Constructing a One-Minute Video from a 2.586 Seconds Burst of Sentinel-2 Imagery to Show the Potential for Estimating Wave-Derived Nearshore Bathymetry by Using a Spatio-Temporal Cross-Correlation Method and a DMD-Based DIA

Abstract

To effectively mitigate environmental changes and to manage coastal environments, a good under- standing of nearshore bathymetry and its evolution is required. The coastal zone is expected to be vulnerable in the foreseeable future due to the combined effects of increased coastal impacts and a growing coastal population. The use of spaceborne remote sensing methods has been developed in recent decades to estimate nearshore bathymetry. Compared to traditional methods, spaceborne remote sensing techniques offer a relatively large spatial coverage as well as frequent temporal mon- itoring. The Sentinel-2 mission (EC/ESA) is potentially able to provide regular bathymetry estima- tions, mainly due to its significant revisit time and freely accessible data.

This research focuses on the development of a spatio-temporal cross-correlation model in order to construct a one-minute wave-representing video. The video is used to show the potential for wave- derived bathymetry estimation, exploiting a DMD-based DIA to invert depth. The model is ap- plied to two distinct datasets: a synthetic FUNWAVE dataset and Sentinel-2 imagery. The Sentinel-2 imagery covers research sites in Saint-Louis, Senegal and Capbreton, France. An application frame- work related to the model is furthermore developed by analysing a set of synthetic sinusoidal waves.

Three model requirements are created based on the research objectives. The model should show wave propagation for at least one minute and, moreover, the waves in the video should accurately represent the wave field. Both requirements are demanded by the use of a DMD-based DIA. Lastly, the resulting video should enable an accurate bathymetry estimation. Based on the requirements, the developed spatio-temporal cross-correlation model includes four model parts: pre-processing, image resolution augmentation, wave characteristics estimation and video construction.

The research has led to a range of insights. A discrepancy between the quality of constructed videos and related bathymetry estimations is observed. The videos as constructed by the model generally show a good representation of average wave propagation for a sufficiently long duration, while the bathymetry estimations are less accurate. The low quality of bathymetry estimations is explained by three main error sources: celerity estimation errors, the applied filtering methods and the way in which the video is constructed. These three error sources together lead to videos that lack pixel-wise detail and therefore decrease the bathymetry estimation quality. The developed application frame- work shows that estimating wave characteristics from Sentinel-2 imagery by means of the model is at the edge of possibilities. In temporal sense, relatively large celerity estimation errors are expected for wave periods lower than 5 s and higher than 7 s. The model is less sensitive for spatial parame- ters: as long as wavelengths are larger than circa 150 m the celerity estimation error is acceptable.

All together, the developed model and related video constructions offer added value, although not for the purpose of a bathymetry estimation by means of a DMD-based DIA. The constructed videos represent wave propagation in an average sense and can therefore be exploited for wave-related purposes, e.g. obtaining wave spectra, estimating dominant wave direction and estimating wave climates. In general, the video offers a way to enlarge the temporal range of Sentinel-2 imagery. It is furthermore concluded that the model is probably more suitable for use in combination with other types of imagery, including satellite imagery with larger burst duration and increased spatial resolution as well as standard video imagery.