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 turbine,
or it can be used to push air through a pipe, which then turns a
turbine.
Large lock gates, like the ones used on canals, allow ships to pass.
If one was built across the Severn Estuary, the tides
at Weston-super-Mare would not go out nearly as far - there'd be
water to play in for most of the time.
But the Severn Estuary carries sewage and other wastes
from many places (e.g. Bristol & Gloucester) out to sea. A tidal
barrage would mean that this stuff would hang around Weston-super-Mare
an awful lot longer!
Also, if you're one of the 80,000+ birds that feeds
on the exposed mud flats when the tide goes out, then you have a
problem, because the tide won't be going out properly any more.