Accuracy of Nearshore Bathymetry Inverted From X-Band Radar and Optical Video Data

Shore-based remote sensing platforms are increasingly used to frequently (~daily) obtain bathymetric information of large (~km²) nearshore regions over many years. With recorded wave frequency Ω and wavenumber k (and hence wave phase speed c = Ω/k), bed elevation zb can be derived using a model that relates Ω and k to water depth. However, the accuracy of zb as a function of the sensor and the method of Ω-k retrieval is not well known, especially not under low-period waves. Here, we assess the accuracy of zb, based on two sensors with their own method of phase speed retrieval, in a dynamic, kilometer-scale environment (Sand Engine, The Netherlands). Bias in zb is systematic. A fast Fourier transform (FFT) method on X-band radar imagery produced zb too shallow by 1.0 m for -15 m ≤ zb ≤ -9 m, and too deep by 2.3 m for zb≥q -6 m. A cross-spectral method on optical video imagery produced zb too shallow by 0.59 m for -10 m ≤ zb ≤ -5 m, and too deep by 0.92 m for zb≥ -1 m. Intermediate depths had negligible bias, -0.02 m for the radar-FFT approach and -0.01 m for the video-CS approach. The collapse of the FFT method in shallow water may be explained by the inhomogeneity of the wave field in the 960 m x 960 m analysis windows. A shoreward limit of the FFT method is proposed that depends on zb in the analysis windows.