• Erosion of cohesive sediments.
  
  • Introduction:
    Risk management of contamination of water conduces to the implementation of models to predict the time and spatial evolution of the concentration of contaminants. In a previous section, it has been seen that contaminants can accumulate in banks and upstream of hydraulic works because they fix on cohesive sediments.
    More generally, fluxes of sediments rise problems of geomorphology: they constitute the river bed itself and lead to the generation of litoral environments or their erosion (cf. activities of the European group Eurosion). Erosion also presents a risk for hydraulic work security: dike overflow (during the Katrina hurricane in US in 2005), rock dam failure (see the article).
    Usually, prediction tools relay on mechanical modellings. During the erosion under a stream flow, there is a coupling between the relative movements of the water and of the sediment which much be described simply. The point is: what is the sediment flux during erosion?
    
    To this end, let us have a closer look on the phenomenon.
  
  
  • Laboratory artificial erosion: HERODE flume
    
    
    This facility is dedicated to study sediment erosion. Sampling campaigns were lead to core sediment in field. Back to the lab, erosion fluxes were measured and interpreted by the Partheniades law.
    It was found that the measured parameters were strongly depending on the consolidation state of the sediment. As experiments are expensive and not really operational, the development of a theory was of high interest.
  
  
  • Cohesion modelling
    Cohesion in sediments mainly comes from the presence of clay particles (and also from the organic matters): mud for instance behaves more like a paste rather than a powder. Among all the interactions that can be source of cohesion, the van der Waals force plays an main role in our experimental conditions.
    Elsewhere, particle present a heterogeneous grain size distribution. The number of interactions is thus multiplied by the number of coordination between a given particle and those surrounding, that was found to be a function of the compaction of the lattice.
    Interparticle distance and coordination being unknown, a simplified modelling was developed to link them to operational parameters: the porosity and the grain size distribution.
  
  
  • Erosion modelling:
    Considering a deposited particle at the interface between a turbulent water flow and the sediment bed, the force balance gave an erosion criterion parameterized by the grain size distribution (known as the famous Shields diagram) and the porosity of the sediment (the new contribution of this work which has been published, cf. Ternat et al, 2008).
    
    
  
  
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© 2009 Fabien Ternat