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September 28, 2016
North Andover Planning Board
1600 Osgood Street
North Andover, MA 01845
Attn: Rebecca Oldham, Staff Planner
RE: Stormwater Review
Princeton at North Andover, 1210 Osgood Street
Dear Ms. Oldham and Board Members:
In follow-up to my July 29, 2016 review comments on the above-referenced project and
our August 24, 2016 meeting with the Applicant's engineer, I have received and
reviewed the September 20, 2016 response letter and the September 17, 2016 revised
Stormwater Report and Site Plans (27 Sheets) submitted by Hancock Associates. I also
requested and received the data from approximately 20 soil borings conducted on the site
in June 2016.
The revised submittal includes a number of revisions to the proposed drainage design; the
most significant of which are that some of the previously proposed infiltration systems
are now proposed as non-infiltrating subsurface detention systems and six Stormceptor
hydrodynamic separators have been added to treat runoff from the commercial portion of
the site. A number of my previous comments have been satisfactorily addressed by this
revised submittal. I do, however, have a number of concerns with the project as currently
proposed, primarily related to the validity of the hydrologic analysis, the design of the
stormwater infiltration systems and the proposed stormwater treatment trains. My
specific comments are outlined below:
1. In the response to my previous comment #3 Hancock indicates that they have
conservatively estimated the entire commercial ground plane to be impervious. I
note though that Subcatchments 110, 115, 350, 360, 364, 370, 371, 381 and 382
in the commercial portion of the site are all still modeled as having curve numbers
less than 98, indicating that some pervious cover is assumed. Only the building
roofs are modeled as being entirely impervious.
2. Page 5 of the Stormwater Report indicates that in order to meet the requirements
of Stormwater Standard 1 the outlet protection provided at the four proposed
stormwater outfalls shall be 6-in fractured stone, 18-in depth, 8 ft wide and 16 ft
long. This is inconsistent with the design detail added to Sheet 24 of the plans,
which calls for a 12-in depth of stone and an apron 10 ft wide and 10 ft long. It is
also not clear whether the outlet protection for System 10 takes into account that
the proposed apron would be on a relatively steep slope.
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3. In comment #6 of my July 29th letter, I recommended that the control points used
in the hydrologic analysis be set at the wetlands on and adjacent to the site. The
intent was to better quantify the impacts to the wetland resource areas, several of
which appear to be isolated and therefore more prone to impact if the hydrologic
regime is altered. While the proposed project may control the peak rate of runoff
discharged from the site, the potential changes in the total volume of runoff
discharged to these resource areas have not been evaluated, as is required by the
North Andover Wetlands and Stormwater Regulations. (The Stormwater Report
summarizes peak volume impacts). Based on the drainage analysis it appears that
the proposed project would lead to an increase in the total volume of runoff
discharged toward the 1-series wetland at the southwest corner of the site, and a
significant decrease in the total volume of runoff directed toward the 3-series
wetland approximately 40 feet to the west of the project site.
4. The North Andover Wetlands and Stormwater Regulations also require that the
existing topography fifty feet beyond the perimeter of the parcel be included on
the plan. This information would be particularly useful in assessing how well
control point"B" in the model represents the flow to the offsite 3-series wetland.
5. There are a number of inconsistencies between the hydrologic analysis and the
site plans that call into question the accuracy of the analysis:
■ The model is based on 4-ft overflow weirs in the outlet structures of all of the
subsurface stormwater infiltration/storage systems; Sheets 17, 18 and 19 of
the plans list the overflow weirs as being 3-ft in diameter.
■ Stormtech System#10 - the model assumes a weir elevation 147.75; the plans
(Sheets 13 and 17) show the weir at el. 147.3.
■ Stormtech System#21 —the model is based on two 4-inch outlet orifices; the
plans (Sheets 13 and 17) call for a single 4-inch orifice.
■ Stormtech System#21 B—the model assumes the bottom of the stone layer at
136.25; the plans (Sheet 21) show it at 136. The model also assumes a single
4-in outlet orifice at el. 137.5; Sheet 13 of the plans calls for a 2-in orifice at
137.5 and Sheet 18 calls for 3-in orifice at 137.3. Sheet 18 also shows the
outlet culvert as being 12-inches, and not 15-in as assumed in the model.
■ Stormtech System#31 —the outlet is modeled as a 15-in pipe at invert 140.9;
Sheet 15 calls for a 12-in pipe with an invert of 143.0, and Sheet 18 calls for a
15-in pipe at 143.0.
■ Stormtech System#32—the overflow weir is modeled at el. 144.75 but the
plans (Sheets 15 and 17) list the invert as 144.8 and the outlet is modeled as a
12-in pipe, Sheet 17 calls for a 15-in outlet.
■ Stormtech System#33 —the model assumes a 1-inch outlet orifice; the plans
(Sheets 15 and 17) call for a 3-inch orifice.
