HomeMy WebLinkAbout12. Stormwater Report Executive Summary - Comprehensive Permit - Application - 0 Prescott Street 10/21/2025 STORMWATER MANAGEMENT REPORT
FOR
EAST MILL RESIDENTIAL DEVELOPMENT
0 PRESCOTT STREET
NORTH ANDOVER, MA 01845
PREPARED FOR:
RCG, LLC
17 Ivaloo Street, Suite 100
Somerville, MA 02143
PREPARED BY:
CIVIL SI R U , LLC
4 HIGH STREET, SUITE 155
NORTH ANDOVER,MA 01845
DATE: OCTOBER 2025
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East Mill Redevelopment, North Andover MA
TABLE OF CONTENTS PAGE
I. STORMWATER MANAGEMENT NARRATIVE
1.0 SITE LOCATION AND DESCRIPTION............................................................ 1
2.0 METHODOLOGY........................................................................................ 1
3.0 SOILS & GROUNDWATER............................................................................ 1
4.0 POINTS OF ANALYSIS................................................................................. 2
5.0 EXISTING DRAINAGE WATERSHEDS............................................................ 2
6.0 PROPOSED DRAINAGE WATERSHEDS.......................................................... 2
7.0 PEAK FLOW RATE .................................................................................... 3
8.0 WATER QUALITY...................................................................................... 3
9.0 GROUNDWATER RECHARGE ....................................................................... 4
10.0 DRAINAGE CONVEYANCE SYSTEM.............................................................. 5
11.0 COMPLIANCE WITH THE MA DEP STORMWATER HANDBOOK........................ 5
12.0 SUMMARY.............................................................................................. 7
II. LIST OF FIGURES
FIGURE 1: USGS PLAN
FIGURE 2: SOIL MAP
0 FIGURE 3: PRE-DEVELOPMENT WATERSHEDS
FIGURE 4: POST-DEVELOPMENT WATERSHEDS
TEST PITS
111. HYDROLOGICAL CALCULATIONS
PRE-DEVELOPMENT HYDROCAD CALCULATIONS
POST-DEVELOPMENT HYDROCAD CALCULATIONS
IV. APPENDIX-1
• MASSACHUSETTS DEP STORMWATER REPORT CHECKLIST
V. APPENDIX-2
0 OPERATION AND MAINTENANCE PLAN (O&M)
(WITH LONG-TERM POLLUTION PREVENTION PLAN)
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East Mill Redevelopment, North Andover MA
1.0 SITE LOCATION AND DESCRIPTION
Civil Design Group, LLC (CDG) has been retained by RCG, LLC to prepare this Stormwater Management
Report for the construction of a residential development and to upgrade associated existing parking areas
located on the East Mill campus on High Street, North Andover, Massachusetts (refer to Figure-1). The project
includes the construction of a 92,000± square foot (footprint)residential building comprised of 296 units and
the recon structi on/en A an cement of existing parking areas including curb, landscaping and stormwater
improvements within a 6.13± acre limit of work area within the site.
According to FEMA flood insurance rate maps community panel number 2500980228G, effective date
07/08/2025, the area of the site to be redeveloped lies within Zone X, which is defined as areas determined to be
outside the 0.2% (500-year) annual chance floodplain. The site does not include bordering vegetated wetlands
and based on available MassGIS information, the site does not lie within an area mapped for rare and
endangered species or certified vernal pools as mapped by the State of Massachusetts' Natural Heritage and
Endangered Species Program (NHESP) or within an Area of Critical Environmental Concern (ACEC). The site
also does not lie within a surface water protection district.
This study presents a comparative analysis of the pre-development and post-development hydrologic
characteristics of the site, and outlines the proposed measures to mitigate flow, provide groundwater recharge,
and improve water quality from the site in accordance with local and state requirements. The proposed best
management practices (BMPs) one water quality unit and one (1) subsurface infiltration system (SIS) to provide
mitigation of the peak flow rates for the 2, 10, 25 and 100-year storm events and to provide infiltration of onsite
impervious areas and treat stormwater to the maximum extent practicable prior to discharging off site.
2.0 METHODOLOGY
Technical Paper 40 (TP-40) was utilized to source the precipitation values Technical Release 55 (TR-55)
methodology was utilized to determine weighted curve numbers (CNs) for each pre and post-development
subcatchment area. Weighted CNs are based on ground cover type and hydrologic soil groups (HSGs). The
times of concentration (Tc's) for each of the existing and proposed watersheds have been calculated. The areas
that do not show a Tc travel path resulted in travel times of less than 6 minutes. CN and Tc values were then
utilized to generate hydrographs using HydroCad 10.0, an industry standard software package that develops a
hydrologic model based on the SCS method and computes peak discharges from rainfall runoff for urban and
rural watersheds.
