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):



Starting Level





Max. water level (ft)


(6.7" rainfall)

Low water




High water





(8.2" rainfall)

Low water




High water




1/2 PMF

(18.1" rainfall)

Low water




High water




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?).


  • Impact of Option 4 on swimming area: Details of seepage if swimming area was filled with water and Res remained low are unknown. Could swimming area be simply deepened instead of seep-proofed? But this would create a drop-off from present beach.
  • Impact of Option 4 on habitat: Res has been at low level during spring and fall migration seasons anyway, creating mudflats for sandpipers, etc. Would these dry out if level kept permanently low? Probably not: would be flooded occasionally by heavy rains.
  • At low water level, about 2/3 of Res is only 5 or 6" deep; remaining 1/3 is about 1 ft. deep. Conservation Com. is checking whether water chestnuts could still be harvested by boat at low level. (Would seeds now stranded on mudflats germinate at all if not refloated?)
  • Cost analysis of options has not yet been done, would be next step, but Options 2 and 3 are probably considerably costlier than Option 4. The difference may well be enough to pay for modifications to the swimming area.
  • Permitting costs (hiring the necessary expertise) would be about the same for any option.
  • Could the watershed be redefined, i.e., some water channeled differently so that it no longer flows into the Res? Would be very controversial and time-consuming. The area studied by Weston & Sampson is already truncated somewhat from the watershed shown by the Mass. GIS because the flow rate is controlled by a 24" pipe from the Thesda Street area.
  • Enlarging the spillway (see options 2 and 3) would require a study of downstream effects.
  • An emergency plan needs to be done promptly regardless of the option chosen.
  • Now that the safety problem has been publicly identified, would the gate have to be kept down (leaving the water level at 153 ft.) next summer regardless of the effect on the swimming area? Or could an interim O+M plan that allows the gate to be raised be developed?
  • If we want Option 4, process of getting it put into effect: Weston & Sampson will finalize the report and present it to the Town Manager, Selectmen, and ConCom. This will take about one or two months. Res Committee wants to see the report so that we can support it.
  • Note that being able to use Option 4 depends on getting DEM to accept a design based on a 500-year storm rather than 1/2 PMS.

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