Laboratory modelling of gravel braided stream morphology

Abstract Hydraulic modelling principles, together with a knowledge of channel pattern thresholds, allow the development of a small scale model of a gravel braided stream with flow characteristics and equivalent dimensions of a natural river. The forms and processes of natural gravel braided rivers are reproduced by imposing a constant flume discharge and slope, and maintaining approximate equilibrium with an adjustable sediment feed. Beginning from a straight trough, braiding is initiated by development of a series of alternating bars and scour pools which produce bends of increasing amplitude, leading finally to channel division. These lobate bars accrete downstream by deposition of bed material at their margins, often in the form of avalanche faces. Together with the scour pools with which they are necessarily closely associated, these bars are the fundamental elements of the channel pattern. Channel migration and division is a response to the development of bars, and these adjustments leave portions of the originally active bars in the form of exposed and eroded remnants. Complex flats built from these lobate forms show varying degrees of preservation of the original depositional units, but the model allows observation of the systematic construction of some flats. Sorting of sediment on active bars with avalanche faces shows a distinct fining downstream. This may be the result of the accretion of fining upwards avalanche faces along the bar margins rather than a ‘winnowing out’ of fine material. The processes and forms observed in the model appear to be very similar to those occurring in natural gravel braided streams during peak flows.