Analysis of the measured data of sediment nourishments in the Rhine River

Abstract

Sediment nourishments have become an attractive alternative to hard measures in counteracting long-term river bed degradation. Due to the large degree of uncertainty concerning mixed sediment morphodynamics, river authorities conduct field studies to collect data that will help in the calibration of morphological models, eventually used to design effective mitigation measures.

In the present study, we analyse measured data from two field experiments in the Rhine river, that monitored the fate of the mixed-size nourished sediment. In the first study case, we analyse data from a 5 year monitoring campaign at the reach below the most downstream weir in the Rhine river. The sieving analyses of the soil samples collected from the bed of the study reach, reveal the concentrations of various tracer granite fractions at several vertical layers, lateral positions and cross sections along the streamwise direction. Second, we studied the early development of the nourishment pilot study initiated in 2016 at Lobith, a reach situated further downstream in the Rhine river. In spite of the early stage of the nourishment, due to intensive monitoring of the pilot, data are collected in dense time intervals by means of bed surveys, radioactivity measurements for the detection of tracer sediment as well as soil samples, revealing the initial morphodynamic trends in the study reach.

In both study cases we predominantly focus on the migration of nourished sediment in all directions and the main physical processes that regulate the feedback between flow and mixed sediment morphodynamics. We conclude that nourished sediment disperses along the streamwise direction, gets mixed into larger depths as it migrates downstream, but also follows certain paths along the mitigated meandering reaches. We found that especially vertical mixing of nourished sediment is a major control on its streamwise migration. Regarding certain of these aspects, the various grain size fractions of the nourished sediment were seen to behave differently. For example ner fractions
migrated downstream faster, but were also mixed in larger depths below Iffezheim. In the present study we assess the in influence of temporal and spatial flow variability, but also the inuence of small and large scale processes which are relevant in mixed sediment morphodynamics. The feedback between
ow and mixed sediment morphodynamics is key in explaining the main physical processes controlling the fate of nourished sediment, yet more work needs to be done to assess which are the dominant small and large scale physical mechanisms when mixed-size sediment feeding in lowland rivers is considered.