The swash zone largely influences nearshore hydrodynamics and morphodynamics through dissipating or reflecting wave energy and controlling whether sediment will be stored on the upper beach or returned to the inner surf zone. It is a region where active beach accretion and erosion occur and beach protection measures such as sand nourishments are often placed. Hence, proper prediction of swash zone beach evolution is required to evaluate beach management scenarios. This paper describes the advances related to swash zone sand transport processes and morphodynamics. We discuss the effects of a variety of physical processes and factors (e.g. bore turbulence, pre-suspended sediment advection, wave-swash interactions, infragravity waves, in (cid:0) /exfiltration, pressure gradient and bed slope) on sand transport in the swash zone. We then focus on practical models of swash zone sand transport which are appropriate for predicting longer term (days to years) beach evolutions. Three types of practical models, i.e. empirical sand transport formulae, sand transport distribution methods and equilibrium models, are identified. The strengths and limitations of these practical models are discussed. The empirical sand transport formulae include the intra-swash formulae and swash-averaged formulae. The intra-swash formulae are more physics-based and can take physical processes into account more explicitly. However, upscaling of them for modelling sand transport and morphological changes over tidal cycles or longer term is problematic due to the difficulty in obtaining reliable and accurate instantaneous swash hydrodynamics(e.g. flow velocities) and due to the error propagation. Swash-averaged formulae can be more suitable for predicting longer-term morphological changes while they still require better parameterisations of important physical processes (e.g. wave-swash interactions). Sand transport distribution methods generally work reasonably well for beach erosion under energetic wave conditions whereas they have the inability to predict the beach recovery under mild wave conditions. Equilibrium models show a potential for predicting the beach evolution under both erosive and accretive conditions well. The equilibrium slope appears to be an essential factor that largely determines the performance of the equilibrium models. This equilibrium slope should depend on wave conditions and sediment characteristics, and a quantitative relationship between them needs further research in order to make the equilibrium models more predictive.
Dean's equilibrium beach profile formula was used to investigate the correlation between the static shoreline position and the incident wave energy. The effect of the longshore sediment transport was neglected, and the results showed the reasonable agreement compared with the field observations of Yates et al.(2009), which were conducted for almost 5 years on southern California beaches, USA. The shoreline response varies with the scale factor of Dean's equilibrium beach profile. This implies that the shoreline response could be simply estimated using the sampled grain size without laborious long-term field work. Therefore, the present study results are expected to be practically used for the layout design of submerged or exposed detached breakwaters although the further work is required for performance verification. In addition, after laborous mathematical reviews, the linear relation between incident energy and shoreline response, which was obtained from Yates’s field study, yielded a clear mathematical equation showing how the beach slope is related to the grain size. Received 27 March 2018, revised 21 April 2018, accepted 23 April 2018 Corresponding author Jung-Lyul Lee: +82-31-290-7519, jllee@skku.edu ORCID: https://orcid.org/0000-0001-9410-9725 c 2018, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Climate change is driving higher coastal water levels, and models project accelerated future sea-level rise and coastal storm intensification. Climate change paired with anthropogenic coastal alterations will drive drastic coastal change worldwide. Composite beaches with mixed sediment sizes warrant detailed study as these exhibit complex dynamics in response to changing hydrodynamics due to the distinct transport thresholds of different sediment types. This study uses a novel multi-method approach to investigate a composite sand-cobble beach in Atlantic Canada experiencing a shortening seasonal sand-covered period. Hydrodynamic forcing and associated morphodynamics were monitored over a focused eight-month period, while satellite-based visual imagery and reconstructed wave data were analyzed over longer periods. Results show that intra-annual wave energy changes drive sand dynamics, with reduced summer wave energy facilitating short-term deposition. Long-term positive trends were identified in late spring wave heights, which likely contribute to the shortening sand-covered period. Seasonal dynamics were overwhelmed by extratropical cyclone Fiona, which made landfall on September 24, 2022, generating significant wave heights up to 6.8 m in the bay, mobilizing sediment, and steepening cobble berms. A new index approach based on visual imagery facilitated the investigation of beach sand appearance/disappearance using the relative redness of sand compared to cobble. Finally, the UAV-based surveys yielded high-resolution orthomosaics and LiDAR-based elevation mapping, and highlighted pronounced longshore variability in erosion and deposition during Fiona. The beach mostly recovered to pre-storm conditions in <4 months, which indicates that proposed beach nourishment activities may only experience temporary success. The longer-term results showing a conversion of sand to cobble suggest that loss of sandy beach habitat is likely to increase, even without shoreline migration or coastal squeeze driven by sea-level rise.
