Author(s): Peter Rosza; Janos Jozsa
Linked Author(s):
Keywords: Bed-load sediment; Armouring process; Shape-factor; Areal sampling
Abstract: If the hydraulic conditions in an alluvial river change in such a way that the bed-load transport capacity gets close to the threshold value of the motion and remains long enough, the sediment transport gradually stops, because the amount of mobile grains gradually decreases to zero. Due to the lack of further sediment supply the final outcome of this process is a stable armoured surface. Such an armoured surface might be so stable that only a flow velocity much higher then the critical one can break it up. Investigating the armoured surface layer, one could discover certain coherence in the arrangement of the particles of different grain sizes. On a surface with such a texture the particles are very stable, preventing each other from getting into motion. A uniform material, of course, is not able to show any fractioned pattern. The wider the grainsize distribution, the more distinct is the pattern of the armoured surface. A stable, armoured gravel surface can be partially broken up by fine fractions flowing over it. The fine material transported over an armoured surface originated from non-uniform initial substrate can not penetrate inside the bed at all. In the case of initially homogeneous distribution fine sand can be found in the subsurface layers. Apart from the initial grainsize distribution the shape-factor of the gravel has also a great importance. The spherical particles can be easier washed out, whereas the disk shaped particles can remain on the surface longer. The initial grain size distribution and the shape-factor of the gravel also play a significant role in the change of the bed resistance.
Year: 1999