SPILLWAYS

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SPILLWAYS
Uses -the safety valve for a dam -discharges
major floods keeps reservoir below
predetermined level 1. Controlled 2.
Uncontrolled (excrest gates) Required
Capacity (Max. Outflow Rate through the
spillway) 1. Spillway design flood (inflow
hydrograph to reservoir) 2. Discharge capacity
of the outlet works 3. Available storage All
depend on type of dam, location, consequences
of failure
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SPILLWAYS
Spillway Design Minimize cost providing -suffici
ent crest length to convey discharge -acceptable
minimum pressures acting on the crest
boundary -acceptable maximum energy head on the
spillway crest -acceptable velocities and flow
characteristics through the spillway
system -acceptable environmental and aesthetic
conditions Most Economic Design -narrow
spillway, high discharge (large energy head at
the crest)
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SPILLWAYS
Types of Spillways a. Overflow spillways b.
Chute spillways c. Side-channel spillways d.
Shaft spillways e. Siphon spillways f. Service
Emergency spillways
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SPILLWAYS
a) Overflow Spillways section of a dam that
allows water to pass over its crest widely used
on gravity, arch, buttress dam Discharge of an
Overflow Spillway Q Cw L
h3/2 where, Q discharge, ft3/s or m3/s Cw
coefficient of discharge which varies with design
and head, dimensionless L length of crest, ft
or m h head on the spillway (vertical distance
from the crest of the spillway to the reservoir
level), ft or m
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Overflow Spillway
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SPILLWAYS
b) Chute Spillways formed by spillways that
flow over a crest into a steep-sloped
open channel chute width is often constant
-narrowed for economy -widened to
decrease discharge velocity c) Side-Channel
Spillways spillway in which flow, after passing
over the crest, is carried away in a channel
running parallel to the crest used in narrow
canyons in which there is sufficient crest
length for overflow or chute is available
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Chute Spillway
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Side-Channel Spillway
Burrinjuck Dam on the Murrumbidgee River near
Yass.
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SPILLWAYS
d) Shaft Spillways water drops through a
vertical shaft in a the foundation material to
a horizontal conduit that conveys the water
past the dam often used where there is not
room enough for other spillways possible
clogging with debris a potential problem screens
and trash racks protect inlet Flow
Conditions - outlet conduit is partly full (weir
flow) or full (orifice flow) - intermediate
condition - submerged inlet (pipe flow)
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Shaft Spillway(s)
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SPILLWAYS
e) Siphon Spillways air vent used automatically
maintain the water-surface elevation large
capacity not needed, good for limited space
At low flow it acts like an overflow spillway
At high flow the siphon action removes the water
thru the structure until reservoir drops
to the elevation at the upper lip of
entrance f) Service and Emergency
Spillways -extra spillways provided on a
project in rare case of extreme floods
(emergency) -used to convey frequently
occurring outflow rates (service)
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SPILLWAYS
Upgrading of Spillways (Increase spillway
capacity) -Raise crest on dam to increase the
head on spillway -Chutes made wider increases
capacity -Use a labyrinth spillway spillway
with sawtooth configuration that adds extra
length to spillway
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SPILLWAYS
Major Damage Caused By Cavitation when water
breaks contact with the spillway surface at
high velocities, reduced pressures cause the
formation of cavities filled with vapor, air,
and other gases in the water when this cavity
reaches a point where the absolute pressure is
much higher, an implosion occurs.
Extremely high pressure result from the collapse
of this cavity, which result in damage to
spillway structure (Pitting the sponge- like
appearance of spillway surface) Smoother,
properly designed ramps prevent water leaving
spillway surface, which reduces cavitation.