Bars in Large Rivers

Previous work in large sand-bed rivers has shown that the bed morphology is dominated by km-scale mid-channel bars together with 2D and 3D dunes. These large braid-bars control local flow and sediment routing and therefore govern channel evolution, planform stability and sedimentary architecture. Laboratory and numerical modelling work and field research in small (less than 1 km wide) sand and gravel-bed braided rivers has demonstrated that channels typically contain a train of migrating lobate, low-relief unit bars that may stall, coalesce and rework to eventually form compound bars. However, remotely sensed data suggest that unit bars may not be so common, or indeed may be absent in large rivers because even at low flow, few if any submerged unit bars can be observed. Potential reasons for barform difference in large rivers are unclear and existing theoretical work on unit bar initiation may not be relevant as these studies have been based on channels with width-depth ratios an order of magnitude lower in large river channels. One of the few sets of flow measurements available for large rivers shows that secondary flow, which may be responsible for unit bar growth, may be absent in large, low-sinuosity channels, with high width:depth ratio, and also where dunes form significant bed roughness. Moreover, theory suggests that a high suspended sediment load, as typifies many of the World’s largest rivers, may impact upon the braid-bar initiation process, because of the phase lag between shear stress and suspended sediment transport rate and possible modulation of turbulence. Whilst there are estimates of total sediment yield for big rivers, there are few distributed datasets that quantify channel-wide bedload and suspended load transport rates, especially as braid-bars evolve and emerge. Indeed, there is, quite simply, a serious lack of quantitative knowledge concerning the basic processes of bar evolution within large braided rivers.