Large perturbations in the coastline, such as the 'Sand Motor' nourishment (∼21 million m³) at the Holland coast, can initiate considerable spatial and temporal changes in the median grain size (D₅₀) of the sea bed on the lower shoreface. The relevance of hydrodynamic conditions for the development of the heterogeneity in D₅₀ at large-scale nourishments was assessed with a numerical model (Delft3D), which required a validation against 2.5 years of D₅₀ measurements. A good representation of the observed spatial pattern of D₅₀ was obtained independent of a 2DH or 3D approach and initial condition for the D₅₀ of the bed. Five sediment size fractions and a multi-layer administration of the bed composition were used. The extent and magnitude of the coarsening of the bed is related to the velocity of the horizontal tide, while a far less pronounced coarsening takes place during energetic conditions (i.e. H_m0≥ 3 m). Differential suspension behaviour between the size fractions, which are all mobilized at the bed, causes a preferential transport of fine sediment (in alongshore direction) away from the Sand Motor at the lower shoreface (i.e. seaward of MSL -6 m). Storm conditions may induce a partial removal of the coarse top-layer due to mobilization of all of the size fractions and mixing with the relatively fine substrate material. Simulations also show that transport of the fine sand fraction extents to much deeper water than for the medium and coarse sand fractions. Models with multiple sediment fractions are therefore required for the assessment of environmental impacts of large-scale coastal structures or land reclamation's and sediment transport on the lower shoreface.

Bed sediment composition, with a focus on the median grain size D₅₀, was investigated at a large-scale nourishment (The ‘Sand Motor’) at the Dutch coast (∼21.5 million m³ sand). Considerable alongshore heterogeneity of the bed composition (D₅₀) was observed as the Sand Motor evolved over time with (1) coarsening of the exposed part of the Sand Motor (+90 to +150 μm) and (2) a depositional area with relatively fine material (50 μm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the measured D₅₀ values was most evident outside the surfzone (i.e. seaward of MSL −4 m). Coarsening of the bed after construction of the Sand Motor was attributed to hydrodynamic sorting processes, because the alongshore heterogeneity of the D₅₀ showed a similar spatial pattern as the mean bed shear stresses. The observed alongshore heterogeneity of the D₅₀ and correlation of D₅₀ with modelled mean bed shear stresses suggest that preferential erosion of the finer sand fractions has taken place. The selective transport of finer sand fractions results in a coarser top layer of the bed at the Sand Motor. The preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport. The measurements also show the impact of a storm, which consists of a ∼40 μm finer D₅₀ of the offshore bed composition in front of the Sand Motor (i.e. where a considerably coarser bed was in place). Additionally, storms may generate a (temporary) zone with fine bed material at the toe of the deposition profile. This means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate.

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.

The numerical model SWASH is used to investigate nonlinear energy transfers between waves for a diverse set of beach profiles and wave conditions, with a specific focus on infragravity waves. We use bispectral analysis to study the nonlinear triad interactions, and estimate energy transfers to determine energy flows within the spectra. The energy transfers are divided into four types of triad interactions, with triads including either one, two or three infragravity-frequency components, and triad interactions solely between sea-swell wave frequencies. The SWASH model is validated with a high-resolution laboratory data set on a gently sloping beach, which shows that SWASH is capable of modeling the detailed nonlinear interactions. From the simulations, we observe that especially the beach slope affects nonlinear infragravity-wave interactions. On a low-sloping beach, infragravity-wave energy dominates the water motion close to shore. Here infragravity-infragravity interactions dominate and generate higher harmonics that lead to the steepening of the infragravity wave and eventually breaking, causing large infragravity energy dissipation. On the contrary, on a steep-sloping beach, sea-swell wave energy dominates the water motion everywhere. Here infragravity frequencies interact with the spectral peak and spread energy to a wide range of higher frequencies, with relatively less infragravity energy dissipation. Although both beach types have different nonlinear interaction patterns during infragravity-wave dissipation, the amount of infragravity-wave reflection can be estimated by a single parameter, the normalized bed slope.