SAR Monitoring of Coastal Changes

Longterm Changes

From the early 1990s on, a number of European (ERS-1/2, EnviSAT, Sentinel-1A/B), Japanese (JERS-1, ALOS-1/2), Canadian (Radarsat 1/2) and German (TerraSAR-X, TanDEM-X) satellite missions have provided a wealth of SAR data that can be used to investigate the radar backscattering from the same (or similar) targets at different frequencies and polarizations, but also to monitor temporal changes.

Exposed intertidal flats along the coast can be seen on both SAR images shown below, along with an extended system of sandbanks and mudflats in the images’ upper (northern) part, sometimes referred to as ‘Xinchuan Gang Shoals’. It is obvious that the wind speed was higher during the first image acquisition, resulting in a stronger radar backscatter from the sea (higher image brightness with respect to the mean image brightness of the mainland). This stronger wind may also be responsible for the distinct textures on the open sandbanks, i.e. the strong variation between dark and bright areas on the exposed flats, likely corresponding to higher and lower sediment moisture, respectively.

The intertidal flats along the coast, in the lower part of both SAR images, underwent only smaller changes during those 25 years between the image acquisitions, and only minor differences can be seen at some places. Major changes, however, can be found in areas of strong coastal developments, which are marked by two letters ‘B’ in the lower small map. Here, extensive land reclamation has resulted in changes of the coastline and therefore, of the overall topography in the intertidal range. The letter ‘G’ in the panel’s center marks the Rudong offshore LNG (liquefied natural gas) terminal that was built in the late 2000s (operational since 2011). The terminal is clearly visible as a bright construction in the center of the Sentinel-1A SAR-C image (lower panel).

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• Two SAR images (105 km × 102 km) of exposed intertidal flats on the Chinese coast (see map). Upper: ERS-1 SAR image, 10 June 1993; lower: Sentinel-1A SAR-C image, 19 December 2018. Letters in the middle right panel mark sites of interest: ‘A’: aquaculture; ‘B’: land reclamation; ‘C’: major tidal channel; ‘G’: gas terminal; ‘W’: windfarms. SAR images © ESA 1993, 2018.
  
  

The next figure shows (25 km × 25 km) details of four SAR images acquired in June 1993 and April 2000 (upper row; ERS-1), and in May 2009 and March 2017 (lower row; EnviSat and Sintinel-A, respectively) that demonstrate these changes in greater detail. It is obvious that the morphological changes in that area between 1993 and 2000 (upper two panels) were smaller than thereafter. The major tidal channel (marked ‘C’ in the small map above) appears in the upper left corner of the EnviSat ASAR image acquired in 2009, and it had extended towards south by the time of the Sentinel-1A SAR-C acquisition in 2017. This development occurred along with a general shift of the intertidal flats towards south-east, resulting in more exposed flats in the depicted area in 2017. Further brightness variations in the EnviSat ASAR image (lower left panel) are due to underwater sandbanks. Similar brightness variations can also be seen in the first ERS-1 SAR image (upper left panel), but are less pronounced, likely because of the higher wind speed at that time.

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• Morphological changes and aquaculture on extended intertidal flats off Rudong County. Details (25 km × 25 km) of two ERS-1 SAR (upper row), an EnviSat ASAR and a Sentinel-1A SAR-C (lower row) images acquired between 1993 and 2017 show that major changes occurred in recent years. The lower right panel depicts the location inside the SAR images shown in Figure 7.4. © ESA 1993, 2000, 2009, 2017.
  
  

Also note that the exposed flats appear more homogeneous on the Sentinel-1A SAR-C image (lower right panel), with less variable radar backscattering (image brightness). Apparently the flats had become flatter and showed a more homogeneous sediment distribution. Strip-shaped patterns can be identified on most of them, indicating extensive aquaculture (mainly seaweed; letters ‘A’ in the map). The growing area of those strip-shaped patterns demonstrates the ongoing intensification of aquaculture in the study area.

SAR sensors can also be used to identify shortterm changes in coastal environments, e.g. on intertidal flats. The next figure demonstrates this, using a series of Sentinel-1A/B SAR images (12.1 km x 5.6 km) of exposed intertidal flats on the German North Sea coast (see the map). A narrow flat, marked by the red ellipses, used to connect the small island of Trischen with the mainland, but has been cut by a new tidal channel that first appeared in July 2018 (middle panel). Within a few weeks, this tidal channel grew in width, under the action of strong local tidal currents.

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• Series of Sentinel-1A/B SAR-C image details of the same spot on the German North Sea coast (small map). Marked by red ellipses is a narrow sand flat that was cut by a newly developing tidal channel. © ESA 2018.
  
  

Theses, Publications

• Gade, M., J. Kohlus, and C. Kost, 2017: SAR Imaging of Archaeological Sites on Intertidal Flats in the German Wadden Sea, Geosci. 7, 105; doi:10.3390/geosciences7040105.

• Gade, M., J. Kohlus, and C. Mertens, 2017: Archaeological Surveys on the German North Sea Coast Using High-Resolution Synthetic Aperture Radar Data. Proceed. 37th Intern. Sympos. Remote Sens. Environ. (ISRSE37), Tshwane, South Africa, 8 - 12 May 2017.
• Gade, M., and J. Kohlus, 2016: After the Great Floods: SAR-Driven Archaeology on Exposed Intertidal Flats. Proceed. ESA Living Planet Symp. 2016, Prague, Czech Republic, 9-13 May, 2016.
• Gade, M., and J. Kohlus, 2015: SAR Imaging of Archeological Sites on Dry-Fallen Intertidal Flats in the German Wadden Sea, Proceed. Intern. Geosci. Remote Sens. Sympos. (IGARSS) 2015, Milan, Italy, 27-31 July 2015.

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