HomeMy Public PortalAbout101 North Beacon_Developers Conf_Stormwater Summary_051019Hydrologic Analysis
101 North Beacon Street, Watertown, MA
(Parcel ID 907-22-0)
Project Summary
The project proponent, 101 North Beacon, LLC. proposes to redevelop one (1) parcel of
land located at 101 North Beacon Street in Watertown, Massachusetts. The proposed
redevelopment will consist of 28 multi -family residential units in a three (3) story
building with access drives, stormwater management systems including infiltration basins
and subsurface infiltration systems, utilities and other related infrastructure ("The
Project").
The Project will be located on one (1) parcel shown on the Town of Watertown
Assessor's Map 907 as Parcel ID 907-22-0 with an aggregate area of 0.73 acres (31,662
S.F.) ("The Site"). The Site is bounded by North Beacon Street to the north, Ladd Street
to the west, Irving Street to the east, and developed residential property to the south as
shown on Figure 1 — USGS Locus Map.
The Project will access the existing utility infrastructure located on Irving Street and
Ladd Street, including sanitary sewer, water, electric, telephone and cable television. The
proposed stormwater management systems will be designed to fully comply with all
standards of the Department of Environmental Protection's Stormwater Management
Regulations and will utilize an on -site subsurface infiltration system and an infiltration
basin to mitigate peak rates of runoff.
The existing and proposed site conditions are illustrated on the project site plans entitled
"Site Design Plans, 101 North Beacon Street, Watertown, MA", Prepared by McKenzie
Engineering Group, Inc. May 10, 2019.
This report contains stormwater runoff calculations for the pre -development and post -
development conditions and includes the sizing of proposed stormwater best management
practices (BMPs). All stormwater management facilities will be designed to mitigate
peak rates of runoff, provide renovation of stormwater and meet the requirements of the
DEP's Stormwater Management Regulations.
Pre -Development Condition
The Site is located within the Town of Watertown's Limited Business (LB) Zoning
District. The majority of the 101 North Beacon Street parcel is comprised of a
commercial buildings with associated parking lot and equipment storage areas. The
existing building will be razed prior to construction. Refer to Figure 4 — Aerial
Photograph.
The existing topography generally ranges in elevation from approximately 24 ft. (NAVD
1988) at the middle and east side of the site adjacent to the building, to elevation 22 at the
western side of the site. The Site generally slopes from the middle of the site toward the
streets located to the east, west, and north.
Review of available environmental databases such as MassGIS reveals that the Site is not
located within a mapped Natural Heritage Endangered Species Area (refer to Figure 5 —
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NHESP Map), FEMA Flood Zone (refer to Figure 2 — FEMA Flood Map), or a Zone II
Wellhead Protection Area.
The Natural Resources Conservation Service (NRCS) has identified the soil on the site as
602, Urban Land with no hydrologic soil group (HSG) included in the description (refer
to Figure 3 — NRCS Soils Map). Hydrologic soil group "D" was assumed for infiltration
calculations due to the hydrologic soil group of the surrounding soils..
The pre -development watershed area analyzed in this report is comprised of
approximately 0.75 acres which includes the Site and a portion of offsite tributary areas
to the south. The watershed consists of three (3) subcatchment areas (1S, 2S & 3S) and
three (3) Design Points (DP -1, DP -2, & DP -3). Refer to the Pre -Development Watershed
Plan WS -1 in Appendix A for a delineation of drainage subcatchments for the pre -
development design condition.
The SCS Technical Release 20 (TR-20) and Technical Release 55 (TR-55) method based
program "Hydrocad" was employed to develop pre- and post -development peak rates of
runoff and volumes. Drainage calculations were prepared for the pre -development
condition for the 2, 10, 25 and 100 -year, Type III Storm events. Refer to Appendix A for
computer results, soil characteristics, cover descriptions and times of concentrations for
all subareas.
Post -Development Condition
The proponent proposes a 28 -unit redevelopment. The Project will consist of a 31,662
S.F. (GFA) building with supporting site infrastructure including sidewalks, access
drives, stormwater management systems, and utilities.
The Project will access existing utility infrastructure located on Irving Street and Ladd
Street including sanitary sewer, water, electric, telephone and cable television. Roof
runoff produced by the constructed building will be captured by internal drains and
conveyed to a proposed subsurface infiltration system located in underneath the proposed
parking lot.
In both the pre -development and post -development condition, an approximate watershed
area of 0.75 acres was analyzed. The objective in designing the proposed drainage
facilities for the Project was to maintain existing drainage patterns to the extent
practicable and to ensure that the post -development rates of runoff are equal to or less
than pre -development rates. Refer to Proposed Watershed Delineation Plan WS -2 in
Appendix B for a delineation of the post -development drainage subareas. The design
points for the post -development design conditions correspond to those analyzed for the
pre -development design condition. All design points are shown on Plan No. WS -2.
