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.

1. The PK. weir design

A totally different design has been studied for five years by Hydrocoop (a non-profit-making international association), and this has been supported by more than 50 hydraulic tests. The target is a structure which:

can be placed on existing or new gravity dam sections;

will allow for specific flows of up to 100 m³/s/m;

can multiply at least by four the flow of a Creager weir; and,

is structurally simple, and easy to build with the local resources of all countries.

Preliminary model tests were done in 1999 at the LNH Laboratory in France (owned by Electricité de France) and in 2002 at Roorke University in India and Biskra University in Algeria. Some shapes were then selected, and are based on:

a rectangular layout somewhat similar to the shapes of piano keys which explains the name "Piano Keys weir" (thus "PK. weir");

an inclined bottom of the upstream and downstream part (the part where the flow enters is known as the inlet, and the other part the outlet); and,

a reduced width of the elements.

Many detailed tests were then done in 2003 on selected shapes at Biskra University and some tests using a very wide flume at LNH.

These detailed tests provided the basis for optimizing the flow increases according to the ratios between length, depth, width and shape of the elements, and particularly according to the ratio (length of walls/length of spillway) N.

The impact of various overhangs has also been studied. Particular attention has been paid to the structural design and construction facilities for selecting the most attractive solutions.

Very simple longitudinal sections have at present been preferred: it is possible that refining these shapes may slightly improve the cost efficiency.

Further studies are now under way in China (at IWHR in Beijing) and India (Roorke University) as there are great possibilities for using PK. weirs in these two countries.

Hydraulic and structural data are given next for two solutions.

Solution A, with similar upstream and downstream overhangs; this favours the use of precast concrete elements, which may be used for specific flows of up to about 20 m³/s/m. It may be preferred for this reason to improve many existing spillways.

Solution B, with only an upstream overhang. Relevant savings are about 10 per cent higher than for solution A, and structural loads are less for high specific flows. It could thus be a very attractive choice for many future large dams.

It is likely that other designs based on the same principles could be more efficient, but the range of cost saving would probably not be very different. It could be interesting to choose, according to local conditions within each country, one or two basic solutions and to standardize drawings for various specific flows.