HEAD DISCHARGE CURVES FOR A MECHANICAL FUSEPLUG (FUSEGATE)SPILLWAY: FLOOD DAMAGE AND GATE OPERATION CONSIDERATIONS AT OTTER BROOK DAM, KEENE, NEW HAMPSHIRE

摘要

This paper expands the idea of a head-discharge curve, to describe a spillway in which the characteristics of a given segment are designed to increase flow once a certain minimum water level has been reached. The work extends the cost comparison calculations that were prepared to evaluate the new retrofit project at Otter Brook Dam. Otter Brook Dam was designed and constructed as a 133-ft high flood reduction earthfill dam, constructed in the late 1950s by the US Army Corps of Engineers, on a tributary of the Connecticut River in New Hampshire, USA. Flood predictions based on Hydrometeorological Reports No. 51 and 52 (Reports issued by NOAA in 1978 and 1982) indicated that the dam would be overtopped under the Probable Maximum Flood (PMF) conditions. The new spillway channel was essentially the old spillway channel, excavated approximately 3 feet deeper than before; the weir crest elevation is unchanged. The weir has six separate sections which are designed to be washed away at key moments in a flooding event, to ensure that the full probable maximum flood can be withstood without a failure of the dam itself. For floods of smaller magnitude, fewer than six of the separate units would fail (tip), and they would leave behind a structure with characteristics different from those of the original spillway weir. The structure as a whole therefore has a quality of "fatigue" in that the response to the "stress" of applied head leads to an increasing "strain" of water discharge as the system is subjected to heads (pool elevations) in excess of the pre-set tipping levels. This study reviewed the one current and six potential states of the new system, developing likely peak reservoir levels and downstream flood costs for various flooding events, noting how peak flow in a given storm, and the associated comparative costs, depend on the starting state (partially failed or whole) of the spillway weir prior to the storm and the amount of available (empty) storage in the reservoir. It was noted that for the gated conduit at the dam during a storm, control of reservoir levels is minimal once the water first Fusegate has tipped. The review indicated where evacuations should be anticipated.