Memo Zengineer Comments response 10-16-19.pdfT REYNOLDS ENGINEERING | CIVIL ENGINEERS | PLANNING, DESIGN, AND PERMITTING SERVICES
152 MAPLEWOOD TERRACE, FLORENCE, MA 01062 | PHONE: 413-387-8078 | FAX: 413-727-3477
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To: Northampton, Conservation Commission.
From: Terrence Reynolds P.E.
Subject: 25 Maple Avenue
Date: 10/17/2019
T Reynolds Engineering (TRE) received the following comments on 10/9/19 from Bucky Sparkle for the
abutter Graham Ridley regarding the Notice of Intent submittal for 25 Maple Avenue. All comments are
followed by responses in bold;
1 Massachusetts Stormwater Management Standards
1.1 Volume 2, Chapter 1
1.1.1 Table 2.3: Setbacks for Infiltration Structures
1.1.1.1 This table establishes 10 feet setback between infiltration structures and property lines. The
proposed setback scales to 5.2 feet, roughly half of the minimum.
The system has been moved to 10' away from the property line.
1.1.1.2 This table established establishes that infiltration systems must be, at a minimum, 10 feet
downstream and 100 feet upstream of adjacent structures. The location of #28 Maple Avenue is not
shown on the plan, though it is close to the infiltration system. This structure should be located to
ensure it will not be impacted by the nearby infiltration system. The vector to the nearest waterway
from the infiltration system runs nearly parallel to the lot line, suggesting that a minimum setback
should be something greater than the minimum 10 feet to protect the neighbor’s property. This is
especially the case as silt fill is between the infiltration basin and the waterway, creating a potential
barrier to lateral movement of subsurface water (i.e. potentially causing a backup against the
neighbor’s foundation).
The system is down gradient of the neighboring garage and is approximately 15-20 feet
down gradient. Fill materials in the area were all found to be sand and gravel as shown in
the soil investigation logs.
1.2 Volume 2, Chapter 2: Infiltration Basins
1.2.1 Table IB.1: Site Criteria for Infiltration Basins
This system is not an infiltration basin as listed in Volume 2, Chapter 2. It is listed as a
sub-surface structure and as such does not have the same design criteria. However,
some of the criteria are worth consideration.
1.2.1.1 This table establishes a 50 foot setback of infiltration systems from slopes steeper than 15%.
The system is 47 feet from such a slope to the northwest.
The system has been designed to be over 50 feet from the steep slope.
1.2.1.2 Also, “a minimum of three samples for each infiltration basin [shall be made]. Samples should
be taken at the actual location of the proposed infiltration basin so that any localized soil conditions are
detected.” No samples were taken within the actual basin area. The site has shown highly variable
soils. Other than the nearby test pit TP-9, the closest 3 test pits (#TP’s 1, 2 & 8) show 42”, 90” and
68” of fill, respectively. No test was performed at the northwest corner of the infiltration basin to
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ensure this area is also not fill. Soils testing should confirm the apparent assumption that soils remain
consistent across the infiltration basin area.
This is intended for basins that have significant size. It is not appropriate for this sub-
surface structure as over sampling can result in too much disturbance of the naturally
occurring pervious material. A sub-grade inspection of the sub-surface system at the time
of construction is recommended to determine if unsuitable materials are present that need
to be removed.
1.2.1.3 Siting criteria also states that the infiltration system should not be placed over fill – another
critical variable that is not fully established by the soil testing.
See above.
1.2.2 Design Guidance
1.2.2.1 The standards state that “For each bore hole or test pit, evaluate the saturated hydraulic
conductivity of the soil, depth to seasonal high groundwater, NRCS soil textural class, NRCS Hydrologic
Soil Group, and the presence of fill materials in accordance with Volume 3.” The stormwater
calculations did not demonstrate method, other than the Rawls Rate, used to evaluate the saturated
hydraulic conductivity, a soil characteristic important to determining true offset to groundwater through
mounding analysis.
Rawles rates are the overall most common method for assigning hydraulic conductivity
rates for infiltrating stormwater systems. This is a standard method listed in the MA
stormwater regulations.
1.2.2.2 Guidance states “Infiltration basins must include an overflow outlet in addition to an emergency
spillway.” This is also reiterated in Northampton Zoning Bylaw 281-6: Stormwater Management
Performance Standards. Should the system fail and water backup, no designated emergency spillway
path is provided to the receiving waters. Such an emergency overflow may impact resource areas.
The system is designed with an outlet control including overflow outlets that allows
stormwater to leave the system when the sub-surface infiltration capacity is exceeded.
Again, this is not a infiltration basin including such things as emergency spillways.
1.3 Volume 3, Chapter 1
1.3.1 “Mounding analysis is required when the vertical separation from the bottom of an exfiltration
system to seasonal high groundwater is less than four (4) feet and the recharge system is proposed to
attenuate the peak discharge from a 10-year or higher 24-hour storm.” The system is designed to
attenuate the peak discharge rate. The bottom of the infiltration basin is elevation 113.5. The closest
test hole, TP-9, shows seasonal high groundwater at 95” below existing grade, or elevation 111.1.
Therefore the separation is less than 4 feet. A mounding analysis should be performed to verify the
system will perform as designed. The Rawls Rate is not adequate for this analysis.
A mounding analysis has been prepared as required.
2 Plan Comments
2.1 There is no detail or sizing for the proposed riprap energy dissipation structure.
A detail has been added to the plans
2.2 No temporary sedimentation basins or material laydown areas are shown on the plans.
Due to the scale of the project and the implementation of the erosion control plan, no
temporary sedimentation basins are anticipated to be needed. laydown areas will be
located within the limits of work.
3 Zoning Comments
3.1 The URC zoning district requires that the scale and massing of a proposed structure be comparable
with the block. The proposal is for a 3-story, 5-unit structure. The adjacent residence, #28 Maple Ave.,
is a 1-story ranch. All other structures are 2 to 2.5 story buildings. The appropriateness of a 3-story
structure is questionable.
Structures on the street do range from 1 to 3 stories and the building design has been
made in keeping with other buildings on the street.
4 Staff Report
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4.1 The installation of pipe and riprap to the existing waterway will disturb buffer zone within 35 feet of
the resource area. The suggestion was made to create a wetland mitigation area in lieu of riprap. This
idea should be seriously considered for the betterment of the resource area and ameliorate the impacts
within the most sensitive buffer zone.
This option has been discussed with conservation and is not feasible.
4.2 As stated by staff, an operation and maintenance plan for the stormwater management system and
plans for snow storage and treatment should be offered by the applicant and discussed by the
Commission.
An O&M plan is included in the stormwater report. A snow storage area is also shown on
the plans.