Effect of unsteady flows on the development and magnitude of bed forms

Sediment transport and associated morphological changes in alluvial rivers occur primarily under
unsteady flow conditions that are manifested as well-defined flood hydrograph events. At present,
typical bed forms generated by such unsteady flows is far less studied and, thus, more poorly
understood, than equivalent bed forms generated under steady flow conditions. In view of this,
the objective of this work is to investigate the development of morphological bed features, and
specifically variability in the length, height and steepness of bed forms that develop in a mobile
coarse-sand bed layer under unsteady flow hydrographs under zero sediment feed conditions. A
series of laboratory flume experiments is conducted within which different flow hydrograph
events are simulated physically by controlling their shape, unsteadiness and magnitude.
Experimental results indicate that different categories of bed forms such as dunes, alternate bars
or transitional dune-bar structures develop within the erodible bed layer when subject to varying
hydrograph flow conditions. Examination of relative importance of three parameters used to
describe the hydrograph characteristics (i.e. asymmetry, unsteadiness and total water work) on
bed form dimensional descriptors (i.e. wavelength, height and steepness) reveals that hydrograph
unsteadiness and total water work are the primary and second-order controls on bed
deformations or corresponding bed form dimensions. By contrast, hydrograph asymmetry
appears to have minimal or negligible influence on bed form development in terms of their type
and magnitude. Based on these findings, a physical model was developed and tested to describe
the effect of unsteady flow hydrographs with varying unsteadiness and total water work on the
nature and size of resulting bed forms that are generated in sand-bed layers.