When two storms quickly succeed one another, it can be called a twin storm. Storm conditions at the coast can lead to higher water levels and waves that can give rise to dune erosion. Therefore, after a storm the dunes are smaller than before the storm. Dune accretion is a much slower process than dune erosion; it can take weeks to months of time for a dune to grow the volume of sand it lost during a storm. If the interval time between two storms is smaller than the time it takes for a dune to gain the same volume of sand it loses during the storm, the second storm can lead to additional impact. So when storms quickly succeed one another, there is not enough time for the dune to significantly grow in between the storms. Due to additive impact, a twin storm might lead to more damage than a single storm with the same return period. In this research it is investigated whether twin storms give more dune erosion than a single storm with the same return period.

To answer this question, the research is divided into two parts. The first part consists of creating representative storm schematisations for single storms and for twin storms using simulated weather data from the European Centre for Medium Range Weather Forecasts (ECMWF). To start with, the storms from the dataset are divided into several clusters. For each cluster, one schematisation is made that represents the storm within that cluster.
In part two, the storm schematisations are used to calculate the dune erosion for each defined storm cluster. The dune erosion is modelled using XBeach, which is a model that is developed to simulate the evolution of the dune profile during extreme storms. The dune erosion will be modelled for two locations along the Dutch coast: Hoek van Holland and Nieuwvliet-Groede.

In this research, it turns out that at Hoek van Holland 25-30% of all storms can be considered a twin storm. At Nieuwvliet-Groede, 15-20% of all storms are twin storms. For Hoek van Holland very few of the constructed twin storms (approximately 10%) give more dune erosion than the constructed single storms. At location Nieuwvliet-Groede approximately 30% of the twin storms cause a larger dune erosion volume than the single storms. In most cases where twin storms result in more erosion than a single storm, this effect can be attributed to deviations in the storm schematisations.

For further research it’s recommended to investigate how the prediction of the time evolution of the water level can be improved. This is relevant because the dune erosion volume is strongly correlated with the water level. Furthermore, it is advised to do more research into the time scales and modelling of dune growth. Storm groups with interval periods of multiple weeks can be investigated with better predictions of dune growth.