Complex variations in sediment transport at three large river bifurcations during discharge waves in the river Rhine

Abstract

River bifurcations strongly control the distribution of water and sediment over a river system. A good understanding of this distribution process is crucial for river management. In this paper, an extensive data set from three large bifurcations in the Dutch Rhine is presented, containing data on bed‐load transport, suspended bed sediment transport, dune development and hydrodynamics. The data show complex variations in sediment transport during discharge waves. The objective of this paper is to examine and explain these measured variations in sediment transport. It is found that bend sorting upstream of the bifurcations leads to supply limitation, particularly in the downstream branch that originates in the outer bend of the main channel. Tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches. Lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport. All bifurcations show evidence of sediment waves, which probably are intrinsic bifurcation phenomena. The complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river. Differences among the studied river bifurcations are mainly due to differences in sediment mobility (Shields value). Because the variations in sediment transport are complex and poorly correlated with the flow discharge, prediction of the sediment distribution with existing relationships for one‐dimensional models is problematic.

Assessment of Methods Used in 1D Models for Computing Bed-Load Transport in a Large River: The Danube River in Slovakia

“…It yields severe underestimation, especially for relatively low discharges. As observed by Habersack & Laronne (2001) or Frings & Kleinhans (2008), a high spatial variability of the grain-size distributions would also induce a larger variability in bed shear stresses but also a much larger variability in bedload transport along the cross section. When using a 1D model, the application of bedload formulae on local parameters and so the distribution of the bed shear stress throughout the section appear to be necessary.…”

Lower Rhine historical flood magnitudes of the last 450years reproduced from grain-size measurements of flood deposits using End Member Modelling

“…The rate of floodplain inundation and the timing of maximum discharge during a discharge wave determine dynamics that are of importance for the transported material; e.g., flow velocities and hysteresis-effects Asselman, 1999;Benedetti, 2003;Berner et al, 2012). In-channel measurements of sediment fluxes and mobile grain-sizes (Frings and Kleinhans, 2008), and analysis of deposited material after the AD 1993 flood (Middelkoop, 1997) indicate a relatively wide range in grain sizes of transported material during a single flood, and among various floods of similar magnitudes. Considerable variability is observed in the coarse event-beds of moderately-sized floods (Fig.…”

Tidal impact on the division of river discharge over distributary channels in the Mahakam Delta

“…We attribute the minor discrepancies between model results and observations to the limited degree in which spatial variations of bed roughness are accounted for. Differences in bed roughness between distributaries may be explained by sediment sorting at the apex (Frings and Kleinhans 2008), which leads to variation in bed material and in the occurrence of bed forms. Other possible causes of discrepancies are the representation of the river banks in the model, which does not capture the geometric complexity of lateral channels, and the limited number of tidal constituents used to force the model.…”