Explain straight drop spillway and ogee spillway.
Answers
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 over-topping 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.
This is the normal reservoir level. If a flood enters the reservoir at this time, the water level will start going up and simultaneously water will start flowing out through the spillway. The rise in water level in the reservoir will continue for some time and so will the discharge over the spillway. After reaching a maximum, the reservoir level will come down and eventually come back to the normal reservoir level.
This is the most common type of spillway provided on gravity dams. The profile of the spillway is ogee or ‘s’ shaped.
The overflowing water is guided smoothly over the crest and profile of the spillway so that the overflow water does not break contact with the spillway surface.
If this is not assured, a vacuum may form at a point of separation and cavitation may occur. In addition to cavitation, vibration from the alternate making and breaking of contact between the water and face of dam may result in serious structural damage.
Hence the upper profile of the ogee is, made to confirm with the lower nappe of a freely falling jet of water over a sharp crested weir, when the flow rate corresponding to the maximum designed capacity of the spillway.
Here, the essential difference between the straight drop spillway and the ogee or overflow spillway should be clearly noted. In the straight drop spillway type, the jet falls clearly away from the face of the spillway and the gap between the jet and face is kept ventilated.
In the ogee or overflow spillway, the falling water is made to glide over the curved profile of the spillway.
A smooth gradual reverse curvature on the downstream face of the spillway is provided. This reverse curve turns the flow on the apron of a stilling basin or into the spillway discharge channel
A chute spillway is a common and basic design which transfers excess water from behind the dam down a smooth decline into the river below. These are usually designed following an ogee curve. Most often, they are lined on the bottom and sides with concrete to protect the dam and topography.