Bedform morphology under combined wave-current conditions in a nearshore environment

Abstract

The shape and geometry of bedforms in the nearshore zone strongly controls the turbulence and flow field above them, and as such, they determine the vertical sand concentration profile and with that the magnitude and direction of sediment transport. Understanding the relations between forcing and resultant bathymetry is therefore of crucial importance for coastal modelling purposes. The relationship between forcing and bedforms is, however, less understood for bedforms that are formed under the combined action of waves and currents (combined flows) than for pure waves or currents. To give insight in to the effect of a dynamic nearshore environment on combined flow bedform geometry, I studied the effect of changing hydrodynamics (within and between tidal cycles) on bedform morphology. Thereby, the accuracy of existing bedform geometry predictors is tested. A one-month measuring campaign was conducted at the beach of Egmond aan Zee to measure hydrodynamics together with small-scale sea-bed bathymetry, using a three-dimensional sonar ripple profiler (3DSRPLS). The sonar-acquired bathymetry images are processed in to Digital Elevation Models (DEMs) and classified in to four different bed states: small-scale 2D ripples, small-scale 3D ripples, large-scale bedforms, and super-positioned bedforms. Results show that a θc – θw bed state stability diagram gives a good segregation of small-scale 2D ripples and large-scale bedforms, that small-scale 3D ripples are stable across a large range of the current strength, and that large-scale bedforms are formed under equal wave and current strengths. Tidal development of hydrodynamics and bedforms shows that bedform height shows a distinct post-high tide arrested development when large scale bedforms develop. This causes a decoupling of forcing and morphology, which is the reason that (a part of) the bedforms are not in equilibrium with the flow. I discuss that this out-of-equilibrium state is the main reason for the dis-ability of the equilibrium predictors to accurately predict bedform geometry. Therefore, future research on nearshore intertidal environments should specifically pay attention to hysteresis effects and its consequence for predicting geometry.