Jantien Rutten

Accuracy of Nearshore Bathymetry Inverted From

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 z_b can be derived using a model that relates Ω and k to water depth. However, the accuracy of z_b 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 z_b, 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 ≤ z_b ≤ -9 m, and too deep by 2.3 m for z_b ≥ -6 m. A cross-spectral method on optical video imagery produced zb too shallow by 0.59 m for -10 m ≤ z_b ≤ -5 m, and too deep by 0.92 m for z_b ≥ -1 m. Intermediate depths had negligible bias, -0.02 m for the radar-FFT approach and -0.01 m for the videoCS approach. The collapse of the FFT method in shallow water may be explained by the inhomogeneity of the wave field in the 960 m × 960 m analysis windows. A shoreward limit of the FFT method is proposed that depends on z_b in the analysis windows.

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Sandbars, submerged ridges of sand parallel to the shoreline, affect surfzone circulation, beach topography and beach width. Under time-varying wave forcing, sandbars may migrate onshore and offshore, referred to as two-dimensional (2D) behaviour, and vary in planshape from alongshore uniform ridges to alongshore non-uniform ridges through the growth and decay of three-dimensional (3D) patterns, referred to as 3D behaviour. Although 2D and 3D sandbar behaviour is reasonably well understood along straight coasts, this is not the case for curved coasts, where the curvature can invoke spatial variability in wave forcing. Here, we analyse sandbar behaviour along the ∼3000 m man-made curved coastline of the Sand Engine, Netherlands, and determine the wave conditions governing this behaviour. 2D and 3D behaviour was quantified within a box north and west of the Sand Engines tip, respectively, using a 2.4-year dataset of daily low-tide video images and a sparser bathymetric dataset. The northern and western sides behaved similarly in terms of 2D behaviour, with seasonal onshore and offshore migration, resulting in a stable position on inter-annual timescales. However, both sandbar geometry and 3D behaviour differed substantially between both sides. The geometric differences (bar shape, bar crest depth and wavelength of 3D patterns) are consistent with computed alongshore differences in breaker height due to refraction. The differences in the timing in growth, decay and morphological coupling of 3D patterns in the sandbar and shoreline are likely related to differences in the local wave angle, imposed by the curved coast. Similar dependency of bar behaviour on local wave height and angle may be expected elsewhere along curved coasts, e.g. shoreline sandwaves, cuspate forelands or embayed beaches. Copyright