The reproduction of seventeen laboratory tests for dune erosion by the surf-beat mode of the two-dimensional horizontal (2DH) model XBeach for storm wave conditions showed that the model overestimates the erosion and thus the deposition volumes. This overestimation becomes particularly significant for very large overtopping rates on coastal barriers, which are commonly induced by extreme storm surges. In this paper, the recent model improvements to overcome this problem are first summarised. Moreover, two physical reasons for these overestimations are identified: (i) The wave nonlinearity effect on sediment transport, which is described in XBeach by a calibration factor for the time-averaged flow depending on the wave skewness and asymmetry and (ii) The considerable excess of the actual shear stress required to initiate the sediment particle motion as compared to that predicted by the common Shields curve. To improve the prediction capability of XBeach in terms of erosion and overwash volumes, two new improvements, related to the two aforementioned reasons for the overestimation, are introduced and implemented in the model. The improved XBeach model is then tested for dune erosion, for barrier breaching as well as for a barrier island erosion and overwash under an extreme storm surge event. The results showed a very good prediction capability of the improved model. Particularly, the second model improvement opens the way toward further model improvements to account for spatially varying soil resistance, which is crucial for reliable prediction of a barrier breaching.

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.

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Europe and many other countries all over the world are often surrounded by coastal defence systems (e.g. protective dunes and dykes) in order to protect coastal areas from threats of wave attack, storm surges and subsequent coastal floods. During moderate sea conditions, wave attack and coastal erosion is limited to nearshore areas and may only cause shore erosion. Under the same conditions, fresh groundwater, which is hydraulically interconnected with seawater, is in equilibrium with the laterally intruding seawaters. Such equilibrium prevails as long as the moderate sea level (MSL) and the hydrogeological conditions at the sea/land boundary are stationary. However, during extreme storm surges, the higher water levels may temporally threaten the coastal defence systems. In fact, shortwaves riding on the temporally rising sea level during storm surge events may directly runup, rundown and/or impact on barriers, possibly causing seaward erosion followed by lowering of barrier’s crest and hence wave overtopping or overflow through combined surge and waves. As a result, barriers may breach, inducing coastal inundation and subsequent vertical saltwater intrusion (VSWI) behind the breached barriers due to the vertical infiltration of inundating seawater into the fresh groundwater. In this study, a new integral physically based methodology is developed to reliably assess the possible implications of extreme storm surges on the safety of coastal barriers and the implications of possible breaching for contamination of coastal aquifers. The integral model is therefore capable to successively simulate breaching/overtopping of coastal barriers forced by storm surges as a hydraulic load, induced flood propagation in the hinterland and subsequent VSWI into coastal aquifers while considering the complexity of these processes and mutual interaction among them. The modelling methodology consists of an improved XBeach code (Roelvink et al., 2009) for hydro-morphodynamics unidirectionally coupled with the SEAWAT code (Langevin et al., 2008) for groundwater flow. The model is applied to a case study in northern Germany, showing that marine floods represent a serious threat to usability of coastal aquifers which are extremely important water resources. Outcomes of model application showed that a coastal flood event of a few hours may contaminate coastal aquifers for many decades, thus reducing the agricultural yield and hindering the sustainable development in coastal areas prone to coastal floods. This study represents, to the knowledge of the author, the first systematic research study that addresses the safety of natural coastal sandy barriers under extreme storm surge conditions together with the consequences of possible barrier breaching and overwash on subsequent flooding and saltwater intrusion into fresh groundwater. Moreover, it is probably the foremost study that attempts to mitigate storm-driven saltwater intrusion through the use and modelling of a subsurface drainage network. Besides improving the agricultural yield in coastal areas, the use of subsurface drainage network significantly reduces the natural remediation interval required for aquifers recovery after a coastal flood event. Moreover, it limits the vertical extent of the salt intrusion. The multiple flow domains and aspects discussed in this research make it a multi-disciplinary study that is quite relevant for the coastal engineering community, for flood risk managers, for coastal hydrologists, for groundwater suppliers as well as for sustainable development planners.

Sand dunes and other natural coastal barriers (e.g. barrier islands) represent important components of the defense system against consequences of storm surges. However, in many coastal systems, major storm surges represent important drivers of coastal erosion. Increased extreme events potentially result in accelerated coastal erosion, coastal barrier breaching, and coastal flooding. The response of a barrier to a storm surge is often determined by mutual interaction among the driving hydrodynamics, the subsequent morphodynamics, and the local geology, including spatial variations of subaqueous bathymetry and subaerial topography. However, the effect of alongshore variability of soil properties on the alongshore varying response is not yet considered. Therefore, this study examines soil parameters that may affect coastal erosion during major storm surges. Moreover, it applies a novel extension of the numerical model XBeach that accounts for spatial variation of soil properties to an artificial dune system of spatially varying soil permeability. Results showed that variability of soil permeability alongshore the dune results in alongshore varying resistance to erosion so that breaches may occur at the locations of less resistance that are corresponding to locations of higher soil permeability. Outcomes of the numerical simulations proved also that reduced soil permeability represents a nature-based solution that increases the resilience of natural defense systems during major storm surges by mitigating rates of coastal erosion.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/1ERwbW5BmYA

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