Flow resistance in open channels with intermediate scale roughness
Date
2007-02-22T11:02:38Z
Authors
Mashau, Mashau Samson
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Abstract
Many environmental and engineering projects require prediction of the velocity of
flow in river channels, in terms of those channel properties and flow characteristics
which induce resisting forces or an energy loss to the flow. Relationships such as the
Manning, Chézy and Darcy-Weisbach equations have been in use for a century or
more. All of them account for resistance with a single coefficient of resistance, and
the central problem is evaluation of this coefficient.
Experimental results by different researchers have shown that Manning’s n varies
strongly with the ratio of flow depth to roughness height. It is constant for values of
this ratio above about 4, but increases significantly for lower values. This suggests
that the equation is not suitable in its original form for the case of intermediate-scale
roughness. The roughness is intermediate-scale if the relative submergence ratio of
flow depth to roughness elements height lies between 1 and 4. The influence of the
roughness elements on flow resistance in this regime is caused by a combination of
both element drag and boundary shear, or friction.
The results of an experimental study with hemispherical roughness elements are
presented, showing how the roughness element size, spacing and pattern influence
flow resistance. For the range of conditions tested, Manning’s n appears to depend on
roughness element size, spacing and pattern.
Description
Student Number : 0100281N -
MSc(Eng) Research Report -
School of Civil and Environmental Engineering -
Faculty of Engineering and the Built Environment
Keywords
intermediate-scale roughness, open channels, flow resistance