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| = SAR Monitoring of Coastal Changes = | = SAR Archaeological Studies = |
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| === 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. |
=== Cultural Traces in the Wadden Sea === In Medieval times, the German North Sea coastline was very different from how it is today: the North Frisian islands did not yet exist, but were still what was called the ‘Uthlande’ (outer lands) and what was part of, or connected with, the mainland. Vast areas along the coast were dominated by swamps, marshes, and swamp forests, which often made any settlements difficult or impossible. In the sparse settlements on the German North Sea coast houses were often built on dwelling mounds, protected by small dikes (the latter being called ‘summer dikes’, because they could effectively provide protection against high water only during summer, when there are usually no storms). Systems of drainage ditches were built to remove the water from the farmlands, thereby allowing for any kind of agriculture. |
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| 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. | On January 16, 1362, after more than 24 hours of severe westerly storm, an immense storm tide flooded the coast, causing the small dikes to break at many places, and eventually causing the death of a great number of cattle and men. As a result of that storm surge, which is known in history as the Saint Marcellus’ Flood or ‘Grote Mandrenke’ (‘great drowning of men’), huge land areas were lost to the sea, and they haven’t been diked ever since (compare the upper and middle panels of Figure 1). Thereafter, it took a long time until new dikes were built to protect the remaining marsh land. The new farmland was characterized by wider plots of land, the dikes enclosed larger polders than in the centuries before, and farmhouses on terps were connected by narrow lanes. |
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| 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).<<BR>><<BR>> | Another major storm surge occurred on October 11, 1634, again killing cattle and men, after the dikes had broken at many places. This second ‘Grote Mandrenke’ (also known as Burchardi Flood) hit the area of North Frisia in an economically weak period, after the plague had caused many deaths only about 30 years before. The island of Strand, in the center of the North Frisian coast, was cut into parts by the flood (compare the middle and lower panels of the figure below), thereby destroying farmland, farms, and whole villages. The Burchardi Flood is still the most-known storm surge in history in the area of the North Frisian Wadden Sea. <<BR>><<BR>> |
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| . {{attachment:SARcoastFig2s.jpg|SAR images of the Chinese coast|width="600"}} | . {{attachment:SARKulturspuren1.jpg|Changes in the German North Sea coast|height="300"}} . ~-Changes in the German North Sea coastline during the past 700 years, after Behre (2009).-~<<BR>><<BR>> |
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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.-~<<BR>><<BR>> | A TerraSAR-X image (11.6 km × 5.2 km) of that area, acquired on December 12, 2012, (at 05:33 UTC, 18 minutes after low tide), is shown below. The islands of Pellworm and Hooge can be seen in the lower and left parts of the image, respectively, and tidal channels and creeks show up dark, because of the low wind speed during image acquisition (4 m/s, blowing from SE; the radar backscattering mainly depends on the roughness of the water surface; therefore, a flat surface at low wind speeds causes low radar backscatter and, thus, dark image areas). The bright features in the right half of the image mark edges of tidal creeks and dry, sandy sediments, but are not of interest herein. However, in the two (1.0 km × 1.0 km) areas marked by the white squares, we found fine, linear structures, which are due to remnants of former land use (before the storm surge of 1634). |
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| 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.<<BR>><<BR>> | . {{attachment:SARKulturspuren2.jpg|SAR image of the area of interest|height="300"}} . ~-TerraSAR-X image of the area of interest, north of Pellworm and east of Hooge. The image was acquired on 12 December 2012. The red squares denote the same locations as in Figure 3. © DLR 2012.-~<<BR>><<BR>> |
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| . {{attachment:SARcoastFig3s.jpg|SAR image details|width="600"}} | The next figure contains 1000 m × 1000 m details of two TanDEM-X images acquired in staring spotlight mode on November 19, 2014, at 17:01 UTC (left panel; 26 minutes after low tide, 3 m/s wind from easterly directions) and on January 20, 2015, at 05:50 UTC (right panel; 37 minutes before low tide, 1.3 m/s wind from easterly directions), respectively, and shows examples of such structures. The location of these 1 km² details is marked by the upper right square in the above figure. The very fine pixel sizes of 26 cm × 26 cm and 28 cm × 28 cm, respectively, allow imaging of residuals of historical land use (houses, ditches, lanes), which usually are too narrow to be delineated on SAR imagery of conventional resolution (with pixel sizes on the order of 10 m). |
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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.-~<<BR>><<BR>> | . {{attachment:SARKulturspuren3.jpg|SAR images of the area of interest|height="300"}} . ~-Subsections (1000 m × 1000 m) of two TanDEM-X staring spotlight scenes acquired on 19 November 2014 (left) and 20 January 2015 (right), north of Pellworm. The linear structures are cultural traces. The letter (A) and the arrows are inserted for comparison. © DLR 2014, 2015.-~<<BR>><<BR>> |
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| 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. | |
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| 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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| . {{attachment:SARBirthofaTidalChannel.jpg|Birth of a tidal channel|height="600"}} . ~-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.-~<<BR>><<BR>> |
Reference: ~-Behre, K.-H., 2009: Landschaftsgeschichte Norddeutschlands: Umwelt und Siedlung von der Steinzeit bis zur Gegenwart. Wachholtz, Neumünster, 308 pp.-~ |
SAR Archaeological Studies
Cultural Traces in the Wadden Sea
In Medieval times, the German North Sea coastline was very different from how it is today: the North Frisian islands did not yet exist, but were still what was called the ‘Uthlande’ (outer lands) and what was part of, or connected with, the mainland. Vast areas along the coast were dominated by swamps, marshes, and swamp forests, which often made any settlements difficult or impossible. In the sparse settlements on the German North Sea coast houses were often built on dwelling mounds, protected by small dikes (the latter being called ‘summer dikes’, because they could effectively provide protection against high water only during summer, when there are usually no storms). Systems of drainage ditches were built to remove the water from the farmlands, thereby allowing for any kind of agriculture.
