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.