Minutes

Macallen Dam Study Committee

Town Council Chambers

Newmarket Town Hall

Thursday, February 11, 2016

11:00 am.

Present: Leo Filion, Peter Wellenburger, Dale Pike, Bill Arcieri, Diane Hardy, Steve Fournier, Rick Malasky. Also, present was Gary Lemay, Project Manager/Engineer of Gomez and Sullivan. Bob Sheehan was excused. Steve Doyon and Jim Weber (Dam Bureau) were also present.

Chairman Wellenburger opened the meeting and introduced the guests from the New Hampshire Department of Environmental Services (NH DES) Dam Bureau, Steve Doyon and Jim Weber and Consultant Gary Lemay, Project Manager/Engineer of Gomez and Sullivan (GSE).

1.           Review of data, methodologies, and analysis related to the 100 year design event projections at         the Macallen Dam - Gary Lemay of Gomez and Sullivan

Gary Lemay gave a presentation. He provided some background on the dam and the project. A summary of the points included:

• The Dam Bureau has issued a Letter of Deficiency (LOD) noting inadequate spillway capacity.
• The dam cannot pass the 100 year design event, which is currently estimated at 10,260 cfs. There are concerns about safety and failure of the dam could result in the loss of lives.
• The Town has issued a Request for Proposals to conduct a stability analysis, which will assess whether the dam is stable under the 100 year design event.
• Modifications to the dam or dam abutments are likely necessary to bring the dam into compliance with the Dam Bureau safety regulations.
• Gomez and Sullivan have  been hired by the Town to conduct this hydraulic study as a precursor to the stability analysis.
• The Lamprey-Oyster ‘flow split’ means changes in dam flow capacity (e.g., dam gates open versus closed, changes in the weir coefficient) can change the balance of flow between the Oyster and Lamprey Rivers (and the dam) during high flows.
  • The ‘flow split’ diversion results in less water flowing to the Macallen Dam than there otherwise would be.
• The purpose of this hydraulic analysis study is to re-assess the 100-year design event using recent survey data and some different hydraulic assumptions than were used as part of the last assessment in February 2013. Some of these factors include:
  • Gate openings
  • Upstream ‘flow split’ survey data
  • The spillway weir coefficient
  • Ineffective areas around the dam
• The analysis will look at current (existing) conditions as well as future conditions (i.e., after modifications to Rt 108 and Longmarsh Road)

Gomez and Sullivan did a comparison of the two hydraulic models (GSE versus W-P model) that are available to conduct this analysis. It was determined that the Committee will proceed with the GSE model which will be submitted to the Dam Bureau for review.  This model has been updated and calibrated with new bathymetry, Lidar, and survey data. We have good flow data and surface elevation data that match very well.  Gary presented a spreadsheet that summarizes the analysis results, which showed how varying the assumptions can affect the 100 year design flood flow and the water surface elevations at the Macallen Dam.  GSE and the Town understand that any modifications to the dam or abutments (to bring it into compliance and address the LOD) cannot increase upstream flooding. Therefore, these discussions need to be had with the Dam Bureau. GSE’s initial analysis results show a 5-10% reduction in the 100-year design event relative to the current 10,260 cfs number.

Gary Lemay has spoken with Tim Mallette of the New Hampshire Department of Transportation (DOT) regarding any immediate plans to make improvements to Hamel Brook. Initial modeling results have indicated changes at Hamel Brook could change the 100 year design event at the Macallen Dam.  Every time there is a major flood event, the road is flooded out at that location. DOT has indicated they don’t have anything on in their plans to upgrade the Hamel Brook crossing at this point. Gary has requested the new road profile for the Route 108 and will see what impact, if any, this will have on the calculations. Also, the Town of Durham will be modifying the culverts on Long Marsh Road.  If we know that these upgrades are happening in the next few years, the question was asked whether we can used data in our calculations to determine what impacts that they will have on analysis and possible solutions.

DES Response

Jim Weber was interested in knowing more about the “tailwater” at the flow split.  What is the elevation at the flow split?  Wright Pierce had estimated the elevation at that location as being 33 feet. Gary Lemay believed the data at that location might have been underestimated and indicated that the elevation at the tail water would be evaluated as part of his work.  Jim Weber stated that he believed that the “flow split” would be critical to determining the total flow split diversion. Gary Lemay agreed and stated it would be analyzed. 

