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The Piano Keys weir: a new cost-effective solution for spillways.
Free-flow spillways are simpler and safer than gated ones, but the low specific flow of their traditional shapes requires high spilling nappe depths and thus huge losses of storage..

THE PIANO KEYS WEIR: A NEW COST-EFFECTIVE SOLUTION

FOR SPILLWAYS

F. Lempérière, Hydrocoop, France

A. Ouamane, Biskra University, Algeria

Free-flow spillways are simpler and safer than gated ones, but the low specific flow of their traditional shapes requires high spilling nappe depths and thus huge losses of storage
(100 x 109m3 worldwide). A new shape of free-flow spillway (the "Piano Keys weir") can increase the specific flow fourfold or more. It could substantially reduce the cost of most new dams and increase, at low cost. the safety and the storage and/or the flood control efficiency of many existing dams.

 

Most existing free-flow spillways have a standardized shape (Creager weir) and are placed upon concrete gravity dam structures.

Their drawback is their low specific flow which is (in m3/s/m) close to 2.2 h 1.5 (h being the nappe depth in metres).

Consequently, the loss of live storage corresponding to the maximum nappe depth may be 20 to 50 per cent, compared with a gated reservoir, even if using longer spillways than with gates.

It is thus very advantageous to increase the specific flow as much as possible. Some tens of existing spillways have been designed accordingly as vertical walls on a flat bottom, with a trapezoidal labyrinth layout which is much longer than the spillway length (often four times.) They usually double the specific flow of a Creager weir. They require 1 or 2 m3 of reinforced concrete to increase the flow by 1 m3/s, and have mainly been used for specific flows in the range of 10 m3/s/m. Exceptionally, the Ute dam in USA increased the specific flow by 30 m3/s/m; it required 60 m3 of reinforced concrete per meter. Apart from its cost for high flows, the main drawback of the traditional labyrinth solution (vertical walls and flat bottom) is that it cannot be used on top of the usual concrete gravity dam sections and requires a substantialy flat area. It can therefore only be used for a few dams and in fact has been used for one per thousand large dams.

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