# Factors Affecting Velocity Distribution in Open Channels

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.

## Velocity Distribution in Open Channels

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. To measure velocity of open channel at required depth, Pitot tube or current meter are used.Â In general, to find average velocity of a particular open channel, velocity at a depth of 0.6 m from free water surface is measured. In the other case, velocity at 0.2 m depth, 0.8 m depth from free water surface is taken and average velocity of these two values is considered as channel average velocity.

Fig 1: Open Channel Flow

## Factors Affecting Velocity Distribution 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. Contour lines of equal velocities in different shapes of channel sections are shown in below figure.

Fig 2: Contour lines of Equal velocities in Different Channel Sections

### 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. If the channel is made of smooth clay or silt, its roughness is very low and water flows faster. In case of artificial channels, smooth finishing is required to maintain required flow velocity. The average velocity in open channels can be calculated using manningâ€™s formula mentioned below. Where, V = Average velocity of channel R = hydraulic radius of channel = Area/Perimeter S = Slope of channel n = Manningâ€™s roughness coefficient Manningâ€™s roughness coefficient values are different for different materials used to construct channels and the values for different materials are tabulated below. Table 1: Â Manning's Roughness Coefficient (n) values for Different Types of Channel Materials
 Channel Surface Material Channel Condition/Description Manningâ€™s Roughness Coefficient (n) Â  Â  Â  Â  Earth Newly excavated 0.018 After weathering 0.022 Gravel 0.025 No vegetation 0.028 GrassÂ  and Weeds 0.030 Presence of Aquatic Plants 0.035 Earth bottom and rubble sides 0.030 Stoney bottom and weedy banks 0.035 Cobble bottom and Clean sides 0.040 Rock cuts Smooth and uniform 0.035 Jagged and irregular 0.040 Â  Â  Â  Concrete Trowel finish 0.013 Float finish 0.015 Finished with gravel on bottom 0.017 unfinished 0.017 Gunite, good section 0.019 Gunite, wavy section 0.022 on good excavated rock 0.020 on irregular excavated rock 0.027 Brick Glazed bricks 0.013 Cement mortar finish 0.015 Asphalt Â Smooth 0.013 Rough 0.016 Â  Â  Wood Planed, untreated 0.012 Planed, creosoted 0.012 Unplaned 0.013 Plank with battens 0.015 Lined with roofing paper 0.014 Masonry Cemented rubble masonry 0.025 Dry rubble masonry 0.032
Below figure represents velocity distribution curves of different shaped channels. If the channel roughness is more, the curvature increases.

Fig 3: Velocity Distribution Curves for Different Channel Sections

### 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.

Fig 4: Different Channel Alignments

### 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.

Fig 5: Channel Slope