Type of Spillways
As indicated above, the spillways are provided to discharge the flood water during rainy season if the water level rise above the designed maximum level in the reservoir. It saves the dam from damage.
Types of Spillway are :
- Solid gravity or overall spill way
- Chute or Trough spillway
- Side channel spillway
- Saddle spillway
- Shaft spillway
- Siphon spillway
1. Solid Gravity or Overall spillway : such spillways are provided in case of gravity dams as shown in Figure A it is built within the body of the dam.
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| Figure B |
Excess water above the full reservoir level flows over the crest in the form of rolling sheet of water. The bucket provided at the end of spillway changes the direction of fast moving water and its energy is destroyed.
2. Chute or Trough spillway : It is similar to overall type of spillway except that at the end of spillway a trough or channel is provided to meet the river channel down stream of the dam. These type of spillways are suitable for construction when the valley is too narrow to accommodate the overall spillway in the body of the dam.
3. Side channel spillway : These type of spillways are provided in a situation when the valley is too narrow to accommodate even the trough spillway. By using side channel spillway the undesirable flow of water over the dam is avoided as shown in Figure B. The flow of water in side channel is either used as a cannel or it meets the river on downstream side.
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| Figure B |
4. Saddle spillway : This type of spillway is shown in Figure C. Such spillways are constructed when any of the above types of spillways are not favourable, There may be some natural depression or saddle on the periphery of the reservoir basin away from the dam. This is used as a spillway as shown in Figure C.
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| Figure C |
5. Shaft spillway : The shape of this type of spillway is like a funnel constructed adjoining the dam . It’s lower end is turned at right angle and then taken out horizontally below the dam. The excess water above the reservoir level spills over the circular crest and then enters into the vertical shaft. Finally, this water flows below the dam and comes out horizontally in a tunnel which meets the downstream water.
6. Siphon spillway : It is constructed in the form of a siphon as shown in Figure D. The crest of the spillway is kept at full reservoir level of water. In case the water rises above F.R.L it enters into the spillway and discharged through the siphon.
If the water level falls below the lower end of the upper limb, the air enters into the lower limb and it stops the siphon action, thus stopping the flow of water from reservoir.
Upper limb is kept slightly below the FRL so that floating debris and foreign material doesnot enter into siphon.
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| Figure D |