On January 16, 1362, after more than 24 hours of severe westerly storm, an immense storm tide flooded the coast, causing the small dikes to break at many places, and eventually causing the death of a great number of cattle and men. As a result of that storm surge, which is known in history as the Saint Marcellus’ Flood or ‘Grote Mandrenke’ (‘great drowning of men’), huge land areas were lost to the sea, and they haven’t been diked ever since (compare the upper and middle panels of Figure 1). Thereafter, it took a long time until new dikes were built to protect the remaining marsh land. The new farmland was characterized by wider plots of land, the dikes enclosed larger polders than in the centuries before, and farmhouses on terps were connected by narrow lanes.
Another major storm surge occurred on October 11, 1634, again killing cattle and men, after the dikes had broken at many places. This second ‘Grote Mandrenke’ (also known as Burchardi Flood) hit the area of North Frisia in an economically weak period, after the plague had caused many deaths only about 30 years before. The island of Strand, in the center of the North Frisian coast, was cut into parts by the flood (compare the middle and lower panels of the figure below), thereby destroying farmland, farms, and whole villages. The Burchardi Flood is still the most-known storm surge in history in the area of the North Frisian Wadden Sea.
Changes in the German North Sea coastline during the past 700 years, after Behre (2009).
A TerraSAR-X image (11.6 km × 5.2 km) of that area, acquired on December 12, 2012, (at 05:33 UTC, 18 minutes after low tide), is shown below. The islands of Pellworm and Hooge can be seen in the lower and left parts of the image, respectively, and tidal channels and creeks show up dark, because of the low wind speed during image acquisition (4 m/s, blowing from SE; the radar backscattering mainly depends on the roughness of the water surface; therefore, a flat surface at low wind speeds causes low radar backscatter and, thus, dark image areas). The bright features in the right half of the image mark edges of tidal creeks and dry, sandy sediments, but are not of interest herein. However, in the two (1.0 km × 1.0 km) areas marked by the white squares, we found fine, linear structures, which are due to remnants of former land use (before the storm surge of 1634).
TerraSAR-X image of the area of interest, north of Pellworm and east of Hooge. The image was acquired on 12 December 2012. The red squares denote the same locations as in Figure 3. © DLR 2012.
The next figure contains 1000 m × 1000 m details of two TanDEM-X images acquired in staring spotlight mode on November 19, 2014, at 17:01 UTC (left panel; 26 minutes after low tide, 3 m/s wind from easterly directions) and on January 20, 2015, at 05:50 UTC (right panel; 37 minutes before low tide, 1.3 m/s wind from easterly directions), respectively, and shows examples of such structures. The location of these 1 km² details is marked by the upper right square in the above figure. The very fine pixel sizes of 26 cm × 26 cm and 28 cm × 28 cm, respectively, allow imaging of residuals of historical land use (houses, ditches, lanes), which usually are too narrow to be delineated on SAR imagery of conventional resolution (with pixel sizes on the order of 10 m).
Subsections (1000 m × 1000 m) of two TanDEM-X staring spotlight scenes acquired on 19 November 2014 (left) and 20 January 2015 (right), north of Pellworm. The linear structures are cultural traces. The letter (A) and the arrows are inserted for comparison. © DLR 2014, 2015.
Reference:
Behre, K.-H., 2009: Landschaftsgeschichte Norddeutschlands: Umwelt und Siedlung von der Steinzeit bis zur Gegenwart. Wachholtz, Neumünster, 308 pp.
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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