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GATE 2025 CE (CE2) – Question 25

Irrigation · Canals, design of weirs on permeable foundation, cross drainage structures · 1 mark · Multiple select

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.

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

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Show answer and explanation

Correct answer: (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.

Explanation

Cut-offs (sheet piles or walls) lengthen the seepage path under the apron, which lowers the exit gradient and prevents piping (B). They must go below the maximum scour level, so their depth is fixed by the scour depth (C). They do not increase the discharge (A) and they do not control the hydraulic jump (D), which is a matter of the stilling basin design.