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. 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.

Open Channel Flow

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 1: Contour lines of Equal velocities in Different Channel Sections

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 manning’s formula mentioned below.

Manning's formula

Where,

V = Average velocity of channel

R = hydraulic radius of channel = Area/Perimeter

S = Slope of channel

n = Manning’s roughness coefficient

Manning’s roughness coefficient values are different for different materials used to construct channels and the values for different materials are tabulated below.

Table 1:  Manning’s Roughness Coefficient (n) values for Different Types of Channel Materials

Channel Surface Material Channel Condition/DescriptionManning’s Roughness Coefficient (n)
 

 

 

 

Earth

Newly excavated0.018
After weathering0.022
Gravel0.025
No vegetation0.028
Grass  and Weeds0.030
Presence of Aquatic Plants0.035
Earth bottom and rubble sides0.030
Stoney bottom and weedy banks0.035
Cobble bottom and Clean sides0.040
Rock cutsSmooth and uniform0.035
Jagged and irregular0.040
 

 

 

Concrete

Trowel finish0.013
Float finish0.015
Finished with gravel on bottom0.017
unfinished0.017
Gunite, good section0.019
Gunite, wavy section0.022
on good excavated rock0.020
on irregular excavated rock0.027
BrickGlazed bricks0.013
Cement mortar finish0.015
Asphalt

 

Smooth0.013
Rough0.016
 

 

Wood

Planed, untreated0.012
Planed, creosoted0.012
Unplaned0.013
Plank with battens0.015
Lined with roofing paper0.014
MasonryCemented rubble masonry0.025
Dry rubble masonry0.032

Below figure represents velocity distribution curves of different shaped channels. If the channel roughness is more, the curvature increases.

Velocity Distribution Curves for Different Channel Sections

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.

Different Channel Alignments

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.

Channel Slope

Fig 5: Channel Slope