Question

Pick one or more CORRECT statement(s) from the choices given below, in the context of upstream and downstream cut-offs provided below the concrete apron of weirs/barrages constructed across alluvial rivers.
 

• A. Cut-offs are provided to increase the rate of flow over the weir / barrage.
• B. Cut-offs are provided to increase the seepage length and prevent failure due to piping.
• C. The bottom level of cut-offs mainly depends on the scour depth.
• D. Cut-offs are provided to ensure occurrence of hydraulic jump within the stilling basin.

Hint:

Cut-offs protect structures against piping and undermining. Their depth is governed by scour analysis, not flow behavior above the structure.

Answer 1

The Correct Option is B, C

Cut-offs are vertical or inclined structural members provided below the floor of weirs or barrages to prevent sub-surface flow that may lead to piping.

- Option (B):
Correct. Cut-offs increase the seepage path length beneath the floor, thereby reducing the exit gradient and minimizing the risk of piping, which is a major cause of failure in hydraulic structures.
- Option (C):
Correct. The depth of cut-offs is designed considering the maximum anticipated scour depth, to ensure that the foundation remains safe even if the bed material is eroded during floods.
- Option (A):
Incorrect. Cut-offs do not influence surface flow rates; they function below the floor and impact sub-surface flow patterns.
- Option (D):
Incorrect. Hydraulic jumps are managed using stilling basins, baffle blocks, and aprons, not cut-offs.