Question

Explain why the velocity distribution over a channel section is not uniform.

Answer 1

In an open channel flow, velocity distribution is non-uniform which means velocity is different at different depths. Various Factors such as channel slope, alignment, shape, roughness etc plays key role in velocity distribution.

An open channel is a conduit which has free water surface exposed to the atmosphere. Rivers, canals etc come under open channel category. Because of free water surface and frictional resistance along the channel boundary velocity distribution is non-uniform in open channels.

Velocity distribution in open channels is mainly depends upon the following factors.

1. Shape of the channel section
2. Roughness of channel
3. Alignment of channel
4. Slope of Channel bed.

1. Shape of the Channel Section

Open channels may be naturally formed or artificially developed. Natural open channels do not have any particular shape and they contain irregular sections while artificial channels are provided with certain designed shapes such as rectangular, circular, trapezoidal, triangular etc.

2. Roughness of Channel

Roughness of channel is the measure of amount frictional resistance offered by channel bed material against flow of water. In natural channels, the flow velocity is affected by the presence of large angular boulders as bed material, vegetation, obstructions etc.

3. Alignment of Channel

The velocity of flow in channel also depends up on the alignment of channel. If the channel is straight there will be no change is velocity with respect to alignment. In straight channels, maximum velocity is generally occurs at 0.05 to 0.15 m depth from free water surface.if it is sinuous or meandering, the velocity will vary at bends. At bend, due to centrifugal action of flow the velocity becomes more at convex side compared to concave side.

4. Slope of Channel Bed

Slope of channel bed or gradient of channel will also effects the velocity of flow in open channels. At steeper gradients, velocity increases while at normal gradients velocity decreases.

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