Canal freeboard definition: The vertical difference between the canal bank top and the design supply water level is called what?

Introduction / Context:

In canal design, providing an adequate vertical margin between the full supply water level and the bank top is essential for safety against wave action, surges, wind setup, and minor operational fluctuations. This margin has a specific name and standard recommended values depending on canal size and location.

Given Data / Assumptions:

• Earthen or lined canal section with defined full supply level (FSL).
• Bank crest elevation is fixed above FSL for safety.
• Operational and environmental variations can cause temporary rises in water surface.

Concept / Approach:

The term for the vertical difference between bank top and design supply level is freeboard. It is not the geometric cross-section element like ‘‘berm’’ (a horizontal shelf on the bank), nor a generic ‘‘height of bank.’’ Freeboard is specifically the safety margin above FSL.

Step-by-Step Solution:

Verification / Alternative check:

Manuals prescribe freeboard based on discharge class and wind fetch; increasing freeboard improves safety but raises earthwork cost.

Why Other Options Are Wrong:

• Berm: A shelf; not a vertical margin above water level.
• Height of bank: Total bank height; not the safety allowance above FSL specifically.
• Crest rise / parapet allowance: Not standard canal terms for this margin.

Common Pitfalls:

• Confusing freeboard with free surface variations due to backwater or transitions.
• Under-estimating freeboard in windy reaches leading to overtopping risk.

Final Answer:

freeboard