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AFTER THE GREAT FLOODS: SAR-DRIVEN ARCHAEOLOGY ON EXPOSED
INTERTIDAL FLATS
Martin Gade (1), Jörn Kohlus (2)
(1) Universität Hamburg, Institut für Meereskunde, Bundesstraße 53, D-20146 Hamburg, Germany,
Email: Martin.Gade@uni-hamburg.de
(2) LKN Schleswig-Holstein, Nationalparkamt, , Email:
ABSTRACT
After major storm surges in the 14th and 17th centuries,
vast areas on the German North Sea coast were lost to
the sea. What was left of former settlements and
historical land use was buried under sediments for
centuries, but when the surface layer is driven away
under the permanent action of wind, currents, and
waves, they appear again at the Wadden Sea surface.
However, the frequent flooding and, thereby, the strong
erosion of the intertidal flats make any archaeological
monitoring a difficult task, so that remote sensing
techniques appear to be an efficient and cost-effective
instrument for any archaeological surveillance of that
area. We show that high-resolution space borne
Synthetic Aperture Radar (SAR) imagery with pixel
sizes well below 1 m² can be used to complement
archaeological surveys and that SAR images from the
German TerraSAR/ TanDEM-X satellites clearly show
remnants of farmhouse foundations and of former
systems of ditches, dating back to the 14th and to the
16th/17th centuries. In particular, the new high-resolution
TerraSAR-X acquisition mode (‘staring spotlight’)
allows for the detection of various kinds of residuals of
historical land use, some of which have been unknown
so far.
1. INTRODUCTION
In Medieval times, the German North Sea coastline was
very different from how it is today (Fig. 1): 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 [2]. 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
Figure 1. Changes in the German North Sea coastline during the past 1100 years (upper left part of each panel), after
[2]
_____________________________________
Proc. ‘Living Planet Symposium 2016’, Prague, Czech Republic,
9–13 May 2016 (ESA SP-740, August 2016)
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 (see the upper middle and upper right panels of
Fig. 1). Thereafter, it took a long time until new dikes
were built to protect the remaining marsh land. The new
farmland was characterized by a dense system of
ditches, 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 centre of the North Frisian coast,
was cut into parts by the flood (cf. the upper right and
the lower left panels of Fig. 1), 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.
Over the following centuries, great parts of this former
agricultural area have been buried under muddy and
sandy sediments, which nowadays form the German
Wadden Sea. This area is being flooded, and falls dry,
once during each tidal cycle, thereby making
archaeological excavations very difficult. However,
under the permanent action of the tidal forces the
muddy and sandy marine sediments are partly driven
away, and traces of former peat digging, drainage
systems, and settlements appear again on the surface
[1]. Since those areas are difficult to reach, and thus to
observe from ground, airborne sensors have proven to
be advantageous for a systematic observation of the
residuals of those historic places [3][4]. Their use,
however, is cost-intensive, which makes high-resolution
space borne sensors an alternative source of data that
can be used by archaeologists for their frequent
surveillance of the area [6].
2. REGIONS OF INTEREST
A map of our area of interest is shown in Fig. 2, with
the location of the Synthetic Aperture Radar (SAR)
image in Fig. 3 inserted. The area of interest lies in the
centre of the North Frisian Wadden Sea, i.e. in an area,
which was most affected by the major storm surges in
the 14th and 17th centuries (see Fig. 1). 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 in Fig. 3. 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 [5], 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 to remnants of former landuse (before the storm surge
of 1634).
3. SAR DATA
A total of 26 TerraSAR-X/TanDEM-X images acquired
in high-resolution spotlight mode between 2008 and
2014 form the basis for our systematic analyses of SAR
signatures of historical land use. The pixel sizes of all
images were on the order of 1 m², or even below,
thereby allowing for the detection of fine structures that
can be attributed to remnants of narrow ditches or
settlements. Those images are complemented by
Terrasar-X/TanDEM-X acquisitions in the new ‘staring
spotlight’ mode, with extremely fine pixel sizes of
0.3 m × 0.3 m and below.
Figure 2. Area of interest on the German North Sea
coast. The black rectangle in the close-up on the
upper left marks the location of the SAR image shown in
Fig. 3.
4. EXAMPLES OF CULTURAL TRACES
FOUND ON SAR IMAGERY
Intertidal flats are highly morphodynamic, and when the
muddy and sandy marine sediments of the flats’ upper
layer are moved away, banks of peat, old clay, and
remnants of farmland and settlements appear again on
the dry-fallen surface. Moreover, the deposition of fine
sediments along those morphologically harder structures
can help pronouncing them, thereby making them
(better) visible in aerial and satellite imagery. Analysing
the high-resolution SAR imagery we found at several
places fine linear structures, which are clearly
anthropogenic.
