Tidal+Barrages

These work rather like a [|hydro-electric]  scheme, except that the dam is much bigger. A huge dam (called a "barrage") is built across a river estuary. When the tide goes in and out, the water flows through tunnels in the dam. The ebb and flow of the tides can be used to turn a, or it can be used to push air through a pipe, which then turns a. Large lock gates, like the ones used on canals, allow ships to pass. 