Andrew Nicholas

University of Exeter, GB

Andrew Nicholas earned a Ph.D. at the University of Exeter, UK in 1994. He specializes in numerical modelling of river and floodplain processes over time-scales ranging from individual floods up to millennia. Methodologically, his interests includes Computational Fluid Dynamics, 2D hydrodynamic and morphodynamic modelling using approaches based on the shallow water equations, and “reduced-complexity” approaches based on simplified abstractions of the governing physics for flow and sediment transport. He has applied these approaches to examine the controls on overbank sedimentation on lowland floodplains, long-term floodplain evolution, the construction of alluvial fans, the dynamics of river morphology across a range of scales (e.g., including bedforms, bars, and channel belts), and the relationships between geomorphic processes and preserved alluvial deposits. He has published >60 journal articles and book chapters on these topics.

Challenges in modelling of river and floodplain evolution

Recent years have seen significant advances in the development and application of morphodynamic models, based on the shallow water equations, to simulate river and floodplain evolution. Despite this progress, significant challenges remain to be overcome if such models are to provide realistic simulations of fluvial system responses to environmental change. This situation reflects a wide range of factors, not least the large number of environmental processes that influence river behaviour, and the fact that these processes operate across a multitude of spatial and temporal scales, many of which cannot be resolved in such models. Here I will discuss some of these challenges by examining the application of morphodynamic models to two contrasting problems. The first application involves the simulation of large sand-bed rivers, which are typically characterized by multiple scales of bed morphology, the finest of which must often be parameterized rather than represented explicitly. High resolution aerial imagery and Digital Elevation Models quantifying dune, bar and channel dynamics in the sandy braided South Saskatchewan River, Canada, will be presented. These data will be used to examine the degree to which depth-averaged morphodynamic models are capable of simulating sand-bed river evolution and responses to high flow events, and to discuss the limitations of existing model process parameterisations. The second application involves the simulation of lowland floodplain construction over periods of centuries to millennia. Hydrodynamic models are regularly used to simulate floodplain inundation and overbank sedimentation during individual floods. However, most existing models of long-term floodplain construction and alluvial architecture do not represent flood hydraulics explicitly, but instead parameterize the effects of hydraulics on processes such as sediment transport and deposition, and river avulsion. Results will be presented from a hydrodynamically-driven model of long-term floodplain evolution, which will be used to explore the controls on channel belt morphology, alluvial ridge construction and avulsion likelihood.

 

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