Atwood North Stormwater Revision Submittal SummaryMEMORANDUM Tighe&Bond
Atwood North Redevelopment Project
Stormwater Management Update
TO: Sarah LaValley, City of Northampton Conservation, Preservation, and Land
Use Planner
FROM: Jean E. Christy, P.E.
Brian S. Huntley, P.E.
COPY: Douglas McDonald, City of Northampton Stormwater Manager; Travis Ward, Development Associates
DATE: May 16, 2017
The Atwood North Redevelopment Project, or as it has previously be known as the
Hospitality North Project, was submitted for review by the City in March 2017. During the
City’s review, it was evident that modifications to the stormwater management system were necessary to meet the requirements of both the Conservation Commission and the Department of Public Works (DPW). On behalf of Development Associates, the Applicant,
Tighe & Bond has prepared the following memorandum to present the changes to the
stormwater management system and how those changes comply with local and state
regulations. A full Stormwater Management Report detailing all the requirements of local
and state regulations has been provided to the Northampton Office of Planning and
Sustainability as well as revised Site Plans.
The modifications to the stormwater design were made to address concerns of the Conservation Department and the Department of Public Works in continuing to hydrate the
existing wetland to the north while meeting the performance standards of the Northampton
Stormwater Ordinance. Challenges with this request include existing topography and
abutting property ownership. The existing wetland to the north ranges from 2-feet to 6-feet
lower than existing elevations along the northern property line of the Atwood project. This existing condition results in a lack of suitable cover over discharge pipes and/or too shallow slopes to properly discharge stormwater directly to the northern wetland. Additionally, raising the site is not appropriate as the site is located almost entirely within Bordering Land
Subject to Flooding (BLSF). Based on field observations, however, the topography around
the northern wetland area is generally flat, and it is presumed that the northern wetland is
hydraulically connected to the western wetland. As such, water feeding the western
wetland will also ultimately feed the northern wetland. It is also important to note that this wetland area is also fed by drainage infrastructure servicing Route 91 immediately west of the subject property.
The existing wetland to the north is located on property not owned by Atwood Partners.
This property is owned by the Massachusetts Department of Transportation (MassDOT).
Work within this property requires an access easement granted by MassDOT to Atwood
Partners.
The changes to the stormwater design include:
Relocation of Bioretention Basin 1 and associated parking area
Conveyance of “clean” roof water to northern wetland
Sheet flow eastern areas to easternmost tip of northern wetland
MEMO Tighe&Bond
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Modifications to the outlet devices at Infiltration Basins 1 and 2 to manage peak
discharge rates
These changes are reflected on the Grading and Drainage Plan, Sheet C-400, of the May 15,
2017 Site Plans. The revisions to the stormwater management system continue to meet the
performance standards set forth by the MassDEP Stormwater Standards (Standards) and the City of Northampton Storm Drain Ordinance.
In general, the project has been designed to comply with the ten Stormwater Standards.
Peak discharge rates for the post-development site do not exceed those of pre-development
conditions. Discharges to jurisdictional area (ie 100-foot buffer zones) have been designed
such that erosive velocities are not anticipated. Groundwater recharge is provided through
the use of infiltration basins and bioretention basins. TSS removal for the project achieves the required 80% through the use of a street sweeping program, deep-sump, hooded catch basins, proprietary water quality units, infiltration basins and bioretention basins.
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