Inner-surf and swash-zone hydrodynamics and sediment transport processes during accretive conditions and local wind forcing

Abstract

The inner surf and swash zones are highly dynamic regions of the nearshore zone, characterized by unsteady, turbulent, sediment- and bubble-laden flows. Hydrodynamic and sediment transport processes occurring in these zones control sand exchange between the surf zone and foreshore, leading to foreshore morphological variation. Increased scientific interest and technological advances have contributed to the more recent advances in understanding of inner-surf and swash-zone processes. However, there is still the need of near-bed measurements of hydrodynamic and sediment transport to identify the role that small-scale inner-surf and swash-zone processes have on the beach during storm beach recovery (accretive conditions) and mesoscale meteorological phenomena (wind forcing mechanism). Therefore, two field-based studies were conducted to acquire detailed observations to address these needs. First, a field experiment was conducted on a steep sloping beach at South Bethany Beach, Delaware, USA in an attempt to quantify the foreshore morphological change during post-storm recovery. Near-bed, highly resolved hydrodynamic and sediment concentration measurements were collected at five cross-shore locations across the foreshore. Suspended sediment transport rates were estimated using instantaneous measurements of flow velocity and suspended sediment concentrations. A spatial and temporal variation of suspended sediment transport rates across the foreshore was identified. The larger transport events resulted from flow interactions that localized suspension and advection of sediment from the point of bore collapse and deposited landward (mostly by the cross-shore component). Net sediment transport and the associated foreshore morphological change were quantified via cross-shore suspended sediment transport rate gradients and an energetics-based suspended sediment transport model. Net suspended sediment transport rate gradient estimates exceeded by two orders of magnitude the net transport quantified via bathymetric difference over each tidal cycle, highlighting the difficulty of predicting transport processes even under weak accretion conditions. This discrepancy implies that all potential mechanisms that enhance sediment transport (e.g. turbulence generated by swash bores, sediment advection) and complete measurements for the full flow duration and throughout the water column are needed to improve sediment transport rate estimates in the inner-surf and swash zone. However, these analyses served to relate the small-scale processes to large temporal and spatial scale accretive patterns. ☐ A second field study was conducted on a microtidal, low wave energy, sea breeze dominated sandy beach in Sisal, Yucatán, Mexico to investigate the effects of local (land/sea breeze) and synoptic (Norte) scale meteorological events on inner-surf and swash-zone dynamics. Flow velocities and suspended sediment concentrations were measured concurrently at three cross-shore locations. The high-resolution data allowed the quantification of bed shear stress, turbulent dissipation rate and sediment transport rates. The change in wind speed and direction induced pronounced changes to the inner-surf and swash-zone dynamics. Field observations showed that strong inner-surf and swash-zone bed shear stresses, turbulence intensity and sediment suspension occur during sea breezes and Norte. Similarities between cross-shore and alongshore hydrodynamic parameters estimated during sea breezes and the Norte indicate that during sea breezes a significant amount of sediment can be mobilized inducing foreshore morphological changes similar to or greater than the effects generated by a short duration small storm. However, despite the milder energy conditions during land breezes, when these coincide with high tides, the estimated instantaneous hydrodynamic and sediment transport parameters often had similar orders of magnitude to sea breezes. These observations suggest that land breeze, under ideal conditions, can mobilize considerable amounts of sediment across the foreshore.