Falls in canals: A fall (drop) in the canal bed is generally provided under which ground and design slope condition?

Introduction / Context:
A fall in a canal is a structure that lowers the canal bed elevation to dissipate energy and maintain the designed water surface profile. It is essential where natural terrain is steeper than the hydraulic gradient desired for stable, silt-free flow.

Given Data / Assumptions:

• Canal of prismatic section designed for a mild bed slope.
• Ground slope may vary along the alignment.
• Objective: maintain designed depth and velocity while preventing erosion.

Concept / Approach:
If the natural ground falls faster than the designed bed slope, the canal bed would otherwise follow the ground and water surface would accelerate, increasing velocity and erosion risk. Providing a fall periodically “steps down” the bed to keep the overall hydraulic gradient at the design value.

Step-by-Step Solution:
1) Compare ground slope (S_g) and designed bed slope (S_d).2) If S_g > S_d, water would gain head and accelerate.3) Introduce a fall to drop bed level and regain target depth/velocity.4) Therefore, falls are justified when S_g exceeds S_d.

Verification / Alternative check:
Canal profiles typically show bed grade lines with intermittent vertical drops (falls) in reaches where the natural surface profile declines too rapidly. This matches field practice and classic designs (glacis, ogee, or broad-crested falls).

Why Other Options Are Wrong:

• Designed bed slope exceeds ground slope: would require cuts or siphons, not falls.
• Slopes the same: no need for a fall if hydraulic grade is achievable.
• Adverse slope (uphill): calls for lifts or pumping, not falls.
• None of these: incorrect because a clear condition is identified.

Common Pitfalls:

• Confusing energy dissipation structures (falls) with cross-regulators or escapes.
• Misjudging the spacing of falls; should be based on longitudinal profile and hydraulic checks.

Final Answer:
When the ground slope exceeds the designed bed slope.