Introduction:
Spillways are structures constructed to
provide safe release of flood waters from a dam to a downstream are, normally
the river on which the dam has been constructed.
Every reservoir has a certain
capacity to store water. If the reservoir is full and flood waters enter the
same, the reservoir level will go up and may eventually result in overtopping
of the dam. To avoid this situation, the flood has to be passed to the
downstream and this is done by providing a spillway which draws water from the
top of the reservoir. A spillway can be a part of the dam or separate from it.
Spillways can be controlled or
uncontrolled. A controlled spillway is provided with gates which can be raised
or lowered. Controlled spillways have certain advantages as will be clear from
the discussion that follows. When a reservoir is full, its water level will be
the same as the crest level of the spillway.
Parameters considered in Designing
Spillways:
Many parameters need consideration in
designing a spillway. These include:
- The inflow design flood
hydro-graph
- The type of spillway to
be provided and its capacity
- The hydraulic and
structural design of various components and
- The energy dissipation
downstream of the spillway
The topography, hydrology,
hydraulics, geology and economic considerations all have a bearing on these
decisions. For a given inflow flood hydro graph, the maximum rise in the
reservoir level depends on the discharge characteristics of the spillway crest
and its size and can be obtained by flood routing. Trial with different sizes
can then help in getting the optimum combination.
Classification of Spillways:
Spillways are ordinarily classified
according to their most prominent feature, either as it pertains to the
control, to the discharge channel, or to some other component. The common types
of spillway in use are the following:
Free Overfall (Straight Drop)
Spillway:
In this type of spillway, the water
freely drops down from the crest, as for an arch dam (Figure 1). It can also be
provided for a decked over flow dam with a vertical or adverse inclined downstream
face (Figure 2). Flows may be free discharging, as will be the case with a
sharp-crested weir or they may be supported along a narrow section of the
crest. Occasionally, the crest is extended in the form of an overhanging lip
(Figure 3) to direct small discharges away from the face of the overfall
section. In free falling water is ventilated sufficiently to prevent a
pulsating, fluctuating jet.
Ogee Spillway:
The Ogee spillway is generally
provided in rigid dams and forms a part of the main dam itself if sufficient length
is available. The
overflow type spillway has a crest shaped in the form of an ogee or S-shape.
The upper curve of the ogee is made to conform closely to the profile of the
lower nappe of a ventilated sheet of water falling from a sharp crested weir
(Figure 6). Flow over the crest of an overflow spillway is made to adhere to
the face of the profile by preventing access of air to the underside of the
sheet of flowing water.
Naturally, the shape of the overflow
spillway is designed according to the shape of the lower nappe of a free
flowing weir conveying the discharge flood. Hence, any discharge higher than
the design flood passing through the overflow spillway would try to shoot
forward and get detached from the spillway surface, which reduces the
efficiency of the spillway due to the presence of negative pressure between the
sheet of water and spillway surface.
An ogee crest apron may comprise an
entire spillway such as the overflow of a concrete gravity dam (Figure 7), or
the ogee crest may only be the control structure for some other type of
spillway (Figure 8).
Chute Spillway:
A chute spillway, variously called as
open channel or trough spillway, is one 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. The control structure for the chute
spillway need not necessarily be an overflow crest, and may be of the
side-channel type as has been shown in Figure 10.
Generally, the chute spillway has
been mostly used in conjunction with embankment dams, like the Tehri dam, for
example. Chute spillways are simple to design and construct and have been
constructed successfully on all types of foundation materials, ranging from
solid rock to soft clay.
Side Channel Spillway:
A side channel spillway is one in
which the control weir is placed approximately parallel to the upper portion of
the discharge channel, as may be seen from Figure 10. When seen in plan with
reference to the dam, the reservoir and the discharge channel, the side channel
spillway would look typically as in Figure 11 and its sectional view in Figure
12. The flow over the crest falls into a narrow trough opposite to the weir,
turns an approximate right angle, and then continues into the main discharge channel.
Shaft Spillway:
A Shaft Spillway is one where 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
nearly horizontal conduit or tunnel (Figure 13). 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. When the inlet is
funnel shaped, the structure is called a Morning Glory Spillway. The name is
derived from the flower by the same name, which it closely resembles especially
when fitted with anti-vortex piers (Figure 14). These piers or guide vanes are
often necessary to minimize vortex action in the reservoir, if air is admitted
to the shaft or bend it may cause troubles of explosive violence in the
discharge tunnel-unless it is amply designed for free flow.
Tunnel Spillway:
Where a closed channel is used to
convey the discharge around a dam through the adjoining hill sides, the
spillway is often called a tunnel or conduit spillway. The closed channel may
take the form of a vertical or inclined shaft, a horizontal tunnel through
earth or rock, or a conduit constructed in open cut and backfilled with earth
materials. Most forms of control structures, including overflow crests,
vertical or inclined orifice entrances, drop inlet entrances, and side channel
crests, can be used with tunnel spillways. Two such examples have been shown in
Figs. 15 and 16. When the closed channel is carried under a dam, as in Figure
13, it is known as a conduit spillway.
Tunnel spillways are advantageous for
dam sites in narrow gorges with steep abutments or at sites where there is
danger to open channels from rock slides from the hills adjoining the
reservoir.
Siphon Spillway:
A siphon spillway is a closed conduit
system formed in the shape of an inverted U, positioned so that the inside of
the bend of the upper passageway is at normal reservoir storage level (Figure
17). This type of siphon is also called a Saddle siphon spillway. The initial
discharges of the spillway, as the reservoir level rises above normal, are
similar to flow over a weir. Siphonic action takes place after the air in the
bend over the crest has been exhausted. Continuous flow is maintained by the
suction effect due to the gravity pull of the water in the lower leg of the
siphon.
Another type of siphon spillway
(Figure 18) designed by Ganesh Iyer has been named after him. It consists of a
vertical pipe or shaft which opens out in the form of a funnel at the top and
at the bottom it is connected by a right angle bend to a horizontal outlet
conduit. The top or lip of the funnel is kept at the Full Reservoir Level. On
the surface of the funnel are attached curved vanes or projections called the
volutes.
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