Sediment size effects on self-organisation behaviour of rip channels

  • Yiqiang Chen

    Student thesis: Doctoral ThesisDoctor of Philosophy


    Rip channels are frequently observed morphological pattern on the sandy beach, which are critical to the erosion of beaches as well as the safety of beach visitors. Field observations indicate the wave conditions and sediment characteristics exert the controlling effects on the states of beaches and determine the development of rip channel system. Although the role of wave conditions on the rip channel dynamics has been extensively studied, little attention has been paid to the effects of sediment characteristics.
    In this thesis, the impacts of grain size of both uniform and non-uniform sediments on the formation and nonlinear evolution of rip channels have been investigated using a nonlinear morphodynamic model that is adapted from the open source program XBeach. To quantitatively describe the impacts of sediment grain size, a number of indicators that represent the dynamical and geometrical characteristics of rip channels have been used. Specifically, the indicators that related to the dynamical properties of rip channels include growth rate, migration speed, response time and saturation time. The indicators that related to the geometrical characteristics of rip channels include predominant spacing and rip channel three-dimensionality.

    For beaches that consist of uniform sediment, the grain size can affect number aspects of rip channel dynamics. It is found that when the grain size increases, both alongshore migration speed and growth rate decreases significantly, while the response and saturation time increases. On the other hand, the influence of grain size on the geometrical properties of the rip channel is much less pronounced, as the predominant wavelength of rip channels hardly changes and the rip channel three-dimensionality only vary slightly, depending on either the variation of grow rate or saturation time. The “global analysis” method that takes the variables over the whole modelling domain into account is then applied to investigate the mechanism underlying the variations of growth rate and migration speed. The results of calculation using “global analysis” method reveal that the variations of growth rate and migration speed are mainly caused by the amount of sediments being stirred up into the water column which is clearly grain-size-dependent.
    Previously, all modeling studies assume that the heterogeneous sediment on sandy beach can be represented by single homogeneous sediment, which implicitly suggest that the actual dynamics of rip channels for heterogeneous sediment are either similar to those within an environment comprised of uniform sediment or equivalent to the linear summation of dynamics determined for individual grains. However, our results of simulations that using heterogeneous sediment show that in some occasion this assumption is not valid, as the values of indicators that concern to the temporal evolution properties of rip channels for heterogeneous sediment can locate outside of the range that restricted by prediction using uniform sediment. This is because the gradient of sediment concentration depends on the spatial distribution of sediment size and is not always the linear summation of the gradient of sediment concentration calculated using uniform sediment grains. Therefore, our results suggest that special care for the spatial distribution of sediment size should be taken when predicting the development of rip channels.
    Date of Award2014
    Original languageEnglish
    SponsorsChina Scholarship Council
    SupervisorPing Dong (Supervisor)

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