Minutes

Macallen Dam Study Committee

Town Council Chambers

Newmarket, Town Hall

Friday, January 29, 2016

11:00 a.m.

Present: Bob Sheehan, Leo Filion, Peter Wellenburger, Dale Pike, Bill Arcieri, Diane Hardy, and Rick Malasky.  Also, present was Gary Lemay, Project Manager/Engineer of Gomez and Sullivan.

Following introductions, Gary Lemay gave an overview of the contract that has been entered into with the Town to follow -up on of some of the questions that were raised related to the hydraulic analysis and design flows that the Committee would like to address before conducting the stability analysis. Gary presented a power point presentation which is attached.

Which hydraulic model should be used? The committee discussed whether the additional work should be based on the Wright Pierce model that was described in the February 2013 report, or the model that was updated and refined by Gomez and Sullivan as part of the 2014 Macallen Dam Removal Feasibility Study. While the models are similar in scope, the 2014 model was updated with new bathymetry data and survey data around NH Route 108 and Longmarsh Road provided by the NH DOT and Durham’s engineering consultant.  It looked at different scenarios, assumptions and conditions at higher flows and provided a more in-depth analysis of the flow split in the vicinity of Moat Island. Gomez and Sullivan will do a comparison of the models so the Committee can see the differences. This can be done through e-mail correspondence. [Update: This was completed on 2/3/2016]

There was an update on the Route 108 Bike path project on the north side of town and a recap of the NH DOT meeting held the week before. Gary Lemay will seek the new road profile from the NH DOT and get an update from Tim Mallette at the State. [Update: Gary spoke with Tim on 2/1/2016 and Tim will be passing along the new Route 108 road profile]. It was noted that Durham has applied for federal emergency management funds for Longmarsh Road improvements. [Update: Gary has requested an updated Longmarsh Road profile from Durham’s consultant]. The flood flows as predicted for Macallen Dam are a “moving target” because of the changes that are anticipated upstream. The changes that are being proposed, however, are in Newmarket’s  favor, including the Hamel Brook and Longmarsh road repairs being anticipated by Durham in the near future.

Gary reviewed the issues that will be looked at as part of the work he will be doing for the Town.

Were the correct gate settings used?  If the gates were closed, this could result in a flood flow at the dam which is nearly 1,000 cfs lower, though the water surface elevation at the dam would be greater.  Opening the dam’s gates lowers the water surface elevation (but increases flow), while closing the gates lowers the 100 year flood flow (but increases the water surface elevation).

What weir “C” coefficient should have been used?  The higher 3.32 coefficient makes the dam more efficient at passing flow and lowers the surface water elevations. This would be in the Town’s favor.

Should the areas adjacent to the dam be modeled as ineffective flow areas? That would not be in the Town’s favor, however, if armoring was done then the Town would not have to “sandbag” the area during major floods. The armoring would create a permanent wall and would not require manpower. Rick Malasky stated that they used 10,000 sand bags to create a four foot wall during one of the major flows.

It was explained that a reconsideration of classification would be moot. Right now the dam is classified as a High Hazard (Class C) dam. This normally requires the dam owner to design to a flood that is 2.5 times greater than the 100-year flood event. However, the State has agreed that the Town only has to design for the Inflow Design Flood (IDF), which is equal to the 100-year flood event under the dam’s current condition.  Even if the dam were to be reclassified as a Significant Hazard (Class B) dam, which the State has already rejected in the past, the design flood would still be the 100-year flood flow.  The State has already allowed the Town to use 100 year flood flows as the IDF. The only change that would have significance would be to re-classify the dam to a low hazard dam, which is highly unlikely given the presence of buildings downstream and a potential threat of damage to downstream properties and residences.

Diane Hardy will check with Steve Doyon of NH DES to see if Thursday, February 11 at 10:00 a.m. would work verses Wednesday, February 17, 2016 for a meeting.  [Update: The committee will meet Steve Doyon on Thursday, February 11 at 10:00 am at the Town Hall in the Town Council chambers. The meeting will be televised on Channel 13].