Falls in canals: which fall tends to maintain its depth (self-regulating water depth across the crest and downstream conditions)?

Introduction / Context:
Falls are energy-dissipating structures on canals to negotiate ground slope while maintaining required water levels. Some fall designs are self-regulating and help maintain depth by adjusting the effective waterway as discharge changes.

Given Data / Assumptions:

• Comparing common fall types: trapezoidal notch fall (Sarda), rectangular notch fall, and low weir fall.
• Focus on depth-maintaining (self-regulating) behavior for varying discharges.

Concept / Approach:
The Sarda (trapezoidal notch) fall uses side slopes and a notched profile so that as discharge varies, the water surface and nappe geometry adjust, helping maintain near-constant depth upstream and ensuring stable downstream energy dissipation. Rectangular notches and simple low weir falls are less self-adjusting with discharge fluctuations.

Step-by-Step Solution:
Identify the fall which varies its effective waterway automatically with discharge.Trapezoidal notch geometry provides changing area–depth relation.Hence it better maintains depth compared to rectangular notches and simple low weirs.

Verification / Alternative check:
Operational experience shows Sarda falls give more stable depth control for distributaries and minors subject to moderate discharge variations, reducing tendency for afflux or drawdown extremes.

Why Other Options Are Wrong:
Low weir fall: Less self-regulating; depth is more sensitive to discharge changes.Rectangular notch fall: Cross-section does not adapt as effectively, giving poorer depth maintenance.All the above: Only the trapezoidal notch fall has the desired self-regulating feature.

Common Pitfalls:

• Assuming any fall with a notch is equally self-regulating.
• Confusing “measuring” falls with “depth-maintaining” falls.

Final Answer:
Trapezoidal notch fall (Sarda type)