National Program For Inspection Of Non Federal Dams American Can Company Dam Ct 00047 Southwestern Coastal Basin Greenwich Connecticut Phase 1 Inspection Report

The dam at the American Can Company building in Greenwich, Connecticut, consists of the north wall of a parking garage which retains earth and the adjacent created pond (North Lake) to a depth of five levels below grade. The height of this wall is 53 feet. There are no visible signs of physical distress and for reasons of fire protection, the water level is monitored very closely. Based on the visual inspection and a review of the technical data available, this dam is judged to be in good condition. This dam is classified as intermediate in size and a high hazard structure in accordance with the recommended guidelines established by the Corps of Engineers. The test flood for this dam is equal to the Probable Maximum Flood (PMF) and has an overflow discharge equal to 83 cfs. The maximum outflow capacity of the overflow weir under a stillwater condition; at the top of the weir opening is equal to 500 cfs, which represents more than 100 percent of the test flood. Rebuilding the headwall at the 48 inch outfall pipe is recommended within a 2 year period so that sedimentation buildup at this location can be monitored and clogging prevented.

The 250-foot-long, 21-foot-high homogeneous earthfill embankment is 12 feet wide at the top and has 3:1 upstream and downstream slopes. Riprap has been provided on the upstream slope to an elevation 2 feet above the wildlife pool surface. The principal spillway is composed of a reinforced concrete drop inlet structure and a 30-inch-diameter conduit through the dam. The crest of the drop inlet riser is at elevation 577.0 NGVD and has a total crest length of 15 feet. There is an ungated low-level outlet on the upstream wall of the riser. This 1 ft. - 10 in. by 1 ft. - 4 in. outlet has an open intake which may be closed, using stoplogs, thus diverting the flow over the weirs on the sides of the intake. The rock cut emergency spillway on the right abutment is 15 feet wide and approximately 245 feet long with a crest elevation of 578.0. The visual inspection of the dam indicated that the structure is in good condition. No indications of seepage through the dam were noted nor was seepage observed at the abutments or along the concrete spillway outlet pipe. There was no discharge from the 6-inch-diameter toe drain outlets during the inspection. The test flood outflow is approximately 1,240 cfs; and the combined capacity of the spillways, with the water surface at the top of the dam, is 1,600 cfs or 129 percent of the routed test flood outflow.

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.

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.

Kendall Company No. 1 Dam is a run of river facility that diverts water into a flume leading to the company's plant. Within the river channel there is a rock-filled, timber crib spillway structure 238 feet long. This overflow structure has a topwidth of 5 feet, and it has 2.9-foot high flashboards on its crest. The height of the dam, measured at the abutments of the spillway overflow section, is 24 feet. Based on visual inspection, the dam is judged to be in poor condition. The major concerns with respect to the integrity of the dam are deterioration of the wood crib abutment structure at the right end of the overflow section; trees growing in the wood crib abutment structure, on the upstream and downstream sides of the right abutment and on the dike; hazards resulting from the existing breach in the dike; lack of adequate erosion protection on the bank of the downstream channel near the left abutment; trees growing on the strip of ground between the dry stone masonry wall, on the left side of the downstream channel, and the right side of the concrete-lined flume on the left side of the valley; and lack of a low-level outlet for the dam. The spillway capacities are, respectively, about 22 percent and 38 percent of the routed test flood outflows. The depths of overtopping of the dam and dike that would occur during the test flood are 4.2 feet with the flashboards in place, and 3.5 feet with the flashboards removed.

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.

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.

The West Branch Dam consists of, from right to left, a 100 foot long earth embankment with an intake structure to a downstream canal, a 107 foot long concrete ogee section, and a 160 foot long earth embankment with a concrete core wall. The intake structure to the downstream canal contains two 42-inch sluice gates which discharge to the canal. The canal is separated from the river channel by an earth embankment which contains a 34 foot long auxilliary spillway. This spillway discharges into the river downstream of the dam. In the past, water from the canal would flow through a 36-inch reinforced concrete pipe (RCP) to a downstream manufacturing plant. The low level outlet, or blow-off, consists of a 48-inch RCP located to the left of the main spillway section and is controlled by a manually operated sluice gate. The dam has an overall length of 370 feet and a maximum height of 22 feet. The dam appears to be in fair condition and requires some work. The West Branch Dam has a tributary watershed of 33.3 square miles. The pond has a water surface area of 5.5 acres and a storage capacity with water level at the crest of the dam of 72 Acre-Feet. The main spillway has a capacity of about 8,200 cfs before overtopping the right portion of the dam.