CHUTE (TROUGH SPILLWAY)
Chute spillways ordinarily
consist of an entrance channel, control structure, a discharge channel, a
terminal structure, and an outlet channel. The simplest form of chute spillway
has a straight centerline and is of uniform width. Often, either the axis of
the entrance channel or that of a discharge channel must be curved to fit
alignment to the topography. in such cases the curvature is confined to the
entrance channel is possible, because of low approach velocity. Where the
discharge channel must be curved, its floor is sometimes super elevated to
guide the high velocity flow around the bend, thus avoiding a piling up of flow
toward the outside of the chute. The base for the channel is usually made of
reinforced concrete slabs, 25 to 50 cm thick. Expansion joints are usually
provided in the chute at intervals of about 10 meters in either direction. The
expansion joints should be made watertight so as to avoid any under-sewage.
Under-drains are also provided, so as to drain the water which may seep through
trough bottom and side walls. These under-drains may be in the form of
perforated steels pipes, clay tiles or rock-filled trenches. If the slope of
the chute can confirmed to available topography, the excavation shall be
minimum. But the slope of the chute must be high enough, and should at least be
able to maintain super critical flow to avoid unstable flow conditions.
A spillway whose
discharge is conveyed from the reservoir to the downstream river level through
an open channel, placed either along a dam abutment or through a saddle, might
be called a chute, open channel, or through type spillway. The chute spillway has
been used with earth-fill dams. Sometimes, even for gravity dams, a separate
spillway is required because of the narrowness of the main valley. In all such
circumstances, a chute spillway is provided. It is lighter and adaptable to any
type of foundations and hence provided easily on earth and rock fill dams.
Factors affecting
the selection of chute spillway are the simplicity of their design and
construction. a chute spillway is sometime known as Waste Weir. If it is
constructed in continuation of the dam at one end, it may be called a Flank
Weir. If it is constructed in a natural saddle in a bank of a river
separated from the main dam by a high ridge, it is called a Saddle Weir.
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