MID-CHANNEL BAR GROWTH AND ITS RELATIONSHIP TO LOCAL FLOW STRENGTH AND DIRECTION

Anabranches of braided rivers typically migrate and avulse across the floodplain to produce new channel junctions, scour and subsequent mid- channel bar growth immediately downstream. Few quantitative studies ha ve been made of this bar development process and the link to change in channel geometry and local flow strength and direction. This paper pr ovides data on the spatial and temporal pattern of surface velocity as mid-channel bar growth is initiated downstream of a fixed junction sc our in a generic scale flume model. The sequence of channel changes is : (i) development of a confluence scour with flow convergence and maxi mum velocity in the channel centre; (ii) exceedance of the local trans port capacity and initial stalling of coarse sediment in the channel t halweg downstream of the scour; (iii) bar growth through entrapment of all sizes of bedload; (iv) change from velocity maximum to minimum an d flow convergence to divergence when the bar height is approximately 55 per cent of the thalweg depth; (v) broadening of the bartop platfor m, a drop in local competence and bankward migration of the two distri butaries whose cross-section and velocity remains approximately consta nt. These flume data and interpretations are compared to descriptions in the literature of the braiding process with particular reference to the flume work of Leopold and Wolman (1957) and Ashmore (1991, 1993). A new model for mid-channel bar growth is presented which helps expla in the long-term development of the confluence-diffluence unit.