Xbeach

Breaching of coastal barriers is a three-dimensional process induced by complex interactions between hydrodynamics, sediment transport and soil avalanching processes. Although numerous coastal barriers are breached every year in many coastal countries, causing dramatic inundations of the nearshore areas, the understanding of the processes and interactions associated with both breaching and subsequent flood propagation is still poor. This might explain why their combined modelling and prediction has not yet been sufficiently addressed. Consequently, barrier breaching and subsequent inundation are still often modelled separately, thus ignoring the strong interaction between breaching and flooding. However, the combined modelling of such strongly coupled processes is crucial. Since the open-source model system " XBeach " consists, among others, of a nonlinear shallow water solver coupled with a morphodynamic model, also including a soil avalanching module, it has the potential to simulate both breaching and subsequent flood propagation together. Indeed, the mutual interactions between hydrodynamics and morphodynamics (including soil avalanching) are properly accounted for. This paper, therefore, aims to examine the applicability of XBeach for modelling coastal barrier breaching and inundation modelling in combination, instead of the current approaches, which address the modelling of each of these two processes separately. The performance of XBeach, in terms of inundation modelling, is assessed through comparisons of the results from this model system (i) with the results from common 1D and 2D flood propagation models and (ii) with observations for barrier breaching and subsequent inundation from a real case study. Besides providing an improved understanding of the breaching process, the results of this study demonstrate a new promising application of XBeach and its potential for calculating time-varying inland discharges, as well as for combined modelling of both dune breaching and subsequent flood propagation in coastal zones.

Low-lying tropical islands are highly vulnerable to the effects of sea-level rise and climate change. Most pressing is the threat posed to their fresh water supplies by wave-induced flooding. This thesis attempts to generalize previous site-specific studies of flooding on coral atolls and apply it in a framework that can be used for early warning systems or long-term climate change impact studies. To do so, a large synthetic database of representative reef properties and hydrodynamics was developed using the numerical wave model XBeach, and then analyzed using a Bayesian probabilistic network. The resulting tool allows us to make real-time flood predictions based on offshore wave or sea level conditions, and the unique characteristics of a given island (e.g. topography). Narrow, smooth reefs with steep fore reef slopes are the most vulnerable to runup. Extreme flooding is associated with anomalously high, resonant low frequency waves, which are more likely to occur on narrow, smooth reefs subjected to extreme swell waves (large wave heights with long periods). These extreme swells are "blue sky" events which originate from distant storms. Thus, they may thus arrive without warning, since they are completely independent of local weather conditions. This bolsters the need for more effective prediction tools. Validation of the results presented in this study is limited by the small number of field observations against which the model can be compared. Thus, there is a need to develop a comprehensive database of reef morphology and hydrodynamics. Offshore wave conditions, water levels, and reef width are the most essential variables for predicting runup, so future research efforts should be directed towards collecting those data. This model can also be used to ask questions like, "which islands will be most severely impacted by climate change?", or "can we increase flood resilience for a given island by restoring its coral reefs?". Hence, there is also potential for its use in guiding decision-makers to allocate limited funding in the places where it will have the most impact.

Numerical cross-shore profile evolution models have been good at predicting beach erosion during storm conditions, but have difficulty in predicting the accretion of the beach during calm periods. This paper describes the progress made in modifying and applying the public domain XBeach code to the prediction and explanation of the observed behaviour of coarse-grained beaches in the laboratory and the field under accretive conditions. The paper outlines in details the changes made to the original code (version 12), including the introduction of a new morphological module based upon Soulsby’s sediment transport equation for waves and currents, and the incorporation of Packwood’s infiltration approach in the unsaturated area of the swash region. The competence of this modified model during calm conditions for describing the steepening of the profile, and the growth of the beach berm is demonstrated. Preliminary results on the behaviour of the beach subject to both waves and tides are presented. Good agreement is found between the model simulations and large-scale laboratory measurements, as well as field observations from a composite beach in the UK. The reasons for the model's capabilities are discussed.

