January 20, 2000 Meeting Notes - Arlington Senior Center
In attendance: Freeland Abbott, Eugene Benson, Richard Bento, Karsten Hartel, Jane Howard, Elizabeth Karpati. David White, others ...
The meeting was facilitated by Richard Bento, DPW Director. The minutes are by Elizabeth Karpati.
Presentation of Reservoir Dam Study by Mohammad M. Kheirallah of Weston & Sampson
The study found erosion at the bottom of the dam from Sickle Brook as well as upstream erosion. Trees on the dam present a problem because they can rip holes in the dam if uprooted by a storm, and because if the water table fluctuates, any roots that grow down to the water table when it is lower will die and rot when it rises and thereby form conduits for water to undermine the dam
Dams are classified according to two characteristics: size and hazard (high, medium, low ; high hazard = fatalities likely if dam is breached). Reservoir dam is classified as intermediate size, high hazard because of Colonial Village just downstream.
Weston & Sampson is still discussing with the Town just what drains into the reservoir. The dam portion of the study does not include any area draining directly into Sickle Brook.
A dam must be designed for "1/2 PMF" = Peak Maximum Flood. According to the Army Corps of Engineers, the PMF for this area is 36+ inches of rain in 72 hours, so the Reservoir dam must be designed for 18+ inches in 72 hours.
The height of the Reservoir dam is 162 ft.; height of gate is 159 ft. when up, 153 ft. when down.
The analysis by Weston & Sampson yielded the following for various flood scenarios (low water means the gate was down, anything over the top of the dam at 162 ft is a breach):
Thus for a 1/2 PMF storm the dam and its water level would make essentially no difference.
Breach analysis includes two scenarios: "dry" = in good weather, and "wet" = during a storm. A breach during a 500-year storm would put Colonial Village under 7 or 8 ft. of water and cause flooding over the lowest terrain all the way to Arlington Center.
The flooded area would be very little larger for a 1/2 PMF storm than for a 500-year storm, because of the steep sides of the low-lying area. Therefore it seems reasonable to design the dam for a 500-year storm; the state seems open to the "ungated" version of this solution. But the emergency plan would still have to be prepared for a 1/2 PMF storm.
Weston & Sampson presented four options which are listed below with what they would involve, what permits they would require, and likely timetables.
1. Do nothing: (this would leave the Town liable for damages in case of a breach). Timetable: 3 months for preparing emergency and operations and maintenance (O+M) plans, which would be required in any case.
2. Complete repairs: remove trees, create 180-ft. wide spillway, re-grade dam and maintain water levels as at present. Permits: Army Corps of Engineers, DEM, DEP, Conservation Commission. Timetable: 24-36 months.
3. Modified repairs: remove trees, create 60-ft. wide spillway, re-grade a section of dam and lower water level in advance of a storm (probably not acceptable to DEM). Permits and timetable as for #2.
4. Isolate swimming area: leave gate down = at 153 ft. year-round: Requires negotiation with DEM to accept designing for only a 500-year storm, and a separate study for modifications to the swimming area (e.g. seep-proofing). Permits same as above but process simpler. However, DEP may require environmental impact report because habitat would be modified by keeping the water level uniformly low. Timetable: 18-24 months for dam (more for modifications to swimming area?).