DPW Stormwater Review Response Letter 10-31-19
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Comments\DPW Stormwater Review Response Letter 10-31-19.docx
October 31, 2019
Ref: 42540.00
City of Northampton
Department of Public Works
Doug McDonald
125 Locust Street
Northampton, MA 01060
Re: Hawley Street Residential Development
Dear Mr. McDonald,
On behalf of our client, O’Connell Development Group, VHB, Inc. submits for your review and use,
responses to comments received as part of the review process for the above referenced project. VHB, Inc.
received the comments via email dated October 4, 2019 regarding your review of the Hawley Street
Residential Development project. VHB’s responses below are bold italic text following the original
comment.
General:
1. No Low Impact Development (LID) designs have been included in the proposed stormwater
management system. The previous stormwater design for this site included rain gardens, an
infiltration system, green roof and permeable pavers. The consideration of LID measures is
required by Mass stormwater standards and by Northampton permits. The permit application
requires the submission of a MA DEP Stormwater Report Checklist which outlines the LID options.
A completed MA DEP Checklist was not submitted. Please submit a complete MA DEP
Stormwater Report Checklist and any other information and revisions regarding stormwater LID
design proposed or describe technical reason that LID measures have not been considered or
designed?
Response: The stormwater design has been updated based on the comments received as
part of this submittal. This new design incorporates LID techniques such as underground
infiltration trenches where the geotechnical report allows based on seasonal high
groundwater. There are two different infiltration systems on site. One is located at the
middle of the site on the south side and the other is located under a grass swale at the far
east side of the site. The stormwater system at the front of the site will continue to be a
storage system only with no infiltration due to the seasonal high groundwater elevation. A
MA DEP stormwater checklist has been completed and is included in the stormwater report.
Hawley Street Residential Development
Ref: 42540.00
October 31, 2019
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2. No soil information has been submitted and is required to be submitted. The documents
submitted state that soil conditions on the property are "poor" and "groundwater elevations were
high" based on previous soil investigation that were not submitted. No infiltration has been
proposed. The test pits that were submitted in 2016 for the site showed B soils (fine sandy loam
and fine loamy sand). Groundwater in one test pit was at 66 inches and not encountered in
another at 104 inches. According to this information, infiltration is feasible. Since the impervious
area is proposed to be increased, a recharge system should be included in the design to provide
sufficient recharge for this increase.
Response: A geotechnical report has been completed which includes on-site soils analysis
and in-situ hydraulic conductivity rates. The stormwater design has been updated based on
this analysis and infiltration systems have been provided.
3. The proposed stormwater system specifies a 24" HDPE pipe connecting to the City's 15 inch drain
line in Hawley Street. This is not acceptable. The DPW will allow a 12 inch RCP connection to the
City's system in Hawley Street. The plans and drainage calculations should be revised to show this.
Response: This was an oversight during the original design. Based on comments provided
by engineering, the pipe connection to the City’s system cannot be larger than the existing
pipe in the road; therefore, we have reduced the proposed overflow pipe to a 15” RCP.
Based on the HydroCAD model, a 12” overflow pipe will exceed its capacity somewhere
between the 10- and 25-year storm event. Based on the design of the system and the
existing invert in the street the slope of the pipe cannot be increased. A 15” pipe will be
able to handle up to and including the 100-year storm event with the pipe only being
between half and 3/4 capacity.
4. Subsurface detention system #1 close to Hawley Street has been specified with 265 feet of 24
inch pipe in the drainage calculations. The plans show only approximately 205 feet of 24 inch
pipe. Either the plans or the drainage calculations should be revised to be consistent.
Response: The stormwater design has been updated based on the comments received as
part of this submittal. Some stormwater has been redirected to different systems. All
systems have been updated with new total lengths of pipe.
5. It appears that there are 4 existing yard drains in the grass area north of the church building. The
plans show the elimination of 3 yard drains and one yard drain is proposed to be utilized to
collect an increased flow from this area. A new catch basin or basins that connects to the
proposed stormwater system is preferred for this area. If the old yard drain is to be utilized,
Hawley Street Residential Development
Ref: 42540.00
October 31, 2019
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additional information must be submitted detailing where this drain flows to, confirm it is not
connected to sewer and if the drain line (4 inch) has adequate capacity.
Response: A demolition plan has been created for the project. A call-out has been provided
on this plan that indicates the contractor to verify destinations of each of the 5 yard drains
and provide the information to the engineer prior to removal. Contractor also to verify if
any other pipes enter the yard drain system.
The plans now show proposed grading at the area north of the church with two new area
drains to collect water from the new development slope and convey it to the proposed
manhole in Hawley Street. The existing yard drains north of the church collected a larger
area under existing conditions than what will be required under proposed conditions. The
new development buildings now occupy a large portion of this area and the stormwater will
be collected from the roof and conveyed to the underground system which creates a smaller
area being collected by the new area drains.
6. The design of subsurface detention #3 does not seem correct. The drainage calculations have
elevations and outlet configurations that do not allow much storage in this system. Please check
the design and calculations and correct as necessary.
Response: The stormwater design has been updated based on the comments received as
part of this submittal. Some stormwater has been redirected to different systems. All
systems have been updated with new design calculations.
Please let me know if you have any questions or require further information.
Sincerely,
Vanasse Hangen Brustlin, Inc.
Jeffrey A. Galarneau, PE
Project Engineer
jgalarneau@vhb.com
Cc: Andrew Crystal, O’Connell Development
Charles Roberts, Kuhn Riddle Architects