Impacts of storm clustering on beach/dune morphodynamics were investigated by applying the state-of-the-art numerical model XBeach to Formby Point (Sefton coast, UK). The adopted storm cluster was established by analysing the observed winter storms from December 2013 to January 2014 using a storm threshold wave height. The first storm that occurred during this period is regarded as exceptionally intense, and the occurrence of such a cluster of events is very unusual. A 1D model was setup for the highly dynamic cross-shore at Formby Point. After initial calibration of the model parameters against available poststorm profile data, the model was used for the simulation of the storm cluster. It was assumed that no beach recovery occurred between adjacent storms due to the very short time intervals between storms. As a result, the final predicted post-storm profile of the previous storm was used as the pre-storm profile of the subsequent storm. The predicted evolution during each storm was influenced by the previous storms in the cluster. Due to the clustering effect, the bed level change is not proportional to the storm power of events within the cluster, as it\ud would be in an individual storm case. Initially, the large storm events interact with the multibared foreshore enabling the subsequent weaker storms to influence the upper beach and lower dune system. This results in greater change at the dune toe level also during less severe subsequent storms. It is also shown that the usual water level threshold used to define dune erosion is over predicted by about 1 m for extreme storm conditions. The predicted profile evolution provides useful insights into the morphodynamic processes of beach/dune systems during a storm cluster (using Formby Point as an example), which is very useful for quantifying the clustering effects to develop tools for coastal management

Impacts of storm chronology within a storm cluster on beach/dune erosion are investigated by applying the state-of-the-art numerical model XBeach to the Sefton coast, northwest England. Six temporal storm clusters of different storm chronologies were formulated using three storms observed during the 2013/2014 winter. The storm power values of these three events nearly halve from the first to second event and from the second to third event. Cross-shore profile evolution was simulated in response to the tide, surge and wave forcing during these storms. The model was first calibrated against the available post-storm survey profiles. Cumulative impacts of beach/dune erosion during each storm cluster were simulated by using the post-storm profile of an event as the pre-storm profile for each subsequent event. For the largest event the water levels caused noticeable retreat of the dune toe due to the high water elevation. For the other events the greatest evolution occurs over the bar formations (erosion) and within the corresponding troughs (deposition) of the upper-beach profile. The sequence of events impacting the size of this ridge–runnel feature is important as it consequently changes the resilience of the system to the most extreme event that causes dune retreat. The highest erosion during each single storm event was always observed when that storm initialised the storm cluster. The most severe storm always resulted in the most erosion during each cluster, no matter when it occurred within the chronology, although the erosion volume due to this storm was reduced when it was not the primary event. The greatest cumulative cluster erosion occurred with increasing storm severity; however, the variability in cumulative cluster impact over a beach/dune cross section due to storm chronology is minimal. Initial storm impact can act to enhance or reduce the system resilience to subsequent impact, but overall the cumulative impact is controlled by the magnitude and number of the storms. This model application provides inter-survey information about morphological response to repeated storm impact. This will inform local managers of the potential beach response and dune vulnerability to variable storm configurations.

Climate change (CC) is likely to affect the thousands of bar-built or barrier estuaries (here referred to as Small tidal inlets - STIs) around the world. Any such CC impacts on the stability of STIs, which governs the dynamics of STIs as well as that of the inlet-adjacent coastline, can result in significant socio-economic consequences due to the heavy human utilisation of these systems and their surrounds. This article demonstrates the application of a process based snap-shot modelling approach, using the coastal morphodynamic model Delft3D, to 3 case study sites representing the 3 main STI types; Permanently open, locationally stable inlets (Type 1), Permanently open, alongshore migrating inlets (Type 2) and Seasonally/Intermittently open, locationally stable inlets (Type 3). The 3 case study sites (Negombo lagoon – Type 1, Kalutara lagoon – Type 2, and Maha Oya river – Type 3) are all located along the southwest coast of Sri Lanka.