Influence of location and instrumentation on wave group characteristics

Wave groups have received a considerable interest in the last decades because of their connection to various coastal and ocean engineering problems, including the damage and capsizing of ships, high forces in mooring lines and the resonance of flexible structures. Studies of wave groups in random seas generally involve the statistical description of runs of one or more high waves. Here, run lengths in intermediate (both relatively deep and relatively shallow) water depths are discussed in comparison with theoretical predictions provided by a Markov chain approach (e.g. Kimura A., Statistical properties of random wave group, Int. Conf. Coastal Engineering, vol. III, 2955-2973). To this end, time series have been selected from wave buoys measurements at two different locations: West coast of Orkney (Scotland, 50 m water depth); Belgian coast (10 m water depth). Note that whereas the first is exposed to North Atlantic swells, the latter is more influenced by fetch limited conditions. Furthermore, at the Belgian coast the wave elevation is also monitored with a fixed (radar) device. Measurements from the latter are used to address possible differences in group activities related to the type of instrumentation. Results indicate that, for similar spectral conditions, the general distributions of the run lengths are remarkably similar for both locations. However, for less steep sea states, it has been observed that the group tendency is more pronounced at the West coast of Orkney due to the more relevant swell activities. For both locations, the empirical findings result in relatively good agreement with probabilistic predictions provided by the Markov chain approach. Analysis of data collected from a fixed (Eulerian) sensor (at the Belgian coast), however, has demonstrated the run lengths measured by a radar device are significantly lower than the ones obtained from the wave buoys