August 27: Conservation Commission Revisits Kohl Condo Proposal; Hydrological Review

Kohl Construction’s proposal to build 20 condo units off North Street goes back before the Conservation Commission on August 27, 6pm in the City Hall Hearing Room. We believe that several key concerns raised by commission members in March have been addressed little if at all in the current proposal. Concerned citizens are urged to attend.

Northampton Conservation Commission
Agenda

Date: Thursday August 27, 2009
Time: 5:30 PM
Place: City Hall Hearing Room (use back door or main Crafts Avenue door) 2nd floor, 210 Main Street, Northampton

For more information: Bruce W. Young, Land Use and Conservation Planner
by****@no***********.gov

…6:00 PM
Continuation of a Notice of Intent filed by Tofino Associates, Inc. and Northern Avenue Homes, Inc. for the construction of twenty-three dwelling units and associated roadways, parking areas, driveways, sidewalks, utilities, landscaping and stormwater management system. Project is proposed to take place in the 100-foot buffer zone of Bordering Vegetated Wetlands. Project location is Northern Avenue, Map Id 25C-12 and 25C-17.

Below is a hydrological review of the Kohl proposal conducted at the request of the Conservation Commission. This review is expected to be discussed at the August 27 hearing.

August 20, 2009                                             via email: by****@no***********.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

Units 5-12 would be in an area that Conservation Commissioner Paul Wetzel objects to building on because the ground is so wet (March 12 hearing)
Wetzel also expressed concerns about how the underground stormwater detention system by Unit 10 would interact with groundwater in and around the wetland
Unit 18 appears to be slated for the same place as a unit that Conservation Commissioner Downey Meyer objected to previously as too close to the wetland
The total amount of disturbance inside the 100-foot wetlands buffer zone would remain high; Meyer objected to this on the version of the proposal reviewed on March 12

Watering-up: Studies of Groundwater Rising After Trees Cut
…”sites most susceptible to [hydrology] changes after clearcutting were the transition ones between the bogs or fens and the uplands.” This appears to describe well the land Kohl proposes to remove trees from. An average rise on the order of 20 cm (7.9 inches) in the water table is plausible. This could put unanticipated strains both on the foundations of the condos and on the stormwater mitigation system, a major component of which is underground. In addition, “the clear-cut of riparian and other wetland vegetation may lead to ecosystem conversion, i.e., to the encroachment…of water-tolerant or of shallow-rooted invasive species.” Conservation and Land Use Planner Bruce Young has shown a keen interest in controlling invasive species on Kohl’s land.

The phenomenon of groundwater rising after tree harvests is common enough to have its own term: “watering-up”…

July 23: Kohl Condo Proposal Goes Back Before the Conservation Commission; New Draft Covenants

Special Permit As Issued to Kohl Construction for North Street Condos

Video and Slides: Planning Board Grants Special Permit to Kohl Condos on 6/25/09

Video: Conservation Commission Reviews Latest Kohl Condo Proposal on 5/14/09
Kohl’s own test pit data (PDF, 735KB) shows areas along and south of Northern Avenue where removing existing fill (necessary to secure the condo foundations) would involve work below the level of seasonal high groundwater…

The potential impact to groundwater flows appeared to concern some members of the Conservation Commission, who called for an independent expert evaluation of how the proposed development would impact the hydrology of the site.

Video: Conservation Commission Meeting of 3/12/09; Deadlock on Kohl Condo Proposal
[Below, Paul Wetzel refers to an earlier version of the condo proposal which called for 23 units. The current proposal calls for 20 units.]

3:00:22… Commissioner Paul Wetzel: “…I’m just going to say something, we’ve spent a lot of time listening… right now as this plan…I’m inclined to deny it. And its just because…if we were looking at whether this development has an impact on the wetlands, I think it’s going to have an impact on the wetlands. And, I see, a number of things, primarily the underground [detention basin] getting in the way of the hydrology connections underground…”

Tropical Storm Floyd Flood Damage Report (1999)
In the map below, the red flag behind View Avenue (the topmost flag) indicates a flood damage report from Tropical Storm Floyd (1999). This area is in the eastern portion of Kohl Construction’s proposed condo site, one of the more elevated portions. We infer that much of Kohl’s property may be at risk from heavy rainfall events.

…The [2004 Flood and Natural Hazard Mitigation Plan] specifically warns against building on Filled Wetlands:

Many areas of the City were developed before the passage of the Massachusetts Wetlands Protection Act of 1972. Historically filled wetlands are commonly related to problems with wet basements, flooding, shifting foundations and failed septic systems. Development in historically filled wetlands should be discouraged through zoning in order to protect health and safety. (p.24)

It is well known that some of Kohl’s land contains fill from work on Market Street, dumped there in the early 1980s. Kohl’s own Request for Determination of Applicability notes, “The wetlands are degraded with masonry and other construction and road building debris.” It seems plausible that parts of Kohl’s property meet the definition of a filled wetland.

Heat and Rain Increasing in Massachusetts: Implications for Infill and the Proposed Landfill Expansion
This [Hampshire Life] article underscores points we have been raising for some time:

Development that encroaches close to wetlands, as does the proposed Kohl North Street condo project, is unwise. We need to give our urban wetlands more space, not less, to help them counteract the prospect of increased stormwater flows as well as droughts. Northampton’s recently enacted Wetlands Ordinance, which encourages “Smart Growth” development as close as 10 feet to wetlands in Northampton’s more urban districts, flies in the face of predicted climate trends…