Wave induced sediment mobility modelling: bedforms, sediment suspension and sediment transport

Dissipation due to bottom friction can be inaccurate in numerical wave modelling from the possible formations of bed-forms on the ocean floor, specifically sand ripples. An algorithm developed by Nielsen (1981) was incorporated into the SWAN wave model (Booij et al., 1999) to increase the accuracy of a model for Lake George that exhibited overestimation of the significant wave height based on observed data. Validation of the roughness coefficients estimated from the algorithm was achieved from laboratory experiments conducted by Babanin et al. (2005) that estimated the roughness coefficient associated with ripples for Lake George sediment, and the velocity where ripples began to form. The new friction subroutine generated ripples at similar orbital velocities and improved the accuracy of the wave model for the Lake George application. Following on from the ripple algorithm, the mobile sediment data was utilized within the SWAN wave model to produce an estimation of the wave driven suspended sediment at various depths based on formulae define in Soulsby and Whitehouse (1997). Comparison of the two dimensional estimation of surface concentrations by Gayer (2006) in the North Sea provided a qualitative verification. Output from the SWAN wave model over an identical spatial domain for the North Sea enabled direct qualitative comparison between the two models, showing similar magnitudes of suspended sediment concentration and patterns along the coastline. The SWAN wave model was coupled with the NLINE sediment transport model (Dabees, 2000) along with the new ripple subroutine and suspended sediment capability, embedded within a graphical user interface. Bathymetry for the SWAN model could be selected graphically, and was extracted from the GEMS bathymetry database. Conversion between the SWAN digital elevation model and the NLINE contour model was achieved by instating and adapting a contouring algorithm developed by Bourke (1987). Contour depths and coastal structures can be defined by the user within the GUI, and modelling output in the form of sediment transport driven contour changes can be viewed in the form of an animation.