Sorting out sand and gravel : sediment transport and deposition in sand-gravel bed rivers

Abstract

The general aim of this PhD-project was to gain better understanding of the sediment transport and depositional processes of sand-gravel mixtures in rivers with subaqueous dunes. The understanding of the fundamental processes of sediment transport and deposition in channel beds is crucial for morphological models. Sand-gravel bed rivers have a mixture of sand and gravel in their beds, and have dunes while the coarsest sediment is near incipient motion. In this thesis, the sediment transport, sorting and deposition processes are studied with field measurements and laboratory experiments. Existing bedform stability diagrams were shown to be valid for bedforms observed in sand- gravel bed rivers and experiments, but new bedform types were identified as well: sand ribbons, barchans and bedload sheets. When the bed surface is armoured, barchans and sand ribbons are dependend on the sediment supply from upstream. This supply is often not predictable from the local hydraulics and sediment characteristics. A predictor for bedload transport of sediment mixtures was developed by extending existing deterministic bedload transport predictors to non-uniform sediment, based on flume experiments reported herein. The near-bed turbulence is modelled stochastically to obtain realistic bedload transport rates at incipient motion. The difference in mobility of small and large grains is represented by hiding-exposure functions. The transport predictor was tested on data from the river Rhine, the Netherlands during a discharge wave in 1998. For the measurements, a new measurement strategy was developed during a discharge wave in 1997, leading to an uncertainty in transport rates of less than 20%. Strong time-lag effects were observed in the field that could not be hindcast by the predictor. Part of the observed hysteresis can be explained by vertical sorting of bedload sediment by the dunes, combined with the time lag between dune height development and the changing flow. A record of vertically sorted sediment is left in the channel bed after a discharge wave, which is the antecedent sorting for the next discharge wave, often called history effect'. The entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height. The vertical sorting is in general fining upward by two processes: by grain flows at the lee side of the dunes, and by grain size-selective entrainment and deposition in the dune troughs (gravel lag deposit). Both processes are studied in detail in additional experiments in both equilibrium and non-equilibrium conditions (discharge events). The implications of these results for modelling are discussed.