Adaptation of coastal structures to mean sea level rise

With the mean sea level (MSL) rise, coastal defence structures will be exposed to wave height, which are larger than the design values, in particular for all the structures built in shallow waters where the depth imposes the maximal wave height due to bathymetric breaking. If MSL rise is one meter, the crest of these structures will have to be raised between two and three meters in order to keep the same overtopping volumes. Moreover the structures will have more severe damages and mass of armour units should be doubled. Statistics moderate the first conclusions because it keeps into account the whole set of events, including in particular shoaling waves. Schematically with the increase of damages, according to the severity of changes the stakeholders will adopt one of the following scenarios: a) repairing the structures as it is b) reinforcing the structures c) demolishing and redesigning the structures d) accepting coastal realignment. Three axes of reinforcement of structures are presented: limiting overtopping by modifying for example the crown wall, improving armour stability by adding an armour layer or by using milder armour slope and reducing the incident wave energy by building a detached low-crested breakwater or by sand nourishment. A curved parapet wall is a very efficient solution for impervious structures. This solution must often be completed by. an additional armour layer for pervious structures. The front reservoir is also a promising solution. Cost benefice analysis (CBA) applied to the city of Le Havre shows that reinforcement becomes economically justified in district of Malraux when MSL rise is 1 m. Redesign and coastal realignment as far as they are concerned are acceptable when MSL rise exceeds 2 m.