Phase velocity of shoaling and breaking waves was in focus of interest at the end of the 1970s. The author had found the phenomenon of an intense anomalous dispersion effect (ADE) existing with respect to storm surge waves, measured at Sylt Island/Germany. In the theory of electromagnetic waves Anomalous Dispersion is known to be an accompanying effect of the resonance phenomenon. Because boundary conditions had not been defined adequately with respect to analogue effects taking place within the coastal zone, the real existence of the ADE with water waves had remained questionable. An ADE, however, recently has been verified by model investigations at Bielefeld University of Applied Sciences, relating to resonance phenomena occurring in a wave tank. Using a special technique of analysing composite spectra (containing information of incoming, reflected and re-reflected waves), it was possible to detect a set of partial standing waves existing coincidentally. As each partial cl
apotis is composed of a number of neighbouring frequency components disposing of nearly equal wave length, such components must obey an anomalous dispersion law dc/df > 0.
At Sylt Island boundary conditions sufficiently comparable to that of the wave tank are formed by a structured long shore bar running roughly parallel to the shoreline. 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 harmonics numbers 1 through 5 of the enclosed water body, resonance documented by the ADE is believed to be responsible for the tremendous coastal recessions of Sylt island due to storm surge occurrences in the past. The existence of resonance absorption effects especially is traced back to the fact that the function of the component lengths L(f) (wave length spectrum), calculated from the anomalous phase.
resonant basin oscillation, anomalous dispersion, Sylt island, resonant absorption
620 Engineering and allied operations
Publications of TU Braunschweig
Also published in :
Die Küste : Archiv für Forschung und Technik an der Nord- und Ostsee, Heft 67, (2003) pp. 51-82