Experimental analysis of braided channel pattern response to increased discharge

Physical models of gravel braided rivers were used to investigate the adjustment of braiding intensity to step changes in channel-forming discharge and the mechanisms by which channel pattern adjustment and maintenance occurs. A braided channel developed at low discharge was subjected to two step increases in discharge between which the channel was given time to develop stable average braiding intensity in response to each steady discharge. Active (with visible bed material movement) and total channel networks were mapped throughout the experiment. Total braiding intensity exceeded active braiding intensity and both adjusted to a stable, average value at each discharge, indicating that channel pattern adjustment to total discharge involves both the active and the total network. Only portions of the total braided channel network developed at a given time, and it formed progressively by migration and avulsion of the (less extensive) active network. At equilibrium, the ratio of active to total braiding intensity stabilized at about 0.4. This stable value may increase with relative mobility of the bed material (stream power relative to grain size). The stable value was achieved via gradual increase of total braiding intensity while active braiding intensity adjusted very quickly to the increased flow. These adjustments are controlled by partial avulsion of the main active channel associated with changes in its sinuosity, and allocation of flow and bed load to secondary anabranches. Braided channel pattern dynamics is closely tied to, and explained by, the local dynamics and symmetry/asymmetry of bifurcations and avulsions.