The dam is located 117km north of New York city in Schoharie county. The 61m-long hole has been made in the dam wall and siphons have been installed to remove a planned 2.3B litres of water per day from the reservoir, with the first two siphons due to be tested on 23 February.
The New York City Department of Environmental Protection (DEP) has said that with the full notch in place, the amount of water that would have to be going over the dam to risk failure is about 2830m3/sec; that would bring water levels to 3m above the crest of the dam. Without the notch, only about 2547m3/sec is required to hit the risk point. During the largest flood on record on 19 January 1996, flow over the dam was 2003m3/sec - at that time water reached 2m above the dam.
It is expected that the entire siphoning project will be completed within three weeks.
The affects of Gilboa dam failing have long been a source of local anxiety, as catastrophic flooding would submerge the area just north of the structure under 13m of water, and it is estimated that floodwaters could spread as far as 96km to Scotia and Schenectady and still be as deep as 3m.
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