COI Funded Project: Investigation of Wave Energy Dissipation and Sediment Transport over a Rippled Seabed by Using Large-Eddy Simulation Driven by Field Data
Project Funded: 2006
In many coastal regions, the seabed is often covered by ripples formed by oscillatory flows due to surface waves. The ripples significantly influence bottom-boundary-layer flow structure and turbulence intensity near the seabed, and therefore have great influence on sediment transport. Also, it has been recognized that the seabed (covered by ripples) is a dominant source of dissipating kinetic energy of water waves. The enhanced dissipation of wave energy due to ripples has to be considered in most larger-scale models of the coastal environment. This is currently done by a parameterization using the Madsen eddy viscosity model with a wave friction factor determined by a single roughness scale. However, some of the assumptions underlying this parameterization may limit its applicability to more realistic field observations. For instance the roughness scale is usually assumed to be smaller than the wave boundary layer, which is often not the case. While the effectiveness of this parameterization has been tested against wave measurements in very temporally and spatially averaged sense, it has not been tested as thoroughly relative to field measurements of turbulence. It is generally agreed that an accurate prediction of turbulent flows over ripples under realistic wave plus current conditions will improve our understanding of the rather complex interactions among ripples, currents, waves, and sediment transport.
Recent developments in computational hardware and in algorithms for computational fluid dynamics have allowed for using Large-Eddy Simulation (LES) to fairly accurately simulate the turbulent flows over ripples. This provides a new opportunity for us to study these research topics in great detail by synthesizing simulation results with field observations. Here we propose to use LES driven by field data (i.e. observed wave forcing and ripple topography) to investigate wave energy dissipation over ripples for various field wave conditions. We will then compare the modeled dissipation rates to previous measurements of turbulence from a Doppler profiler. The ultimate aim of this project is to provide better parameterization of energy dissipation of non-monochromatic waves over orbital scale ripples for modeling efforts of larger-scale coastal processes. Also, a LES sediment transport model will be developed. With this model, we will examine an important question in cross-shore sediment transport, i.e. the relative roles between onshore bedload and offshore suspended load, the two observed modes of sediment transport over ripples, under skew-waves measured in field experiments. These cross-shore transport processes are responsible for delivering (or removing) sediment from the inner shelf to the nearshore where sediment can become part of the beach/sand bar system.