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July 29, 2016
North Andover Planning Board
1600 Osgood Street
North Andover, MA 01845
Attn: Rebecca Oldham, Staff Planner
RE: Stormwater Review
Princeton at North Andover Apartment Homes
Dear Ms. Oldham and Board Members:
Per your request, I have reviewed the following materials submitted in support of the Site
Plan— Special Permit Applications for the above-referenced project:
■ Special Permit— Site Plan Review Application for an approximately 40,000 sf
commercial development on approximately 4.3 acres of land known and
numbered as 1210 Osgood Street, Princeton Development LLC—Petitioner,
submitted by John T. Smolak, Esq.
■ Special Permit— Site Plan Review Application for a market rate rental residential
community on approximately 9.5 acres of land known and numbered as 1210
Osgood Street, Princeton Development LLC—Petitioner, submitted by John T.
Smolak, Esq.
■ Stormwater Report in Support of: Princeton at North Andover Apartment Homes,
prepared for Princeton Properties by Hancock Associates and stamped 6/21/16.
■ Permit Site Plan(13 Sheets), Princeton at North Andover Apartment Homes,
prepared for Princeton Properties by project team members in cooperation with
Hancock Associates, dated 6/17/16.
The following additional materials were not part of the application package but were
provided to me by Hancock Associates upon request:
■ Electronic copy of the 6/17/16 Grading and Drainage Plan with test pit locations
shown.
■ Table of inverts, HSA — Princeton at North Andover Apartment Homes, dated
7/20/16
I also conducted a brief site visit to the property to observe existing drainage patterns.
My primary focus in this technical review is on the overall stormwater management
approach and design concepts used in the project, as well as its compliance with the
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Town of North Andover's Site Plan Review regulations, the Massachusetts Department
of Environmental Protection (DEP) Stormwater Management Standards and Regulations,
and the North Andover Wetlands Bylaw. It is my understanding that a Notice of Intent
(NOI) application for the project has been or will be filed concurrently with the
Conservation Commission. My review is aimed at assisting both boards in their
respective reviews of the project.
The project site is an approximately 13.8-acre parcel on the west side of Osgood Street in
North Andover. It is currently undeveloped and mostly vegetated. Drainage is overland to
the north and west toward several wetland areas on and adjacent to the site. The proposed
project calls for a mixed use development comprised of approximately 40,000 square feet
(sf) of commercial development, four apartment buildings, a leasing office and pool
facility, and associated landscaping, parking and driveway areas. As proposed, runoff
from the impervious areas of the site would be drained through a closed drainage system
to sixteen subsurface storage/infiltration systems, with overflow discharged toward the
wetlands via stone aprons.
My comments on the proposed design are outlined below:
1. Contrary to statements made in the Stormwater Report, the proposed project does
not constitute Environmentally Sensitive Site Design or incorporate Low Impact
Development (LID) techniques in the management of stormwater from the site,
both of which entail minimizing impervious surfaces, disconnecting flow paths
and using natural drainage systems to treat and recharge stormwater as an
alternative to "curb and gutter" closed systems such as the one proposed. While
the proposed plan does largely maintain existing drainage patterns it is not clear
that any serious consideration was given to incorporating such measures in the
design of the project, as is required by Stormwater Standard 1.
2. The test pit locations and drainage system inverts that were provided to me as
supplemental materials are a fundamental component of the drainage design and
need to be included on the permit plans.
3. The plans should clearly identify the pervious areas on the proposed site. The
Landscape Plan identifies the proposed plantings and some areas to be loamed
and seeded, but it is not clear how the areas immediately surrounding the
buildings are to be treated and whether the plan is consistent with the runoff
calculations.
4. The Stormwater Report includes sizing calculations for each of the proposed
storm drain outlets in accordance with Stormwater Standard 1, however this
information is not shown on the proposed plan and there is no design detail for the
outlet aprons.
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5. Proposed Stormtech Systems #21, 33 and 34B are not labeled on the Grading and
Drainage Plan and Stormtech System #3 8 is incorrectly identified as a second
System#3 6.
6. In order to better quantify impacts on wetland resources I recommend setting the
control points used in the hydrologic analysis at the wetlands themselves and
including the two onsite wetland areas along the western boundary and the two
offsite wetlands to the west and north of the site. It should also be clarified
whether any of these wetland areas are hydrologically connected.
7. The drainage analysis assumes that all of the runoff from eighteen subcatchments
on the post-development site is conveyed through the closed drainage system to
the subsurface chambers for flow attenuation in all design storms up to and
including the 100-year event. It therefore needs to be demonstrated that the
catchbasin grates, roof drains and storm drains are designed to capture and convey
the 100-yr flow.
