Abstract
The wide spatial and temporal coverage of remotely sensing images is an essential asset to analyze the morphological behaviour of fast-changing coastal environments such as estuarine systems. This paper investigates the reliability of intertidal topography mapping around the Authie Bay, a macrotidal estuarine system located on the northern coast of France. A Satellite-Derived Topography technique is developed by relating the green band reflectance of Sentinel-2 images to rapid variations in topography. This method is well suited to small sedimentary structures of the coastal zone with a 0.30 to 0.35 m height accuracy of the constructed Digital Elevation Model (DEM). For the more complex estuarine configuration, the waterline method was applied and resulted in the construction of DEMs with a height accuracy of 0.35 to 0.38 m. Video animations and records of Authie meander positions along transects are created from Sentinel-2 and Landsat satellite archives (1984–2020). These materials allow to highlight a sedimentation phase at the east side of the spit since 2015. It constrains the main channel towards the eastern bank, thus promoting coastal erosion. The monitoring of a severe erosion phase throughout 2019 shows a 130 m retreat of the coastline. Topographic map differentiation led to the detection of a sedimentation anomaly upstream of the bay, probably linked to this erosion event.
Highlights
Exposed to natural processes and human activities, estuarine systems are subject to morphological changes
Since the reflectance ρ B03 is minimal on the low tide waterline, no offset is needed, and the height assigned to the low tide (LT) waterline is the height from the observations zobs ( LT ) = −1.1 m
Satellite-derived bathymetry is known to be very effective in retrieving water depths in the nearshore zone but is hampered by turbidity in fast moving areas, such as estuarine systems, where it loses accuracy
Summary
Exposed to natural processes and human activities, estuarine systems are subject to morphological changes. The deposition of sediment from the sea changes the morphology of the estuary, which in turn modifies the tidal currents and the sediment import, leading to a morphodynamic feedback [3,4]. Since sea level rise is expected to hamper estuary infilling, the morphological response of estuaries is uncertain and may depend on the size of the estuary [5]. A long-term sustainable management requires a better understanding of the morphological changes in estuaries and coastal environments. This implies an accurate bathymetric information with a high temporal frequency in dynamic areas. Traditional methods consist in field surveys with ship based multi-beam echosounder or airborne Lidar systems, which provide high accuracy measurements within a few centimeters [6]. Remote sensing techniques, with increasing accuracy, are emerging as a complementary and low-cost alternative to conventional methods
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