Sediment resuspension on beaches: response to breaking waves

Abstract

Field data of time-averaged suspended sediment concentration (ssc) are presented from three different UK beaches and a variety of wave conditions. The data are collated into a single analysis to examine the mechanisms associated with sand resuspension in the surf zone. Emphasis is placed upon the relative contribution of bottom and surface generated turbulence in controlling suspended sediment concentration distribution. Inside the surf zone, vortices induced by breaking waves and bores are the main mechanism for sediment resuspension; this process dominates over bottom boundary layer processes. Time-averaged reference concentration, defined as the ssc at 3.5 cm above the bed, does not correlate with bottom-induced shear stress or predicted ripple steepness. Both reference concentration and vertical distribution of sediment are controlled by the breaking wave (plunging, spilling) characteristics. Within the inner surf zone, hydraulic jumps (associated with strong offshore flows) are responsible for sediment resuspension. Vertical distribution of sediment is parameterised using a convective type time-averaged suspended sediment concentration profile. Reference concentration is highly correlated with wave breaking type, as defined by the local breaker parameter. The mixing parameter depends also on the wave breaking type and may be expressed as a fraction of the local water depth.