Around the world, there are 58 Small Island Developing States (SIDS). What these SIDS generally have in common is that they are very susceptible to different natural hazards, among which coastal flooding. In order to address this issue, adaptation measures are needed. In order to prioritize adaptation efforts when considering these islands at a large scale, it is important to obtain an overview of which islands are most vulnerable to coastal flooding. This requires a large-scale assessment of coastal flooding for these islands. Such a large-scale assessment introduces different challenges, that were looked into in the hydraulic engineering part of this thesis. A first challenge is the availability op topography data for these islands. For most islands, only satellite-based DEMs are available. The accuracy of different satellite-based DEMs was assessed by comparing them to more accurate elevation data for 11 different islands. It could be concluded that in general, the TanDEM-X DEM is the most accurate for terrains with milder slopes, and the ALOS DEM for terrains with steeper slopes. Furthermore, the DEM error was found to be strongly correlated with forested and builded areas. Especially the DEM error in builded areas poses a problem for coastal flood assessments, as these are the main areas of interest in such assessments. Therefore, a building-correction method based on data from Open Street Map (OSM) was proposed and implemented. It could be concluded that the building-correction can both in- and decrease the DEM error, depending on the other error sources that are present in the DEM. A second challenge that is introduced when upscaling coastal flood assessments, is the used method for flood modelling. In the context of this thesis, a simple flood model (called the JBIW model) was developed. This model is a combination of the Janssen-Battjes model for short wave dissipation and the IW-method, which was originally developed for large-scale river flooding calculations. The JBIW model was tested in 1D and compared to other flood models. The results indicated that the simple flood model can be used to obtain a first estimate of coastal flooding, but is not accurate enough to predict exact values of the maximum water levels. Furthermore, the JBIW model was implemented in 2D for the island of Ebeye. The model was combined both with accurate elevation data and with building-corrected satellite-based DEMs. The results indicated that the error introduced by the use of the simple JBIW model were much smaller than the errors introduced by the use of the satellite-based DEMs. This indicates that further research focusing on the upscaling of coastal flood assessments for reef-fronted SIDS should focus on obtaining more accurate elevation data for these islands.
Apart from allocating resources to the SIDS in the most efficient way, it is important that the resources that are allocated to a certain island are used effectively. There are different ways to help these islands. An interesting approach is to focus on increasing the adaptive capacity of the SIDS. In the context of the science communication part of this thesis it was investigated whether the simple JBIW model could be used to increase the adaptive capacity to coastal flooding in the context of São Tomé. In order to do this, a theoretical framework was developed that aims to provide practical guidance in the assessment of the (barriers to the) adaptive capacity of a certain system. This framework was applied to the context of São Tomé to map the adaptive capacity to coastal flooding of the system and obtain an overview of the most important barriers to this adaptive capacity. Subsequently, it was assessed which barriers could be addressed with a tool based on the JBIW model. These barriers were used as the starting point for an initial design of the tool interface.

Schiermonnikoog is a Dutch barrier island with wide beaches and dunes which are both of importance for recreation as for safety. However, after decades of coastal accretion, in the last years, a strong coastline retreat is observed at the island head in the northwest. In tidal inlets, various forcing conditions of tides, waves, winds and estuarine circulation can all influence the pathways and magnitude of sediment transport. The dominance of any one of these processes can have a controlling influence on the morphology of the delta, inlet and island coast. In the case of Schiermonnikoog it is unclear what drives the recently observed coastal erosion. Therefore, the relation between the ebb-tidal delta development and the coastal transgression has to be studied. With knowledge on the behavior of the system, well-founded decisions can be made about the policy and the management in the area. In this thesis, the newly developed Delft3D Flexible Mesh (FM) modeling suite has been used to set-up and calibrate a morphostatic model. This model consists of the coupled D-Flow FM (Kernkamp et al., 2011) and D-Waves modules (SWAN, Booij et al., 1999), and can adequately hindcast measured water levels, wave heights and the expected sediment transports in the investigated area at Schiermonnikoog. Model uncertainties are overcome by incorporating the entire Dutch Wadden Sea in the model schematization. The unstructured grid allows for a local increase of the resolution in the area of interest. By analyzing the influence of individual physical processes on the system, it is found that wave action is the main driver of the migration of the approaching sand shoal. With the present-day bathymetry, the flood channel in front of the island coast no longer experiences significant tidal currents due to its hydraulic inefficiency. This indicates its upcoming abandonment, which would allow for the attachment of the sand shoal to the island coast. However, wave-driven currents and related sediment transport through the channel obstruct a closure. Similar to the situation at a the neighboring barrier island Ameland in 2014, a divergence point in the channel is visible at the location where the largest waves reach over the approaching sand shoal. The resulting gradient in the wave heights and consequent gradient in the alongshore sediment transport can be linked to the observed coastal erosion. This shows the importance of the shape of the sand shoal for the coastal dynamics in the system. To better understand the final stage before the abandonment of a flood channel, a conceptual model is presented.