CONDUIT AND TUNNEL SPILLWAY
It is a spillway in
the form of a closed channel used to convey the discharge around or under the
dam. The closed channel may take the form of vertical or inclined shaft, a
horizontal tunnel through earth or rock, or a conduit constructed in open cut
and back filled with earth materials. Most form of control structures can be
used with conduit and tunnel spillways. Ample aeration must be provided in this
spillway in order to prevent a make-and-break symphonic action which would
result if some part of the tunnel or conduit tends to seal temporarily because
of an exhaustion of air caused by surging of the water jet, or by wave action
or backwater to guarantee free flow in the tunnel, the ratio of the flow area
to the total tunnel area is often limited to about 75%. Air vents may be
provided at critical points along the tunnel or conduits to insure an adequate
air supply which will avoid unsteady flow through the spillway.
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