Abstract
Currents associated with the nearshore cell circulation, including rip currents, redistribute beach sands into a variety of rhythmic topographies, but do not produce longshore sand transport continuously along the shoreline. Waves breaking at an angle to the beach generate longshore currents flowing parallel to the shoreline. These currents in turn interact with the wave surf to produce a longshore transport of sand. A simple equation has been found with which this longshore current velocity can be evaluated for the mid-surf position where data are available. Theoretical relationships have been formulated for the complete longshore current distribution across the beach width, but data are lacking. However, the distribution can be made to agree with the available data at mid-surf by the proper selection of the drag coefficient. Equations have also been obtained for the evaluation of the sand transport rate, caused either by waves breaking at an angle to the shoreline or by longshore currents generated in other ways. This gives the total sand transport rate. Theoretical relationships have been determined for the distribution of longshore sand transport across the beach width, but again data are lacking to test the equations. The distribution can be calibrated such that, when summed across the beach, it gives the correct total sand transport rate.
Submerged artificial sandbars (SABs) are increasingly favored as an efficient method for beach nourishment, helping to mitigate beach erosion and maintain the sediment transport budget, thus promoting ecological stability. A comprehensive understanding of the morphodynamical evolution of the submerged artificial sandbar-beach profile is essential. This study employs a scaled-down moving-bed flume test to examine the dynamic geomorphological evolution of the submerged artificial sandbar-beach system under regular wave conditions. Twenty experiments, each with different wave heights and periods representative of the Zhejiang coast, were conducted. The study investigated the profile evolution and wave parameters of the submerged artificial sandbar-beach model. The results reveal low wave reflection in front of the sandbar, indicating that under the tested wave conditions, the sandbar primarily serves as a sediment source rather than effectively reflecting wave energy. In all scenarios, the top of the sandbar migrates shoreward, and the sandbar’s contour shape becomes asymmetric. For smaller wave periods, localized erosion occurs in the troughs, while larger periods result in complete siltation. Continuous sediment transport into the trough characterizes the beach face, with sediment movement controlled by wave height and period. A linear or nonlinear relationship between sandbar erosion depth, beach slope, and wave parameters (height, period) is established, providing an empirical model for SAB-beach profiles. These findings enhance the understanding of SAB-beach systems and offer valuable insights for coastal protection and disaster prevention in engineering applications.
Abstract Analogue wave records and associated wave spectra from field measurements of wave transformation across two nearshore bar systems in Georgian Bay, Ontario, show considerable growth of energy in the higher harmonics; in addition, there appears to be a regular spatial pattern of energy transfer between the peak frequency and the first higher harmonics with a length dimension equal to the spacing between the bars. The characteristic peak period of moderate and severe storm waves affecting the two beaches (4.0 and 7.0 s) and the mean nearshore slopes (0.005 and 0.01) are used as input to a mathematical model which describes the non-linear evolution of a shallow water wave, and the interaction between the first and second harmonics. In the model, the mass transport velocities associated with the primary wave and the second harmonic are used to calculate net sediment flux across a two-dimensional profile. Model predictions of bar number and spacing, starting with an initially planar slope, correlate well with the field measurements, for the two sets of wave conditions and mean slopes; in addition the model is able to predict profile changes reflecting seasonal changes in wave climate.
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