The presence of marine pollutants such as marine plastics has increased significantly over the last decades and poses a major environmental problem, in both the coastal and offshore area. Marine pollutants are transported, mixed and diffused in the ocean, which means the understanding and modelling of marine transport is key for mitigation purposes (Moulton et al., 2022). Additional to large scale and planetary currents that play a major role in marine transport, free surface waves, internal gravity waves in density stratified fluids and the Coriolis force due to the rotation of the Earth are also fundamental drivers of transport that need to be accounted for. The fundamental fluid mechanics processes associated with these are often not resolved in large-scale models, but are instead included in a parametrised form. However, some fundamental processes associated with wave-induced currents (e.g., Stokes drift) in rotating, density-stratified fluids with a free surface remain unclear and untested. In addition, parametrisation for different environments, forcings and time scales must be developed and tested before being implemented into models for them to reliably predict transport, accumulation and storage of marine pollutants. For this purpose, the Delta Transport Processes Laboratory (DTPLab) is being developed at TUDelft Hydraulic Engineering Laboratory. This laboratory pioneers the combined experimental study of surface waves, density stratification and Coriolis forces in a single laboratory. The DTPLab was designed with a multi- users and purposes vision, with interchangeable facilities and state-of-the-art measurement devices. This paper presents the DTPLab facilities (under construction) and equipment that make this laboratory unique in the world, and describes, as an example of what is feasible, a novel experiment that will be performed in this lab.</jats:p>@en

Laboratory investigations of beach morphology change under wave action are undertaken to gain insight into coastal processes, design coastal structures and validate the predictions of numerical models. For the results of such experiments to be reliable, it is necessary that they are repeatable. The equilibrium beach concept, that beach morphology will evolve to a quasi-static equilibrium shape for a given forcing suggests that experiments should be repeatable to some degree. However, sediment transport in turbulent breaking and broken waves is complex and highly variable and the level of repeatability at different temporal and spatial scales is challenging to measure, as such, previous work has restricted comparisons to small numbers of waves. Here we use the results of two identical, 20-h large-scale wave flume experiments to investigate the repeatability of sediment transport and beach morphology change under waves at timescales down to individual swash events. It is shown that while flow characteristics from identical swash events are very repeatable, the sediment transported can be very different in both magnitude and direction due to differences in turbulence, sediment advection and morphological feedback. Over longer periods containing multiple matching swash events however, the beach responds in a very similar manner, with the level of morphological repeatability increasing with time. The results also demonstrate that gross swash zone sediment transport remains high even as a beach profile approaches quasi-equilibrium, but the proportion of individual swash events that cause large sediment fluxes (>±7.5 kg/event/m) reduces with time. The results of this laboratory study indicate that beach morphology change has a level of determinism over timescales of several minutes and longer, giving confidence in the results from physical modelling studies. However, the large differences in sediment transport from apparently identical swash events questions the value in pursuing numerical predictions of sediment transport at the wave-by-wave timescale unless the reversals in sediment transport between apparently near identical swash events can also be predicted.

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A dynamic revetment is a cobble-gravel berm constructed around the high tide wave runup limit. These structures mimic composite beaches, which consist of a lower foreshore of sand and a backshore berm constructed of gravel or cobbles, which stabilizes the upper beach and provides overtopping protection to the hinterland. Dynamic revetments contrast with static coastal defense structures as they are “dynamic” and are expected to reshape under wave attack. The idea of a dynamic structure made of smaller material than a classic riprap type of revetment is not new, and previous laboratory and field research has investigated the use of dynamic revetments for coastal protection (Allan et al., 2016). However, applications of such a structure remain sporadic and often differ to each other in terms of their design and aim. Furthermore, few existing dynamic revetments have been monitored to evaluate their function, and therefore, the way they behave at the particle and global scale remains poorly understood. In this study, 2D Lidar, GPS topography, particle tracking, and sub-surface particle composition data were collected on the dynamic revetment at North Cove, Washington State, USA, during a spring tidal cycle. Together they enabled investigation of the revetment dynamics and evolution in response to waves and water levels. In addition, this monitoring is proposed as a suitable protocol for short and long-term monitoring of dynamic revetments.@en

Infragravity (hereafter ‘IG’) waves are ocean waves related to the presence of groups in incident shortwaves. The importance of IG waves in the nearshore and their role in beach morphodynamics have been widely documented over the last few decades (Bertin et al., 2018) while in contrast, studies on characterisation of IG waves and related impacts in tidal inlets are scarce and have only recently begun to be addressed (Williams and Stacey, 2016). Nonetheless, existing studies tend to show that IG waves in tidal inlets can play an important role in the short and long-term hydro-morphodynamics of coastal lagoons. The current study presents the first large scale field measurements and analysis of IG waves propagation and characteristics in a meso-macro tidal lagoon/inlet.@en

Tidal inlets are transitional area between the ocean and an estuary or a lagoon. They are characterised by very complex hydro-morphodynamics processes, resulting from the interaction between tidal currents and waves with large and rapid water level changes. The coastline around the inner part of the inlet and more generally the entire lagoon often host important economic and touristic activities. In these environments, the studies of hazards associated with energetic events are mainly based on modelling approaches. However, the phase-averaged numerical models typically used for large-scale coastal applications exclude infragravity waves (long waves ranging from several tens of seconds to several minutes, noted hereinafter IG) and phase-resolving model are too computationally expensive for such large domains. Therefore, the propagation of waves from the open sea to the lagoon and in particular the characterisation of IG waves are still poorly understood (Bertin et al., 2018). To study the amplitude and spatial variability of IG waves in the Arcachon lagoon during storms, the Xbeach model (Roelvink et al., 2009) was implemented in surfbeat mode (SB mode).@en

