Influence of surface waves on sand wave migration ans asymmetry

IJzer, Steven (2010) Influence of surface waves on sand wave migration ans asymmetry.

Abstract:The seabed of shallow shelf seas is rarely flat. When sediment is in good supply and tidal flows are sufficiently strong, wavelike subaqueous sediment structures, called sand waves, may occur. As they may migrate up to several tens of meters each year and as their height may reach up to 1/3 of the local water depth, sand waves may severely influence human offshore activities, such as navigation. This makes it important to understand the physical processes that shape and change sand waves, in order to improve management strategies of expensive operations such as e.g. the dredging of shipping lanes. Sand wave migration was until now explained as the result of interaction between a steady Z0 current with the oscillatory M2 tide, see e.g. Németh (2003) or as the result of interaction between higher harmonics of the tides with sufficient phase lags, e.g. M4 with M2. See Besio et al. (2004). In field data, temporal variation in the direction of migration and the asymmetry of sand waves were found to be related to none tidal residual currents, such as a Monsoon-driven current found in the Adolphus Channel in Australia by Harris (1989). Surface wave action may also have an influence as Van Dijk & Kleinhans (2005) found that the dynamics of sand waves may depend on a balance between the relative influence of surface waves and currents. We found in a pre and post storm study that sand waves were lowered and that the asymmetry was changed. In this study, a morphodynamic model and empirical wave and current data were used to investigate the relative importance of the waves with respect to the effect of the currents. It was found that an increasing angle of waves with respect to the residual current increasingly limits sand wave migration and increases the horizontal asymmetry. The effect increases for increasing height of surface waves.
Item Type:Essay (Master)
UK Hydrographic Office
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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