Comparison of Three Techniques to Monitor Bathymetric Evolution in a Spatially Extensive, Rapidly Changing Environment

Abstract:

The wide variety in spatial and temporal scales inherent to nearshore morphodynamics, together with site-specific environmental characteristics, complicate our current understanding and predictive capability of large (~ km)-scale, long-term (seasons-years) sediment transport patterns and morphologic evolution. The monitoring of this evolution at all relevant scales demands a smart combination of multiple techniques. Here, we compare depth estimates derived from operational optical (Argus video) and microwave (X-band radar) remote sensing with those from jet-ski echo-sounding in an approximately 2.5 km² large region at the Sand Engine, a 20 Mm³ mega-nourishment at the Dutch coast. Using depth inversion techniques based on linear wave theory, frequent (hourly-daily) bathymetric maps were derived from instantaneous Argus video and X-band radar imagery. Jet-ski surveys were available every 2 to 3 months. Depth inversion on Argus imagery overestimates surveyed depths by up to 0.5 m in shallow water (< 2 m), but underestimates larger water depths (> 5m) by up to 1 m. Averaged over the entire subtidal study area, the errors canceled in volumetric budget computations. Additionally, estimates of shoreline and subtidal sandbar positions were derived from Argus imagery and jet-ski surveys. Sandbar crest positions extracted from daily low-tide time-exposure Argus images reveal a persistent onshore offset of some 20 m, but do show the smaller temporal variability not visible from jet-ski surveys. Potential improvements to the applied depth-inversion technique will be discussed.