HomeMy WebLinkAbout2016-10-04 Stormwater Management Report 9/22/16 STORMWATER MANAGEMENT REPORT
FOR
WEST MILL REDEVELOPMENT
4W HIGH STREET
NORTH ANDOVER, MA 01845
PREPARED FOR:
R4✓G, LLC
17 Ival.00 Street, Suite 100
Somerville, MA 02143
PREPARED BY:
21 HIGH STREET, SUITE 207
NORTH ANDOVER,MA 01.845
DATE: AUSGUST 25, 2016
Rev. 09122116
p: 978.794.5400 f:978.965.3971 www.ccigengineering.com
West Mill Redevelopment, North Andover MA
TABLE OF CONTENTS PAGE
I. STORMWATER MANAGEMENT NARRATIVE
1.0 SITE LOCATION AND DESCRIPTION............................................................ !
2.0 SOILS......................................................................................................
3.0 PEAK FLOW RATE MITIGATION AND GROUNDWATER RECHARGE.................. I
4.0 WATER QUALITY IMPROVEMENTS.............................................................. 2
5.0 DRAINAGE CONVEYANCE SYSTEM............................................................. 4
6.0 COMPLIANCE WITH THE MA DEP STORMWATER HANDBOOK........................ 4
7.0 SUMMARY................................................................................................. 6
II. LIST OF FIGURES
• FIGURE 1: USGS PLAN
• FIGURE 2: SOIL MAP
• FIGURE 3: BIORETENTION AREA WATERSHEDS
III. HYDROLOGICAL CALCULATIONS
• `PREVIOUSLY' PROPOSED HYDROCAD CALCULATIONS WITHIN LIMIT OF WORK
• `NEW' PROPOSED HYDROCAD CALCULATIONS WITHIN LIMIT OF WORK
IV. HYDRAULIC CALCULATIONS
• PIPE DESIGN
0
p: 978-794-5400 f:978,965.3935 wwwxdgengineering.eom
West 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 parking lot towards the rear of Building#36 located on the West Mill campus at
4W High Street,North Andover, Massachusetts (refer to Figure-1). The project includes the construction of 108
parking spaces, curb, landscaping and stormwater improvements within a 1.25-± acre limit of work area and
located within the area denoted as `Area of Future Development' on the original permitting plans submitted by
Horsley Witten Group, dated December 2015. As such, this project expands upon the previously submitted plan
set to provide clarity to the development area as originally denoted, which will include vehicular and pedestrian
connectivity to the central parking area improvements currently under construction. This project lies within a
portion of the Building#37 footprint, therefore, a reduction of impervious coverage will be realized within the
limit of work area and also within the various resource buffer zones associated with Osgood Pond.
This study expands upon the previously submitted "Stormwater Analysis and Drainage Report" (`initial
drainage report') dated July 2015, submitted by Horsley Witten Group. As such, this report maintains the same
methodology, points of analysis and considers the peak flow rates and groundwater recharge standards satisfied
due to the decrease in impervious area. Due to the change in impervious coverage fi•om rooftop to pavement,
this report focuses on water quality treatment within the project area in accordance with the Town of North
Andover and the Massachusetts Department of Environmental Protection (DEP's) requirements. The proposed
best management practices (BMPs) as outlined in this report include two (2)bio-retention areas to provide
water quality treatment for the parking lot runoff and have been sized to accommodate storm events up to the
100-year. These BMPs also provide infiltration to onsite impervious areas and treat stormwater to the maximum
extent practicable prior-to discharging off site.
According to FEMA flood insurance rate maps community panel number 2500980209F, effective date
07/03/2012, the area of the site to be redeveloped does not lie within a floodplain. Portions of the project area
lie within the wetland buffer zones as previously delineated under the current Order of Conditions foe the
subject property. Based on MassG1S, the site is not located 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).
2.0 SOILS
According Natural Resources Conservation Service Web Soil Survey(Figure-2)the underlying soils are
classified as Urban Land. The surrounding soils are Canton fine sandy loam (HSG-B), Charlton fine sandy loam
(HSG-B) and Woodbridge fine sandy loam(HSG-C). According to the initial drainage report, five test pits were
performed at the site in November 2014, and estimated seasonal high groundwater was approximated at
elevation 73.S to 77.5 and subsoil infiltration rates were determined to range between 1.2 and 2.6 in/hr. This
stormwater report utilizes an HSG rating of C and an infiltration rate of 2.41 in/hr for purposes of sizing the bio-
retention basins.
