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

摘要

The general aim of this PhD-project was to gain better understanding ofudthe sediment transport and depositional processes of sand-graveludmixtures in rivers with subaqueous dunes. The understanding of theudfundamental processes of sediment transport and deposition in channeludbeds is crucial for morphological models. Sand-gravel bed rivers have audmixture of sand and gravel in their beds, and have dunes while theudcoarsest sediment is near incipient motion. In this thesis, the sedimentudtransport, sorting and deposition processes are studied with fieldudmeasurements and laboratory experiments.udExisting bedform stability diagrams were shown to be valid for bedformsudobserved in sand- gravel bed rivers and experiments, but new bedformudtypes were identified as well: sand ribbons, barchans and bedloadudsheets. When the bed surface is armoured, barchans and sand ribbons areuddependend on the sediment supply from upstream. This supply is often notudpredictable from the local hydraulics and sediment characteristics.udA predictor for bedload transport of sediment mixtures was developed byudextending existing deterministic bedload transport predictors toudnon-uniform sediment, based on flume experiments reported herein. Theudnear-bed turbulence is modelled stochastically to obtain realisticudbedload transport rates at incipient motion. The difference in mobilityudof small and large grains is represented by hiding-exposure functions.udThe transport predictor was tested on data from the river Rhine, theudNetherlands during a discharge wave in 1998. For the measurements, a newudmeasurement strategy was developed during a discharge wave in 1997,udleading to an uncertainty in transport rates of less than 20%. Strongudtime-lag effects were observed in the field that could not be hindcastudby the predictor. Part of the observed hysteresis can be explained byudvertical sorting of bedload sediment by the dunes, combined with theudtime lag between dune height development and the changing flow. A recordudof vertically sorted sediment is left in the channel bed after auddischarge wave, which is the antecedent sorting for the next dischargeudwave, often called history effect'. The entrainment and depositionuddepth of the sediment depends on the dune trough level below the averageudbed level and therefore on the dune height.udThe vertical sorting is in general fining upward by two processes: byudgrain flows at the lee side of the dunes, and by grain size-selectiveudentrainment and deposition in the dune troughs (gravel lag deposit).udBoth processes are studied in detail in additional experiments in bothudequilibrium and non-equilibrium conditions (discharge events). Theudimplications of these results for modelling are discussed.