1087 westhampton rd stormwater excerpt.pdf1087 Westhampton Road, February 18, 2021
Northampton, Massachusetts Stormwater Management Report
Berkshire Design Group Page 2
The proposed development is the construction of a single-family home and driveway on a vacant
forested lot at 1087 Westhampton Road.
Berkshire Design Group has prepared a Stormwater Management plan for the site in compliance
with the Massachusetts Stormwater Handbook. This report summarizes the design of the system
and documents how the design complies with those standards.
I. Introduction
The proposed site is a 17.5 acre mostly wooded parcel with some steep grades, wetlands to the
south, east and northeast and a new development to the west. The proposed work will include the
construction of an 875’ driveway to a single-family home. Stormwater improvements are proposed
at the house site and along the driveway.
The parcel is located on the north side of Westhampton Road about 900 ft west of Glendale Road.
From Westhampton Road the planned driveway follows the eastern property line for approximately
400’ and then slopes up a steep gradient to the house site.
Soil Data
NRCS Soil Survey
The NRCS Soil Survey reports that the on-site soils are variable with Hinckley loamy sand (Hydrologic
Soil Group – HSG A) near Westhampton Road and various fine loamy sands (HSG B-D) on the
remainder of the parcel.
The USGS Surficial Geology survey indicates that the majority of the parcel is part of a drumlin
landform making till the soil parent material. The portion of the property near Westhampton Road
includes sand and gravel outwash coarse deposits. The NRCS Soil Report for the site is attached in
Appendix A.
Subsurface Exploration
A total of six test pits were completed on January 22, 2021. The soil evaluation report is attached as
Appendix B. The test pits indicated that the soils included loamy sand and sandy loam in the upper
parts of the lot with varying depths to estimated seasonal high groundwater (ESHGW). At the top of
the site relatively shallow ESHGW was found at 30”. Further down along the driveway ESHGW was
deeper at about 60”. Just above and below the steeper portion of the site groundwater was just
below the surface. Near Westhampton Road soils were sandy, characteristic of outwash, and
ESHGW was estimated at 48”.
Site Limits
Site limits were based both on the parcel lines as well as the contributing drainage area to the west.
Based on the site topography, runoff flows in three directions.. Study points were established west
of the driveway at the wetland on Westhampton Road (Study Point 1), east of the driveway at the
lower bordering vegetated wetland and behind the existing homes (Study Point 2) and on the north
property line at the upper bordering vegetated wetland (Study Point 3) to model pre and post peak
runoff (see Figures 1 and 2).
1087 Westhampton Road, February 18, 2021
Northampton, Massachusetts Stormwater Management Report
Berkshire Design Group Page 3
II. Existing Conditions
An Existing Conditions and Hydrology Plan is shown on Figure 1. The existing hydrology was
analyzed as four major drainage areas, E1 through E4 with 3 study points. Because runoff from the
newly developed area to the west is a major contributor to the site runoff, it was included in the
overall analysis.
Drainage area E1 includes the area tributary to the Westhampton Road wetland. Area E1 is
comprised of a portion of Westhampton Road, a portion of a common drive, steeply sloped forested
areas as well as meadow within the wetland. The wetland includes two City of Northampton catch
basins, one on the east side adjacent to the proposed driveway and a bigger structure to the west,
designated as Study Point 1.
Drainage area E2 includes two of the lots off the common drive as well as a portion of the common
driveway, an area which is tributary to a detention basin located west of the proposed site and
draining onto it. Although the detention basin outlet appears to currently drain to Study Point 2, its
drainage area historically (prior to the development of the common drive) appears to have drained
to the Westhampton Road wetland. As such, the existing modeling directs this flow to Study Point 1.
Drainage area E3 includes the portion of the parcel which drains to Study Point 2, a bordering
vegetated wetland located behind four homes on Westhampton Road.
Drainage area E4 includes the northern part of the project site, an area which drains to Study Point
3, a bordering vegetated wetland which drains north and then (off the property) west.
III. Proposed Conditions
The proposed conditions plan is provided on Figure 2 along with the proposed hydrology. The
proposed work maintains existing stormwater flow patterns and includes a rain garden, wet swale
and three small infiltration basins to treat and infiltrate runoff from the driveway and portions of
the proposed dwelling roof.
The proposed site is modeled with four drainage areas matching the patterns found in existing
conditions.
Areas P1A and P1B replace area E1. Area P1A is the undisturbed area which includes part of the
common drive and wetlands. Area P1B includes approximately 540’ of the new driveway and the
stormwater improvements.
Area P2 is equivalent to area E2, encompassing the drainage area to the existing common drive
detention basin. As in the existing model this drainage area is routed to Study Point 1.
