Cross-shore sand transport by infragravity waves as a function of beach steepness

Abstract

Two field data sets of near-bed velocity, pressure, and sediment concentration are analyzed to study the influence of infragravity waves on sand suspension and cross-shore transport. On the moderately sloping Sand Motor beach (≈1:35), the local ratio of infragravity wave height to sea-swell wave height is relatively small (H_IG/H_SW0. When the largest sea-swell waves are present during negative infragravity velocities (bound wave, negative correlation r₀), most sand is suspended here, and the infragravity sand flux q_IG is offshore. When r₀ is positive, the largest sea-swell waves are present during positive infragravity velocities (free wave), and q_IG is onshore directed. For both cases, the infragravity contribution to the total sand flux is, however, relatively small (IG/H_SW>0.4), most sand is suspended during negative infragravity velocities, and q_IG is offshore directed. The infragravity contribution to the total sand flux is considerably larger and reaches up to ≈60% during energetic conditions. On the whole, H_IG/H_SW is a good indicator for the infragravity-related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance.