Abstract
The morphodynamic processes in coastal areas are affected by tidal currents and the sea state, leading to transport of sand along the seafloor. Conversely, the tidal currents are influenced by changes in the bottom topography. The continuous observation of areas of high morphodynamic activity is important in order to warn of such changes in the flood stream situation and to avoid further change or loss of land. Therefore, there is urgent need for remote sensing methods which allow the retrieval of hydrographic parameters with high spatial resolution. Using a nautical X-band radar, image sequences of the water surface are acquired. The radar backscatter from the ocean surface is modulated by the surface wave field, and in addition in coastal areas by hydrodynamic interactions of the tidal current with the variable bottom topography. The bottom signatures are static during a single measurement, whereas the wave signature, whose spatio-temporal evolution is given by the dispersion relation of surface gravity waves, is dynamic. These signatures are separated by frequency filtering. A nautical X-band radar was mounted on the island of Sylt in the German Bight in the periods from February to June 1997 and from December 1998 to April 1999. The observed area is of high interest because morphological changes, altering the flood stream situation, have taken place in recent years and are still in progress. The static radar signatures have altered in a noticeable way between the two observation periods. A method was developed to estimate the near-surface current, the water depth and local surface-wave spectra from the dynamic radar signature, which is inhomogeneous in coastal areas due to the effects of wave refraction and diffraction, with high spatial resolution.
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