Storm Surf Phenomena at the Western Coast of Sylt Island
Based on existing surge data measured in the coastal area of Westerland/Sylt, physical principles of storm surf are investigated using the model of resonant basin oscillations including reflection and anomalous dispersion phenomena. Contrary to respective previous publications, which are dealing with the averaged anomalous dispersion effect (ADE) considering nearly the whole energy containing frequency range, particular attention has been paid on special spots of resonance. The latter are characterized by maximum gradients dc/df > 0, which even better comply with the analogue of resonant absorption of electromagnetic waves. Two different model conceptions had been found appropriate with respect to (partial) standing waves forming elements of resonant basin oscillations. In natural field conditions: Incident waves from the sea (stimulator) resonating with partial standing half-waves in a definable water basin (resonator), and in a scale model: The wave maker (stimulator) resonating with partial standing quarter-waves in the wave tank (resonator). At high energetic storm surges, besides multimodal energy spectra even a broad unimodal energy-spectrum can be represented by multiple neighboring spots of resonances together with alternating sequences of normal and anomalous dispersion. The occurrence of partial standing waves of considerable extent is supporting the author's finding of a „storm wave resonance“. Local sand removal rates have been estimated as 1.5m/h at heavy storm surges.