Areas P3 (A-D) are tributary to Study Point 2. P3A is an undeveloped sloped area west of the
driveway conveyed east through a culvert. P3D is the undeveloped area east of the driveway which
sheet flows to the bordering vegetated wetland and Study Point 2. P3B and P3C include portions of
the driveway draining to infiltration pools which treat, infiltrate. In large storm events, the pools will
overflow and sheet flow to the Study Point.
Drainage area P4A and P4B drain to Study Point 3. P4A includes the front of the house roof, the top
of the driveway and apron. This area is directed to a filtering rain garden for treatment. The rain
garden underdrain is directed to Study Point 3, as is the overflow weir. Area P4B is the remaining
area which includes a portion of the house roof. This large area drains north to Study Point 3.
1087 Westhampton Road, February 18, 2021
Northampton, Massachusetts Stormwater Management Report
Berkshire Design Group Page 4
Detailed analysis of lower driveway
In addition to the overall drainage area analysis presented above, a detailed analysis of the lower
driveway area (540 linear feet) draining to Westhampton Road was completed to show that existing
flows are not exceeded at this location. The study point for this analysis is an existing catch basin
just east of the proposed driveway. The existing area draining to this location is shown on Figure 1
and the proposed area is shown on Figure 2. The area is bounded by the property line to the east
and the top of the wet swale to the west. Results of the analysis are presented below.
Water Quality
Proposed development includes total impervious areas of 15,720 sf, of which 4,220 sf is the building
roof.
The proposed design provides treatment with sheet flow off of the driveway to a gravel diaphragm,
followed by overland flow to several different BMPs including a filtering rain garden, infiltration
areas, and a wet swale followed by infiltration.
IV. Calculations and Design
Water Quantity
Drainage calculations were performed on HydroCAD Stormwater Modeling System version 10.0
using Soil Conservation Service (SCS) TR-20 methodology. The SCS method is based on rainfall
observations, which were used to develop the Intensity-Duration-Frequency relationship, or IDF
curve. The mass curve is a dimensionless distribution of rainfall over time, which indicates the
fraction of the rainfall event that occurs at a given time within a 24-hour precipitation event. This
synthetic distribution develops peak rates for storms of varying duration and intensities. The SCS
distribution provides a cumulative rainfall at any point in time and allows volume-dependent routing
runoff calculations to occur. These calculations are included in Appendix C. Rainfall values are taken
from the latest NRCS-rain tables and are listed in Table 1.
The watershed boundaries for calculation purposes are divided according to the proposed site
grading and the parcel boundary. The curve numbers (CNs) for the existing and proposed sub-
catchment area are based on the soil type and the existing and proposed cover conditions at the
site. The time of concentration (Tc) for large sub-areas with extensive pervious areas is calculated
using sheet flow, followed by shallow concentrated flow and channel flow if warranted. Small sub-
area TCs are set at a minimum of 6 minutes. Reach routes are added for culvert discharges which
occur at significant distances above study points. Conservatively the Storage-Indication method is
used for reach routing.
Calculations were performed for the 2-, 10-, and 100-year frequency storms under existing and
proposed conditions. The results of the calculations are presented in Table 1 below. Appendix C
presents the HydroCAD output reports.
1087 Westhampton Road, February 18, 2021
Northampton, Massachusetts Stormwater Management Report
Berkshire Design Group Page 5
Table 1. Runoff and Volume Summary Tables
Point of Analysis
2-Year Storm
3.07”
10-Year Storm
4.47”
100-Year Storm
7.68”
Peak Flow Rate(cfs) Peak Flow Rate(cfs) Peak Flow Rate(cfs)
Existing – Study Pt 1 3.66 9.19 26.68
Proposed – Study Pt 1 3.63 8.83 27.09
Existing – Study Pt 2 1.17 3.50 10.45
Proposed – Study Pt 2 1.06 3.11 9.16
Existing – Study Pt 3 0.50 2.02 7.18
Proposed – Study Pt 3 0.43 1.78 6.84
Point of Analysis
2-Year Storm
3.07”
10-Year Storm
4.47”
100-Year Storm
7.68”
Volume Volume Volume
Existing – Study Pt 1 0.668 1.386 3.326
Proposed – Study Pt 1 0.657 1.336 3.264
Existing – Study Pt 2 0.183 0.446 1.236
Proposed – Study Pt 2 0.177 0.423 1.160
Existing – Study Pt 3 0.144 0.403 1.242
Proposed – Study Pt 3 0.145 0.383 1.149
The analysis shows that peak flow rates for the site are maintained below existing conditions with
the exception of a slight increase for the 100-year storm at Study Point 1. All volumes are decreased
in the post development condition due to the proposed stormwater infiltration practices