Storm Wave Resonance Controlled by Hollow Block Structures
The formation of long shore bars in front of sandy beaches often is assumed to act as a shore protecting feature only. At Sylt Island/Germany, however, boundary conditions formed by a structured long shore bar, running roughly parallel to the shoreline, are found to be the reason for intense resonance absorption effects at storm surge conditions as well. Incoming waves interact with the water level deflections in the trough located between the bar and the beach in such a way that frequency components match a limited number of possible harmonics of the enclosed body of water. As there are significant energy densities to be found in the wave energy spectra at harmonic numbers 1 through 5 of the enclosed water body, this phenomenon is believed to be responsible for the tremendous coastal recessions at this island due to storm surge occurrences in the past. The existence of resonant seiching modes is deduced from intense low frequency anomalous dispersion effects (ADE), which had been found previously. Wave tank investigations also demonstrating the combined effect of resonance and anomalous dispersion are reported in detail. Based thereon the former field investigations, executed on Sylt Island, are analysed again. As a consequence of the kind of resonances found, extreme water level deflections are expected to occur not only at the beach face but also at the landward slope of the long shore bar. That is why a special type of protective structure, consisting of hollow concrete blocks, is suggested to be placed on top of the ridge.