Schweiger, C.; Koldrack, N.; Kaehler, C., and Schuettrumpf, H., 0000. Influence of nearshore bathymetry changes on the numerical modelling of dune erosion. Coastal dunes are an essential protection measure at the German Baltic Sea coastline. As the breaching of dunes during a storm is very complex and detailed data is still rare regarding the evolution, width and the depth of the breach and how the hinterland will fill through a breach and empty after drawdown, the aim of the collaborative project PADO is to investigate how a real dune breaches during a storm and how the hinterland is flooded. Within the scope of a physical model test, a large-scale research dune was built at the beach of Rostock-Warnemuende in Germany. During the modelling, the nearshore bathymetry was measured monthly and a high variability was observed. The objective of this study was to investigate the influence of a varying initial nearshore bathymetry on the results of coastal erosion modelling. Therefore, four XBeach models which differed only in nearshore bathymetry were set up and forced with a storm surge event from October 2017. The results were analyzed with regard to the computed post-storm bed level. Using the nearshore bathymetry of July and November 2017, large amounts of the research dune are eroded. The eroded sediment minor decreases when the nearshore bathymetry of September 2017 is applied. On the contrary, the sole use of freely accessible bathymetry data leads to significant decrease in the computed dune erosion. This leads to the conclusion, that the initial nearshore bathymetry can have a significant influence on the computed post-storm bed level. Especially when only freely accessible bathymetry data can be used, deviations between post-storm measurements and computed dune erosion can occur. This might be due to out-of-date data or a too coarse resolution. The results also show that pre-storm bathymetric measurement does not necessarily have to have a high resolution in order to achieve reliable results.

This paper presents and validates a novel root model which accounts for the effect of belowground biomass on dune erosion volumes in XBeach, based on a small-scale wave flume experiment that was translated to a larger scale. A 1D-XBeach model was calibrated by using control runs considering a dune without vegetation. Despite calibration, a general model–data mismatch was observed in terms of overestimated erosion volumes around the waterline. Furthermore, the prediction of overwash had to be induced by increasing the maximum nearshore wave height within the XBeach simulation. Subsequently, applying the root model resulted in a good agreement with the belowground biomass cases, and the consideration of spatially varying rooting depths further improved the results. Predictions of the root model while using locally increased friction coefficients were in line with the aboveground and belowground biomass cases. However, the effect of the root model on the erosion predictions varied among the hydrodynamic conditions, so further improvements are required. Therefore, future research should focus on quantifying the effects of land-based biomass and individual plant characteristics, such as root density, on dune erodibility at large scales, along with their influences on the temporal evolution of dune scarping and avalanching.

Besides the natural protection of coastal dunes against storm surges (Mehrtens et al., 2023), blue-grey-hybrid dune structures play a key role in strengthening the protective mechanism of dunes against erosion, while imitating their natural landform appearance and function (Nordstrom 2019). Although artificial dunes with hybrid core enhancements are widespreadly implemented along many coasts worldwide (Winters et al., 2020), limited work addressed the effectiveness and potential enhancement of specific dune reinforcement strategies in a comparative fashion. An exception to this statement is the work by Figlus et al (2015), who conducted a series of experiments with three incorporated dune cores: a T-wall, a clay levee and an armour stone revetment. Nonetheless, a benchmark study on the efficacy and comparative performance of hybrid dune reinforcement strategies, with an identical parameter frame remains unavailable to date.

A large part of the Baltic Sea coastline in the north-eastern provinces of Germany (Mecklenburg-Vorpommern and Schleswig-Holstein) is protected by coastal protection dunes, which are constructed and maintained to preserve a quasi-natural beach environment. The latest IPCC report predicts a significant sea level rise even in the Baltic, together with an increase in storm surges within the next decades. Sand dune constructions are particularly affected by these developments, in close connection to the increasing problems of sediment mining for beach nourishment and dune maintenance. In the BMBF research project PADO (Processes and Implications of Dune Breaching at the German Baltic Sea Coast), scientists from Rostock University and RWTH Aachen University investigated together with various partners how constructed sand dunes would fail during a storm surge, focusing on the erosion and the initiation and development of a breach. Therefore, a large-scale field experiment was installed at the beach of Rostock-Warnemünde, which was instrumented and surveyed in high resolution during the storm surge in November 2018, leading to the collapse of the dune, using innovative measurement systems, including terrestrial laser scanning and photogrammetry. The measurements were used to run a numerical model (XBeach) to simulate dune breaching for three different sections of existing sand dunes at the coast of Mecklenburg-Vorpommern. Additional investigations were made to analyse the filling and emptying of the polder areas behind the coastal protection dunes and the consequences of the floods with respect to saltwater intrusion. In this paper, the field experiment and the results from measurements and simulations are compared to one of the real reference dunes, and the transfer of results to a dune breach incident near Rostock in 2019 is discussed.