Field measurements of vertical suspended sand concentration profiles in the surfzone in Egmond aan Zee, the Netherlands

Abstract

The interaction of nearshore waves and currents are the dominant forcing to cross-shore sand transport on sandy beaches. In the nearshore surf zone, suspended sediment transport attributes a significant amount of the total transport. The distribution of suspended sand concentration depends on the complex interactions between waves and currents. A good understanding of the time-averaged vertical suspended sediment concentration profile is important in sediment transport calculations. In this study, the influence of hydrodynamics and bedforms on vertical suspended sand concentrations profiles under field conditions is analyzed. Measurements of time-averaged vertical suspended sediment concentration profiles with associated hydrodynamics and bedforms were made in the surf zone near Egmond aan Zee in October 2013. The vertical suspended sediment concentration profiles were measured with 7 optical backscatter sensors placed on top of each other. The bedform configuration was measured using a 3D ripple scanner. Conflicting to models from literature, results show that the shape of the vertical suspended sand concentration in the surfzone was independent on neither hydrodynamics nor bedform type. All vertical suspended sand concentration profiles, reference concentrations and eddy diffusivities showed similar values for all types of bedforms and hydrodynamic conditions. This may be caused due to the time lag effects in bedform development. Under calm weather conditions, when environmental conditions were not dynamic enough, bedforms can not adjust instantaneously with the hydrodynamic changes. This provides higher sediment concentrations than expected. This tidal asymmetry may have major effects for modelling suspended sand transport in the surf zone.