Estimation of the nearshore bathymetry from high temporal resolution Sentinel-1A C-band SAR data - A case study

The study of the changes in swell properties in the nearshore domain due to interaction with the sea-bottom to infer the bathymetry has deserved some attention in the last few years. Satellite Remote Sensing images, in particular Synthetic Aperture Radar (SAR), have been used to produce a directional wave spectrum by Fast Fourier Transform (FFT), from which the wavelength and wave direction of the ocean surface waves can be retrieved. These wave properties can be directly used in the linear dispersion relation to estimate the water depth. The present work takes advantages from the very short revisit times of the Sentinel-1A with C-band (5.405 GHz) SAR images with 10 m of spatial resolution to study the repeatability of this methodology for deriving the nearshore sea-bottom morphology, using a case study in the Aveiro region, located in the Portuguese West Coast. This site is exposed to high energetic wave climate of the North Atlantic Ocean. Overall, a set of four images were selected for the analysis. The robustness of the FFT methodology to calculate the wavelength and wave direction from SAR images was analysed through calculating their standard deviations. The errors of estimating the local bathymetry from these quantities was accessed which allowed to quantify the water depth limits of application of this methodology. The set of images was used to establish a bathymetric estimation that merges the derived bathymetry of each image. The computed bathymetry was quantitatively compared with the measurements made in 2013 and available at RATA Observatory. The relative error of the water depth ranges between 6% and 10%, but increases for the higher depths and depends on the accuracy of the computed wavelength at deep-waters. The merged derived bathymetry also provides a better approximation for the measured bathymetry than those derived from the individual images.