The continual sea-level rise predicted over the next century poses a significant threat to coastal regions. Preserving the coastline will require innovative coastal protection techniques and structures. A potential defence is to introduce artificial gravel beaches (Loman et al., 2010), or dynamic revetments (Bayle et al., 2020), a gravel berm placed at the high tide berm(Bayle et al., 2020). A key feature of these types of defences is their ability to maintain the berm elevation relative to the water level. As the water level rises, the structure is overtopped by waves which drives sediment up and over the berm crest peak. This leads to elevation gain for the berm and limits the landward excursion of waves. Berm crest behaviour has only been recorded at frequencies that do not allow wave-by-wave analysis of the process and the resultant ‘snapshot’ profiles may not be representative of the true variability of gravel berms. Through application of continuous 2-D Lidar monitoring on several active gravel berms this paper addresses this gap. It presents new findings related to sediment transport at the berm crest, the morphodynamic behavior under wave attack and a conceptual model for berm state.@en

The swash zone is a highly dynamic region of the nearshore in terms of both hydro- and sediment dynamics. Previous work has demonstrated that the majority of swash events transport only small amounts of sediment and net beachface volume change over several hours tends to be small. However, a small number of individual swash events can deposit or remove hundreds of kilograms of sediment per metre width of beach. These events are typically associated with swash flows that involve one or more highly turbulent swash-swash interactions, causing enhanced suspension and transport of sediment (Blenkinsopp et al. 2011). The timing and location of these interactions is complex and small changes in either can lead to very different local flow conditions. The complexity of these flows make sediment transport prediction on a swash-by-swash basis very challenging, and raises the question whether deterministic physical and numerical modelling of swash sediment transport is warranted.@en

Pressure on the coastline is escalating due to the impacts of climate change, this is leading to a rise in sea-levels and intensifying storminess. Consequently, many regions of the coast are at increased risk of erosion and flooding. Therefore coastal protection schemes will increase in cost and scale. In response there is a growing use of nature-based coastal protection which aim to be sustainable, effective and adaptable. An example of a nature-based solution is a dynamic cobble berm revetment: a berm constructed from cobble and other gravel sediments at the high tide wave runup limit. These structures limit wave excursion protecting the hinterland from inundation, stabilise the upper beach and adapt to changes in water level. Recent experiments and field applications have shown the suitability of these structures for coastal protection, however many of the processes and design considerations are poorly understood. This study directly compares two prototype scale laboratory experiments which tested dynamic cobble berm revetments constructed with approximately the same geometry but differing gravel characteristics; well-sorted rounded gravel (DynaRev1) and poorly-sorted angular gravel (DynaRev2). In both cases the structures were tested using identical wave forcing including incrementally increasing water level and erosive wave conditions. The results presented in this paper demonstrate that both designs responded to changing water level and wave conditions by approaching a dynamically stable state, where individual gravel is mobilised under wave action but the geometry remains approximately constant. Further, both structures acted to reduce swash excursions compared to a pure sand beach. However, their morphological behaviour is response to wave action varied considerably. Once overtopping of the designed crest occurred, the poorly-sorted revetment developed a peaked crest which grew in elevation as the water level or wave height increased, further limited overtopping. By comparison, the well-sorted revetment was characterised by a larger volume of submerged gravel and a lower elevation flat crest which responded less well to changes in conditions. This occurred due to two processes: (1) for the poorly-sorted case, gravel sorting processes moved small to medium gravel material (D₅₀<70mm) to the crest and (2) the angular nature of the poorly-sorted gravel material promoted increased interlocking. Both of these processes led to a gravel matrix that is more resistant to wave action and gravitational effects. Both revetments experienced some sinking due to sand erosion beneath the front slope. The rate of sinking for the well-sorted case was larger and continued throughout due to the large pore spaces within the gravel matrix. For the poorly sorted revetment in DynaRev2, sand erosion ceased after approximately 28 h due to the development of a filter layer of small gravel at the sand-gravel interface reducing porosity at this location, hence a larger volume of sand was preserved beneath the structure. Both designs present a low-cost and effective solution for protecting sandy coastlines but from an engineering viewpoint it appears better to avoid well-sorted gravel material and greater gravel angularity has been seen to increase crest stability.

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The effects of climate change and sea level rise, combined with overpopulation are leading to ever-increasing stress on coastal regions throughout the world. As a result, there is increased interest in sustainable and adaptable methods of coastal protection. Dynamic cobble berm revetments consist of a gravel berm installed close to the high tide shoreline on a sand beach and are designed to mimic naturally occurring composite beaches (dissipative sandy beaches with a gravel berm around the high tide shoreline). Existing approaches to predict wave runup on sand or pure gravel beaches have very poor skill for composite beaches and this restricts the ability of coastal engineers to assess flood risks at existing sites or design new protection structures. This paper presents high-resolution measurements of wave runup from five field and large-scale laboratory experiments investigating composite beaches and dynamic cobble berm revetments. These data demonstrated that as the swash zone transitions from the fronting sand beach to the gravel berm, the short-wave component of significant swash height rapidly increases and can dominate over the infragravity component. When the berm toe is submerged at high tide, it was found that wave runup is strongly controlled by the water depth at the toe of the gravel berm. This is due to the decoupling of the significant wave height at the berm toe from the offshore wave conditions due to the dissipative nature of the fronting sand beach. This insight, combined with new methods to predict wave setup and infragravity wave dissipation on composite beaches is used to develop the first composite beach/dynamic revetment-specific methodologies for predicting wave runup.@en