4.1. Remnants of drainage systems and lanes
An example of very high resolution SAR imagery of
archaeological sites is shown in Fig. 4. The small
section (again, 1000 m × 1000 m, corresponding to the
lower left square in Fig. 3) of a TanDEM-X staring
spotlight scene was acquired on November 21, 2014, at
05:41 UTC (low tide; 2 m/s wind from easterly
directions) and shows many bright and dark parallel
lines all over the image centre. The distance of those
lines is between 10 m and 20 m, again, indicating that
they are remnants of a former mesh of ditches built for
the drainage of the farmland. The ditch residuals are
marked by denser (harder) sediment causing higher
surface roughness which, in turn, results in higher radar
backscattering. However, we also note that, once the
space in between is partly filled with sandy sediments,
some of the lines may also appear dark (seen in the
image centre of Fig. 4).
Fig. 5 shows a photograph taken during low tide on
May 14, 2009, from a dike on the northern coast of
Figure 3. TerraSAR-X image of the area of interest, north of Pellworm and east of Hooge. The image was acquired on
12 December 2012. The white squares on the bottom and upper right denote the locations of the SAR image details
shown in Figs. 4 and 5 and in Fig. 7, respectively. © DLR 2012.
Figure 4. Subsection (1000 m × 1000 m) of a TanDEM-
X staring spotlight scene acquired on November 21,
2014, north of Pellworm island and showing in its lower
centre the same area as in Fig. 9. © DLR 2014
Pellworm island. In the image centre dark parallel
structures can be delineated, which correspond to those
seen on the SAR images. Clearly visible are the
differences in sediment composition of the linear
structures and the surrounding sandy flats. Also visible
is remnant water in between the dark lines (and in
several puddles all over the place), which results in a
stronger contrast between the parallel bright lines and
the dark area in between (Fig. 4).
Finally, Fig. 6 shows a reconstruction of a historical
lane, with ditches on either sides, which can be found on
the intertidal flats north of Pellworm and which may
cause structures like those observed in the SAR
imagery. Residuals of fossil farmland structures, mostly
of ditches, but also of lanes or dikes, cannot be observed
through their relief of less than 10 cm. Instead, it is the
sediments on the lost pastures that are different from
those in the linear structures of ditches. Typical Wadden
sediments on the flat sand banks consist of marine fine
sand, which had been the basic compound of the old
marsh land and which is still a major part of the coastal
environment. In contrast, the surface of the fossil
ditches is different: in the centre pillow-like sediments
can be found, while the ditch edges are often stabilized
by fossil roots and other plant material connected with
the sediment (Fig. 6). This causes narrow ridges of only
10 cm to 20 cm width, which can still be found today
and which show up on SAR imagery, if its spatial
resolution is high enough.
4.2. Remnants of settlements
Fig. 7 and Fig. 8 are the same 1000 m × 1000 m details
of two TanDEM-X images acquired in staring spotlight
mode on November 19, 2014, at 17:01 UTC (26
minutes after low tide, 3 m/s wind from easterly
directions) and on January 20, 2015, at 05:50 UTC (37
minutes before low tide, 1.3 m/s wind from easterly
directions), respectively, and show examples of such
structures. The location of these 1 km² details is marked
by the upper right square in Fig. 3. 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).
Clearly visible are linear and rectangular structures in
both lower left image centres, close to the tidal creek,
along with groups of parallel vertical lines at different
Figure 7. Subsection (1000 m × 1000 m) of a
TanDEM-X staring spotlight scene acquired on
November 19, 2014, north of Pellworm island. The
linear structures are cultural traces, i.e., remnants of
former settlements and land use. The letter (A) is
included for better comparison with Fig. 8. © DLR 2014
Figure 6. Reconstruction of a historical lane with
ditches on either side. Photograph and sketch: J.
Kohlus.
Figure 5. Photograph taken from a dike on Pellworm
island and showing manifestations of historical land use
as dark parallel lines. Photograph: M. Gade.
locations in both image centres.
The mean distance between those parallel lines is about
15 m, thus indicating that they are remnants of former
ditches and drainage channels. In both SAR images the
letter (A) marks the same location; note that the parallel
vertical lines to the south and south-west of this location
are visible only in the younger SAR image (Fig. 8),
whereas a diffuse bright band in the older SAR image
(Fig. 7), left of (A), indicates that this area was still
covered by sandy sediments in November 2014. This
comparison illustrates the strong morphodynamics on
intertidal flats, and also demonstrates the need for a
frequent monitoring of those archaeological sites.