■ Stormtech System#38B—the model assumes a 15-in outlet at 151.3; Sheet 14
calls for a 12-in outlet at 153.9, and Sheet 18 calls for a 15-in outlet at 151.8.
■ Stormtech System#39B—the model assumes a 15-in outlet at 153.3; Sheet 14
calls for a 12-in outlet at 156.0.
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■ Closed System#11 —the model is based on the overflow weir at 148.9 and an
18-in outlet; the plans (Sheets 13 and 19) show the weir elevation at 150.0 and
a 15-in outlet.
■ Closed System#34—the model assumes a 2-in orifice at 142.1, a weir
elevation at 145.8 and a 12-in outlet culvert; Sheet 13 calls for a 3-in orifice at
142.3, and Sheet 19 specifies a weir elevation of 145.7 and a 15-in outlet.
■ Closed System#36—The model assumes a 5-in orifice at 142.3, two 8-in
orifices at 144.1 and a weir elevation of 146.9; Sheet 19 calls for a 9-in orifice
at 142.2, a 9-in orifice at 143.7 and a weir elevation of 146.5. The model also
assumes that the header row of the system is a 60-inch storage pipe consistent
with the design detail on Sheet 19, whereas Sheet 13 specifies an 18-in
header.
■ Closed System#3 7 is also assumed by the model to have a 60-in header and a
system invert of 147; Sheet 14 calls for three 12-in headers and Sheet 22
shows the invert of the system at 146. The model also assumes a 6-in orifice at
147.1, three 8-in orifices at 148.6 and a weir elevation of 149.9 whereas Sheet
19 shows the 6-in orifice at 147.3, the three 8-in orifices at 148.8 and the weir
at 150.5.
■ Closed System#38 —the model assumes a 2-in orifice at 147.1 and two 6-in
orifices at 149.1; Sheet 13 lists the lower orifice as being 6-in.
■ DMH 31 is modeled as having an 18-in outlet at 142.8; Sheet 15 calls for a
12-in outlet at 143.3.
■ CB 25 is modeled as having a 15-in outlet; Sheet 15 shows a 12-in outlet.
■ CB 3 is modeled as having a 15-in outlet; Sheet 13 shows a 12-in outlet.
6. The project site is not one with soils that have not been assigned to a Hydrologic
Soil Group by NRCS, as is suggested by the response to my previous comment
#14. The soils on the site have been mapped by NRCS as Paxton and Woodbridge
fine sandy loam, with HSG designations of C and C/D, respectively. This is
consistent with the logs from the twenty soil borings conducted on the site and is
acknowledged on page 3 of the Stormwater Report, which states:
"The vast majority of the site is poorly drained soils, as confirmed through
significant on site test pit verification (see appendix). Therefore, the
Hydrologic Soil Group (HSG) designation of C is used throughout the
stormwater modeling. "
Hancock does in fact base the calculation of runoff curve numbers and recharge
requirements on the assumption that the site soils are HSG C. However, in
calculating the amount of stormwater that would exfiltrate from the ten proposed
infiltration structures during the design storm events, and consequently the
amount of runoff that would overflow from the systems, they assume Rawls
infiltration rates characteristic of HSG A and B soils instead of the HSG C soils
mapped and confirmed. It remains unclear what they are basing this assumption
on as no soil test pits were even performed in eight of the ten proposed infiltration
system locations (#21, #21 B, #31, #32, #3 5, #3 5B, #3 8B and #3 9B) — the plans
call for verifying seasonal high groundwater and textural classification prior to
construction. The two test pits at infiltration system #10 did not go as deep as the
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bottom of the proposed system, and none of the test pit logs provide any
indication that any sort of detailed soil textural analysis was performed. They also
do not characterize the nature of the existing fill on the site. While it is true that a
number of the proposed infiltration systems would be constructed either entirely
or partially in additional fill which may or may not have a higher infiltrative
capacity than the native soils (the nature of the fill material has not been specified
on the plans), the infiltration rate used in the design should be based on the more
restrictive native C soils underlying the fill.
7. Sheet 20 of the site plans indicates that the estimated seasonal high groundwater
elevation (ESHGW) at Infiltration System#10 is at or below elevation 141.0. This
is inconsistent with Test Pit SW-2 conducted at the proposed infiltration system
location, which encountered redox indicative of ESHGW at elevation 144. The
bottom of System #10 (143.25) would thus be within the seasonal high
groundwater table.
8. The ESHGW elevation listed on Sheet 20 for Infiltration System #33 (139.3) is
based on Test Pit SW-17, approximately 30 ft to the south of System #33. The
ESHGW elevation at Test Pit SW-14, about 35 ft east of the system was observed
to be nearly two feet higher (143.2) and suggests that the system may not have
adequate separation to groundwater.