3.0 SOILS & GROUNDWATER
According to the Natural Resources Conservation Service Web Soil Survey (Figure 2), the underlying soils on
the site are classified as Urban Land and fine sandy loam. Based on test pits that were performed in August
2025, the underlying soils are primarily a sandy loam (HSG-B), therefore, this stor nwater report utilizes a HSG
rating of B for both the existing and proposed conditions for the purposes of capturing surface runoff curve
numbers (CN). Furthermore, the proposed infiltration best management practices (BMPs) utilize an infiltration
rate of 2.41 in/hr, which is representative of the sandy loam soil.
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East Mill Redevelopment, North Andover MA
4.0 POINTS OF ANALYSIS
Points of Analysis (POAs) are discharge points or lines that convey runoff from the study area via overland
flow or through drain pipes. The pre-development and post-development areas of disturbance drain to four(4)
POA's listed and described below and shown on Figures 3 and 4.
TABLE-1: POINTS OF ANALYSIS
POINT OF ANALYSIS DESCRIPTION
POA-1 A comparison point representing the existing culvert conveying
runoff from the existing drainage swale towards High Street POA-2 .
POA-2 A comparison line along the westerly portion of the limit of work
adjacent to the High Street/Prescott Street intersection that receives
runoff via overland flow.
POA-3 A comparison line along the easterly portion of the site that collects
runoff via overland flow and discharged to the abutting property.
POA-4 A comparison line along the southerly portion of the site that collects
runoff via overland flow and discharged to the abutting property.
5.0 EXISTING DRAINAGE WATERSHEDS
The existing watersheds are delineated based on topography, physical characteristics, drainage networks and
limit of disturbance within the site limits and collect and direct stormwater towards the POAs. There are
potential upstream areas that flow onsite, however, these areas flow through the property are not subject to be
captured into the drainage or stormwater management system, therefore, these areas have been excluded from
the stormwater analysis. As such, the total study area for the site is 6.l 3± acres and is divided into four (4)pre-
development watersheds as described below:
Subcatchment EX-1: The 2.69-acre watershed is comprised of rooftops, pavement and grass areas. Runoff
travels via overland flow in a southerly direction towards POA-1.
Subcatchment EX-2: The 2.68-acre watershed is comprised of pavement, grass and wooded areas. Runoff
travels via overland flow in a westerly direction towards POA-2.
Subcatchment EX-3: The 0.38-acre watershed is comprised of grass and wooded areas. Runoff travels via
overland flow in an easterly direction towards POA-3.
Subcatchment EX-4: The 0.24-acre watershed is comprised of pavement, grass and wooded areas. Runoff
travels via overland flow in a southerly direction towards POA-4.
6.0 PROPOSED DRAINAGE WATERSHEDS
Similar to the existing watersheds, the proposed watersheds are delineated based on topography, physical
characteristics, drainage networks and limit of disturbance within the site limits and collect and direct
stormwater towards the POAs. This area is divided into seven (7)post-development watersheds described
below:
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East Mill Redevelopment, North Andover MA
Subcatchment PR-1: The 0.69-acre watershed is comprised of pavement and grassed area. Runoff travels via
overland flow in a southerly direction towards POA-1.
Subcatchment PR-2: The 2.47-acre watershed is comprised of pavement, grassed and wooded areas. Runoff
travels via overland flow in a westerly direction towards POA-2.
Subcatchment PR-2A: The 0.24-acre watershed is comprised of rooftop area. Runoff is collected via roof
drains and discharged into the SIS. The outlet associated with the SIS flows towards POA-2.
Subcatchment PR-2B: The 0.20-acre watershed is comprised of pavement and grassed areas. Runoff is
collected via the onsite drainage system and discharged into the SIS. The outlet associated with the SIS flows
towards POA-2.
Subcatchment PR-2C: The 1.88-acre watershed is comprised of rooftop area. Runoff is collected via roof drains
and discharged towards POA-2.
Subcatchment PR-3: The 0.35-acre watershed is comprised of grass and wooded areas. Runoff travels via
overland flow in an easterly direction towards POA-3.
Subcatchment PR-4: The 0.31-acre watershed is comprised of pavement, grass and wooded areas. Runoff
travels via overland flow in a southerly direction towards POA-4.
