**Most commonly used types of spillways are summarized as follows:**Side

channel spillway:

Provided in narrow valleys, where required crest length is not available for
overflow spillway, or in a less wide stream where it is advantageous to leave
the central portion of the stream for the power house. It is generally of
trapezoidal section. The difference between side channel and chute spillway is
that in chute spillway, the water flows at right angle to the weir crest after
spilling over it, whereas in a side channel spillway, the flow of water after
spilling over the crest is turned by 90 degrees such that it flows parallel to
the weir crest.

Emergency or breaching spillway:

Provided for an extra safety of dams and main spillway. Emergency situation may
arise when incoming discharge exceeds the maximum flood discharge or even when
the gate of spillway malfunctions due to flood. In such cases the water level
will exceed beyond the maximum water level and there will be danger of
overtopping. In such a situation emergency spillway comes into action. The crest
level of emergency spillway is at the Maximum Water Level (MWL). Emergency
spillway is the fuse plug type and comes into action as soon as water level
reaches to MWL.Chute spillway/ trough spillway:
These types of spillway are provided through the abutments of the dam when it is
not possible to pass floods over the dam as in the case of the earthen or
rock-fill-dams. The chute spillways are simple in design and constructions and
are adaptable in almost all foundation conditions. Water passes over the crest
of the spillway into trough or chute having a steep slope. The crest is usually
wide and then the channel narrows for economy. The end is then again flared or
widened for reducing velocity.

The chute spillway may have following sections:

  • The entrance channel
  • The control structure
  • The conveyance structure

Free over fall spillway:

It could be the sharp crested spillway where water falls in the form of free
nape water. Its downstream face is vertical or nearly vertical. The flow nape
drops at the free jet and may cause the severe erosion at D/S due to the high
energy possession. There may be formation of plunge pool. Usually the flow is
discharge as the free jet in case of the low head and also when the firm bed
exists at D/S. the space below the free nape should have the ventilation
otherwise there will be variation of the discharge due to the fluctuation of the
flow caused by decrease of the air pressure. The flow will gradually take the
air inside that nape and it will ultimately coincide with the D/S face of the
weir.Ogee shape spillway:

If the surface if the weir is made, following the shape of the lower surface of
nape of free over-fall spillway with the proper provision of the ventilation,
then it is termed as the ogee spillway. In case of the flow with the designed
head water glides smoothly above the free surface of the spillway so the
coefficient of discharge of discharge is more than that of the free fall
spillway.With ogee shape spillway there will be no formation of the negative
pressure above the surface of low up to designed head, but when the flow exceeds
the designed head there is possibility of separation of flow and consequently
formation of negative pressure.

When the head is more than the designed head, the suction pressure will cause
the increase of the discharge where as in case of flow with the head less than
the designed head the nape will be suppressed to the ogee surface and Cd will be
decreased due to increase of the frictional resistance.