The impact of extreme storms on the coast is one of the costliest forms of natural disasters (Kron [1
]; Bertin et al. [2
]). In heavily urbanized coastal areas, such as the Mediterranean (in general) and the Catalan coast (in particular), where properties, infrastructures and businesses are located close to the shoreline, this kind of event usually results in the damage or destruction of exposed assets (Jiménez et al. [3
]). These effects are the integrated consequences of two main storm-induced coastal hazards: inundation and erosion. In this context, an accurate assessment of the magnitude, location and extension of these hazards is becoming an essential part of the risk management process (e.g., Ciavola et al. [4
]; Van Dongeren et al. [6
], Jimenez et al. [7
]; Plomaritis et al. [8
], Harley et al. [9
]) and, in this sense, the use of process-oriented models to forecast storm-induced morphodynamic changes under given scenarios is now standard (e.g., Roelvink et al., [10
]; McCall, [11
]; Van Dongeren et al. [12
] and references therein, Dissanayake et al. [13
]). Most of the studies on testing state-of-art morphodynamic process-based models have addressed cases characterized by straight coastlines and gentle slopes (i.e., conditions close to the comfort zone of the models) (e.g., McCall, [11
]; Harter and Figlus [14
]). However, applications to estimate costal hazards in hihgly curvilinear environments (e.g., deltaic cuspate coasts) have seldom been tested (e.g., Roelvink et al., [15
]; Valchev et al., [16
]; and Dissanayake et al. [13
]). Furthermore, the effect of testing models based on surf-beat (i.e., the infragravity wave band) on steep slopes and coarse sediment has been recently undertaken mainly in 1D applications (e.g., Vousdoukas et al. [17
]; Elsayed and Oumeraci [18
]) but rarely so in fully 2DH (2-dimensional, depth-averaged) simulations.
Within this context, the magnitude of storm-induced hazards on a highly curvilinear coast by using XBeach is assessed in the present study. The relevance and main aim of this work is twofold: first, from a general standpoint, to test the use of Xbeach on a highly curvilinear coast characterized by coarse sediment reflective beaches, and second, from the local standpoint, to analyze the sensitivity of an already identified hotspot, the Tordera Delta (NW Mediterranean) (Jiménez et al. [7
]), to assess storm impacts for different storm direction scenarios. Thus, the largest recorded storm in the area is used as base case scenario. It occurred in December 2008 and had the typical incoming direction of current climate conditions where eastern (E) incoming storms dominate (e.g., Mendoza et al. [19
]). Existing storminess projections under climate change scenarios for the Western Mediterranean do not predict any increase in wave height (e.g., Lionello et al. [20
]; Conte and Lionello [21
]), but some projections identify potential changes in wave direction (Cases-Prat and Sierra [22
]). Due to this and to the great sensitivity of cuspate coastlines to wave direction resulting from their curvature (e.g., Slott et al. [24
]; Johnson et al. [25
]), the study aims to assess the potential effects of changing wave direction on extreme storm-induced hazards for the Tordera Delta. The hypothesis to be tested is that changes in wave direction may cause large variations in the magnitude of storm-induced hazards. Other studies have included the sensitivity to incoming storm direction in their assessments, such as those by Mortlock et al. [26
] in Australia, or de Winter and Ruessink [27
] in Holland.
The article is arranged as follows: the second section introduces the study site and the data used, describes the Sant Esteve 2008 event, which is used as the base case storm-scenario, and presents the methodological part, i.e., the used morphodynamic model and the comparative assessment framework descriptions; the third section presents obtained results; and finally, the discussion and concluding remarks are presented in the fourth section.
