Breakwaters on steep foreshores

Abstract

Designing breakwaters on steep foreshores faces some problems. At present, it is not known what the influence of the steepness of the bottom slope is. Design formulae for armour layers don't take the bottom steepness into account in a direct manner. In theory, in irregular waves, wave average heights in shallow water become higher as the bottom becomes steeper, but clear formulae don't exist. In this report, an attempt has been made to investigate the influence of the slope of the sea bottom on the stability of an armour layer. The two main questions were: is there an influence of the foreshore slope at all? if yes, how large is this influence? In a literature study, no answer could be found to these questions, so a model investigation has been done in order to measure the influence of the foreshore steepness. In the investigation, two foreshore steepnesses have been compared (1:30 and 1:8) and four different wave periods have been used. In most experiments, the armour existed of stone. Two comparisons have been made: one was with the same wave conditions at the wave board, the other was with similar wave conditions close to the toe of the breakwater. The experiments showed a clear influence of the steepness of the foreshore: with equal offshore wave conditions, the damage is significantly higher at a steep foreshore. if the wave conditions near the breakwater are known, even then the damage on the steep foreshore is higher than on the mild foreshore. interlocking units showed more rocking. As, for the last case, the wave heights and the wave spectra were equal at the toe, these cannot explain the differences. Therefore, another explanation has been tried to be found. It appeared that the wave shape, or more precise: the steepness of the wave front, changes. On the steep foreshores, the wave fronts are steeper. This may explain the larger damage levels at the steep foreshore. At present, there are no wave theories that can calculate such a steepness of the wave front. Therefore, an empirical correction factor has been derived, which can take several values, depending on the situation. Until more research has been done on this topic, it is recommended to pay special attention to areas with steep foreshores if model tests are performed in the breakwater design process.