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Types of spillways
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.
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