4. Discussion and Conclusions
In this study, the potential effects of changing wave direction for the storm-induced hazards on a highly curvilinear coarse sandy coastline have been assessed. This sensitivity test has been selected because although storminess projections under climate change scenarios for the Western Mediterranean do not predict any increase in wave height (e.g., Lionello et al. [20
]; Conte and Lionello [21
]), some existing projections identify potential changes in wave direction (Casas-Prat and Sierra [22
]). These changes in wave direction may have significant implications for coastal sediment transport and coastal stability, as has been confirmed for the interannual changes influenced by El Niño (e.g., Barnard et al. [60
]). Moreover, regarding cuspate coastlines such as the study area, their greater sensitivity, due to their curvature, results in even more significant implications (e.g., Slott et al. [24
]; Johnson et al. [25
The tested hypothesis is that changes in wave direction may cause large variations in the magnitude of storm-induced hazards. This effect has also been addressed in other studies such as those by Mortlock et al. [26
] and de Winter and Ruessink [27
], which specifically analyzed the effects of changes in wave direction on the storm-induced hazards in the SE Australian and Holland coasts respectively. To this end and to isolate the influence of wave direction, we used a recorded long-return period storm as a base case scenario and we built test scenarios just by changing wave direction while maintaining the other wave parameters as recorded during the base storm (wave height and period).
In any case, tested conditions have not been designed to be used as climate change induced projections, as that may require the proper forecasting of regional wave conditions under given climate scenarios (e.g., Casas Prat and Sierra [23
]). These have to be considered from the perspective of coastal risk management, in which a set of possible conditions are analyzed to characterize coastal vulnerability and resilience to inform risk management under uncertainty (see e.g., Hinkel et al. [61
] for an application of this perspective to analyze sea level rise). In the study area, the current storm wave conditions depend on direction, with largest wave height and power being associated with NE-E waves, whereas S storms are less frequent and present a smaller associated power (e.g., Sánchez-Arcilla et al. [62
]; Mendoza et al. [19
]). To assess the potential variability on storm-induced hazards, tested scenarios were built by just changing wave direction while the remaining recorded parameters (representative of a worst case scenario, according to recorded conditions) were maintained.
This analysis has been performed by using the SWAN and XBeach models to simulate storm-induced hazards. Both models were calibrated by using data recorded during the impact of an extreme storm recorded in December 2008, which is used as the base case scenario. Although it is desirable to use more than one event to properly calibrate/validate the models (e.g., Ranashinge [63
]), data availability during storm conditions was restricted to this event. However, on the positive side, it has to be considered that this storm was the largest event recorded in the area and representative of extreme storms with a very long return period (Mendoza et al. [19
]) under current climate conditions. The SWAN model was very successful in simulating wave conditions during the development phase of the storm up to the pass of the peak of the storm, with the larger differences between measured and simulated waves being detected during the relaxation phase of the storm, when most of the induced changes had already occurred. The default parametrization of the XBeach model had to be adapted for application at the side to represent the effects the coarse-sand environment. Sediment transport was limited by using the sedcal parameter, avalanching was limited by increasing the critical slope, wave asymmetry was increased as suggested in literature for steep slopes (Elsayed and Oumeraci [18
]) and groundwater effects were included. Gamma and delta wave breaking parameters were also tuned (Table 1
). Calibrated parameters setup for XBeach in the study area led to a BSS score of 0.68 in spite of the out-of-comfort tested conditions (i.e., highly curvilinear coast, steep beach, coarse sediment). Although the predictive skill was very good for the northern and southern beaches, the model performance was better in the northern domain (BSS = 0.75) than in the southern one (BSS = 0.60), since this last area presented a significantly larger obliquity to wave direction during the storm, and a more complex bathymetry.
