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National Program for Inspection of Non-Federal Dams. Poquonock Dam (CT 00231), Poquonock River Basin, Groton, Connecticut. Phase I Inspection Report.

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Poquonock Reservoir Dam is a stonewall-earth structure about 285 ft. long, with a maximum height of about 12 ft. It was constructed in 1901 and is said to incorporate an earlier dam. The reservoir above the dam serves as a head pond and equalizing storage facility for the City of Grotons water supply system. A 90 ft. length of the masonry wall and embankment, which is 3.25 ft. lower than the remainder of the dam, serves as a spillway. The only operative outlet is the intake to the pumping plant adjacent to the dam. The maximum storage capacity of the reservoir to top of dam is about 1,660 acre-ft. and the drainage area is about 14 square miles. The reservoir is about 1.61 miles long with a surface of 184 acres at spillway crest elevation. The upstream slope of the embankment has become eroded by wave action above the present limits of riprap. There is also some embankment erosion near the spillway inlet walls. Brush and marsh growth have become established both upstream of the spillway and in the downstream channel. Minor seepage is evident in several locations on the face of the dam and at the end of the downstream riprap slope, while more serious leakage was noted through an abandoned filter and pumping plant. Both the dam and its appurtenant structures are judged to be in generally good condition. Provided that the stoplogs were not in place, the test flood would overtop the dam by about 2.5 ft., the total outflow being about 5,800 cfs. The spillway is adequate to pass an outflow corresponding to about 30 percent of the test flood this figure would be lower with the stoplogs installed on the spillway crest.

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  • Hydrology, Limnology and Potamology
  • Civil Engineering

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