Poate, T.G., Masselink, G., McCall, R.T., Russell, P.E., Davidson, M.A., 2014. Storm-driven cusp behaviour on a high energy gravel beach. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 645–650, ISSN 0749-0208.Gravel and mixed sand-gravel beaches are characterised by steep reflective profiles which provide effective forms of wave absorption and therefore coastal defence to many mid-latitude regions, including northwestern Europe and North America. In the UK the combination of energetic wave conditions and large tides creates very dynamic and responsive morphology often dominated by cuspate features. Recent storm-responsive field campaigns at Loe Bar, Cornwall, UK, have captured highly energetic wave conditions (Hs = 2.5–5.8 m) using temporary video camera installations, low tide 3D topographic surveys with real time kinematic GPS, local tide level measurements and inshore directional wave data. Characterised by fine gravel (D50 = 3 mm) and a steep reflective profile (tanβ = 0.118), the barrier at Loe Bar is exposed to an annual 10% exceedence significant wave height Hs10% of 2.4 m arriving predominantly from the southwest (Atlantic Ocean) and shore-normal to the beach. Under medium-wave conditions (Hs = 2–3 m), contrasting cusp behaviour was recorded with accretion and erosion, principally, through horn growth and decay (bed-level change Δz = c. 1 m). During more energetic conditions (Hs = 5.8 m), the morphological response is more consistent and the waves drive erosion of the lower profile causing bed-level changes over a tide in excess of 1.5 m. Very rapid recovery to pre-storm bed levels is observed with defined cusp evolution occurring within 12 hours during the falling limb of the storm as incident wave energy decreases. The unique gravel cusp dataset suggests free behavior due to cusp morphodynamic feedback, rather than hydrodynamic forcing, plays an dominant role in cusp evolution.
A one-dimensional hydrostatic version of the XBeach model (Roelvink et al., 2009) is applied to hindcast swash morphodynamics measured during an accretive, and an erosive tide at Le Truc Vert beach (France) in early spring 2008 (Masselink et. al, 2009; Blenkinsopp et al., 2011). Swash hydrodynamics are solved by applying the nonlinear shallow water equations, and sediment transport rates are obtained from a combined intra-wave Nielsen and Bagnold type transport model. Reasonable predictions of morphological change in the swash were obtained. Nevertheless, the model underpredicts the water level setup and/or wave run-up during the accretive tide, which is hypothesized to be related to 2D-effects
The effect of the longshore dimension on dune erosion is examined numerically with a 2DH process-based model XBeach. Exploratory simulations are presented to examine longshore effects due to directionally spread waves, longshore variation in topography and longshore variation in bathymetry. The simulations reveal that alongshore sediment exchange during a storm surge affects the cross-shore development of the foreshore and can locally increase or decrease storm impact on dunes substantially. This finding illustrates that dune erosion on natural coasts (which are never cylindrical or subject to longcrested waves) is an inherent 2D process for which 1D cross-shore models do not suffice without further assumptions.
Since a submerged breakwater (SBW) was built and the beach was nourished at Anmok in the east coast of Republic of Korea at October, 2014, the shoreline in the lee of the SBW has accreted about 25 m during the first seven months after construction. The shoreline evolution showed two distinct patterns which were studied in this paper. A strong local accretion behind the submerged breakwater was observed in March 2015 and a smoother shoreline with accretion that extended up to Gangneung Harbor breakwater in May 2015. The UNIBEST coastline model (developed at Delft Hydraulics) was applied for the investigation of the observed shoreline undulation patterns which were generated by alongshore sediment transport gradients that were induced by the SBW and nourishment. Nearshore wave conditions were computed for this purpose at nine locations in the nearshore with the Delft3D-wave model. Two detailed wave scenarios with different crest height of the SBW were taken into account to represent the transmission of waves at the SBW which were validated with field measurements. The coastline model is able to reproduce the observed shoreline evolution patterns when wave transmission at the SBW is represented well, which is the case for a wave scenario with a lowered effective crest level of the SBW. Initially, accretion takes place predominantly at the northern side of the scheme, which was similar to the observed shoreline shape of the March 2015 situation. This local accretion is a result of the low sediment transport capacity behind the SBW due to sheltering of the wave energy, which initially hinders the redistribution of sediment to the South (i.e. area in-between Gangneung Harbor and SBW). After some months, a redistribution of sediment will take place behind the SBW which results in a smoother shoreline pattern which is similar to the May 2015 situation. The rate of change of the shoreline accretion is controlled by the absolute transport rates at the coast (i.e. wave energy), but is often of lesser importance since an adjustment towards a new shoreline equilibrium may take place within relatively short time scales (i.e. months to a few years). In addition to the wave energy, it was found that the relative angle of the incoming waves (α) is most relevant for the final shoreline shape. The shoreline evolution at future SWB structures may therefore be predicted by precisely estimating the α directly after construction of the SBW.
<p>Integrated modelling approaches for the evolution of the entire dune-beach system have become increasingly sought-after, not only for management purposes, but also to allow better understanding of the feedbacks between processes and scales and a closer approximation of where critical system thresholds may lie. The effective reproduction of both destructive and constructive processes over a broad spectrum of temporal scales is crucial to any, such, integrated approach. Recent improvements of the XBeach-Duna model regarding approximation of nearshore processes were tested using in-situ data from the Emma storm impacts on a reflective beach (Praia de Faro, in S. Portugal). The model results compare well with measured post-storm and recovered profiles, showing high model skill under both erosive and constructive regimes. Building from this event-scale analysis, a gradual increase of temporal windows in simulated forcing conditions, through wave schematisation, is presented and discussed in terms of optimisation between gains in simulation time and losses in geomorphic change information. This methodological approach and findings are the basis that will allow passing on to dependable, long-term simulations of the beach-dune system evolution.</p><p>&#160;</p><p><em>Acknowledgements: The work was implemented in the framework of the ENLACE project (ref. 28949 FEDER), funded by FCT (Funda&#231;&#227;o para a Ci&#234;ncia e a Tecnologia)</em></p>
Many coral reef-lined coasts are low-lying with elevations less than four meters above mean sea level. Climate-change-driven sea-level rise, coral reef degradation, and changes in storm wave climate will lead to greater occurrence and impacts of wave-driven flooding. This poses a significant threat to their coastal communities. While greatly at risk, the complex hydrodynamics and bathymetry of reef-lined coasts make flood risk assessment and prediction costly and difficult. Here we use a large ($>$30,000) dataset of measured coral reef topobathymetric cross-shore profiles, statistics, machine learning, and numerical modelling to develop a set of representative cluster profiles (RCPs) that can be used to accurately represent the shoreline hydrodynamics of a large variety of coral reef-lined coasts around the globe. In two stages, the large dataset is reduced by clustering cross-shore profiles based on morphology and hydrodynamic response to typical wind and swell wave conditions. By representing a large variety of coral reef morphologies with a reduced number of RCPs, a computationally feasible number of numerical model simulations can be done to obtain wave runup estimates, including setup at the shoreline and swash separated into infragravity and sea-swell components, of the entire dataset. The predictive capability of the RCPs is tested against 5,000 profiles from the dataset. The wave runup is predicted with a mean error of 9.7\% -- 13.1\%, depending on the number of cluster profiles used, ranging from 312 to 50. The RCPs identified here can be combined with probabilistic tools that can provide an enhanced prediction given a multivariate wave and water level climate and reef ecology state. Such a tool can be used for climate change impact assessments and effectiveness of reef restoration projects, as well as for the provision of coastal flood potential predictions in a simplified (global) early warning system.
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