9. Stormtech System#3 9B is mislabeled as System#3 9 on Sheets 14 and 18.
10. Based on the plans (Sheets 14 and 18) the primary lines into Closed Systems
#38B and#39B would be several feet below the bottom of the systems.
11. The revised Stormwater Report includes capture area adjustment calculations as
required to demonstrate compliance with Stormwater Standard 3 (Recharge).
According to the calculations on page 7, however, only 60 percent of the total
impervious area of the site would be tributary to the proposed recharge systems so
the project does not in fact meet the requirements of Standard 3. Volume 3 of the
MA Stormwater Handbook clearly states that in no case shall runoff from less
than 65% of the site's impervious cover be directed to the BMPs intended to
infiltrate the Required Recharge Volume.
12. Sheets 17 through 21 provide specifications for the fill to be used within the
proposed Stormtech systems, however I was unable to locate any specifications
for the fill to be used beneath a number of the systems (see previous comment
19). All such fill should be clean and should have an infiltration capacity equal to
or greater than that of the native soils.
13. In their response to my previous comment 21 Hancock indicates that they are
unaware of any MassDEP Stormwater breakout requirements. Table 1B.1 in
Volume 2, Chapter 2 (p.88) of the Massachusetts Stormwater Handbook lists a
number of setback requirements for infiltration basins, including a minimum of 50
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feet from any slope greater than 15 percent. As proposed, Infiltration System #10
would be immediately adjacent to a 64 percent slope and the bottom of the system
would be approximately 7.25 feet above the toe of the slope. Breakout of the
infiltrated flow could not only impact the stability of the slope but also would
defeat the intent of the groundwater recharge provided and short-circuit the flow
to the adjacent wetland. Similarly, Infiltration System #32 is within 10 feet of a
retaining wall, the base of which is several feet below the bottom of the system.
As per the details on Sheet 16, the retaining walls have weepholes and/or a
drainage pipe aimed at relieving the hydrostatic pressure behind the wall.
Daylighting of this infiltrated flow would also be counter to the intent of the
groundwater recharge provided by the system.
14. As currently proposed, runoff from the paved portions of the residential site and
some of the building roofs would undergo treatment in proprietary Stormtech
Isolator Rows. Given that the flow through the Isolator Rows would subsequently
be infiltrated, I concur that this treatment train would meet or exceed the 80
percent TSS removal requirement of Stormwater Standard 4. I do not believe this
to be the case for the flow through Infiltration System #10 and possibly System
#33, though, as those systems do not appear to have adequate separation to
groundwater.
15. It appears that Hancock misunderstood my previous comment#23. The roof areas
do not require pretreatment prior to infiltration under Standard 3, however
Standard 4 does require that the runoff from all impervious areas of the site
undergo treatment. For those roof areas that discharge directly to the infiltration
systems (those that provide adequate groundwater separation) it just needs to be
demonstrated that the required water quality volume, 1/2 inch over the impervious
area, is captured and infiltrated. For the remaining roof areas another means of
treatment needs to be identified.
16. The revised plans call for six inline Stormceptor hydrodynamic separators, in
conjunction with deep sump catchbasins, to treat the pavement runoff from the
commercial area of the site. The TSS calculations assume 77 percent removal in
the Stormceptor units. Proprietary separators are listed in Vol. 1, Ch. 1 of the
Stormwater Handbook as a suitable pretreatment BMP for Land Uses with Higher
Potential Pollutant Loads (LUHPPLs), but they do not qualify as treatment BMPs.
Volume 2 of the Stormwater Handbook specifically states that proprietary
separators should only be used as pretreatment devices for new construction. The
flows through this treatment train would not subsequently be infiltrated, hence an
additional treatment BMP is needed.
17. The 77 percent TSS removal rate previously assigned to proprietary separators
under the Massachusetts STEP Program and cited in the Stormwater Report were
sunsetted effective January 1, 2011, when MassDEP transitioned to a stormwater
performance rating system (see attached). Based on more recent field monitoring
data from the UNH Stormwater Center, the International Stormwater BMP
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Database, the USGS, and EPA's Environmental Technology Verification Program
the TSS removal rate for Stormceptors and similar hydrodynamic separators in
field conditions is more likely in the range of 30 percent to 50 percent. All of
those studies were based on "raw" stormwater however, so the removal rate for
units downstream of deep sump catchbasins would be even lower.
18. Design details for the proposed Stormceptor units should be added to the plans.
19. Catchbasin 23 should be placed in an offline configuration to prevent mobilizing
of accumulated sediments.
20. The Conceptual Landscape Plan does not appear to have been revised per the
response to my previous comment #29. In addition to the concern regarding
deeply rooted trees and shrubs over several of the proposed subsurface infiltration
systems I note that several of the landscaped areas are designated snow storage
locations and should be planted accordingly.
