Numerical modelling of the sorting and transport of non-uniform sediments in the swash zone
Pedrozo-Acuña, Diego (2013) Numerical modelling of the sorting and transport of non-uniform sediments in the swash zone. PhD thesis, University of Nottingham.
The sorting and transport of different sediment fractions and the resulting beachface evolution over the swash zone of a beach, is numerically investigated within the framework of the shallow water theory and the active layer theory. The shallow water and Exner equations, along with the volume fraction (sorting) equation given by the active layer theory compose the system which is numerically solved using an uncoupled approach, i.e., the model assumes that changes in bed level (and volume fraction) do not have an effect on the flow. Two different numerical methods are applied to solve the system depending on the type of flow tested (constant current or swash flow); a classic Finite Differences Method and a hybrid Finite Difference-Method of Characteristics (FD-MOC) are then used respectively. The numerical model is first tested for the case of a sand dune composed of two different sediment fractions subjected to a constant current. Comparison between simulations from the model and results given by Hudson (2001) solution (which only considers one sediment fraction) showed that the composition of the bed is crucial for the subsequent bed evolution. This case served to verify that the equations were solved correctly and some interesting features of the kinematics of the system were observed. The model is then applied to the case in which a single Shen and Meyer (1963b) swash event acts over a plane sloping beach composed of two different sediment fractions in the same proportion. Different values of the A (sediment mobility parameter) are investigated taking as a base point the value calculated by Kelly (2009). The results show that the behaviour in time of Pfa (fine volume fraction in the active layer) directly depends on the difference between sediment mobility parameters (Af and Ac) for the fine and coarse fractions; and this in turn results in an effect on the beachface evolution. The finer the mixture of sediments involved the bigger the bed change. Similar behaviour is obtained when the model is tested for the case in which a non-breaking wave swash is acting over the beach profile, although some interesting differences are noted mainly due to the different driving hydrodynamics.Finally, realistic A parameters for the fine and coarse fraction are defined linking grain diameters that can be found on real beaches to the sediment parameters used in the model.Numerical tests for both type of swash flows (breaking and non-breaking wave) are implemented, in which different initial Pfa distributions are considered. The results from these simulations confirmed the crucial role played by the initial distribution of sediments on the beach evolution; it was observed that a kink in the bed (a sort of swash bar/trough) formed around the middle part of the swash zone for the cases in which the initial distribution of sediments showed a maximum or minimum in that area. Comparing the results given by the model when two sediment fractions are considered, with those when a mean value (A¯ = (Af+Ac) / 2 )which is a common practice in coastal engineering models) showed that the inclusion of the two sediment fractions is crucial in order to get better predictions.
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