Fig. 9 shows an aerial photograph of the same intertidal
flat north-east of Pellworm, taken on July 29, 2009, at
low tide. Here, it is obvious that the linear structures
origin from foundations of former settlements. In
addition, several dark spots are visible, which origin
from former wells, pits, cisterns, etc. Those spots can
also be found on SAR imagery (as bright spots, though;
see Fig. 7 and Fig. 8), if the spatial resolution is high
enough (in SAR images of lower resolutions they could
easily be confused with speckle noise typical for SAR
imagery). For an easier comparison with the above SAR
images three arrows marking the same spots have been
included in both the aerial photograph (Fig. 9) and the
second SAR image (Fig. 8). Also visible is the sandy
sediment, by which those structures were buried for
long, and which was driven apart by the action of
currents and waves. A close comparison of the aerial
photograph with the high-resolution SAR images
reveals that, during the (more than) five years between
the two kinds of acquisitions, parts of the residuals were
already lost, due to the permanent erosion,
sedimentation, and morphological changes.
5. CONCLUSIONS
High-resolution SAR images can be used to
complement archaeological surveys on intertidal flats on
the German North Sea coast. Here, signatures of both
former settlements and remnants of former systems of
ditches and of peat cutting, dating back to periods
before major storm surges in the 14th and 17th centuries,
can be found on high-resolution TerraSAR/TanDEM-X
images. In this respect, best results were obtained (i.e.,
strongest and clearest signatures were found) when SAR
images acquired in the new ‘staring spotlight’ mode,
with pixel sizes on the order of 0.1 m², were used.
In many cases, the observed signatures of former
ditches are due to different sediment types, which in
turn are due to the actual ditch morphology. Fossil roots
and other organic material may result in denser and
harder sediments, which may be directly sensed by the
space borne SAR, or which may cause additional
sedimentation (i.e., deposition of sandy sediments) that
can be seen on SAR imagery. We also note that
different sediments may cause different biological
productivity, and are therefore often marked by benthic
organisms, which may cause different surface roughness
patterns. It is those patterns that are sensed by the high-
resolution X-Band SAR.
The archaeological sites presented herein have already
been subject to previous studies [3][6]. However, the
new high-resolution TerraSAR-X acquisition mode
(‘staring spotlight’) allows for the detection of various
kinds of residuals of historical land use, some of which
have been unknown so far.
Figure 9. Aerial photograph of exposed intertidal flats
north of Pellworm, taken in July 2009. Residuals of
former settlements, close to a tidal creek (upper left),
can be clearly seen. The arrows were included for
easier comparison with the SAR image shown in Fig. 5.
Photograph: B. Hälterlein, LKN.
Figure 8. Same as Fig. 7, but TanDEM-X staring
spotlight scene acquired on January 20, 2015. © DLR
2015. The letter (A) marks the same area as in Fig. 7,
the arrows were included for easier comparison with
the aerial photograph shown in Fig. 9.
6. ACKNOWLEDGEMENTS
The authors are grateful to those colleagues
participating in DeMarine-U’s sub-project 4, who
contributed to the results presented herein. DMU
received funding from the German Ministry of
Economy (BMWi) under contract 50 EE 0817, TSX
data were provided by DLR under contract COA0118.
7. REFERENCES
1. Bantelmann, A. (1967). Die Landschaftsentwicklung
an der schleswig-holsteinischen Westküste,
Wachholtz, Neumünster, 97 pp.
2. Behre, K.-H. (2009), “Landschaftsgeschichte
Norddeutschlands: Umwelt und Siedlung von der
Steinzeit bis zur Gegenwart“, Wachholtz,
Neumünster, 308 pp.
3. Gade, M., Kohlus, J. (2011). Archeological Sites on
Intertidal Flats in the German Wadden Sea’.
Proceed. 4th TSX Science Team Meeting,
Oberpfaffenhofen, Germany, 14-16 February 2011,
DLR Publications.
4. Gade, M., and Kohlus, J. (2015). SAR Imaging of
Archaeological Sites on Dry-Fallen Intertidal Flats
in the German Wadden Sea. Proceed. Intern.
Geosci. Remote Sens. Sympos. (IGARSS) 2015,
Milan, Italy.
5. Gade, M., Alpers, W., Melsheimer, C., Tanck, G.
(2008). Classification of sediments on exposed
tidal flats in the German Bight using multi-
frequency radar data. Remote Sens. Environ., 112:
1603-1613.
6. Gade, M., Melchionna, S., Stelzer, K., Kohlus, J.
(2014). Multi-Frequency SAR Data Help
Improving the Monitoring of Intertidal Flats on the
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