When the tail-water depths in the river downstream of a spillway are quite low such that the tail-water curve at all discharges lies below the post jump depth curve, then the energy dissipation can be affected best by _________a)a roller bucketb)a ski-jump bucketc)either roller or ski-jump bucketd)a sloping apronCorrect answer is option 'B'. Can you explain this answer?

Question

Answer

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.

Answer

Explanation:

When the tail-water depths in the river downstream of a spillway are quite low such that the tail-water curve at all discharges lies below the post-jump depth curve, the best way to dissipate energy is by using a ski-jump bucket.

1. Tail-Water Depth and Post-Jump Depth Curve:
- The tail-water depth refers to the depth of water downstream of the spillway.
- The post-jump depth curve represents the depth of water after the hydraulic jump occurs downstream of the spillway.
- In this situation, the tail-water depths are low, and the tail-water curve lies below the post-jump depth curve.

2. Energy Dissipation:
- Energy dissipation is the process of reducing the energy of the flowing water.
- In spillways, energy dissipation is crucial to prevent erosion and damage downstream.
- Different energy dissipation structures or measures can be used, depending on the flow conditions.

3. Roller Bucket vs. Ski-Jump Bucket:
- A roller bucket is an energy dissipation structure that consists of a series of rollers or baffle plates.
- It is effective in dissipating energy when the tail-water depths are relatively high.
- However, in this case, the tail-water depths are low, and a roller bucket may not provide efficient energy dissipation.

- A ski-jump bucket, on the other hand, is a curved structure that creates a hydraulic jump and dissipates energy.
- It is designed to handle low tail-water depths and is more effective in such conditions.
- The ski-jump shape encourages the formation of a hydraulic jump, which helps in energy dissipation.

4. Sloping Apron:
- The sloping apron is another energy dissipation measure commonly used in spillways.
- It is a concrete slope downstream of the spillway that helps in dissipating energy.
- However, in this situation, where the tail-water depths are low, a sloping apron may not provide sufficient energy dissipation compared to a ski-jump bucket.

Conclusion:
- When the tail-water depths in the river downstream of a spillway are quite low and lie below the post-jump depth curve, the best way to dissipate energy is by using a ski-jump bucket.
- The ski-jump shape encourages the formation of a hydraulic jump, which efficiently dissipates energy in low tail-water conditions.

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