8. The building roof areas are modeled as being unconnected (e.g. draining across
pervious surfaces) in the post-development drainage analysis; this is inconsistent
with the plan, which calls for roof drains directly connected to the subsurface
structures.
9. According to the plan, the roof drainage from each of the proposed buildings on
the site would be discharged at a single location on one side of the building. It
should be confirmed (e.g. through architectural drawings) that this is consistent
with design of the roofs and that all of the roof runoff from each building can be
conveyed to that single location.
10. It is unclear from the plans how the runoff from the proposed carports would be
handled.
11. The assumed flow path for post-development subcatchment area 300 is not
consistent with the grading shown on the plan and artificially increases the Tc for
the subcatchment. In general, the post-development Tc's should not be greater
than the pre-development ones.
12. The drainage analysis is not consistent with the outlet control structure detail for
the subsurface Stormtech systems shown on Sheet 10 of the plan. The plan shows
a weir with a 3-ft crest and four 4-inch orifices, while the drainage analysis
assumes 4-ft weirs in all sixteen outlet structures, and a variety of configurations
of 3-inch, 4-inch, 4.3-inch, 5-inch and 6-inch orifices. All such design
assumptions need to be accurately reflected on the permit plans.
13. The drainage analysis assumes that eleven of the proposed subsurface systems are
comprised of Stormtech MC-3500 chambers and that five of the systems are
comprised of MC-4500 chambers. Only the MC-3500 chambers are shown on the
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. 1151detail sheet. The design detail also does not include the distribution manifolds or
specify the amount of stone beneath the system.
14. As is identified in the Stormwater Report, the predominant soils on the site are
mapped by NRCS as Paxton and Woodbridge fine sandy loams. These soils are
relatively poorly drained; the Paxton series is assigned a Hydrologic Soil Group
(HSG) classification of C, and the Woodbridge soils are classified HSG C/D, or
only marginally suitable for infiltration. The soil test pits conducted on the site
appear to confirm the mapped soils, and no additional grain size analyses or
infiltration testing data were provided. I also note that the runoff calculations and
the compensatory recharge calculations are based on HSG C soils. It is not clear,
therefore, why both the drainage analysis and the drawdown calculations assume
the Rawls infiltration rate for HSG B sandy loam (1.02 in/hr) for eleven of the
sixteen proposed subsurface infiltration systems, and the Rawls rate for HSG
loamy sand (2.41 in/hr) for infiltration system #21. These assumed infiltration
rates are an order of magnitude greater than the Rawls rates for the mapped soils
(0.17 to 0.27 in/hr) and significantly overestimate the amount of runoff that will
be infiltrated during the design storm events.
15. I also note that based on the test pit data provided, four of the subsurface systems
where exfiltration is assumed (Stormtech Systems 31, 32, 33 and 34) would not
meet the 2-ft separation requirement between the bottom of the system and
estimated seasonal high groundwater (ESHGW). No soil tests were conducted in
another six of the proposed infiltration areas (Stormtech Systems 21, 31 B, 34B,
3 5B, 3 8B, 3 9) where exfiltration is assumed. Two of those systems (38B and 3 9)
are several feet below existing grade and would likely not have adequate
separation to ESHGW.
16. The drainage analysis should not assume any storage volume in that portion of
any of the systems that is below ESHGW.
17. The infiltration system elevations listed on pages 7 and 8 of the Stormwater
Report are not consistent with the table of inverts that I was provided. In addition,
the separation to ESHGW should be measured from the bottom of the system (e.g.
the stone layer) and not the invert of the lowest structure, thus several of the
recharge systems listed would not provide the required separation. The model
assumes a foot of stone beneath each of the proposed infiltration systems.
18. In accordance with the requirements of Stormwater Standard #3, the recharge
calculations should include a capture area adjustment to account for impervious
areas that are not tributary to the proposed recharge structures.
19. A number of the proposed infiltration systems are to be constructed in fill. The
plans should specify any fill material to be used beneath the systems.
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20. A portion of Stormtech System 10 is located within the 50-ft buffer to the onsite
wetland. The MA Stormwater Handbook requires infiltration structures to have a
minimum setback of 50-ft from wetlands.
21. Based on the relative inverts there appears to be potential for breakout of
infiltrated flows downgradient of Stormtech Systems 10 and 21.
22. As proposed, stormwater treatment for the impervious areas on the site would be
provided by the subsurface Stormtech Isolator Rows, for which 80 percent TSS
removal credit is being claimed in the water quality calculations. I note that the
Isolator Row is considered a proprietary treatment/pretreatment device and that no
TSS removal rate has been assigned by MADEP, therefore third party verification
of the stated removal rate must be provided. Although it was checked off in the
Stormwater Checklist, no such third party verification was included in the
materials I reviewed. The verified removal rate should also reflect the fact that
some of the Isolator Rows would not be receiving raw runoff, but rather flow
which has already undergone some sediment removal in upgradient deep sump
catchbasins. (Most third party testing is based on raw runoff). I further note that
the treatment provided in an Isolator Row system does rely on exfiltration through
the woven geotextile that surrounds the chambers; hence it will not function
effectively in those locations where adequate separation to groundwater is not
provided. This would include proposed Stormtech Systems 11, 36, 37 and 38 as
well as those identified in my previous comment# 13.
