29-03-2014, 11:48 AM
Chute Spillways
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General
The Chute Spillway is an obvious choice wherever the foundation strata pose difficult
problems. Chutes are also employed in canals for conveying water from a higher to
lower elevation with a consequent energy dissipation. The U.S.B.R. specifies broadly,
that when canal drops are about 60 m the Chute discharge carriers should be
employed.The principle hydraulic Elements of a Chute Spillway are: (1). the inlet
(approach) channel and a high coefficient crested spillway (control structure). (2). Chute
(3). Energy dissipator (or Terminal Structure).
Control Structure
The control structure should have a proper approach channel. It is usually located on
the flanks where the height of body wall either of masonry or concrete of spillway is
considerably small. The Crest gates for flood control if necessary may be provided.
Water overflowing the spillway is let into the chute.
Chute Discharge Carrier
The Chute portion will be a steep channel to convey water from a higher to lower
elevation ( i.e. to the natural river course at very high velocity. The cross section of the
Chute may be rectangular or trapezoidal).
Energy Dissipators
These are located at the downstream end, after the fall is completely negotiated and in
the vicinity of the natural stream. It may include Chute blocks, baffle blocks, stilling
basin, end sill and side (training) walls. It is preferable to keep them verticle on water
side for the satisfactory formation of Hydraulic jump. When the velocity at entry of stilling
basin is high, Chute and baffle blocks are omitted.
Location, Alignment
Chute Spillways are used mostly in the case of earthen dams. The important features of
chute spillways are their adoptability for any type of foundation condition and overall
economy effected due to the use of the material excavated for the chute portion for the
embankment. Chute spillways can be built on foundations ranging from solid rock to soft
clay.The location of spillway (control structure) will be on the flank and its alignment
depends on the location and topography. The simplest alignment is a straight with
constant width. Varying widths or curves in alignments lead to complex flow situation
(Example: Hat creek). Under certain circumstances the axis of entrance channel as well
axis of chute will have to be curved. In such cases it is better to have the curved
entrance channel to have low velocities. The bottom slab of the curved channel may
require to be elevated to accomadate the super elevation effect of curved channel.
Usually the control structure (spillway) is built in line with the axis of the main dam.
Cross waves
When the surface configuration of supercritical flows in rectangular channels may be
visualised by considering a curved section of walls placed by a sequence of short
chords each one of which is deflected relative to the preceding one by a small angle
∆θ , for a convex and concave portion. The disturbance lines caused by convex and
concave walls at a wave angle given by β1 = Sin -1
The depths can be found all along the wave line. However, the method of
characteristics can be applied to study these in curves. The diagrams given by Ippen
cannot be used for determining the flow conditions for negative deflection angles,
since negative shock waves are not possible on the assumption of Hydro Static
pressure. ∆n negative disturbance lines diverge. Steep fronts commonly be produced
by concentrating the lines at their origin - the equivalent of introducing the sharper
curvatures. For an abrupt change of wall alignment, the resulting curvature would be
infinite. Near the wall discontinuity the surface configurations will not agree with
actual measurements owing to the necessarily sharp curvatures of the theoretical
streamlines, good agreement is nevertheless obtained at some distance from the
wall, where streamline curvatures are decreased.