Experimental Study On Scour And Transport Of Cohesive Sediment After Deposition And Consolidation

Fine cohesive sediments are small in particle size but large in specific surface area. They widely exist and gradually consolidate in the reservoir, estuarine areas. Therefore they have important impact on the riverbed evolution and governance of these areas. For instance, after a long time of consolidation, the deposits in reservoir areas will become dense and resistant against erosion, namely easily to consolidate and difficult to be scoured. The large numbers of issues of riverbed and riverbanks erosion in alluvial rivers can start with the cohesive sediment erosion problems. This can be seen from the phenomena of "river bottom tearing scour" in hyper-concentrated flood in the North Yellow River and Weihe River. In the basin’s hydropower resources development and governance process there will be many cohesive sediment movement law-related issues. The basic law of motion of cohesive sediment deserves more attention. Researches on erosion and movement laws of consolidated cohesive sediment have important theoretical and practical significance.This dissertation conducted a series of experimental studies to enhance the understanding of cohesive sediment erosion and transport. The contents include the following parts:(1) Review of the background and significance of the scour issues of cohesive sediment. On the basis of predecessors’ work, the main research contents are decided.(2) The relationship between deposition properties and erosion characteristics in different areas and flow conditions is analyzed. The impact of physical composition and deposition properties on scour of cohesive sediment is discussed.(3) By indoor experiment, the consolidation process of cohesive sediment is studied. The variation of dry density versus consolidation time is formulated. Based on flume test results, the impact of consolidation duration, sediment particle size, dry density, flow conditions on cohesive sediment erosion mechanism is investigated(4) Aggregation and flocculation of cohesive sediment are introduced and their impacts on consolidation, incipient, erosion are analyzed. Furthermore, y modified flume experiments, law of motion after the incipient of the aggregated cohesive sediment is investigated. Analysis of suspended load variation versus the flow conditions and sediment composition provide a preliminary insight of the conversion mechanism between different transport modes of cohesive sediment.(5) Disscusion on the existing problems of this research and suggestion for further work are proposed.