Abstract

In order to assess the role of competence in the braiding process, a technique was devised to measure point instantaneous bed-shear stresses based on the principle of equating particle moments under controlled conditions. In particular, the effects of channel form and stage changes on bed-shear stress are examined. Measurements in nine different cross sections at two stage levels indicate a distinct contrast between parent channels and distributaries. High stresses were recorded in both at higher discharge, but on reduced stage, the stress in the parent channel decreased appreciably but remained moderately high in the distributaries. This spatial pattern of competence changes with stage was further demonstrated by the analysis of hydraulic exponents of discharge which implied a cross-over effect in sediment-transporting ability. Distributaries at high stage appear to become a sedimentary bottleneck relative to the parent channel, thus providing a mechanism for bar growth. Morphological developments demonstrated two superimposed forms of braiding, one consequent upon channel widening and loss of competence, and the other, inherent fluid instability of high-energy flow, producing channel wandering.

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