Opposite hysteresis of sand and gravel transport upstream and downstream of a bifurcation during a flood in the River Rhine, the Netherlands

Abstract

At river bifurcations water and sediment is divided among the downstream branches. Prediction of the sediment transport rate and divisionthereof at bifurcations is of utmost importance for understanding the evolution of the bifurcates for short-term management purposes and forlong-term fluvial plain development. However, measured sediment transports in rivers rarely show a uniquely determined relation with hydrodynamicparameters. Commonly a hysteresis is observed of transport rate as a function of discharge or shear stress which cannot be explained with thestandard sediment transport predictor approach. The aim of this paper is to investigate the causes of hysteresis at a bifurcation of the lowerRhine river, a meandering river with stable banks, large dunes during flood, and poorly sorted bed sediment. The hydrodynamics and bedsediment transport were measured in detail during a discharge wave with a recurrence interval larger than 10 years. Surprisingly, the hysteresisin bedload against discharge was in the opposite direction upstream and downstream of the bifurcation. The upstream clockwise hysteresis iscaused by the lagging development of dunes during the flood. The counter-clockwise hysteresis downstream of the bifurcation is caused by acombination of processes in addition to dune lagging, namely 1) formation of a scour zone upstream of the bifurcation, causing a migrating finesediment wave, and 2) vertical bed sorting of the bed sediment by dunes with avalanching lee-sides, together leading to surface-sediment finingand increased transport during and after the flood. These findings lead to challenges for future morphological models, particularly forbifurcations, which will have to deal with varying discharge, sediment sorting in the channel bed, lagging dunes and related hydraulic roughness.