Subsurface Infiltration System
Runoff from subcatchment 4S, runoff from the western portion of the proposed parking
lot as well as roof runoff produced from the proposed building will be captured in a
subsurface infiltration chamber system shown as pond 1P on Plan No. WS -2. The
stormwater will be infiltrated into the surrounding soil without an overflow to the
municipal drainage system located on Ladd Street. The subsurface infiltration chamber
system was designed to accommodate peak flow generated by all storms up to the 100 -
year storm event. Refer to the site plans for the drainage system design. All BMPs shall
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be supported by a comprehensive Construction Phase Pollution Prevention Plan and
Erosion Control Plan and Post -Development BMP Operation and Maintenance Plan.
Bioretention Area
Runoff from subcatchment 5S, produced by the eastern portion of the proposed parking
lot and surrounding buffer will be captured in a bioretention pond shown as pond 2P on
Plan WS -2. The stormwater produced by the eastern portion of the parking area will flow
through a peastone diaphragm filter strip located at a curb break. The combination of the
peastone diaphragm filter strip and bioretention area allows for 90% Total Suspended
Solids (TSS) removal. Stormwater will then flow through the soil media in the
bioretention area before it is infiltrated into the soil below.
Stormwater Best Management Practices (BMP's)
Treatment stream for the Phase I development shall consist of deep sump hooded catch
basins, a first defense unit, subsurface infiltration system and a bioretention pond with
pretreatment to achieve the required removal of a least 80% of the total suspended solids
(TSS) and mitigate the anticipated pollutant loading. Refer to the TSS Removal
Worksheets in Appendix D for TSS removal rates.
Erosion and Siltation Control
Silt sock erosion control barriers will be placed where indicated on the plans prior to the
commencement of any construction activity. The integrity of the silt sock erosion control
barrier will be maintained by periodic inspection and replacement as necessary. The silt
sock erosion control barrier will remain in place until the first course of pavement has
been placed and all side slopes have been loamed and seeded and vegetation has been
established. Operation and Maintenance Plans including pollution prevention and
erosion and sedimentation controls will be prepared in conjunction with the final
construction plans as required by the DEP Stormwater Management Regulations and
NPDES.
Compliance with Stormwater Management Standards
Standard 1 — No New Untreated Discharges
The site development is designed so that new stormwater conveyances do not discharge
untreated pavement runoff into, or cause erosion to, wetlands.
Standard 2 — Peak Rate Attenuation
In the pre -development and post -development stormwater analysis, the watershed area
analyzed was approximately 0.75 acres consisting of the subject parcels to be developed
and offsite tributary areas. Refer to the Pre -Development Watershed Plan WS -1 for a
delineation of drainage subareas for the pre -development design condition and refer to
Post -Development Watershed Plan WS -2 for a delineation of drainage subareas for the
post -development design condition.
Drainage calculations were performed by employing SCS TR-20 methods for the 2, 10,
25, and 100 -year Type III storm events. Refer to Appendix A and B for computer results.
The stormwater management systems were designed to accommodate peak flows
generated by a 100 -year storm event.
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The peak rates of runoff and volumes for the pre -development condition are as follows:
Design Point
2 Year Storm
10 Year Storm
25 Year Storm
100 Year Storm
(CFS)
(CFS)
(CFS)
(CFS)
Ladd St.
(DP -1)
1.20
1.79
2.10
2.57
N. Beacon St.
(DP -2)
0.64
0.96
1.13
1.38
Irving St.
(DP -3)
0.48
0.71
0.84
1.02
Design Point
2 Year Storm
10 Year Storm
25 Year Storm
100 Year Storm
(AC -FT)
(AC -FT)
(AC -FT)
(AC -FT)
Ladd St.
(DP -1)
0.092
0.139
0.164
0.201
N. Beacon St.
(DP -2)
0.049
0.074
0.088
0.108
Irving St.
(DP -3)
0.037
0.056
0.066
0.081
The peak rates of runoff and volumes for the post -development condition are as follows:
Design Point
2 Year Storm
10 Year Storm
25 Year Storm
100 Year Storm
(CFS)
(CFS)
(CFS)
(CFS)
Ladd St.
(DP -1)
0.12
0.21
0.26
0.34
N. Beacon St.
(DP -2)
0.18
0.30
0.36
0.45
Irving St.
(DP -3)
0.13
0.21
0.26
0.32
Design Point
2 Year Storm
10 Year Storm
25 Year Storm
100 Year Storm
(AC -FT)
(AC -FT)
(AC -FT)
(AC -FT)
Ladd St.
(DP -1)
0.009
0.015
0.019
0.025
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N. Beacon St.
(DP -2)
0.013
0.021
0.026
0.033
Irving St.
(DP -3)
0.010
0.016
0.019
0.024
Comparisons between pre -development and post -development calculations indicate that
post -development peak rates of runoff and volumes are less for all design points. Refer to
site plans for the drainage system design. All BMPs shall be supported by a
comprehensive Construction Phase Pollution Prevention and Erosion Control Plan and
Post -Development BMP Operation and Maintenance Plan.
Standard 3 — Groundwater Recharge
Runoff will be infiltrated by both a subsurface infiltration system, and bioretention pond, which
will meet the Stormwater Guidelines for infiltration:
• Infiltration structures will be a minimum of two (2) feet above seasonal high
groundwater.
