129 Pleasant_Supplemental Sub 8.5x11GZA Engineers and
GeoEnvironmental, Inc.Scientists
An Equal Opportunity Employer M/F/V/H
1350 Main Street
Suite 1400
Springfield, MA 01103
413-726-2100 (phone)
413-732-1249 (fax)
www.gza.com
MEMORANDUM
TO: Douglas Dick – LDa Architecture & Interiors
FROM: Steve D’Ambrosio, P.E. – GZA GeoEnvironmental, Inc.
DATE: October 24, 2014
FILE NO.: 15.0166433.00
RE: Proposed Stormwater Management Strategy
129 Pleasant Street
Northampton, MA
LDa Architects is proposing to construct a 13,700 SF (building footprint) housing complex
on approximately 0.74 acres located at 129 Pleasant Street in Northampton, MA.
Currently the property is home to Northampton Lodging. The stormwater management
strategy for the proposed redevelopment is to direct surface runoff from the majority of the
site to two (2) proposed leaching catch basins connected to an underground infiltration
trench, with a piped direct-connection overflow into the City’s existing drainage system in
Pleasant Street utilizing an existing stormwater connection.
The project’s stormwater management system is comprised of approximately 490 L.F. of
new drainpipe, two (2) leaching catch basins, four (4) area drains, and one (1) subsurface
infiltration trench.
Runoff from the proposed building roof will be collected and piped to the proposed
subsurface infiltration trench along the site’s western property line (see plan set Drawing
C1.01). The infiltration trench consists of a 12-inch diameter perforated pipe encased in
crushed stone. Roof runoff will enter the infiltration trench via Drainage Manhole #1
(DMH-1). Additionally, runoff from the paved and landscaped areas west of the proposed
building will be directed to one of two proposed leaching catch basins (CB-1 & CB-2).
The leaching catch basins, like the roof drain, are connected to the subsurface infiltration
trench.
The subsurface infiltration trench has been preliminarily designed to retain and infiltrate
the proposed project’s Water Quality Volume and the Required Recharge Volume, as
prescribed by the Massachusetts Department of Environmental Protection’s Stormwater
Management Standards (see GZA’s Preliminary Drainage Calculations, dated October 14,
2014). To promote groundwater recharge, the proposed drain manhole directly
downstream of the subsurface infiltration trench (DMH-2) will include an interior baffle
and will function as an outlet control structure. The top of the baffle will be coincident
with the top of the subsurface infiltration trench. During storm events that produce runoff
volumes in excess of the subsurface infiltration trench’s available storage and infiltrative
capacity, the stormwater management system will backwater, but the water is expected to
129 Pleasant Street – GZA Project #: 15.0166433.00
Page 2 of 2
remain on property, with shallow ponding anticipated around the two proposed leaching
catch basins. During the backwater condition, stormwater will flow over the top of the
baffle within DMH-2, to DMH-3, and then to the City’s existing piped drainage system
within Pleasant Street. DMH-3 connects the proposed stormwater management system to
the City’s drainage system via an existing stormwater connection. Surface ponding is not
expected to exceed elevation 100.4 during the NRCS 100-year, 24-hour storm event.
Runoff from the landscaped area east of the proposed building, between the building and
the existing bike path, will be directed to a series of area drains (AD-1, AD-2, AD-3, &
AD-4). These drains are proposed to connect to the City’s piped drainage system via
DMH-3.
UP
SDDOHWG
WCOMM
GMETERGASCONCRETE & BRICK WALKVAULT(FORMERLY N 45°04'05" E)100.05'99.5'10099.5+
101.2+5%
100.94+
+99.7
99.7+
+101.5
+100.08
+99.9
101+ LP98.75
+100.91
5%1.8%101100+TW 102.41
100
FRONT SECTION OF
EXISTING WALL REMOVED BY CITY
EXTEND BRICK PAVING
TO WALL
PROPOSED NEW EXTENSION
OF WALL BY CITY LDate:Scale:129 Pleasant Street129 Pleasant StreetNorthampton, MA 0106010/14/14L2.021"=10'-0"Landscape Concept - Alternative Entry