Stability Criteria for Coarse Sediment in Natural Streams

Shields dimensionless shear stress has been widely used in the field of fluvial hydraulics for the stability of mobile bed channels. Some researchers have suggested an alternative, the Movability Number, which includes more hydrodynamic information. This study evaluates the two quantities in the determination of stability for channel beds containing coarse sediment. Data obtained in natural channels from previous studies are analyzed for their stability. Three criteria are obtained from existing formulae: (i) Shields parameter, (ii) settling velocity, and (iii) stream power. Second and third criteria are based on Movability Number. The first two are comparable whereas the third one provides smaller critical sediment size considerably as compared to the first two. The settling velocity criterion based on Movability Number can be thought as the potential replacement of the Shields approach, however further study with larger data set can lead to more breakthroughs.