Bed load transport and incipient motion below a large gravel bed river bend

Abstract

A new data set of bed load measurements in a cross-section at the exit of a river bend is presented. Data are analyzed to identify processes that contribute to the morphodynamic stability of gravel bed meanders. It is shown that boundary shear stress and bed material texture are strongly coupled, resulting in an almost equal mobility at incipient motion over the bend point bar in relation to channel flow stage. Conversely, for conditions above bankfull an excess of fine sediment towards the inner-bank, likely related to more intense crosswise flux and grain size sorting, results in size selective transport in relation to the local bed material. We suggest that bed armoring and structuring, as well as crosswise sediment flux, add stability to the outer-bank pool, while the point bar is eroded by large floods and restored by moderate flows. Results reveal the strong feedback of processes at different scales promoting stability at bends of gravel bed rivers.