23. Calculations are needed to demonstrate that the entire water quality flow from all
impervious areas on the site, including roof areas, would be directed through the
proposed treatment BMPs and not bypassed.
24. The Stormwater Checklist indicates (under Standard 5) that the proposed land use
is covered by the NPDES Multi-Sector General Permit (MSGP). I do not believe
this to be the case since it is not an industrial use, however the proposed project
may constitute a Land Use with Higher Potential Pollutant Loadings (LUHPPL)
with moderate to higher concentrations of oil and grease if the projected traffic
generation is greater than 1,000 vehicle trips per day. If that is the case, the
treatment train for the pavement runoff on the site would need to include an oil
grit separator, a filtering bioretention area, a sand filter, or equivalent. The
treatment train currently proposed would not meet this requirement.
25. It should be confirmed that the wetlands on and adjacent to the site do not drain to
Lake Cochechewick and that the requirements of Stormwater Standard 6 (Critical
Area) do not apply.
26. The proposed plan calls for upwards of ten feet of fill across much of the project
site. The Board may want to consider requesting cut and fill calculations for the
proposed development, as well as specifications for any fill to be brought onto the
site.
27. I recommend against the placement of geotextile fabric on the bottom of the
proposed infiltration systems, as it can be prone to clogging. A drainage net per
Stormtech's Tech Sheet #5 or a filter layer of graded stone are acceptable
substitutes for the fabric.
28. The Conceptual Landscape Plan shows shade trees and shrubs over several of the
proposed subsurface infiltration systems. Deeply rooted plantings should be
avoided.
29. As is indicated in the Stormwater Checklist, the proposed project would entail the
disturbance of more than one acre of land and be subject to EPA's NPDES
Construction General Permit (CGP). Prior to the initiation of work the selected
contractor will need to file a Notice of Intent for coverage under the CGP, and
prepare a Stormwater Pollution Prevention Plan (SWPPP) to be implemented
during construction. This requirement should be clearly noted on the plans. The
Planning Board and/or Conservation Commission may also want to require proof
of filing of the NOI and the opportunity to review the SWPPP prior to the start of
work.
30. The Erosion & Sediment Plan should include a detailed construction sequence
and should address stabilization of the 2:1 slope between Stormtech System 10
and the adjacent wetland and protection of proposed infiltration areas during
construction.
31. The minimum stone size on the proposed construction entrance should be 1-1/2 to
2 inches.
32. The Layout and Materials Plan shows several proposed snow storage locations
between the developed areas of the site and the wetlands. Snow stockpiles should
be located outside of the buffer zone and upgradient of stormwater BMPs, so that
the meltwater undergoes treatment prior to discharge.
33. The plan should show inspection ports brought to grade on each row of
subsurface chambers.
34. I have the following comments on the Operation and Maintenance (O&M) Plan
included in the Stormwater Report:
■ Short-term, construction related measures should be removed from the O&M
Plan and included in a separate construction SWPPP. The long-term plan
should be structured as a standalone document to be maintained on site.
■ The plan should identify the owners and parties responsible for the ongoing
maintenance of the stormwater system. If the commercial and residential
portions of the site are to have different owners separate plans may be
required.
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■ The O&M Plan should clearly identify the inspections and maintenance tasks
that are to be performed for each pretreatment and treatment BMP, as well as
the outlet aprons. Simply attaching design guidelines from the MA
Stormwater Handbook and the manufacturer's cut sheets is not adequate.
■ Subsurface storage/infiltration systems should be inspected at least once a
year following a storm of one inch or more to ensure that they are fully
drained within 72 hours.
■ The Long Term Pollution Prevention Plan should be expanded to include
pavement sweeping, landscape maintenance, snow storage and any other
source controls associated with the proposed land uses.
■ The plan should also include a maintenance checklist and a simple figure
showing the locations of all stormwater BMPs to be maintained as well as
designated snow storage locations.
I appreciate the opportunity to assist the North Andover Planning Board and
Conservation Commission with the review of this project, and hope that this information
is suitable for your needs. Please feel free to contact me if you or the applicants have any
questions regarding the issues addressed herein.
Sincerely,
EGGLESTON ENVIRONMENTAL
Lisa D. Eggleston, P.E.
C: Jennifer Hughes, Conservation Coordinator