Test: Energy Dissipation - Civil Engineering (CE) MCQ

Energy dissipation bucket called ski-jump bucket is used when the tail-water depth is insufficient or low at all discharge. It requires sound and rocky river bed. Water may shoot up out of the bucket and fail harmlessly into the river at some distance downstream of the bucket.

Most of the kinetic energy is destroyed by creating a condition suitable for the formation of a hydraulic jump. Sometimes the depth of tail-water may be more than that necessary to create the hydraulic jump. The depth of water can be reduced to create a hydraulic jump by providing a sloping apron.

Hydraulic jump is generally accompanied by large scale turbulence dissipating most of the kinetic energy of the super-critical flow. It is the most suitable method because the energy is lost in the impact of the water against water. Most of the kinetic energy is destroyed by creating a condition suitable for the hydraulic jump.

A flared inlet called morning glory is often used in large projects. The horizontal tunnel is either taken through the dam body or below the foundations. Chute blocks, dentated sills and baffle piers are all auxiliary devices which help in energy dissipation.

Chute blocks are a row of small projections like teeth of saw and are provided at the entrance of the silting basin. It produces a shorter length of jump and stabilizes the flow. Hence, they improve jump performance.

A ski-jump bucket is also called flip bucket is used for energy dissipation when tail-water depth is insufficient or low at all discharge. A part of energy dissipation takes place by impact and some of the energy is dissipated in the air by diffusion or aeration.

At low discharges, the jump will be drowned and at high discharges tail-water depth is insufficient. When TWC lies above the JHC at low discharges and below the JHC at high discharges, the solution is the provision of sloping apron partly above and partly below the river bed. The horizontal apron and end-sill are also provided.

When the TWC is lying above the JHC at all discharges, the problem can be solved by-

• By constructing a sloping apron above the river bed
• By providing a roller bucket type of energy dissipator.

In this case, the jump is formed at the toe will be drowned by the tail-water and little energy will be dissipated.

The energy dissipation in the jump depends upon the Froude number, if this Froude number is higher, the greater energy dissipation can take place.

When TWC lies below the JHC at all discharges, the expected solution is –
i. Provision of a ski-jump bucket
ii. A sloping apron below the river bed of length 5 (y₂ – y₁)
iii. Construction of a subsidiary dam below the main dam.