Channel Morphology and Bed Load Pulses in Braided, Gravel-Bed Streams

ABSTRACT

Small-scale hydraulic laboratory models based on Froude modelling criteria were used to investigate: i) the influence of gradient and discharge on the average cross-sectional geometry and channel pattern of braided streams; ii) the temporal variability of braided stream morphology at constant gradient and discharge; iii) the relationship between this temporal variability in morphology, the occurrence of bed-load pulses and the nature of bed-load transfer processes in gravel-bed braided streams.

Repeated transect measurements of braided channels, formed at several different combinations of slope and discharge, show that, despite large temporal and spatial fluctuations, mean active channel width and depth show an orderly relationship equivalent to the classical downstream hydraulic geometry. Similarly, for a given particle-size distribution, braiding intensity increases with increasing discharge and total stream power, as predicted from theory.

Temporal fluctuations in channel form parameters correlate with periodic pulses in bed load over a time scale of hours in the models (probably equivalent to days or weeks in prototype streams). The pulses are generated internally by local degradation and aggradation. Increasing bed-load input to a stream reach causes local aggradation, increased braiding intensity and an increase in the number of migrating, avalanche-face, unit bars. These groups of migrating bars are the primary means of downstream transfer of bed-load pulses. Transfer tends to occur as a downstream progression from one aggradational area (complex bar) to another, through an intervening confluence zone. This provides the framework of longer-term braided stream mechanics within which detailed, reach-scale investigations of short-term channel changes and bar development can be conducted.