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Gilboa: Questions and answers on the integrity of the dam

Gov. Andrew Cuomo got this aerial view of the Gilboa Dam on Aug. 28, 2011, during a tour of flood-ravaged Schoharie County. (Courtesy of Governor's Office)
Gov. Andrew Cuomo got this aerial view of the Gilboa Dam on Aug. 28, 2011, during a tour of flood-ravaged Schoharie County. (Courtesy of Governor's Office)
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The day after Tropical Storm Irene’s floodwaters had already receded, a rumor spread that sent people scrambling for high ground. Somebody erroneously said the dam broke, referring to the massive Gilboa Dam that looms upstream of the Schoharie Valley. The dam holds back 17.6 billion gallons of water that's used to quench the thirst in New York City — a volume so great that a failure in Gilboa would mean catastrophe for communities from the ...


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comments

howard
August 26, 2012
7:36 p.m.

[ Flag Post ]

Thank you for a well prepared article. The questions asked and answers given were satisfactory to the extent of their scope. One major point of continuing concern is item "I" in the list of the project's elements that are expected to be performed in the future reconstruction of the Gilboa Dam/Schoharie Reservoir. Item "I" states, "maintain and monitor existing instrumentation". Dam Concerned Citizens, Inc. has asked NYCDEP to install load cells; devices that measure retention or downward force exerted by the post-tensioned anchors on selected sections in the Gilboa Dam Concrete spillway. Some portions of the 1324' long masonry spillway are more susceptible to sliding failure than other monoliths (sections) of the structure. This is due to variables in the quality of the bedrock upon which the dam spillway rests. Post-tensioned anchors are a reliable means of adding additional stability to existing structures, but the cables are subject to elements such as corrosion stress due to expansion and contraction, hydrostatic uplift and hydraulic energy exerted against the upstream face of the dam, especially in time of flooding. As a result, post-tensioned anchors do lose retentive strength over time. As shown on page 9-23, table 9-4a entitled "Minimum recommended instrumentation for existing dams", the Federal Energy Regulatory Commission (FERC) expects that dams under its supervision meet the requirement that at least 10% of the post-tensioned anchors in a dam have measuring devices, ie. load cells, contained in their structure. The Schoharie Reservoir/Gilboa Dam, one of the six West of Hudson operations owned by NYCDEP is not subject to FERC supervision. This is due to the fact that the dam and the waters it impounds are not being used to generate hydroelectricity. At present, there are no load cells in place in the anchors at the Gilboa Dam spillway. Instead, load cells are in anchors located downstream of the dam in rock not subjected to any of the stresses as the anchors in the dam are. It was fortunate that the 79 anchors in the Gilboa Dam were in place during the flood of 8/28/2011. We can only speculate what might have happened had they been absent. In order to monitor the actual status of anchor integrity in the Gilboa Dam, "sentinel" or load cell equipped anchors should be installed in the dam spillway now or existing anchors retrofitted with load cells, while dam repairs are under way. For more information regarding "instrumentation needed at the Gilboa Dam" go to www.dccinc.org.

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