3.0 PEAK FLOW RATE MITIGATION AND GROUNDWATER RECHARGE
As indicated above,this project expands upon the previously submitted application for the center parking lot
project. Despite the fact that this parking lot project lies within a portion of the Building#37 footprint, the
majority of this area was classified as `grass' in the previously submitted HydroCAD analysis. As such,this
stormwater management reclassifies the cover types based on the proposed improvements within the limit of
1
p: 978-794-5400 f: 978.965.3935 www.edgengineering.coEn
West Mill Redea,eloRnaent,North Andover Mai
work. As depicted in the plan set,this project primarily drains towards two (2)bioretention areas and both
discharge into DMH2.4.0 and eventually into Osgood Pond, which was the original point of analysis. As such,
this report utilizes the same point of analysis to compare runoff rates from the previously approved cover types
vs. the new cover types runoff within this project's limit of work. The HydroCAD outputs enclosed in this
report provide detailed land coverage calculations resulting in peak flow rates as summarized below:
TABLE 1: PEAK FLOW RATE COMPARISON
POINT 2-YEAR STORM 10-YEAR STORM 25-YEAR STORM 100-YEAR STORM
OF EVENT EVENT EVENT EVENT
ANALYIS 3.6"124-HR 4.8"/24-HR 5.7"/24-HR 7.1"124-11R
PRE(CFS) POST(CFS) PRE(CFS) POST(CFS PRE CFS POST(CFS) PRE(CFS) POST(CFS)
POA-1 3.79 3.61 5.14 4.21 6.14 4.52 7.69 6.65
(modifted)
As indicated above,the peak flow rates under the new condition are less than the previously approved
conditions, thereby, the peak flow rates for the entire site are considered met.
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 proposed net
decrease in impervious area. Nevertheless, as outlined below, two (2)bioretention areas are utilized to collect,
treat and recharge runoff to groundwater.
4.0 WATER QUALITY IMPROVEMENTS
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 two (2) bio-retention areas, the water quality
volume from the watersheds contributing to the each BMP will undergo treatment. As such, all of the non-
rooftop impervious area will be captured and undergo treatment to achieve a minimum of 80% TSS removal
rate. As depicted in Figure-3, subcatchraents DA2b and DA-5, corresponding to bio-retention areas#3 and#9,
respectively, collect 0.93± acres of non-rooftop impervious area and will be treated to a minimum of 80% TSS
removal (see below), thereby meeting the requirement. Refer to the TSS Removal Calculation Worksheet
below.
The proposed project includes the construction of 108 parking spaces, curb, landscaping and stormwater
enhancements towards the rear of Building#36, and as mentioned above, expands upon the center parking lot
project including vehicular and pedestrian connectivity to the previous phase. This project will be located within
the Building#37 footprint, and therefore, a reduction of impervious coverage will be realized within the limit of
work area and within the various resource buffer zones associated with Osgood Pond as illustrated below.
2
p: 978-794-5400 f:978.965.3935 www.cdgengineering.coni
West Mill Redevelopment,North Andover MA
TABLE-2: LAND COVERAGE CALCULATIONS
Land Cover Type Existing___ Proposed
Building 0.96±Ac. 0.00±Ac.
Pavement/Concrete 0.23±Ac. 1.01±Ae.
Wooded/Open Space 0.06f Ac. 0.24±Ac,
Total 1.25±Ac. 1.25±Ac.
Reduction of Impervious Area 0.18-1 Ac.
TABLE-3: REDUCTION OF IMPERVIOUS AREA WITHIN RESOURCE BUFFER ZONES
Area Proposed
0'-25' Wetland Non-Disturb Zone 365t SF
25'-50' Wetland No Build Zone 1,285±- SF
50'-100' Wetland Buffer Zone 3,106± SF
Beyond 100' Wetland Buffer Zone 3,340± SF
Total 8,096±SF
BIORETENTION AREA SIZING CALCULATIONS
Two (2) bioretention areas were selected to treat the average annual TSS load from stormwater runoff under the
post-development condition. Bioretention Areas are shallow underdrained areas sized to treat the 0.5-inch storm
event(min) for water quality treatment of runoff from the driveway, walkways, and parking areas. The shallow
landscape within the bioretention area is designed to filter runoff sediment and facilitate pollutant uptake
through vegetation that will be planted within the bioretention areas.
TABLE-4: WATER QUALITY VOLUME (WQV) CALCULATIONS
Watershed % Drainage Area Im ervious Area WQV Re uired*
Impervious SF Ac. SF Ae. CF Ae.
DA2b 96% 15,229 0.350 13,094 0.300 546 0.012
DA5 80% 34,513 0.792 27,566 0.633 1149 0.026
"WQV=(0.5 inch rainfa11/I2)x Impervious Area(SF or Ae)
TABLE-5: BIORETENTION AREA SIZING CALCULATIONS
Bio Watershed '1A7QV df K limax hf tf Surface Surface Forebay Forebay WQV
Area (af) (ft) (ft/day) (in) (ft) (days) area area voluuue req'd volume treatment
req'd prov'd (10%WQV) provided provided
s (sf)* (cf) (c (af)
3 DA2b 0.012 2 1 6 0.25 1.67 279 327 55 56 0.014
9 DA5 0.026 1.5 1 6 0.25 1.67 566 900 115 162 0.040
*Required surface area (sf)— WQVx df
[k x (hf+df)x tf]
3
p: 978-794-5400 f:978.965.3935 www.edgejigiiiecring.com
West Mill Redevelaplaent,North 4ndover MA
TABLE 6: TSS REMOVAL CALCULATION WORKSHEETr
TREATMENT TRAIN-1 TT#1 :DA2B&DAS 0.93:1= ACRES OF NON-ROOFTOP IMPERVIOUS AREA
TSS Removal Starting TSS Amount Remaining Load
BMP Rate Load Removed (BxC) (C-D)
(A) (B) (C) (D) (E)
Sediment Forebay 0.25 1.0 0.25 0.75
Bioretention Area 0.90 1 0.75 0.68 1 0.07
Total TSS Removal= Summation of(D) = 93%
5.0 DRAINAGE CONVEYANCE SYSTEM
The proposed storrnwater conveyance system was designed to collect and convey runoff from developed areas
to the associated stormwater management system BMP's described in this report. The drainage system consists
of two (2)bio-retention areas each including an overflow outlet,underdrains and associated piping. Using the
rational method to determine peak runoff flows,the proposed conveyance system is designed for the 25-year
storm event.
