Open Channel Flow MCQ with answer and solution

Concept

Velocity Distribution in Open Channel Flow

Formula

No mathematical formula exists for this conceptual observation, as it describes empirical velocity distribution influenced by boundary shear stress and air resistance.

Step-by-step solution

1. Recognize that friction along the channel bed and side walls slows down the flow near the boundaries. 2. Note that air resistance and secondary circulation currents slightly reduce the flow velocity at the very top of the free water surface. 3. Conclude that the maximum velocity occurs just below the water surface (typically at a distance of 0.05 to 0.25 of the total depth from the surface).

Explanation

In an open channel, friction along the channel bed and walls slows down the water near the boundaries. Additionally, air resistance and secondary flow currents near the top water surface slightly reduce the velocity at the very top. As a result, the maximum flow velocity occurs just below the free water surface.

Shortcut

Use the mnemonic "Not at the Top, Not at the Bottom": Friction at the bed slows the bottom, and air resistance/secondary currents slow the absolute top, placing maximum velocity just below the surface (5% to 25% depth).

Common mistake

Many students mistakenly choose Option D believing maximum velocity occurs at the very top surface due to minimal bed shear, ignoring the effects of surface air resistance and secondary currents that push the maximum velocity slightly below the free surface.