North Street Hydrology Report
August 20, 2009 via email: byoung@northamptonma.gov
Northampton Conservation Commission Northampton City Hall
210 Main Street Northampton, MA 01060
RE: Hydrological review
North Street Condominiums Northampton, MA
Dear members of the Commission,
New England Environmental, Inc. and Doucet & Associates, Inc. have completed a
hydrological assessment of the North Street Condominiums project, proposed for land
with frontage on North Street and Northern Avenue in Northampton. This review was based on materials contained in the Notice of Intent for North Street Condominiums,
Northampton, MA prepared by Tofino Associates, Inc. and Northern Avenue Homes, Inc.
dated April 15, 2009. Public data sources, such as MassGIS and the Web Soil Survey,
were also employed. A thorough site inspection, including some auguring of soils, was
conducted on August 5, 2009. No additional field testing was included within the scope of this review. It is our understanding that the Conservation Commission
has previously
determined that the North Street Condominium project satisfies all MassDEP and City of
Northampton Stormwater Management Standards, and has approved the wetland
delineation as depicted on the project plans, so our review was limited to assessing the
potential impacts of the project upon the hydrology of wetland resources in the area, and within the 100 foot buffer zone to Bordering Vegetated Wetlands.
The Applicant has asked that in addition to the proposed layout (last revised on July 13,
2009), that this review also consider the effect of: (1) a change to full basements instead
of frost walls; and (2) the effect of the 23 unit plan that was previously considered instead of the current 20 unit plan.
The project proposes alterations within the 100 foot buffer zone. No alteration is
proposed within the first 10 feet (the “no work zone” depicted on plans), and no
development within the first 35 feet. Between 10 and 35 feet from wetlands, removal of
invasive alien species and planting of native species, including trees and shrubs, is
proposed. Therefore, we are concerned with the hydrological aspects of this project
where work is proposed within the Conservation Commission’s jurisdiction, that is,
between 10 and 100 feet from the Bordering Vegetated Wetlands (BVW). Under existing
conditions, this area is almost entirely wooded, with the exception of the mown lawn
behind the house at 8 View Avenue and a small open area near the northwestern end of
Northern Avenue. The area between 35 and 100 feet is proposed to be cleared of existing
vegetation, which will decrease evapo-transpiration. Also within this area, development
will include creation of impervious surfaces, which will increase stormwater runoff.
Other proposed changes, including infiltration enhancements, conversion of existing lawn
to woody vegetation, construction of building foundations, and the native planting plan, will have lesser effects.
The soil within the buffer zone on this site is primarily mapped as Amostown-Windsor
silty substratum-Urban land complex. Amostown and Windsor soils have permeable
surface layers ranging from fine sandy loam to loamy sand. Soil pits dug on this site by
Berkshire Design Group and Cold Spring Environmental Consultants found soil profiles
which were consistent with the mapped soils, with some pits showing permeable or very
permeable soils to a greater depth than is typical for the mapped soils. They also found
extensive fill on the northeastern sections of the buffer zone.
The Northampton Conservation Commission has made clear its mandate to protect the
wetland resources of the City. Either an increase or decrease in water elevation may
affect a wetland system. Employing available information, we have endeavored to predict possible consequences of this development which may result in either a
reduction
or an increase in water levels within the BVW. These two cases are presented below.
Ground Water Impediment:
We understand that concerns have been raised about the possibility of the proposed
foundations restricting ground water flow from upland portions of the watershed to the
wetlands. Seasonal High Ground Water was discovered to be shallow in several test pits,
especially in the southwestern corner of the site. Certainly any foundation type, be it
frost wall or full basement, will impede the existing flow of ground water. Typically the
result is a rise in ground water elevation on the upgradient side of the foundation wall, or
‘mounding’. This increase in hydraulic head will eventually create enough pressure to
convey ground water around or under its impediment. It is possible for some mounding
to cause hydraulic breakout at the surface. While this condition will not ultimately deny
the conveyance of ground water to the wetland system, the applicant may wish to
consider crushed stone backfill in any upgradient foundation trenches to help facilitate the conveyance of ground water around and under the structure.
Volumetric Increase of Runoff/Ground Water:
Local areas, especially on the upgradient side of proposed structures, may see an increase in ground water elevation, or mounding, as described above. The increase
in impervious
area and the reduction of tree cover will result in increased storm water runoff if left
unmitigated. It is our understanding that the Conservation Commission has previously determined that the North Street Condominium project meets all applicable Stormwater
Management Standards. As such, peak runoff rates for the proposed project do not
exceed existing rates, and adequate groundwater recharge is provided as prescribed by
MassDEP. The increase in impervious area and loss of tree cover is accounted for in the Applicant’s HydroCAD model by the SCS Curve Number (CN) for each subcatchment.
The Curve Number is based on the soil type, ground cover, and other factors. High CN
values (up to 100) indicate complete runoff with little retention, and low numbers
indicate high retention and reduced runoff. Even a modest increase in the Curve Number
can produce a dramatic increase in the amount of runoff. Although peak runoff rates have been mitigated in the proposed development, the total volume of runoff
is increased.
This means that the wetland system will receive more stormwater overall, either via
runoff or recharged ground water, but not at a rate faster than what happens today.
Increased water volume could lead some to conclude that the water level of the wetland
system would rise, also resulting in an overall rise in ground water in the buffer zone.
However, it is our opinion that Millyard Brook provides a natural relief point that will keep the wetland water level in the same range as current conditions.
It is our opinion that none of the options described above: 20 units or 23 units, frost walls
or full basements, will produce a negative effect on the conveyance of groundwater to the
wetland system. It is our recommendation that a French Drain or crushed stone be placed
on the upgradient side of all proposed foundation walls to help alleviate any potential issues with ground water mounding around the proposed units.
We appreciate the opportunity to review this project for the Northampton Conservation
Commission, and look forward to discussing the hydrological implications of the
development in the public hearing. In the meantime, if there are any further questions or
concerns, please do not hesitate to call.
Sincerely yours,
Doucet & Associates, Inc. New England Environmental, Inc.
Chris Stidsen, P.E. Bruce Griffin, PWS, CPSS