The effect of breaking-induced turbulence on sediment entrainment in the surfzone

Abstract

In the surfzone sediment is suspended into the water column both due to turbulence generated at the bottom boundary layer (BBL) and turbulence generated at the surface boundary layer (SBL). In this research the impact of the source of turbulence under breaking waves on sediment suspension in the surfzone is investigated using the Turbulent Kinetic Energy (TKE) and the Suspended Sediment Concentration (SSC). The TKE was measured at three different heights above the bed. SSCs were measured at 3, 5, or 7 different heights above the bed depending on the cross-shore location. The Feddersen and Williams (2007) differencing method was used to derive turbulence from three dimensional velocity data. Both the TKE and SSC are related to cross-shore position. Under breaking waves the TKE increases towards the water surface, which indicates that the surface generated turbulence is dominant over turbulence generated at the sea bed. Conditions with non-breaking waves show an increase of TKE towards the sea bed, indicating the dominance of the bottom boundary layer in the generation of turbulence in the water column. Breaking-induced turbulence increases the near bed SSC and increases the vertical mixing of sediment in the water column. Despite the fact that both the TKE and SSC are related to cross-shore position and thus the degree of breaking waves, there is no clear relation between the short-wave energy dissipation and the percentage of TKE events which can be related to SSC events.