21. Proposed dumpster locations should be shown on the plans.
22. In response to my previous comment #30 Hancock indicates that the Erosion and
Sediment Control Plan has been revised to include a detailed construction
sequence and address stabilization of the 2:1 slope between Stormtech System 10
and the adjacent wetland. I was, however, unable to locate these elements in the
revised submittal. I also note that as a result of the design revisions the slope
adjacent to Stormtech System#10 appears to have gotten steeper.
23. I have the following comments on the revised O&M Plan:
■ The Long Term Pollution Prevention Plan should address deicing and snow
management. The snow storage locations on the revised Layout and Materials
Plan do not appear to be adequate for the site.
■ In response to my previous comment regarding the ownership and parties
responsible for the ongoing maintenance of the stormwater system and
whether two separate O&M Plans will be required Hancock has indicated that
this is still being worked out and requested that the final resolution be
conditioned. I just want to point out to the board that the drainage systems for
the commercial and residential portions of the development are not segregated
and that the proposed property line actually bisects several of the subsurface
structures, thus easements may also be needed.
■ The deep sump catchbasins and Stormceptor units should be cleaned a
minimum of once per year, not just when sediment thresholds are reached.
■ 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. If they are not, corrective action must be taken.
■ Per my previous comments the plan should also include a maintenance
checklist and a simple figure showing the locations of all stormwater BMPs to
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be maintained as well as designated snow storage locations. These were not
included in the revised O&M Plan.
Once, 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
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Lisa D. Eggleston, P.E.
w/attachment
C: Jennifer Hughes, Conservation Coordinator
The ommonwea t o 9l4assac usetts
� = Eavecutive Office of Energy and Environmentaf
Affairs
,�4 100 Cambridge Street, Suite 900
Ooston, W,4 0,2114
TRANSITION TO A STORMWATER PERFORMANCE RATING SYSTEM
Background
Government and industry efforts to prevent pollution from stormwater have come a long way since 1998 when
this Executive Office was a partner in the Massachusetts Strategic Envirotechnology Partnership (STEP) and
STEP issued independent technology assessment reports on the performance of three proprietary stormwater
control products.Due to funding cutbacks,STEP no longer exists as a program to evaluate new technologies or
to update existing reports. This creates an uneven playing field, with no new technology vendor able to go
through a STEP review or obtain any of the marketing advantages that the STEP fact sheets or assessment
reports offer. The existing reports are static, allowing for no additional models to be evaluated or for updated
data collection protocols to be applied.
Also, while the STEP program was unique in its day, commonly accepted criteria for collecting data and
evaluating performance information have changed in ten years.The Commonwealth partnered with other states
(in a Technology Acceptance and Reciprocity Partnership or TARP) to create a performance demonstration
pathway that relies on a common methodology. This uniform method, the TARP Stormwater field testing
protocol,is a contemporary,scientifically credible and defensible method that is today recognized as the current
standard evaluation tool in this state and in others.Other protocols may be deemed equivalent by MassDEP and
as technologies develop and science evolves,TARP may be replaced with a newer evaluation tool.
Stormwater Performance Ratings in Massachusetts using TARP
When data from the TARP field studies become available, Massachusetts' staff will evaluate the results and
findings of other states to make a regulatory performance decision on how the product meets relevant state
stormwater standards. The state will issue a regulatory determination letter based on completion of adequate
field studies which comply with TARP or equivalent testing. The state is drafting a regulatory finding for one
technology that has completed the TARP field testing, and more are expected this year. These findings will be
posted on MassDEP's website. MassDEP's written regulatory finding will replace all STEP documentation as
described below.
STEP Sunset-Transitioning to the Stormwater Performance Rating System
In order to recognize the vendors that participated in STEP in good faith and to provide an orderly transition to
the final performance rating system, the STEP fact sheets and technology assessment reports will remain in
effect until either the completion of the TARP path to a final stormwater performance rating or the vendor
wishes to withdraw its STEP fact sheet and report findings. In any case, the state will recall the STEP
Technology Fact Sheets on January 1,2011.
The STEP fact sheets and verification reports only determine removal efficiencies of total suspended solids
(TSS) and not other parameters such as nutrients, metals or oil and grease. All technologies verified by STEP
must be sized in accordance with the information published in the STEP assessment report and accompanying
fact sheets. A study is underway to examine different methods to convert water quality volume to an equivalent
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flow rate as required for adequate system sizing. When that study is completed, MassDEP may notify the
public that there needs to be a change in the sizing practices and this may affect the validity of some or all of the
STEP reports.
Once the stormwater technology fact sheets are recalled,they will no longer be relied on for making regulatory
decisions at any level of government. The cover letters to the assessment reports never had any regulatory
meaning and should never be used as evidence of performance efficiency. The STEP Technology Performance
Assessment Reports will remain available on www.mastep.net.
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