National Program For Inspection Of Non Federal Dams Mianus Filter Plant Dam Ct 00040 Southwestern Coastal Basin Greenwich Connecticut Phase I Inspection Report

The dam at the Mianus Filter Plant is a stone masonry dam built in the 1870s and is a source of water supply for the surrounding area. The water treatment facility was renovated in 1954. The dam is approximately 130 feet long, 31 feet high and has a top width of 7 feet. The outlet works for the dam consist of a 65 foot long spillway, a 30 inch supply main to the water treatment facility, a low level blowoff, and two 4 foot wide by 5 foot high flood control gates. Based on the visual inspection, the review of the 1954 renovation plans and past operational performance, the dam is judged to be in FAIR condition. However, there is a concern about the seepage zone at the downstream face of the dam, which requires further study. The dam is classified as SMALL in size and a HIGH hazard potential structure in accordance with recommended guidelines established by the Corps of Engineers. The test flood for this dam is 1/2 the Probable Maximum Flood (PMF). The test flood has an outflow discharge equal to 14100 cfs and will overtop the dam in a stillwater condition. The maximum outflow capacity of the spillway under stillwater condition is 680 cfs which represents approximately 5 percent of the test flood. It is recommended that the owner take the following actions: Monitor the seepage on the downstream face, remove the trees from the upstream channel bed and develop and implement a formal emergency warning system.

The dam at Nash Pond is constructed of stone masonry, is approximately 105 feet long, 25 feet high and has a top width of 5 to 7 feet. There have been no significant modifications to the dam since the dam was completed in 1879. The impounded water is primarily used for recreation. The present owner of the dam could not be determined during this report and it is recommended that the State of Connecticut ascertain the ownership of the dam. Based on the visual inspection and past operational performance, the dam is judged to be in FAIR condition. Slight seepage from the masonry joints was noted on the downstream face and some of the joints need repointing. The valve controlling discharge to the 30 inch pipe outlet self-opened in the Summer of 1978 and was subsequently reclosed. A low-level blowoff is believed to exist through the base of the dam. This dam is classified as SMALL in size and a HIGH hazard potential structure in accordance with recommended guidelines established by the Corps of Engineers. The impoundment storage at the top of the dam is 114 ac.-ft. and the maximum height of the dam is 25 feet. Failure of the dam would result in the loss of more than a few lives and excessive economic loss to the downstream urbanized area, two commercial buildings, 2 - 3 residential homes, and an apartment building. The test flood for this dam is 1/2 the Probable Maximum Flood (PMF). The test flood has an inflow equal to 2650 cfs and an outflow discharge equal to 2570 cfs with a stillwater elevation of 64.0 which will overtop the dam by 3.8 feet in a stillwater condition. The maximum outflow capacity of the spillway under stillway conditions is 250 cfs which is 10 percent of the test flood.

The dam at New Canaan Reservoir is an earth embankment approximately 445 feet long, 37 feet high and having a top width of 21 feet. Built originally about 1898 with subsequent modification from 1929 to 1942, this dam impounds water used by New Canaan, Connecticut. The New Canaan Water Company presently owns and operates the dam including its water works facilities. Based on the visual inspection and past operational performance, the dam is judged to be in fair condition. Seepage was noted on the downstream face. The emergency outlet works have not been operated in some time. The two service bridges were found to be in need of repair. Trees were recently cut on the downstream face and the slope is in an unvegetated state. This dam is classified as small in size and a high hazard potential structure in accordance with the recommended guidelines established by the Corps of Engineers. The test flood for this dam is the Probable Maximum Flood (PMF). The test flood has an outflow discharge equal to 1820 cfs and will overtop the dam by 1.1 feet in a stillwater condition. The maximum outflow capacity of the spillway under stillwater conditions is 240 cfs, which is approximately 13 percent of the test flood. It is recommended that the following items be studied further: The location, condition, and operability of various outlet works; downstream seepage; the composition and adequacy of the dam embankment; the stumps from recently cut trees on the downstream face of the dam; and the deteriorated condition of the service bridge to the gate house.

This dam is an earth embankment structure with a steel sheet pile core. It is approximately 650 feet long, 24 feet high with a top width of 15 feet and has a centrally located 200 foot long spillway. The original dam was built about 1890 and was substantially expanded to its present configuration in 1965. The official name of the impoundment created by the dam is Pope's Pond, although it is also known as Streets Pond as shown on the USGS maps. Based on the visual inspection, review of construction plans and past operational performance, the dam is judged to be in FAIR condition. Minor seepage was noted on the downstream face of the east embankment. Some vegetation was noted in the joints of the spillway. The single control valve for the by-pass pipe is on the downstream side of the dam. The test flood for this dam is the Probable Maximum Flood (PMF). The test flood has an inflow equal to 4575 cfs and an outflow discharge equal to 4475 cfs at a stillwater elevation of 371.5 which will not overtop the dam (1.5 feet freeboard). The maximum outflow capacity of the spillway under a stillwater condition is 7600 cfs, which is 170 percent of the test flood.

Putnam Reservoir dam consists of a rolled earth embankment with a concrete core wall, built originally in 1893 and modified in 1906 and 1922. The dam section is 640 feet long with a maximum height of 35 feet. The spillway is 55 feet long with an Ogee crest. Based on the visual inspection of the site, review of available information and past performance of the dam, the dam is judged to be in good condition. The maximum spillway capacity at top of dam is 35 per cent of the peak inflow rate of the test flood. Therefore, the test flood cannot be passed by the spillway without overtopping the dam. The overflow will be 1.1 feet above the top of the dam.

This dam is a masonry arch approximately 115 feet long, 45 feet high with an average top of dam width of 8 feet. The dam was built about 1867 and presently serves no function except an aesthetic one. Based on the visual inspection and past operational performance, the dam is judged to be in Fair condition. Seepage was noted on the downstream face. The emergency outlet works are inoperable and there is no method of lowering the water level. This dam is classified as INTERMEDIATE in size and a High hazard potential structure in accordance with recommended guidelines established by the Corps of Engineers. The test flood for this dam is the Probable Maximum Flood (PMF). The test flood has an outflow discharge equal to 25,600 cfs and will overtop the dam by 17.0 feet in a stillwater condition. The maximum outflow capacity of the spillway under a stillwater condition is 1,100 cfs, which is 4 percent of the test flood. It is recommended that the following items be studied further. The downstream leakage, the toe and the upstream face, and the spillway capacity.