The application of a radar-based depth inversion method to monitor near-shore nourishments on an open sandy coast and an ebb-tidal delta

The application of a radar-based depth inversion method to monitor near-shore nourishments on an open sandy coast and an ebb-tidal delta

Gawehn, M.A. (TU Delft Coastal Engineering; Deltares)
van Dongeren, Ap (Deltares)
de Vries, S. (TU Delft Coastal Engineering)
Swinkels, Cilia (Deltares)
Hoekstra, Roderik (Deltares)
Aarninkhof, S.G.J. (TU Delft Hydraulic Engineering)
Friedman, Joshua (Deltares)

Hydraulic Engineering

2020

Coastal management in the Netherlands has the aim to defend coastal zones by preventing flooding and mitigating erosion. To that end, large-scale nourishments are placed in the nearshore, which are supposed to dynamically preserve the coastal zone over a timescale of years. To assess their effectiveness, these nourishments are monitored over large areas and long durations. As repetitive, in-situ measurements become too expensive, remote sensing offers an attractive alternative, mapping depth and near-surface current fields via depth inversion algorithms (DIA). However, the information that can be derived from remotely-sensed data is subject to improvement. In this study a 3D-FFT based DIA named XMFit (X-Band Matlab Fitting) is introduced, which is robust, accurate and fast enough for operational use. Focusing on depth estimates, the algorithm was validated for two case studies in the Netherlands: (1) the “Sand Engine”, a beach mega nourishment at a uniform open coast, and (2) the tidal inlet of the Dutch Wadden Sea island Ameland, characterizing a more complex coast. Considering both sites, the algorithm performance was characterized by a spatially averaged depth bias of −0.9 m at the Sand Engine and a time-varying bias of approximately -2 – 0 m at the Ameland Inlet. When compared to in-situ depth surveys the accuracy was lower, but the time resolution higher. Depth estimates from the Ameland tidal inlet were produced every 50 min by an operational system using a navigational X-Band radar to monitor the placement of a 5 million m³ ebb-tidal delta nourishment – a pilot measure for coastal management. Volumetric changes in the nourishment area over the year 2018, occurring at 7 km distance from the radar, were estimated with an error of 7%. Depth errors statistically correlated with the direction and magnitude of simultaneous near-surface current estimates. Additional experiments on Sand Engine data demonstrated that depth errors may be significantly reduced using an alternative spectral approach and/or by using a Kalman filter.

Coastal zone
Depth inversion
Nourishment
Remote sensing
Tidal inlet
X-band radar

http://resolver.tudelft.nl/uuid:c6b28112-9023-49ce-b9fa-e21b3ef289a4

https://doi.org/10.1016/j.coastaleng.2020.103716

2022-05-11

0378-3839

Coastal Engineering, 159

Accepted Author Manuscript

Institutional Repository

journal article

© 2020 M.A. Gawehn, Ap van Dongeren, S. de Vries, Cilia Swinkels, Roderik Hoekstra, S.G.J. Aarninkhof, Joshua Friedman