As demonstrated by recent tropical cyclone events, including U.S. Hurricanes Harvey, Irma, and Maria (2017), and Florence (2018), the destructive potential of flooding driven by wind, precipitation, and coastal surge coupled with growing exposure of people and property along coastlines is leading to unprecedented damage from coastal storms. In this paper, we demonstrate the ability of the recently developed Super-Fast INundation of CoastS (SFINCS) model to delineate the depth and extent of flooding during Hurricane Harvey in Houston, Texas. The model was validated against water level time-series at twenty-one United States Geological Survey (USGS) observation points and 115 high water mark locations. FEMA depth-damage curves were used to estimate building and content damages from the combined flood sources (e.g., pluvial, fluvial, and marine) and total losses are compared against insurance claims registered with the U.S. National Flood Insurance Program (NFIP) and a depth grid produced during the U.S. Federal Emergency Management Agency’s (FEMA) Preliminary Damage Assessment (PDA). The results suggest that Harvey may have caused upwards of $8.3 billion USD in uninsured residential loss within the model domain. Comparison against FEMA’s PDA indicates that the SFINCS model predicts much larger total losses, indicating that the incorporation of spatially-distributed pluvial hazards into the modeling method is critical for identifying high-risk areas and supports the need for further flood risk analyses in the region.

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Hurricanes impose great threats on coastal communities in terms of hazard and impact. Recent hurricanes like Harvey (Texas, 2017) and Idai (Mozambique, 2019) emphasize the global character of this threat. In the U.S., Hurricane Harvey tied with 2005's Hurricane Katrina as the costliest tropical cyclone on record, inflicting $125 billion in damage. Harvey hovered above the state of Texas for six days in 2017, making it the longest landfalling storm in Texas history. During this time, over 600 millimeters of rain had fallen in most of the Houston area, with extreme observations showing 1500 millimeters of rain. This, in combination with surge and river discharges, caused inundation for over one-third of Houston. Current generation coastal flood early warning systems are often not designed to take into account the compound effects of pluvial, fluvial and marine flooding (e.g. the ADCIRC + SWAN model deployed by the Coastal Emergency Risks Assessment group ignores pluvial flooding). Moreover, the advanced models applied in these systems are computationally demanding and can therefore not be used in probabilistic real-time forecasting applications in order to include uncertainty in meteorological conditions. Furthermore, despite the importance of tropical cyclone rainfall, this field is not completely understood. Tropical cyclone rainfall distributions are dependent on best track data parameters (e.g. storm motion velocity), but also on environmental elements (e.g. topography). Current modelling approaches are either computational inefficient (e.g. large-scale climate models), are location-specific or are dependent on parameters not always available in archives. Therefore, there is need for a simple generic parametric rainfall model. Main focus of this research was the derivation of a methodology for assessing the joint probability of fluvial, pluvial and marine flooding. For this study, a case study is carried out for Houston, Texas. The TCWiSE tool is used to generate a set of synthetic hurricane tracks based on historical data and a Monte Carlo sampling method. A derived parametrization of tropical cyclone rainfall is used to generate a spatial tropical cyclone rainfall field. Furthermore, a Delft3D-FM model is used to generate offshore water level time-series for the synthetic hurricanes. A combination of SFINCS and Delft-FIAT is used to make an assessment based on both hydrodynamics and exposure for every single generated hurricane. The model train is capable of carrying out a flood risk assessment, derive flood maps for given return periods (e.g. 1 in 100-year flood) and make an exposure assessment for the joint occurrence of pluvial, fluvial and marine flooding. This framework can potentially be a useful tool for future policy- and decision making.

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.

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.

Small island developing states are among the most vulnerable areas to the impact of natural hazards and climate change. Flooding due to storm surges and extreme waves, coastal erosion, and salinization of freshwater lenses are already a serious threat and could lead to irreversible consequences in the coming decades. Reef flat mining is one of the most common practices to source the required material for the implementation of coastal protection measures, but concerns remain that partial removal of the protective reef could increase wave loading on the islands. However, the available data and knowledge on the effects of these mining pits are currently very limited. This study provides new insights on the effects that pits may have on nearshore hydrodynamics and wave runup. Results are based on a large numerical data set of fringing reefs, derived using the validated XBeach nonhydrostatic+ process-based model. Model results indicate that excavation pits cause a decrease in infragravity wave energy around the fundamental mode of the reef, which is partly caused by reduced wave transmission. Additionally, changes in sea and swell wave energy are attributed to reduced transmission, “a decrease” in wave dissipation, and (triad) wave–wave interaction. Furthermore, in 13% of all modeled cases, an increase in wave runup is observed, mainly due to more sea and swell wave energy reaching the shoreline. This probability is lowest for narrow pits relative to the reef flat width or pits located further from shore.

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