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National Program for Inspection of Non-Federal Dams. Great Hill Reservoir Dam (CT 00087), Lower Housatonic River Basin, Seymour, Connecticut. Phase I Inspection Report.

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The dam is a 210 feet long concrete gravity structure with a central concrete round crested ogee weir 40 feet in length, 3.0 feet below top of dam. The dam has a maximum height of 41 feet and a crest width of 6.0 feet. The gate house is adjacent to the left side of the spillway. The regulating outlets include a 16 inch direct supply main and a 20 inch low level intake, which outlets at the toe of the spillway. Based upon the visual inspection and past performance of the dam, the condition of the dam is generally good. The dam appears stable with no signs of movement or settlement. Visual inspection did not disclose an unstable condition due to seepage through the foundation or instability of the dam foundation or abutments. Based upon our hydraulic computations, the spillway capacity is 810 cubic feet per second, which is equivalent to approximately 13 percent of the Test Flood. Peak inflow to the reservoir is 6,600 cubic feet per second peak outflow Test Flood is 6,400 cubic feet per second with the dam being overtopped by 3.6 feet. The spillway is not adequate and will pass only 810 cfs at elevation 290 top of dam. The average downstream flood stage along Fourmile Brook to its confluence with the Housatonic River will be 10 feet for an outflow of 13,300 cubic feet per second. The major impact of such a flood stage would be to wash out the bridge at Route 34 and another masonry arch located 100 feet upstream of Route 34. Before being washed out, back up would undoubtedly occur effecting a day nursery and at least one dwelling. Thus damage to life and property can occur in the vicinity of Route 34, on the east Bank of the Housatonic River, one mile below the dam.

Subject Categories:

  • Hydrology, Limnology and Potamology
  • Civil Engineering

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