Step 1: Understanding the Concept:
This question asks for the name of the design feature in an open channel that acts as a safety margin to prevent water from spilling over the top of the banks.
Step 2: Detailed Explanation:
Let's define the terms related to channel design:
(A) Free board: This is the vertical distance between the designed full supply level (the maximum water surface elevation during normal operation) and the top of the channel banks. It is intentionally included in the design as a safety margin. Its purpose is to accommodate fluctuations in the water level caused by unforeseen events like waves generated by wind, surges from gate operations, or inflow rates slightly higher than designed, thus preventing the channel from overtopping and failing. This perfectly matches the description in the question.
(B) Hydraulic slope: This is another term for the energy gradient or the slope of the water surface in a channel under uniform flow. It is a factor in determining the velocity of flow, not a safety feature against overtopping.
(C) Hydraulic radius (R): This is a geometric property of the channel cross-section, defined as the cross-sectional area of flow (A) divided by the wetted perimeter (P). $R = A/P$. It is used in flow calculation formulas like the Manning equation.
(D) Wetted perimeter (P): This is the length of the channel boundary that is in contact with the water in a given cross-section. It is used to calculate the hydraulic radius.
Step 3: Final Answer:
The safety margin provided by the extra height of the channel banks above the normal water level is called freeboard. Therefore, option (A) is the correct answer.