6.0 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)
requirenicnts. Below is a summary of how the design complies with each applicable DEP standard.
Standard 1: No new stormivater 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 and
subsurface drainage connection points will remain consistent with the existing condition.
Standard 2: Stor•trawrtter tttrrttagettient systertrs shall be designed so tliat post-rleveloptrrertt peak discharge
rates do not exceed pre-development peals discharge rates.
This study expands upon the previously submitted "Stormwater Analysis and Drainage Report"(`initial
drainage report') dated July 2015, submitted by Horsley Witten Group. As such, this report maintains the same
methodology,points of analysis and considers the peals flow rates and groundwater recharge standards satisfied
due to the decrease in impervious area.
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,
stornrwater hest management practices, and good operation and maintenance. At a ttrinimurn, the annual
recharge from the past-development site shall approximate the annual recharge front pre-development
conditions based on soil type. This standard is suet when the stortnwater tnanagenient system is designed to
`TSS Removal Rate calculation includes non-rooftop impervious surfaces.
4
p: 978-794-5400 f: 978.965,3935 wwwxdgengineering.com
Mest Mill Redevelopment, North Andover"A
infiltrate the required recharge voluure as determine in accordance with the Massachusetts Storrsrrvater
Handbook.
This study expands upon the previously submitted"Stormwater Analysis and Drainage Report"('initial
drainage report') dated July 2015, submitted by Horsley Witten Group. As such, this report maintains the same
methodology,points of analysis and considers the peak flow rates and groundwater recharge standards satisfied
due to the decrease in impervious area.
Standard 4: Stormwater jnanagewent 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, two (2) bio-retention areas with sediment forebays are proposed.
Onsite impervious areas will be treated beyond 80%.
Standard 5: For land uses with higher potential pollutant loads, source control and pollution prevention
shall be implemented in accordance with the Massachusetts Stormwater Handbook to eliminate or reduce
the discharge of stornrivater runoff from such land uses to the maximum extent practicable.
No area on the proposed site is designated as a land use with higher potential pollutant loads.
Standard 6: Stormwater discharges within the Zone II 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 and pollution prevention measures and the specific structural stormwater best management
practices determined by the Department to be suitable far managing discharges to such areas, as provided in.
the Massachusetts Stormmwater Handbook.
Not applicable.
Standard 7. A redevelopmentproject is required to meet the following Storrnrvater Management Standards
only to the rnaxinium extentpracticable:Standard 2,Standard 3, and the pretreatment and structural best
managementpractice requirements of Standards 4, 5, and 6. Existing stormwater discharges shall comply
with Standard l 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 Standards required for new development to the maximum extent
practicable. The proposed stormwater management improves the existing conditions through the reduction of
impervious area and the addition of bioretention areas. The proposed stormwater management system is
designed to treat the 0.5-inch stormwater event, therefore, exceeding the requirements for Standard 7.
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, and pollution 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.
5
g: 978-794-5400 f: 978965.3935 www.cdgengineering.com
West Mill Redevelopmew,North Adover MA
Standard 9: A long-terns operation and maintenance plan shall be depeloped and implemented to ensure
that storrnivater ntarurgemerat systems function as designed.
The originally submitted Operation and Maintenance Plan will be expanded upon to include the additional areas
and drainage system outlined in this report.
Standard 10: All illicit discharges to the storinivater rnanagernent system are prohibited.
The proposed stormwater management system does not include any illicit discharges.
7.0 SUMMARY
The stormwater management system for the proposed redevelopment includes measures for collecting,
conveying, treating and controlling stormwater runoff from the site. The stormwater management system
incorporates two (2)bio-retention areas with forebays to adequately treat runoff frorn the proposed
redevelopment area and is designed in accordance with the DEP Storniwater Management Policy to the
maximum extent practicable. The system has been design to capture and treat 0.5 inch(minimum) of
stormwater runoff from the impervious surfaces in order to maintain or improve water quality conditions as
compared to existing conditions. Groundwater recharge and peak flow rate mitigation will occur by means of
reduced impervious area and infiltration through the soil media of the proposed bio-retention areas.
Comprehensive computations and calculations with supporting figures and plans are attached.
6
p:978-794-5400 F:978.965.3935 www.edgengineering.coni