Jim Weber asked if FEMA (Federal Emergency Management Agency) would have some involvement in the Project, if you are changing 100 year flood elevations.  There was discussion about FEMA and whether they would be weighing in on this. Steve Doyon indicated that in other projects they have had to apply for a LOMA (Letter of Map Amendment). Diane Hardy noted that NH DOT had recently challenged the validity of the Preliminary Flood Maps that were issued by FEMA in 2014 due to discrepancies in the data. Gary Lemay believes that FEMA rejected NH DOT’s challenge and proceeded with the map revisions. Jim Weber thought that any solution would have to be viewed as a balancing the amount of water that goes over the dam verses what is transferred to the Oyster River. Durham is faced with similar capacity issues at their dam, so a holistic approach will be necessary in evaluating this. FEMA may want to be involved in any final decisions.

It was noted that there are different standards and models used to determining the 100 year flood flow depending on whether you are talking to FEMA, USGS, or the NH DES Dam Bureau. The technology and terminology is confusing.  NH DES prefers to use the term “100 year design event.”

Dale Pike wanted to know if we are heading in the right direction from NH DES’s perspective.  One of the things that was clear from the three (3) interviews that the Town conducted with the engineering firms who responded to the Town’s RFP is that there is ambiguity. We want to make sure, we are using the correct figures going forward. He said he was encouraged when Chuck Corliss, the former dam bureau representative, suggested that “armoring” would be a possible solution to meeting required design flows. The Town’s position is that it wants to make minimal changes to the dam infrastructure given its age, historic significance and the costs of modifications. Dale also commented that the feedback from NHDES was to keep the gates in place and work on the abutments, as opposed to extensive modifications to the dam to avoid the re-design of the dam.

Steve Doyon stated that Gary’s points make perfect sense regarding the weir coefficient and, the opening or closing of the gates. You want the model to be as accurate as possible. The flow-split upstream is critical and needs to be discussed. The ineffective area question is a valid one and should be looked at. Jim said he would like to revisit the left abutment area and whether it should be considered effective or ineffective. Generally the Town is on the right track. Some of these changes are in the Town’s favor and some are not. Regardless of the model changes, it is very important to know the 100-year design conditions are accurate, especially for some of the design alternatives, including raising the abutments.

Gary Lemay stated that the Town doesn’t want to have to re-do work that has already been done. Steve Doyon didn’t think the Breach Analysis will have to be re-opened, but perhaps the inundation mapping may need to be updated for the Emergency Action Plan (EAP.)

Next Steps

Bill Arcieri asked what the process was going forward. The Dam Bureau representatives stated that the cfs number will likely change as part of the solution.  For next steps, Gary Lemay will make the model available to the State and provide a brief write-up. There will likely be a follow-up meeting with Gary for him to walk them through the model and answer any questions. The Macallen Dam Study committee would like to be present at that meeting.

Dale Pike asked if the Town will have to check with Wright Pierce to see if the gates were open or closed during their modeling. NH DES did not feel that was necessary, as they would evaluate that information based on the updated model that Gary Lemay has provided and the results drawn from his analysis.

Two committee members joined the group at 11:03 a.m. due to a slight miscommunication about the time.  Member Leo Filion introduced himself and reviewed his credentials. Newly appointed member Kathryn Nelson was introduced and expressed her interest in working on this issue. She was welcomed to the Committee. Leo summarized the Committee’s reasons for wanting a re-evaluation of the 100 year design event. He raised several questions regarding the models and why they didn’t reflect actual conditions that were observed in the field during the recent flood events in 2006 and 2007.   Gary Lemay stated that the Committee has decided to use the Gomez and Sullivan model, based on a higher weir coefficient, a more accurate depiction of the flow split, new survey data, and different assumptions.  The analysis is based on theoretical assumptions and not necessary on actual “real time” conditions. Specifically, the model simulations must be run with the dam gates closed, even though the Town would never intentionally leave them closed during a flood event of any size.

 Gary Lemay asked if the State would allow them to run the model based on future conditions, as we know conditions will change in the next few years.  The NH DES felt that they could only accept an analysis based on current conditions, but they are willing to have those discussions with the Town about future impacts.  Steve Doyon indicated that schedule is important.  Any solution in the future would be more efficient if Long Marsh Road improvements are made by the Town of Durham. The actual conditions in 2006 and 2007 will not be weighed into the model. The NHDES will treat this as a permit application. We are not looking at what impact various solutions (such as raising the abutments) will have on the flood flows at this time. That will occur in the next phase of engineering as well as assessing  whether the existing dam is stable enough for overtopping (with raised abutments) as opposed to increasing the spillway length or raising the dam, which would likely be more costly to implement. We want the approach that will proof to provide the best results in the most cost-effective manner. There was a clarification that this type of evaluation is considered at part of the permitting process.

At the end of this process, there will be agreement on what the 100 year design flow (the IDF), the water surface elevation at the dam will be, and which model will be used going forward to evaluate different engineering solutions (e.g. raising the right abutment and/or armoring to allow overtopping) and knowing that the dam will be stable enough to withstand those modifications.

The meeting was adjourned at 11:35 p.m.