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