Surface particle sizes on armoured gravel streambeds: effects of supply and hydraulics

摘要

Most gravel-bed streams exhibit a surface armour in which the median grain size of the surface particles is coarser than that of the subsurface particles. This armour has been interpreted to result when the supply of sediment is less thanthe ability of the stream to move sediment. While there may be certain sizes in the bed for which the supply is less than the ability of the stream to transport these sizes, for other sizes of particles the supply may match or even exceed the ability of the channel to transport these particles. These sizes of particles are called 'supply-limited' and 'hydraulically limited' in their transport, respectively, and can be differentiated in dimensionless sediment transport rating curves by size fractions. The supply- and hydraulically limited sizes can be distinguished also by comparing the size of particles of the surface and subsurface. Those sizes that are supply-limited are winnowed from the bed and are under-represented in the surface layer. Progressive truncation of the surface and subsurface size distributions from the fine end and recalculation until the size distributions are similar (collapse), establishes the break between supply- and hydraulically limited sizes. At sites along 12 streams in Idaho ranging in drainage area from about 100 to 4900 km~2, sediment transport rating curves by size class and surface and subsurface size distribuions were examined. The break between sizes that were supply- and hydraulically limited as determined by examination of the transport rate and surface and subsurface size distributions was similar. The collapse size as described by its percentile in the cumulative size distribution averaged D_(36) of the surface and D_(73) of the subsurface. The discharge at which the collapse size began to move averaged 88 per cent of bankfull discharge. The collapse size decreased as bed load yield increased and increased with the degree of selective transport.