The obtained results show a very high sensitivity of storm-induced processes, i.e., inundation and erosion, to changes in storm wave direction. With respect to inundation, expected changes in hazard magnitude are very significant, especially in the southern part of the study area, since its morphology is characterized by a lower berm, and its low-lying unprotected hinterland makes this area sensitive to storm flooding (Jiménez et al. [7
]). Thus, as storm waves turn from the base case (C0) to the south, the inundated surface along this southern beach dramatically increases due to its direct exposure to the south. On the contrary, a potential shift of wave direction to the N will have a positive impact on inundation in this area, since it will be more sheltered from wave action. At the northern beach, the largest increase in inundation hazard verifies under C20+ and C40+ scenarios when waves face nearly orthogonally to the coastline, although due to local morphology, the affected surface is much lower than in the southern beach. The hinterland of the study area is mostly occupied by agriculture land and, in the outer fringe just behind the shoreline, by campsites. In this sense, to transfer the potential change in hazard magnitude to changes in damage risk, it should be important to consider not only the change in direction but also its seasonality. Thus, risk may vary dramatically between the summer season (when the campsite facilities are used by visitors) and the rest of the year when only installations will be affected (e.g., Merz et al. [64
]). An analysis of the risk associated with storm-induced inundation for different storm conditions can be seen in Sanuy et al. [32
Similarly, storm-induced morphodynamic changes are more sensitive to directional changes on the southern beach, where the magnitude of the changes is larger. The beaches at the south of the river mouth present a larger spatial variability than those in the north due to the presence of a local boundary condition in form of a revetment at the shoreline. This revetment, which modifies local longshore transport, significantly enhances downcoast erosion under storm conditions. This induces a southwards directed longshore sediment transport while simultaneously promoting the accumulation of upcoast sediment. This contrasting behavior is particularly observed in the base case scenario which seems to represent the optimum conditions for longshore sediment transport in the area, thus inducing the largest changes in the surroundings of the structure.
In a particular case, under C40− and C20− scenarios, when wave direction turns north, the beach sector just south of the river experiences an important sediment accumulation due to the apparently efficient transfer of sediment from the northern beach across the river mouth and the partial barrier effect of the existing revetment.
The magnitude of the erosional response along the two control sectors in the northern beach is similar, although a higher variability is detected in the area closest to the river mouth. In general, there is a slight increase in erosion rates as wave direction turns south. This variation should be indicative of the role of longshore sediment fluxes during storm conditions. Thus, as the controlled northern area is just besides the river mouth, where there is another structure acting as a boundary condition, the increase in longshore sediment transport as waves turn S (scenarios from C20+ to C80+) will increase sediment losses, which will be transported further to the north. This behavior is currently observed in the northernmost part of this beach (out of the control zone in Figure 10
) which experiences sediment accumulation under the impact of southern storms.
As expected, changes in the magnitude of overwash deposits follow observed changes in inundation, i.e., they increase as wave direction turns to the south, with maximum values around C20+ and C40+. The exception to this is the predicted changes in the southernmost sector, which present the largest overwash for C80+ conditions. The spatial variability in the northern beach is significantly lower than in the south, with small variations in magnitude across the tested range. Moreover, and reflecting the observed differences in inundation, the magnitude of overwash deposits is much higher in the southern sector.
Finally, and as a concluding remark, this analysis has shown that storm-induced hazards along a highly curvilinear coast are extremely sensitive to changes in wave direction. This means that even under a climate scenario of relatively steady storminess (wave power and frequency), a potential shift in wave direction may significantly change hazard conditions and, in consequence, need to be accounted for in robust damage risk assessments. To this end, an analysis such as the one presented here also permits an assessment of how coastal geomorphology modulates induced changes. In the study area, the low-lying nature of the southern beach and its orientation with respect to the current dominant storm direction make this area much more sensitive to directional changes. This is especially relevant from the coastal management standpoint because this area has been already identified as a hotspot for storm impacts under current conditions. The use of detailed process-based models has permitted the identification and quantification of the drastic increase in sensitivity when anthropogenic perturbations are present along the coast. These perturbations act as boundary conditions modifying local hydrodynamics and associated transport. For the case study analyzed here, the obtained results clearly identify the hazardous potential of the existing revetment in the southern beach, which has also been identified under current conditions, suggesting that its removal will soften the estimated morphodynamic response.