SPILLWAYS
A spillway is structure
constructed at dam site, effectively disposing off the surplus water from
upstream to downstream. Just after the reservoir filled up to the normal pool
level water starts flowing over the spillway crest (which is generally kept at
normal pool level). Depending upon the inflow rate, water will start rising
above the pool level, and at the same time it will be let off over the
spillway.
FUNCTION
Spillways are
provided for storage and detention dams to release surplus or flood water which
cannot be contained in the allotted storage space, and diversion dams to bypass
flows exceeding those which are turned into the diversion system. In addition
to provide sufficient capacity, the spillway must be hydraulically and
structurally adequate and must be located so that spillway discharges will not
erode or undermine the downstream toe of the dam. The spillway's bounding
surfaces must be erosion resistant to withstand the high scouring velocities
created by the drop from the reservoir surface to tail water, and usually some
device will be required for dissipation of energy at the bottom of the drop.
The frequency of spillway use will be determined by the runoff characteristics
of the drainage flows will result during flood and periods of sustained high
runoff when the capacities of other facilities are exceeded. At diversion dams
where storage space is limited and diversions are relatively small compared to
the normal river flows, the spillway will be used almost constantly.
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LOCATION OF SPILLWAY
A spillway can be
located either within the body of a dam, or at one end of it or entirely away
from it, independently in a saddle. If a deep narrow gorge with steep banks,
separated from a flank by a hillock with its level above the top of the dam (as
shown in figure), is available the spillway can be best built independently of
the dam.
Under such circumstances, a
concrete or an earthen dam can be constructed across the main valley and
spillway can be constructed independently into the saddle. Sometimes a concrete
or masonry dam along with its spillway can be constructed in the main valley,
while the flanks are closed by earthen dikes or embankments. A separate
independent spillway is generally preferred for earthen dams, although due to
non availability of sites, a concrete spillway sometimes constructed within or
at one of the ends of an earth dams as shown in Fig
ESSENTIAL
REQUIREMENTS OF A SPILLWAY
The
essential requirements of a spillway, as discussed above, may be summarized as
follows:
i.
It must have adequate discharge
capacity.
ii.
It must he hydraulically and
structurally safe.
iii.
The surface of the spillway must be
erosion resistant.
iv.
The spillway must be so located that
the spillway discharge does not erode or undermine the downstream toe of the
dam.
v.
It should be provided with some device
for the dissipation of excess energy
vi.
The spillway discharge should not
exceed the safe discharge capacity of the downstream channel to avoid its
flooding.
REQUIRED
SPILLWAY CAPACITY
The
required spillway capacity is usually determined by flood routing. The spillway capacity should be equal to the maximum outflow rate determined by
flood routing. The following data
are required for the flood routing:
i.
Inflow flood hydrograph, indicating the rate of
inflow with respect to time. It is the same as the design flood hydrograph of
the spillway.
ii.
Reservoir-capacity curve, indicating
the reservoir storage at different reservoir elevations
iii.
Outflow discharge curve, indicating the rate of outflow through spillways at elevations.
By
flood routing, the maximum outflow rate and the maximum rise in water surface
can be determined.
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