Las ondas de infragravedad, conocidas también como ondas largas o de baja frecuencia, dominan el espectro de energía del oleaje en aguas poco profundas y en la zona de lavado, lo que juega un papel importante en el transporte de sedimentos cercano a la costa, así como en la inundación y los procesos erosivos de las playas ante eventos extremos.
Los rompeolas son obras diseñadas para proteger la costa o abrigar un área de agua de la acción del oleaje, generalmente ubicados paralelos a la costa, siendo catalogados por su cota de coronación como emergidos (rebasables o no rebasables) y sumergidos.
En las últimas décadas los rompeolas sumergidos han incrementado su popularidad debido al bajo impacto que desde el punto de vista estético y paisajístico introducen en las playas. Sin embargo, estudios recientes muestran que en la mayoría de los casos donde fueron construidos se reportaron procesos erosivos, demostrando que los mecanismos hidrodinámicos y de transporte de sedimentos son completamente diferentes a los que se generan ante los rompeolas emergidos y que los criterios que actualmente se emplean para predecir la respuesta de la costa ante la construcción de rompeolas emergidos no pueden utilizarse en su diseño.

Introduction. A series of numerical experiments was carried out to study the mechanism of storm deformations of the coastal zone relief and the redistribution of different sediment fractions after the parameters of the wind waves directed almost normally to the coastline, are changed. Data and methods. The lithodynamic processes taking place at the Western Crimea coastal zone in the region of the Lake Bogaily bay-bar were investigated using the XBeach mathematical model. The in-situ data on the bottom relief and the sediments granulometric composition obtained during monitoring observations in the Western Crimea coastal zone performed by the Marine Hydrophysical Institute of RAS was used as the model input parameters. The granulometric composition was preset as a mixture of three components distributed along the profile of the underwater coastal slope; at that the ratios of volume concentrations were different. Results. It is shown that influence of the storm waves results in intensive beach erosion, strong alteration of the profile of the underwater coastal slope upper part and formation of an accumulative body. Main changes of the initial profile and redistribution of the sediment fractions are observed during the first 3-6 hours and depend on the wave period. The rate of the water edge retreat and the values of the coastal zone relief deformation were quantitatively assessed for various time periods of the wave action. Position of the marine boundary of the site within which significant motion of sediments and their accumulation during a storm took place was calculated. Discussion and conclusion. Possibility of the bay-bar crest erosion is analyzed for the wind wave' different periods. It is shown that the volume concentrations of different components of the mixture change mainly after the fine fractions are rapidly removed to the seaward part of the bottom profile and the coarser fractions are redistributed in the upper part of the underwater coastal slope. The obtained results provide a possibility of indirect determining the conditions for blocking the alongshore sediments transport in case of building a hydraulic structure. Acknowledgements: the investigation is carried out within the framework of the state task on the theme No. 0827-2018-0004 "Complex interdisciplinary investigations of the oceanologic processes conditioning functioning and evolution of the Black and Azov seas' ecosystems of the coastal zones" (code "Coastal investigations") and at the RFBR partial support within the framework of the research project No. 18-35-00230.

Flooding and coastal erosion is a frequent and widespread natural hazard in the UK. With increasing urban development pressures on flood systems and changing global climate, frequency and severity of flooding is rapidly increasing. Temporal clustering of flood events in time has a critical effect in reducing the ability of systems to recover to the pre-flood ‘norm’, leading to amplified flood damages in the second flood event. This can be considered as system memory, the timeframes of which will be highly variable and range from days to years according to system components. The current assumptions of flood risk are based on single-event simulations and fail to consider the spatio-temporal dynamics of morphological memory due to coastal erosion. The floodMEMORY project attempts to address this issue through a continuous flood simulation framework based on temporal clustering of flood drivers.

This paper reports progress made in modifying and applying the X-Beach code to predict and explain the observed behaviour of coarse grained beaches. In a previous study a comparison of beach profile evolution measured during large scale experiments under constant water level with numerical model simulations was made. This placed particular emphasis on the tendency for onshore transport and profile steepening during calm conditions (Jamal et al., 2010). The present paper extends that investigation to study the influence of the advection of surf processes induced by tidal water level variations effects, on gravel beach profile evolution. The parameter values and numerical model used in the simulation is similar to that presented previously. It is assumed that, to good approximation, the groundwater interface inside the beach follows the tidally modulated water level. The results obtained from the model shows that the model provides reasonable simulations of beach profile change in a t...