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The loads resulting from impact pressures are among the most dangerous loads for coastal structures. The impact of a breaking wave on a coastal structure generates impact like loads which are characterised by very high but short pressure peaks often followed by pressure oscillations. Coastal structures which are not constructed with dumping materials will exhibit joints due to their design. These joints usually are sealed to prevent the intrusion of water. Settlements, displacements or weathering of the sealing material can lead to cracks in these joints. If cracks are located near to the impact area of breaking waves a possible propagation of the impact pressures into the cracks can result in high loads inside the structure thus endangering the structural integrity.

By means of extensive physical model tests it is demonstrated in this thesis that inside cracks with small heights close to the impact area considerable pressures are generated. The maximum pressures at the crack end are of the same magnitude as the impact pressures in the impact area on the front face of the breakwater model. For the analysis of the very complex pressure time histories also fluid mechanics of pipelines describing the water hammer phenomena are consulted. The analysis shows that the pressure time histories are mainly caused by the generation of driven and self-induced oscillations of the water and air volumes trapped inside the crack. Only for certain boundary conditions pressure propagation comparable with water hammer propagation is observed. For these cases high pressures are generated in the front part of the crack which are rapidly attenuated inside the crack.

The most important consequence for the design and the maintenance of coastal structures is that additionally to the external loads on the structure internal loads must be considered. The verification of the global structural safety is not sufficient. It is rather necessary to examine the integrity of the structure which is endangered by high loads in cracks.

Completing the findings of this thesis recommendations are elaborated for the design of new coastal structures and for the maintenance of existing coastal structures. The main attention in the design and in every maintenance strategy should be turned to the securing of a long lasting compound effect of the elements of the structure. This is especially important for elements which are situated above horizontal joints in the impact area of breaking waves.


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