Reservoir Consultant's Presentation

On Thursday, January 20'th, the consultant hired by the town to assess the state's report on the Res Dam presented his preliminary findings on the hydrologic and hydraulic requirements on the dam and the town's alternatives in addressing the state's requirements. This note is intended to summarize the situation for those who were unable to be at that meeting.


In 1998, the state Office of Dam Safety hired contractors to inspect and assess the safety of the dams of the Commonwealth. The inspection of the Arlington Res dam found a number of problems, from relatively minor repairs to the concrete of the spillway, to vegetation partially blocking the emergency overflow spillway, to possible seepage through the dam , to the fact that trees and shrubs were growing on the dam. (Trees and shrubs on an earthen dam weaken it and actually reduce its strength.) The inspection report recommended and essentially required corrective actions, including cutting down all the trees and removing their roots; the estimated cost was several hundred thousand dollars. The swimming area at the Res is separated from the Res itself by a permeable berm, and requires the water in the Res to be at a high level to maintain the level of the water in the swimming area; it drains out if the water in the Res is drained.

Consultant's Report.

The consultant confirmed most of the observations of the state inspection, and further defined what the town would have to do to meet the requirements of the office of dam safety for earthen dams whose breach could cause fatalities and serious flooding.

The consultant identified and evaluated three different storm scenarios using computer modeling.

  • The first scenario was the peak maximum flood as defined by the federal government; the formal requirements on a dam are that it be able to pass half of the peak maximum flood (PMF) and hold back the remainder (a dam is considered to have breached if water from a storm event flows over the dam, except at the spillway). In the case of the Res, that amounts to passing 3100 cubic feet of water per second, far more than the existing spillway can handle and also more than Mill Brook can pass without flooding.
  • The second scenario was that of the 500 year storm event.
  • The third scenario was the 100 year storm event.
The rainfalls in a 72 hour period that were associated with the three storm events were 18.07", 8.2", and 6.7", respectively. Since we had actually experienced a 100 year event, the actual results of that storm were used to calibrate the model for the other two events.

Dam performance under the three scenarios was assessed with dam in both the "low water "(gate open) and "high water" (gate closed) states.

The results of the simulated dam performance in the three scenarios and under the two conditions was measured, and used to generate alternatives for the town to consider in meeting state requirements and the need for downstream safety. In the 100 year storm, the dam did not breach, whether the initial state was with the gate closed or open. In the 500 year storm, the dam did not breach if the event occurred when the Res was in the "low water" state, but did breach if the Res was in the "high water" state. In the peak maximum flood, the dam breached in either case. One of the key points of this analysis is that the Res Dam is almost irrelevant in the scenario of the peak maximum flood; the 500 year event is a much more realistic baseline on which to identify changes needed to the existing dam, and it is thought that this would be acceptable to the Office of Dam Safety.

The consultant considered four alternatives for the Res dam in light of these results.

  1. The first is, as always, to simple do nothing, and continue to keep the Res at the "high level" in the summer when the high level is needed for the swimming area and at the "low level" in the rest of the year. The problem with this is that it may not be feasible - the state could force repair and rebuilding of the dam - and it would require the assumption of potentially enormous liability for flooding that could occur downstream. (Not to mention an unacceptable risk to residents.)
  2. The second alternative is to rebuild the Res dam to meet all requirements. This would require construction of an 180 foot concrete spillway, tearing out all the trees, and raising the height of the dam several feet and managing the Res as we now do. It has the advantage of meeting all requirements and not affecting the swimming area, but is by far the most expensive alternative, would eliminate the path around the Res unless a bridge were built over the spillway, and requires removal of all the trees and an environmental impact statement with its associated cost.
  3. The third alternative is to modify the Res dam to meet some requirements, and use water level management action to meet others. This would require building a 60 foot spillway, tearing out all the trees, and lowering the water level whenever there was a forecast of a significant storm event. It would be less costly than the first alternative, but would impact the swimming area whenever the water was lowered and would have the other disadvantages of the first alternative. It would also be less apt to be acceptable to the Office of Dam Safety, which does not like having to rely on a human and mechanical devices to act to change water levels when needed.
  4. The forth alternative is to make minor repairs to the dam, leaving the trees as they are, keep the Res in the "low water" state all the time, and isolate the swimming area from the Res water level by lining it or making the berm impermeable. This in essence changes the Res dam from a flood control structure to a temporary impoundment, and has the least impact on the environment of the Res since the Res is now in the "low water" state most of the year. It also looks as if it is both the cheapest alternative and the quickest to implement, although none of the alternatives has been thoroughly costed out at this point.

Remaining actions include developing an Emergency Action Plan and an Operations and Maintenance Plan for the Res (required under all scenarios and for all alternatives), verifying with the Office of Dam Safety that planning for the 500 year storm is acceptable, and developing cost information for the alternatives.

Discussion and questions from those present at the meeting concurred that the fourth alternative looked like the best choice, that something ought to be done about the swimming area regardless of what was done with the Res dam, and that there would need to be a transition period in which the Res used operations and maintenance to protect against floods until whichever alternative was chosen could be implemented.


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