clarify if the grading for contours 243 and 244 are being adjusted under proposed
conditions.
RESPONSE:The limit of work has been revised to include the construction entrance and laydown
area.A note has been added to Sheet C3.00 that clarifies that the proposed 243 and 244 contours
should tie into existing grade with a maximum of a 3:1 slope.
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August 18, 2022
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Comment 8e. The Applicant has proposed a field verification walk after the stormwater management design is
approved. The Planning Board and/or Conservation Commission may choose to include this site walk as a condition of
approval.
RESPONSE: Understood.
Comment 9c. The Applicant is amenable to requiring a signed O&M Plan as a condition of approval. The Planning
Board and/or Conservation Commission may choose to include receipt of the O&M Plan as a condition of approval.
RESPONSE: Understood.
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 atJkgg!1gd1zQybh=or 617-607-0094.
Sincerely,
VHB
Jeffrey Koetteritz, P.E.
Project Manager