25 Maple ReviewBucky Sparkle, PE
378 Main St., #2, Easthampton, MA 01027
617.271.4004 or zengineerbucky@gmail.com
9 October 2019
Project:25 Maple Street, Northampton, MA
Client:Graham Ridley
36 Wilson Ave
Northampton MA 01060
(413) 222-2721
g.ridley@comcast.net
Subject:Review of Gu/Geng Notice of Intent, 25 Maple Street
Dear Graham,
At your request, I have reviewed documents submitted to the Northampton Conservation Commission
regarding a 5-unit multi-family housing development at 25 Maple Avenue. I have identified several
points relative to the proposal which seem deficient or worthy of further investigation. In list form,
these are:
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.
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).
1.2 Volume 2, Chapter 2: Infiltration Basins
1.2.1 Table IB.1: Site Criteria for Infiltration Basins
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.
1.2.1.2 Also, “a minimum of three samples for each infiltration basin [shall be
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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 ensure this area is also not fill. Soils testing should confirm the
apparent assumption that soils remain consistent across the infiltration basin area.
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.
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.
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.
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.
2 Plan Comments
2.1 There is no detail or sizing for the proposed riprap energy dissipation structure.
2.2 No temporary sedimentation basins or material laydown areas are shown on the plans.
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.
4 Staff Report
4.1 The installation of pipe and riprap to the existing waterway will disturb buffer zone
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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.
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.
Please let me know if you have any questions.
Brightly,
Bucky Sparkle, PE
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