Remote determination of bathymetric changes using ground based radar
The determination of seabed bathymetry is a task commonly carried out by survey vessels using an echo sounder and satellite positioning systems. During the present study a method was developed to map the bathymetry of shallow water areas rapidly and remotely using radar image sequences of waves from two types of shore-based radar systems: an X-band marine radar (9.8 GHz) recording waves up to 2 km from the shore and a high resolution mm wave radar (77 GHz) with a range of approximately 200 m for measuring waves in the surf zone. The physical properties of the wave field were determined from the radar images. Using wave theory, these properties were then used to determine the local water depth. Following investigation of several wave theories, a dispersion equation that approximates the effects of both frequency and amplitude dispersion was found to be the most appropriate. The wavelengths of the waves imaged by the radar were mapped over a range of wave frequencies using a Fourier transform based algorithm for determining the wavelength of 2D waveforms in short lengths of data. The results were used with the selected depth inversion algorithm to infer the water depth matching the observed wave behaviour. Data from two sites were documented in detail: Faro in the Algarve region of Portugal, and Teignmouth in south west England. Radar data from these sites were processed to determine the bathymetry and the results compared with conventional surveys. The water depths derived from the short range mm wave radar data using the non-linear wave dispersion equation showed a mean offset of less than 0.1 m and the standard deviation of the scatter was of the order of 0.2 m. The water depths derived from the longer range X-band radar data again showed a mean offset of the order of 0.1 m and a slightly larger scatter standard deviation of 0.3 m for depths of up to 15 m. The water depths derived from the Teignmouth X-band radar data showed a similar pattern, with a mean offset of less than 0.1 m and a standard deviation of slightly over 0.2 m for water depths of up to 10 m. Radar data obtained in 1999 and 2003 from a tidal inlet in the barrier island system near Faro, Portugal were analysed to obtain bathymetric maps. Comparison of these two bathymetries showed the natural migration of the inlet and the associated bathymetric changes recorded by other researchers using conventional survey methods. In addition, the bathymetric changes evident in the radar derived bathymetries identified deposition to depths exceeding 0.5 m over a wide area of the ebb delta. Further, the data revealed extensive down-drift erosion to a depth of more than 1.5 m. These new observations showed that the balance between accretion and erosion plays an important role in the mechanisms driving inlet migration. This had not been documented previously owing to the practical difficulties of surveying the site by conventional methods. The changes in sediment volume estimated from the two sets of radar data suggest that more than 85% of the sediment supplied by longshore transport is being sequestered in the flood and ebb deltas, resulting in the erosion of the down-drift barrier island and enhanced rates of inlet migration. The techniques developed during this study are now firmly established for future use in coastal studies, and a number of projects are planned to include the use of X-band radar for this purpose.