• Utilize the "Simple Dynamic" method for sizing the storage volume, which takes into
account the fact that stormwater is exfiltrating from the infiltration basin at the same
time that the basin is filling.
• Hydraulic conductivity are based on soil data from the Geotechnical Report and
values developed from Rawls, Brakensiek and Saxton, 1982, Estimation of Soil
Water Properties, Transactions of the American Society of Agricultural Engineers,
vol.25, no. 5.
• Refer to Appendix D for infiltration and drawdown calculations and Appendix F for
soil data.
Groundwater Recharge Volume
Stormwater System
Soil
Type
Target Depth
Factor (F) (in)
Total Impervious
Area (sf)
Required Recharge
Volume (cf)1
Provided
Recharge
Volume (cf)2
D
0.10
20,908
210
1P
(Subsurface Infiltration)
D
2,756
2P
(Bioretention Pond)
D
1,826
210
4,582
1.
Required Recharge Volume = Target Depth Factor X Impervious Area (d+Kt) [Simple
dynamic method]
2. Provided Recharge Volume = Volume Provided from Bottom of System to peak elevation
The subsurface infiltration system will provide both water quality treatment and recharge. Per Standard
4, Water Quality, the BMP must be sized to treat or hold the Target Volume, the larger of the Required
Water Quality Volume and the Required Recharge Volume. The Required Water Quality Volume is
based on the half -inch of runoff and the Required Recharge Volume is based on 0.10 -inches (Soil Type
D); 0.10 inches if greater than 0.50 inches, therefore the Target Volume is the Required Recharge
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Volume of 210 cubic feet. Refer to Appendix D supplemental calculations.
The proposed bioretention pond has been designed to completely drain within 72 hours. Refer to the
peak rates of runoff and volumes table for the post -development condition for groundwater recharge of
2, 10 & 25 -year storms The drawdown analysis is based on the required recharge volume exfiltrating
at the Rawls Rates based on the soil textural analysis conducted at the proposed exfiltration location.
Standard 4 — Water Quality
The Long -Term Pollution Prevention Plan has been incorporated into the Post -Development
Operation and Maintenance Plan. Refer to Appendix E for BMP Operation and Maintenance
Plans.
The stormwater management system design calls for the installation of 4' -deep sump catch basins
with hooded outlets to collect runoff from the western half of the proposed parking area as well as
roof runoff. This runoff will then be treated by a first defense unit and conveyed to the subsurface
infiltration system. Stormwater runoff from the eastern half of the parking area will sheet flow
into a bioretention pond and will be pretreated by a peastone diaphragm. Removal rates for all
paved surfaces are:
Deep Sump Catch Basins 25%
Propriety Devices (First Defense Unit) 70%
Subsurface Infiltration System 80%
Bioretention Pond w/Pretreatment 90%
The drainage system will be designed to comply with the Standards of the DEP Stormwater
Management Policy. A treatment stream consisting of deep -sump catch basins with hooded
outlets and a first defense unit will ensure that the 95% TSS removal (total suspended solids) is
removed prior to discharge to the infiltration facilities and to ensure that 80% TSS removal is
accomplished. Runoff directed to the bioretention pond will be treated by a filter strip, 90% TSS
removal will be achieved. The proposed treatment stream will renovate the stormwater and
improve the water quality by promoting the settlement of sediments and pollutants before runoff
is released into the existing drainage system. Refer to Appendix D for TSS Removal Calculation
Worksheets.
Water Quality Volume Required vs. Volume Provided
Stormwater System
Impervious
Contributing Area
(ft2)
Required Water
Quality Volume'
(ft3)
Provided Water
Quality Volume
(ft3)
1P (Subsurface
Infiltration)
17,476
728
2,756
2P (Bioretention
Pond)
3,432
143
1,826
1. Required Water Quality Volume = Impervious Area (sq.ft) X (0.50") X (lft/12")
Standard 5 — Land Use with Higher Potential Pollutant Loads (LUHPPL)
The proposed project is not considered a land use with higher potential pollutant loads.
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Standard 6 — Critical Areas
The proposed project does not lie within a critical resource area.
Standard 7 - Redevelopments and Other Projects Subject to the Standards only to the maximum
extent practicable
The proposed project is considered a mix of new development and redevelopment project and as
such meets the Stormwater Management Standards to the maximum extent practicable.
Standard 8 — Construction Period Pollution Prevention and Erosion and Sedimentation Control
A detailed Erosion and Sedimentation Control Plan will be included in the Design Plan Set prior
to the commencement of construction activity. The project will require a Construction General
Permit and the Stormwater Pollution Prevention Plan (SWPPP) will be prepared prior to
commencement of construction activities that will result in the disturbance of one acre of land or
more. A Construction Phase BMP Operation and Maintenance Plan has been provided as a basis
for the SWPPP. Refer to Appendix E.
Standard 9 — Operation and Maintenance Plan
The Long -Term Operation and Maintenance Plan is provided in Appendix E.
Standard 10 — Prohibition of Illicit Discharges
To the best of the applicant's knowledge, no illicit discharges exist on site and no illicit
discharges are anticipated. Measures to prevent illicit discharges are included in the Long -Term
O&M Plan.
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