River floods are becoming increasingly devastating because of climate change (more frequent and extreme rainfall), population growth and the increasing economic importance of river basins. This situation requires maintenance and strengthening of flood-defence systems. Adding certain types of vegetation at precise locations for their positive impact may be a cheaper, more flexible, and more environment-friendly way to strengthen dikes than the traditional increase in height. However, this nature-based (NB) option is not yet widely implemented due to the lack of precise knowledge of the potential of vegetation effects and their uncertainty. This study uses a probabilistic method to better understand the effects of vegetation by including vegetation in the computation of the failure probabilities of Dutch river dikes. A framework was established to combine all these vegetation effects simultaneously in the computation of the total failure probability, considering different magnitudes of each effect. This enables the consideration of a wide range of vegetation scenarios, from which conclusions were drawn. Overall, this thesis provides a useful and versatile tool for assessing the influence of vegetation on dikes that has a lot of potential and can be easily enhanced in the future.

The United States of America have a safety standard for flood protection of 1/100 year. However the flood protections in the Washington D.C. do not comply with this requirement. During this study the levees in Washington D.C. area were analysed and it was found, that in order to comply with the 1/100 safety standard the levees in the NationalMall needs to be heighten by 0.5mand those near the Anacostia river by 1.5m. The safety standard of 1/100 imposed in the United States of America is not based on exact calculation, therefore another design method was applied, which uses an optimal safety level based on the damage cost of floods and investment cost of flood protections. The calculation of the optimal return period was based on the ’Standaardmethode 2017’ and it was found that the optimal return period at the NationalMall levee is 1/263 years. At the southern bank of the Anacostia river this optimal return period is 1/373 year. With these return periods new levees were designed in order to ensure the safety of the area of Washington D.C. There were two new levees needed around the National mall. The first was situated north of the Lincolnmemory along the Potomac river and the second replaces the temporary flood defence by the 2nd street SW and had an L-shape facing the Potomac river. The Anacostia levee was stretched on the East side in order to comply with the new 1/373 year safety standard. The old and new National mall levees needed a height of 3.5 m and those by the Anacostia needed a height of 8.5 m. It is important to take cautionwith these new levees as some houses needed to be removed. An attempt was made at modelling the above mentioned floods in ADCIRC to improve the accuracy of the design water levels and to verify the final design. However, the model showed instabilities, preventing the results from being used. Explanations for these instabilities are the incoming water not being properly ramped up, the large vertical gradients in shallow areas and the upstream and downstream boundaries being too close to each other.