SHAFT SPILLWAY
A drop inlet or a
shaft spillway also called morning glory is a spillway in which the water
enters over a horizontally positioned lip, drops through a vertical or sloping
shaft, and then flows to the downstream river channel through a horizontal or
near horizontal conduit or tunnel. The structure may be considered as being
made up of three elements; namely, an overflow control weir, a vertical
transition, and a closed discharge channel. Where the inlet is funnel shaped,
this type of structure is often called a "Morning Glory" or
"Glory Hole" spillway.
Discharge characteristics
of the drop inlet spillway may vary with the range of head. The control will
shift according to the relative discharge capacity of the weir, the transition,
and the conduit or tunnel. A drop inlet spillway can be used at dam sites in
narrow canyons where the abutments rise steeply or where a diversion tunnel or
conduit is available for use as the downstream leg. This type of spillway may
be adapted when the possibility of an overflow spillway and a trough spillway
is ruled out because of non-availability of space due to topography. The
discharge through the shaft spillway does not increase at such a high rate as
it increases in weir type spills. Hence if the un-estimated high floods occur,
the shaft spillway may not prove as useful as a weir type spillway.
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