Laboratory wave flume experiments in coastal engineering and physical oceanography are widely used to provide an improved understanding of morphodynamic processes. Wave flume facilities around the world vary greatly in their physical dimensions and differences in the resulting distortion of the modelled processes are reconciled using scaling laws. However, it is known that perfect model-prototype scaling of all hydro and morphodynamic processes is rarely possible and there is a lack of understanding to what extent distorted models can be used for direct morphological comparison. To address this issue, distorted scale laboratory flume experiments were undertaken in three different facilities, with the aim to measure and compare beach profile evolution under erosive waves and increasing water levels. A novel approach was developed to transform and scale the different experimental geometries into dimensionless coordinates, which enabled a direct quantitative comparison of the beach profile evolution and sediment transport rates between the differing distorted experimental scales. Comparing results from the three experiments revealed that the dimensionless scaled morphological behaviour was similar after the same number of waves – despite very different degrees of model distortion. The distorted profiles appeared to be suitable for comparison as long as a modified version of the Dean number is maintained between them. The new method was then validated with two further published datasets, and showed good agreement for both dimensionless profile shape, dimensionless sediment transport and morphodynamics parameters. The new approach scales the sediment transport by the square of the runup, proportional to HL, rather than H2, and yields good agreement between the datasets. It is further shown that the new scaling method is also applicable for comparing distorted profile evolution under water level increase, as long as the water level is raised in a similar way between the experiments and by the same total increment relative to the significant wave height (Δh/Hs).@en

Beaches around the world continuously adjust to daily and seasonal changes in wave and tide conditions, which are themselves changing over longer time-scales. Different approaches to predict multi-year shoreline evolution have been implemented; however, robust and reliable predictions of shoreline evolution are still problematic even in short-term scenarios (shorter than decadal). Here we show results of a modelling competition, where 19 numerical models (a mix of established shoreline models and machine learning techniques) were tested using data collected for Tairua beach, New Zealand with 18 years of daily averaged alongshore shoreline position and beach rotation (orientation) data obtained from a camera system. In general, traditional shoreline models and machine learning techniques were able to reproduce shoreline changes during the calibration period (1999–2014) for normal conditions but some of the model struggled to predict extreme and fast oscillations. During the forecast period (unseen data, 2014–2017), both approaches showed a decrease in models’ capability to predict the shoreline position. This was more evident for some of the machine learning algorithms. A model ensemble performed better than individual models and enables assessment of uncertainties in model architecture. Research-coordinated approaches (e.g., modelling competitions) can fuel advances in predictive capabilities and provide a forum for the discussion about the advantages/disadvantages of available models.

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Laboratory wave flume experiments have been used to provide improved understanding of beach profile evolution under different wave and water level conditions. However, the understanding of the processes involved in the evolution of beach profile under Sea Level Rise (SLR) toward equilibrium is unclear. Two similar, but distorted experiments were performed at large and medium scale in order to study the qualitative morphological changes involved in beach profile evolution under SLR. Both experiments showed similar beach profile evolution. The profile change predicted by the Profile Translation Model (PTM) and the Bruun Rule underestimated the observed reatreat in both experiments. The length of the active beach profile increased under SLR. For the large scale experiment, the reflection coefficient of the beach decreased while the vertical runup increased significantly. The beachface changed faster than the outer surf zone, making the beach more dissipative.@en

In a changing climate, sea level rise and projected regional–scale changes in storminess may increase the vulnerability of sandy coastlines to coastal erosion and flooding. As a result, there is increased interest in the development of adaptable, sustainable and effective coastal protection measures to protect these highly variable sandy coastlines. One such example is a dynamic cobble berm revetment; a “soft–engineering” solution (i.e., not fixed) consisting of a cobble berm constructed around the high tide wave runup limit, that has the potential to stabilise the upper beach, provide overtopping protection to the hinterland and translate with water level rise. However, there have been limited applications of dynamic cobble berm revetments to date, and there is a lack of understanding about the efficacy of this coastal protection to current and changing waves and water levels. This study details a prototype–scale experiment conducted to test the behaviour and performance of a dynamic cobble berm revetment as a form of coastal protection against erosive waves and water level increase. Results from the experiment showed that the revetment was “dynamically stable” under wave action as a consistent global shape was retained even though individual cobbles were mobilised under every swash event. Although the front slope and the crest responded to the incident wave condition, the net rate of change was always an order of magnitude lower than the gross rate of change. Tracking of individual cobbles using Radio Frequency Identification (RFID) technology showed that stability of the revetment was likely maintained by rollover transport of cobbles onto the crest, as the revetment moved upward and landward under water level rise. The presence of the revetment reduced the vertical and horizontal runup as well as the retreat of the upper beach. The experimental results presented suggest that a dynamic cobble berm revetment could be a cheap, efficient and low environmental impact engineering solution for protecting sandy coastlines in a changing climate. Some preliminary design guidelines for coastal engineers are also drawn from this experiment.

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A large-scale laboratory experiment was completed to investigate the performance of a dynamic cobble berm revetment designed to provide sustainable coastal protection under wave attack and a rising water level. The experiment demonstrated the inherent stability of the dynamic revetment, which was observed to be reshaped by every wave but retained its overall shape throughout the experiment with almost no loss of material. By comparing with a sand beach case, it was found that the revetment reduced shoreline retreat during an erosive wave condition and reduced runup excursions, thus eliminating erosion landward of the revetment. Overall the experiment provided significant new understanding of dynamic revetments and demonstrated the potential for dynamic revetments to provide low cost, robust coastal protection.@en