Infragravity-wave dynamics in shallow water: energy dissipation and role in sand suspension and transport

Abstract

Infragravity waves (20-200 s) receive their energy from sea-swell waves (2-20 s), and are thought to be important to beach erosion during storms, when they can reach up to several meters in height. Numerous studies have observed that on sandy beaches infragravity waves can lose a large part of their energy close to shore. The causes of this energy dissipation are unclear and are currently considered to be either by nonlinear energy transfers back to sea-swell frequencies, by infragravity-wave breaking, or a combination of the two. In addition, the influence of infragravity waves on the suspension of sand, and the resulting cross-shore sand transport, are not well understood. The overarching aim of this thesis is to improve the understanding of infragravity wave dynamics in the nearshore zone of sandy beaches, with a focus on their energy dissipation and role in sand suspension and transport. New field data observations indicate that the infragravity waves dissipated up to 80% of their energy close to the shoreline (water depths < 0.7 m) and depended both on the particular infragravity-wave period and the local beach slope, with less dissipation on steeper beaches, and with longer-period infragravity waves. Overall, infragravity-wave breaking seems to be the main cause for the large energy losses, as it occurs in very shallow water, and non-linear energy transfers back to sea-swell waves were not observed. Energy transfers involving infragravity frequencies were investigated in more detail with a high-resolution lab dataset of wave fields propagating over a gently sloping beach. During infragravity-wave energy dissipation, infragravity-infragravity interactions dominated, and forced strongly asymmetrically shaped infragravity waves, leading to their breaking close to shore. The numerical model SWASH was subsequently used to study the various effects of beach steepness, profile shape and offshore wave conditions on the type of nonlinear energy transfer during infragravity-wave dissipation, and in particular beach steepness was seen to affect infragravity-wave energy loss. On gentle slopes, where infragravity-wave energy dominates the water motion close to shore, infragravity-infragravity interactions dominated and caused large infragravity energy losses. On the contrary, during infragravity-wave energy dissipation on steeper slopes, sea-swell energy dominated the water motion everywhere, and infragravity waves interacted with the sea-swell wave spectral peak, and relatively little infragravity-wave dissipation occurred. To study the effect of beach slope on cross-shore sand suspension and transport by infragravity waves, two field data sets were analyzed, obtained at beaches contrasting in bed slope. On the gently sloping beach the ratio of infragravity- to sea-swell wave height (H­IG/HSW) is typically larger than 0.4, and sand is suspended under offshore directed infragravity-wave velocities. The resulting offshore infragravity transport contributes up to 60% of the total cross-shore transport. On the steeper sloping beach, H­IG/HSW is typically lower than 0.4 and sand is suspended on the infragravity timescale by sea-swell waves. The correlation between the sea-swell wave group and infragravity orbital velocities (r0) then determines whether infragravity-wave sand transport is offshore or onshore directed. During these conditions, the infragravity-wave component contributes for less than 20% to the total cross-shore transport.