Advanced topics in sediment transport modelling: non-alluvial beds and hyperconcentrated flows

Civil and environmental engineers frequently face sediment transport issues such as local scouring, sedimentation in reservoirs, erosion after floods or dam breaching flows as well as long term aggradation or degradation of riverbed (Dewals et al. 2010b; van Rijn 2007; White 2001). Such sediment related problems are of huge importance in most projects of river engineering, calling for structures to be designed considering sediment transport issues from the very early stages of project development. Sustainable operation rules also need to be developed, both in short and long term perspectives. As a result of the complexity of the governing physical processes and the significant uncertainties affecting input data, modeling tools with a genuine predictive capacity, such as comprehensively validated numerical models, constitute key elements to provide quantitative decision-support in project design and developments. Sediment transport has been studied from a physical point of view for almost two hundred years but is not yet fully understood (Frey and Church 2009). In particular, while the Navier-Stokes and continuity equations represent a generally accepted mathematical description of fluid flow, there is no comparable model for the complete interaction of flow, sediment transport and bed evolution (Spasojevic and Holly 2008). Therefore, sediment transport remains a challenging topic of research today, since a unified description of processes is still to be achieved. In this quest, both new experimental approaches and more advanced numerical models have a part to play; the former providing new insight into fundamental processes while the later enabling to upscale the results for real-life applications. In this chapter, we first present an original two-phase flow model for the water-sediment mixture, acting as a unified basis for all our subsequent developments. Next, we focus on two topics in which we have made original contributions, namely sediment routing on alluvial and non-alluvial beds and modelling of transient hyperconcentrated flows. In both cases, we use our original two-phase flow modelling framework to derive specific governing equations, for which we detail an appropriate finite volume numerical scheme and demonstrate their validity through a number of test cases.