We investigated quasi-2D sand ripple geometry (i.e., ripple height, ripple wavelength, and ripple asymmetry) on a mound subject to the influence of waves, currents, and combined wave-current flows. The results of this study quantify how ripple geometry is influenced by bed slope and combined wave-current flows. The geometry of the ripples is shown to depend on the combined wave-current flow ratio and the local bed slope. Under wave-only conditions, the wave-driven ripple length and height decreased as a function of depth and local slope. Under combined wave-current conditions, the ripples increased in height and wavelength on the stoss slope of the mound, and decreased on the lee slope of the mound. Existing ripple geometry predictors, developed for combined flows on flat sand beds, were unable to predict ripple geometry on the sloped bed accurately. We propose correction factors for ripple geometry predictors to account for slope effects and combined wave-current flow conditions. Applying the correction factors significantly improves the predictor performance for predicting ripple height, wavelength, and asymmetry on sloping beds.

A field campaign was carried out at a sheltered sandy beach with the aim of gaining new insights into the driving processes behind sheltered beach morphodynamics. Detailed measurements of the local hydrodynamics, bed-level changes and sediment composition were collected at a man-made beach on the leeside of the barrier island Texel, bordering the Marsdiep basin that is part of the Dutch Wadden Sea. The dataset consists of (1) current, wave and turbidity measurements from a dense cross-shore array and a 3 km alongshore array; (2) sediment composition data from beach surface samples; (3) high-temporal-resolution RTK-GNSS beach profile measurements; (4) a pre-campaign spatially covering topobathy map; and (5) meteorological data. This paper outlines how these measurements were set up and how the data have been processed, stored and can be accessed. The novelty of this dataset lies in the detailed approach to resolve forcing conditions on a sheltered beach, where morphological evolution is governed by a subtle interplay between tidal and wind-driven currents, waves and bed composition, primarily due to the low-energy (near-threshold) forcing. The data are publicly available at 4TU Centre for Research Data at: https://doi.org/10.4121/19c5676c-9cea-49d0-b7a3-7c627e436541 (Van der Lugt et al., 2023).

Mean sea-level rise (MSLR) will induce shoreline recession, increasing the stress on coastal systems of high socio-economic and environmental values. Localized mega-nourishments are meant to alleviate erosion problems by diffusing alongshore over decades and thus feeding adjacent beaches. The 21-st century morphological evolution of the Delfland coast, where the Sand Engine mega-nourishment was built in 2011, was simulated with the Q2Dmorfo model to assess the Sand Engine capacity to protect the area against the effects of MSLR. The calibrated and validated model was forced with historical wave and sea-level data and MSLR projections until 2100 corresponding to different Representative Concentration Pathways (RCP2.6, RCP4.5 and RCP8.5). Results show that the Sand Engine diffusive trend will continue in forthcoming decades, with the feeding effect to adjacent beaches being less noticeable from 2050 onward. Superimposed to this alongshore diffusion, MSLR causes the shoreline to recede because of both passive-flooding and a net offshore sediment transport produced by wave reshaping and gravity. The existing feeding asymmetry enforces more sediment transport to the NE than to the SW, causing the former to remain stable whilst the SW shoreline retreats significantly, especially from 2050 onward. Sediment from the Sand Engine does not reach the beaches located more than 6 km to the SW, with a strong shoreline and profile recession in that area, as well as dune erosion. The uncertainties in the results are dominated by those related to the free model parameters up to 2050 whilst uncertainties in MSLR projections prevail from 2050 to 2100.

We investigate pathways of sediment diffusion for a Gaussian-shaped sand mound subjected to monochromatic waves. Our unique results nearly close the sediment budget by quantifying each of the sediment transport processes responsible for mound diffusion associated with sediment flux due to slope driven transport and ripple migration. Downslope ripple progression was observed as ripples formed at the mound top advanced down the side slopes in a direction perpendicular to wave propagation. Once ripples formed on the sides of the mounds the ripples became pathways for sediment flux from the top to the bottom of the mound, persisting even after ripples reached the base of the mound as sediment avalanching due to gravity and mound slope. Lateral ripple migration caused ripples to migrate along the sides of the sand mound in a direction parallel to wave propagation. Once ripples reached the base of the mound, lateral migration of ripples caused spreading of sand around the sides of the mound. Lateral ripple migration was largely driven by ripple splitting caused by a large downslope sediment flux from the center of the mound that generated ripples with longer wavelengths than wave orbital hydrodynamics could support. To restore equilibrium between sediment and flow conditions, ripples with longer wavelengths continuously split and migrated laterally around the mound. Our results reflect the importance of slope driven transport, bed fluidization, and ripple dynamics on the larger scale diffusivity and suggest that slope driven and ripple driven sediment fluxes should be more explicitly included in sediment transport formulations.

In their recently published paper, Delgado-Fernandez et al. (2019) critically review the limitations and dangers of the relatively recent shift towards dune rejuvenation management in North-western Europe. We would like to comment on the paper from the Dutch perspective.

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth).

The phenomenon of bore merging is investigated using two high-resolution laboratory experiments including bichromatic and irregular wave conditions. The locations at which waves start merging are identified and the hydrodynamic conditions in the vicinity of the merging points are examined. Bore merging takes place in the inner surf zone for all conditions considered. The infragravity- to short-wave height ratio is close to or larger than one at the merging point, indicating that bore merging occurs in a part of the surf zone that is already dominated by the infragravity waves. Our data analysis is supplemented by numerical simulations that confirms the importance of infragravity waves in the occurrence bore merging. Moreover, our simulations suggest that bore merging has a very limited effect on the infragravity wave field. This casts doubts on the importance of bore merging as an infragravity wave generation mechanism.

The development of dunes is characterized by the alternating effects of erosion during storm events and growth during milder conditions. The quantification of dune growth due to aeolian processes has received some attention but uncertainty remains on where the dune sand, which accommodates growing dunes, originates from in the coastal profile. In this paper we hypothesize that sediment eroded from the upper beach is the main sediment supply for aeolian sediment transport governing dune growth. To test this hypothesis we have analyzed morphological profiles collected monthly at three different field sites (Noordwijk, Vlugtenburg and Narrabeen) during several years. No significant erosive trend due to aeolian processes was found at the upper beach at this temporal resolution. We conclude that it is highly unlikely that the main supply for aeolian sediment transport governing dune growth was located at the upper beach. The intertidal zone might be a relevant alternative source of sediment in the cross shore profile.