7.0 PEAK FLOW RATE MITIGATION
The stormwater management system is designed so that post-development peak discharge rates do not exceed
pre-development peak discharge rates for the 2-year, 10-year, 25-year and 100-year, 24-hour storm events. Peak
flow rates for the pre-development and post-development conditions are illustrated below:
TABLE 2: PEAK FLOW RATE COMPARISON
POINT 2-YEAR STORM 10-YEAR STORM 25-YEAR STORM 100-YEAR STORM
OF EVENT EVENT EVENT EVENT
ANALYIS 3.15"/24-HR 4.83"/24-HR 6.16"/24-HR 8.94'24-HR
PRE(CFS) POST(CFS) PRE(CFS) POST(CFS PRE(CFS) POST(CFS) PRE(CFS) POST(CFS)
POA-1 6.25 0.91 10.07 1.85 13.05 2.63 19.23 4.29
POA-2 7.54 6.89 14.11 13.80 19.68 19.24 31.80 31.06
POA-3 0.03 0.03 0.29 0.26 0.57 0.53 1.29 1.19
POA-4 0.56 0.45 1.10 0.87 1.53 1.22 2.45 1.95
8.0 WATER QUALITY
The redevelopment program includes measures to treat runoff from impervious areas prior to discharging
offsite. New stormwater controls have been incorporated into the design that result in a reduction in annual
stormwater pollutant loads from the site. Through the use of structural and non-structural BMPs, the water
quality volume from the watersheds contributing to the proposed drainage system will undergo treatment.
Currently, the total project area contains approximately 3.06± acres of impervious (pavement) area or 49% of
the site. The redevelopment program increases the impervious area to 3.09± acres of impervious or 50% of the
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East lllill Redevelopment, North Andover 1v1A
site, therefore the majority of the of the project area is considered a redevelopment and the balance will be
treated as new development. Subcatchments PR-2A includes clean rooftop area, therefore, this watershed will
be discharged directly to the subsurface infiltration system. The garage portion of the building totals 0.60 acres
of non-rooftop impervious area and will be treated to at least 80% TSS removal (see attached water quality
calculation). This watershed area exceeds the 0.03 acres of new impervious area from the project. Subcatchment
PR-IA includes a reduction of overall and impervious area as compared to its existing counterpart, therefore,
it's reasonable to assume that the post-development runoff traversing through the existing swale will be treated
to at least the same level as the pre-development condition. The following BMPs were selected to treat the
average annual TSS load from stormwater runoff under the post-development condition.
• Deep Sump Hooded Catch Basins
Stormwater runoff from proposed pavement areas will be directed via curbing and site grading to catch
basins with deep sumps and hooded outlets. The catch basins will trap and remove sediment and larger
particles from the stormwater and will improve the performance of subsequent BMP's. The sumps will
be a minimum of 4' in depth and a regular inspection and cleaning schedule has been proposed to ensure
optimal effectiveness. When properly designed and maintained, catch basin sumps are effective in
reducing the sediment and pollutant load in runoff.
• Hydrodynamic Separator
Hydrodynamic Separators are designed to remove heavy particles, floating debris and hydrocarbons
from stormwater. Stormwater enters the system where floatables and oils are separated prior to the
clarified stormwater runoff discharging to an outlet pipe. Since the Hydroworks units are designed to
treat the required flow without overflow, bypass, surcharge, or scouring, and since they include a built-
in bypass mechanism to accommodate high flow storm events, they are considered "offline" units under
the DEP policy as proposed.
• Subsurface infiltration Systems (SiS)
Subsurface infiltration systems are designed to detain and infiltrate runoff from the site. The system is
comprised of RetainiT concrete structures and crushed stone. Proper pretreatment (44% TSS removal
min.) is required to in order to achieve 80% TSS. Runoff from the design storm is stored until it
exfi ltrates through the soil of the basin floor.
9.0 GROUNDWATER RECHARGE
The DEP Stormwater Management Policy addresses the importance of recharging groundwater and reducing
surface runoff. For a redevelopment project, the net increase in site impervious area must be infiltrated to
approximate the annual recharge from pre-development conditions. On this site, there is a net increase in
impervious area equal to 0.03± acres. The required recharge equals a depth of runoff corresponding to the soil
type multiplied by the net increase in impervious area for each soil type in the post development condition.
Using a target factor of 0.35 inches for HSG-B, the total required recharge volume is as follows:
Rv = (F)x (newly created itpervious area)
where,
Rv=Required recharge volume (cubic feet)
F = Target depth factor corresponding to the HSG.
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East lllill Redevelopment, North Andover 1v1A
Rv= 0.35-inch x 0.03 acres x (43,560 ft2/acre) x (1 ft/12 inch) = 38 cubic feet
The available storage within the SIS infiltration basin below the lowest outlet total 2,350± cubic feet, thereby
exceeding the required recharge volume. The soil borings indicate that groundwater is at least 10' below the
existing ground surface (EL 91.00±), as such the proposed design accommodates 2' separation to groundwater
for the SIS. Below are the drawdown calculations for the infiltration BMP's summarizing that each drawdown
time will be less than 72 hours as follows:
Drawdown calculation — Depth
K
Where k=2.41 in/hour
Subsurface Infiltration System -1 Drawdown calculation = 36 in = 15 hours < 72 hours = OK
2.41 in/hr
10.0 DRAINAGE CONVEYANCE SYSTEM
The proposed stormwater conveyance system was designed to collect and convey runoff fi•om developed areas
to the associated stormwater management system BMP's described in this report. The drainage system consists
of one (1) subsurface infiltration system, catch basins, yard drains, manholes and associated piping.
