University of Twente Student Theses


Modelling river dune splitting

Lansink, J. (2007) Modelling river dune splitting.

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Abstract:River dunes are submerged bed forms on the bottom of alluvial channels. They form as a result of the complex interaction of water flow and sediment. River dunes influence the water level, by creating additional bed roughness as a result of form drag and can form a threat to shipping activities. Therefore they deserve the attention of river basin managers. The relevance of physical modelling in contrast to the use of equilibrium predictors or empirical models lies in the applicability in extreme events. Equilibrium predictors do not consider the time component of dune development. Hysteresis for dune height is observed to be strong during a flood wave, which makes the use of equilibrium predictors inaccurate. Empirical modelling depends on calibration with data. The availability of data for extreme events is limited or absent, so the need for physical modelling is strong. The Dune Development (DuDe) model is developed to describe river dune evolution by combining two-dimensional vertical (2DV) flow equations with a sediment transport formula using a parameterization of flow separation to avoid complex turbulence modeling inside the flow separation zone. Dune growth, migration and merging of river dunes is described well by the DuDe model. Model simulations have shown that the model did not predict equilibrium dimensions. Dunes did not reach a finite length and height. Therefore, flume experiments have been conducted in Auckland (New Zealand) to observe the process of dune development, especially regarding dune splitting. Dune splitting in the form of initiation of superposed sand wavelets is seen as the mechanism to obtain equilibrium dune dimensions. Superposed sand wavelets develop a flow separation zone and can decrease the migration rate of underlying dunes and even cease migration when they scour the crest of the underlying dune. Therefore the implementation of superposed sand wavelets is seen as the solution to improve the prediction of equilibrium dune dimensions.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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