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.

The Clarence River catchment is located in the state of New South Wales (NSW), on the east coast of Australia. The lower Clarence Valley is an area covering approximately 1000 square kilometers and is located on the downstream part of the Clarence River. Due to heavy rainfall, the Clarence River discharge can increase from an average 160 m3/s to 20000 m3/s. As a result, water levels rise significantly leading to severe floods in the Clarence Valley. The main urban areas in this region, Grafton, South Grafton and Maclean, are located in narrowing river bends which makes them particulary vulnerable to flooding during high water levels.

The main goal of this report is to present flood mitigation measures to reduce the impact of flooding in the urban areas of the Clarence Valley, based on the Duthc flood mitigation strategy called 'Room for the River'. Consequently the following research question was formulated:

How can the impact of flooding on the urban areas in the Clarence Valley be reduced by increasing the storage capacity of floodplains?

In order to answer the research question, the following project approach is applied. Six areas were identified, based on a fieldvisit and an extensive preliminary study, to implement flood mitigation measures and assess existing flood defences. Part of these flood defences are the Swan Creek Floodgate and the reinforced concrete levee wall of Maclean, which will be investigated on their performance. A fully calibrated numerical floodmodel provides input for the hydrological analysis. The model represents the current situation in the Valley. Scenarios are created by applying topographic adjustments. The new scenarios are implemented into the numerical model and the effectiveness on flood mitigation in urban areas is assessed by comparing the results of a 5, 20 and 50 year Average Reccurance Interval flood event to the current situation during one of these flood events.

By making use of the proposed floodplains and improving the performance of existing flood defences, the flood defence system of the Clarence Valley can be extended. It can be concluded that it is possible to reduce the impact of flooding in the urban areas of the Clarence Valley by increasing the storage capacity of floodplains around Grafton. Therefore, the usage of a ’Room for the River’ strategy can be a solution to the problems the Clarence Valley is facing, and possibly might be applicable to more flooding-vulnerable areas in Australia.

Since 2003 there is a cooperation between the Hydraulic Engineering department of Delft University of Technology and Bulgarian universities. The cooperation focusses on exchange of knowledge and the development of the coast in the area of Varna. Dutch and Bulgarian students get the possibility to gain experience in data collecting, processing and interpreting. Repeating this fieldwork every year in the same area will provide an overview of the coastal development in the Varna area. The students will act as consultants for local hotel owners at the Varna coast. Their work consists of measuring hydraulic aspects in the project area and making a rehabilitation plan for the St. Elias Marina. Data collection consist of inventory material near site, beach measurements, wave measurements, profile measurements, quarry analysis and a bathymetric survey. The rehabilitation plan contains the development of sub-areas in the St. Elias Marina like the peninsula, north beach, south beach and the breakwater.