11.0 COMPLIANCE WITH THE MASSACHUSETTS DEP STORMWATER HANDBOOK
This study presents a comparative analysis of the pre-development and post-development hydrologic
characteristics of the site, and outlines the proposed measures to mitigate flow, provide groundwater recharge,
and improve water quality from the site. The best management practices (BMPs) outlined in this report include
measures to meet the municipal and the Massachusetts Department of Environmental Protection (DEP)
requirements. Below is a summary of how the design complies with each applicable DEP standard.
Standard l: No new stormwater conveyances may discharge untreated directly to or cause erosion in
wetlands or waters of the Commonwealth.
The proposed stormwater conveyance system does not include any new untreated discharges. The overland
connection points will remain consistent with the existing condition.
Standard 2: Soormwater management systems shall he designed so that post-development peak discharge
rates do not exceed pre-development peak discharge rates.
As indicated above and within the supporting HydroCad calculations, the stormwater management system is
designed so that post-development peak discharge rates do not exceed pre-development peak discharge rates.
Standard 3: Loss of annual recharge to groundwater shall be eliminated or minimized through the use of
infiltration measures including environmentally sensitive site design, low impact development techniques,
stormwater best management practices, and good operation and maintenance. At a minimum, the annual
recharge from the post-development site shall approximate the annual recharge from pre-development
conditions based on soil type. This standard is met when the stormwater management system is designed to
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East Mill Redevelopment, North Andover 111A
infiltrate the required recharge volume as determine in accordance with the Massachusetts Stormwater
Handbook.
There is a net increase of impervious area and the corresponding required volume of runoff will be recharged to
groundwater.
Standard 4: Stormwater management systems shall be designed to remove 80% of the average annual post-
construction load of Total Suspended Solids (TSS).
To aid in removal of total suspended solids, deep sump hooded catch basins and water quality units are
proposed. Net increase of onsite non-rooftop impervious areas will be treated beyond 80%.
Standard 5: For land uses with higher potentialpollutant loads, source control andpollution prevention
shall be implemented in accordance with the Massachusetts Stormwater Handbook to eliminate or reduce
the discharge of Stormwater runoff from such land uses to the maximum extent practicable.
Not applicable.
Standard 6: Stormwater discharges within the Zone H or Interim Wellhead Protection Area of a public
water supply, and stormwater discharges near or to any other critical area, require the use of the specific
source control andpollution prevention measures and the specific structural stormwater best management
practices determined by the Department to be suitable for managing discharges to such areas, as provided in
the Massachusetts Stormwater Handbook.
Not applicable.
Standard 7. A redevelopment project is required to meet the following Stormwater Management Standards
only to the maximum extent practicable:Standard 2, Standard 3, and the pretreatment and structural best
management practice requirements of Standards 4, 5, and 6. Existing stormwater discharges shall comply
with Standard 1 only to the maximum extent practicable. A redevelopment project shall also comply with all
other requirements of the Stormwater Management Standards and improve existing conditions.
The proposed site complies with the applicable standards to new development and redevelopment. The
proposed best management practices improve upon the existing conditions and mitigate for the proposed
development impacts.
Standard 8: A plan to control construction-related impacts including erosion, sedimentation and other
pollutant sources during construction and land disturbance activities (construction period erosion,
sedimentations, andpollution prevention plan) shall be developed and implemented.
The `Demolition and Erosion Control Plan' outlines and depicts measures to control construction related
impacts including erosion, sedimentation and other pollutant sources during construction and land disturbance
activities.
Standard 9: A long-term operation and maintenance plan shall be developed and implemented to ensure
that stormwater management systems function as designed.
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East Mill Redevelopment, North Andover MA
An Operation and Maintenance Plan (O&M)has been developed for each site that outlines maintenance
requirements to ensure longevity of BMP's. See Appendix A.
Standard 10: All illicit discharges to the stormwater management system are prohibited.
The proposed stormwater management system does not include any illicit discharges and furthermore the O&M
plan includes an illicit discharge statement.
12.0 SUMMARY
The stormwater management system for the proposed redevelopment includes measures for collecting,
conveying, treating and controlling stormwater runoff from the site. Post-development peak runoff rates have
been attenuated for the 2, 10, 25 and 100-year storm events and the collection system has been adequately
designed to convey the contributing runoff. The stormwater management system incorporates both structural
and non-structural BMP's to adequately treat and recharge runoff from the proposed redevelopment area and is
designed in accordance with the DEP Stormwater Management Policy to the maximum extent practicable.
Comprehensive computations and calculations with supporting figures and plans are attached.
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