uired to produce electricity from tidal power have become highly refined. Tidal energy is extremely predictable with regard to both timing and output. Its development is only practical though in areas with large-tides and good construction sites. One measure used to determine the suitability of an area for tidal electricity production is the Gibrat site ratio. This ratio is defined as the "length of the dam in meters to the annual energy production" (Sanders, 1991, p. 275). Smaller ratios are more desirable. The ratio for the tidal power plant at La Rance, France, for example, is 0.36. Hence, the best basins are connected to the sea through narrow channels and contain a lot of water. It also helps if the basin is in resonance with the tidal period.
Most approaches to tidal power utilization involve the hydraulic head created between high and low tides. Modern systems generally employ a barrage, or dam. This structure can be built out of concrete or steel caissons (Laughton, 1990, p. 27). Caissons may be prefabricated onshore, floated out to the site, and then sunk into place. Different caissons may incorporate either sluiceways or turbines: the sluice gates may allow for water flow between the ocean and the tidal basin during flood tide; the turbines generate electricity (Sanders, 1991, p. 273).
There are a variety of different ways to operate a tidal barrage. "Ebb generation" is perhaps the most common and the simplest. As in the early tidal mills, water passes through sluices to fill the tidal basin. Then, the sluice gates are
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