Evaluation of Damage in Tetrapod Breakwaters

ABSTRACT

The objective of this paper is to evaluate damage in breakwaters armoured with tetrapods using physical modelling data. The hydraulic laboratory tests reproduced the complex geometry of a rubblemound breakwater armoured with tetrapod concrete blocks and subjected to wave breaking action. The tests included both flume (2-dimentional) tests and wave basin (3-dimentional) tests using several combinations of input parameters, such as wave heights, wave periods and wave directions, water level elevations and tetrapod sizes. The results were considered as representative for comparison with published technical literature guidance.

During the physical model tests, the hydraulic stability of the tetrapod-armoured breakwater was evaluated, and the damage was measured for every design storm segment using the ‘flickering’ technique (which measures tetrapod rocking and displacements with repeated photography). The results obtained from the laboratory tests are in general similar to those predicted using published literature regarding the stability coefficient (KD) of Hudson’s formula (USACE, Coastal Engineering Manual, 2002). In some test cases, the model stability coefficients were lower than the recommended value, which means that the published guidance yields under-designed block sizes. This confirms our paper recommendation for a compulsory requirement of physical model studies for the final design of tetrapod-armoured breakwaters. Values of the ratio between incident wave height and design wave height corresponding to the no damage condition (0% to 5% damage level) were reported, and the results showed that wave breaking height and angle of wave incidence play a major role in obtaining accumulated damage progression of the breakwater armour protection during storms.