Relative level of undersluice (scouring sluice) crest — typical depth of undersluice crest below the canal head regulator crest is generally kept about:

Introduction / Context:
In diversion headworks, undersluices (scouring sluices) are provided near the canal offtake to pass heavy silt and to control the approach flow. Their crest is intentionally kept lower than the canal head regulator crest to preferentially pass sediment-laden lower layers and to maintain a silt-free approach to the regulator.

Given Data / Assumptions:

• Standard layout with a weir/barrage, undersluices near the canal bank, and the canal head regulator.
• Undersluice crest is lower than the regulator crest by a modest margin.
• Objective: efficient sediment exclusion and pond control.

Concept / Approach:

Keeping the undersluice crest about 1.0–1.5 m below the regulator crest promotes flushing of silt-laden bottom layers while sustaining an adequate pond level for canal drawal. A typical examination value used is ~1.2 m, balancing sediment control and operational headlosses.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

Physical/CFD models of approach flow confirm improved bottom-layer withdrawal and reduced deposition near the regulator with a suitably lower undersluice crest.

Why Other Options Are Wrong:

0.20 m is too small to create effective silt withdrawal; 2.20–4.90 m are excessive for typical barrages and may waste head or complicate operation.

Common Pitfalls (misconceptions, mistakes):

Assuming “the lower the better”; over-lowering can impair pond control and gate operations; site-specific tuning is still needed.

Final Answer:

1.20 m