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 (HIG/HSW<0.4), and sand fluxes are related to the correlation of the infragravity‐wave orbital motion with the sea‐swell wave envelope, r0. 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 qIG is offshore. When r0 is positive, the largest sea‐swell waves are present during positive infragravity velocities (free wave), and qIG is onshore directed. For both cases, the infragravity contribution to the total sand flux is, however, relatively small (<20%). In the inner surf zone of the gently (≈1:80) sloping Ameland beach, the infragravity waves are relatively large (HIG/HSW>0.4), most sand is suspended during negative infragravity velocities, and qIG 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, HIG/HSW is a good indicator for the infragravity‐related sand suspension mechanism and the resulting infragravity sand transport direction and relative importance.

Transfer and dissipation of energy during wave group propagation on a gentle beach slope

“…[] and de Bakker et al . [], they attributed the lf wave breaking to the growth in asymmetry of the lf wave due to self‐self interaction ( ). However, the results presented in this paper suggest that for a decreasing number n of short waves forming the group, the component also becomes a subharmonic of the coupling between primary wave and the wave group components.…”

“…The analysis presented in this paper is therefore consistent with previous studies [Van Dongeren et al, 2007;de Bakker et al, 2015de Bakker et al, , 2016aBattjes et al, 2004] suggesting lf wave breaking as the mechanism behind the sharp reduction in ILW amplitude. In Van Dongeren et al [2007] and de Bakker et al [2016aBakker et al [ ,2016b, they attributed the lf wave breaking to the growth in asymmetry of the lf wave due to self-self interaction ( f g ; f g È É ! 2f g ).…”

“…The groupiness of high frequency (hf) waves, short gravity waves of period O(10 s), and the presence of associated low frequency (lf) motions, O(100 s), are characteristic features of random wave conditions propagating to the coastline. These wave features can have important implications in terms of sediment transport and beach morphology [ Aagaard and Greenwood , ; Baldock et al ., ; Alsina et al ., ; de Bakker et al ., ] or in the operability and safety of coastal infrastructures such as harbors [ Bowers , ].…”

“…For some lf components and beach slope combinations resulting in small values of b, the nearshore dissipation of lf waves is attributed to long wave breaking [Battjes et al, 2004;Van Dongeren et al, 2007;de Bakker et al, 2014de Bakker et al, , 2016ade Bakker et al, , 2016b suggesting lf energy saturation at the shoreline. Laboratory data [Van Dongeren et al, 2007] and numerical simulations [de Bakker et al, 2016a] have confirmed that with decreasing depth on low-sloping beach conditions the lf wave self-self interactions may dominate energy transfer causing the lf wave front to steepen and eventually break.…”

Sand Suspension and Transport During Inundation of a Dutch Barrier Island

“…The STMs were post‐calibrated in a recirculation tank with sand samples collected from the bed at the frame location. Background noise in concentration time series was removed as the fifth percentile of each 15‐min block (Aagaard & Greenwood, ; Brinkkemper et al, ; De Bakker et al, ), and resulting values <0 kg/m 3 were set to zero. Substantially higher concentrations at sensors closer to the surface than near the bed suggest the presence of air bubbles.…”