Field trip North Sea – Hallig Hooge - Coastal-Change.Org

Field trip North Sea – Hallig Hooge
5 th September 2012
The 3rd joint IGCP588 / INQUA 1001 conference
Christian-Albrechts-University Kiel
04.09.2012 – 10.09.2012
Figure 1: Westerwarft on Hallig Hooge

Field guide North Sea – Hallig Hooge 05.09.20121
Written by:
Dr. Matthias Deicke
Dipl. Geogr. Malte Schindler
Stud. BSc. Geosc. Anna Arsenijevic
Guidance on the tidal flats:
Anna Arsenijevic
Georg-August-Universität Göttingen
Geoscience Center Göttingen
Dep. Sedimentology/Environmental Geology
Goldschmidtstraße 3
D - 37077 Göttingen
Mail to:
mdeicke@gwdg.de
malte.schindler@geo.uni-goettingen.de
anna.arsenijevic@googlemail.com
Conference contact:
Prof. Dr. Karl Stattegger
Dr. Klaus Schwarzer
Christian-Albrechts-University Kiel
Institute of Geosciences
Sedimentology, Coastal and Continental Shelf Research
Otto Hahan Platz 3
24118 Kiel
http://www.gpi.uni-kiel.de/Sedimentologie/
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Field guide North Sea – Hallig Hooge 05.09.20121
1 Technical information 5 th September 2012
1.1 Time schedule
10:00 to 11:15 Ferry from Schlüttsiel to Hallig Hooge
11:15 Pick up our bicycles
11:15 to 12:30 Guided trip to the tidal flat surrounding Hallig Hooge
13:00 Hanswarft and Schutzstation Wattenmeer e.V.
13:45 Test sites of the research project SAHALL II
(Sediment Accumulation HALLigen)
14:15 Visit of the church (Kirchwarft) and storm surge pale
14:45 Coffee and tea break in the historic inn “Blauer Pesel”
15:45 to 17:00 Ferry back to Schlüttsiel
1.2 General information
Please, consider possible unstable weather and take some rainwear. Concerning the trip to
the tidal flats it is common to walk barefooted. Feet can be cleaned with fresh water after
the walk. Only a very limited amount of rubber boots can be provided by the “Schutzstation
Wattenmeer e.V.”.
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Field guide North Sea – Hallig Hooge 05.09.20121
2 The North Frisian Wadden Sea
2.1 Overview
Located witin the German Bight and extending from the Netherlands in the West to
Denmark in the North the Wadden Sea is an unique area of micro- to macrotidal conditions
with barrier islands, large tidal flats, estuaries and tidal channels. With an area of
approximately 9000 km 2 it is the largest tidal flat of the world in the temperate zone. Though
the Wadden Sea itself is mainly affected by natural processes, the coastline is mostly formed
by man. Due to the unique habitat characteristics a variety of highly specialized organisms
create a complex ecosystem. As the high primary production rate of the Wadden Sea is very
high the area provides nutrition for millions of fish and birds. The Wadden Sea of Schleswig-
Holstein became a National Park in 1985. In 2009 the entire Wadden Sea of Schleswig-
Holstein, Lower Saxony and the Netherlands was nominated as UNESCO World Heritage.
North Sea
x Schlüttsiel
x Hooge
Figure 2 (left): Satellite image of the German North Sea coast. The tidal flats between MLW-line and
mainland can be seen clearly (Source: HZG). Figure 3 (right): Satellite image of the North Frisian
Wadden Sea and the Halligen.
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Field guide North Sea – Hallig Hooge 05.09.20121
2.2 Regional geography and geology
The latest period of landscape evolution of the North Frisian Wadden Sea started at the end
of the weichselian glacial period. Melt water streams delivered vast amounts of sediments
from eastward directions onto the area. In the holocene a sea level rise of approx. 60 m took
place triggerd by eustatic processes. During the Calais- and Dünkirchen transgressions, thick
sediment layers (up to more than 30m) accumulated on the former pleistocene surface.
7.000 yr BP the rate of sea level rise decreased significantly. Short periods of regression
occurred, forming peat and brackish sediments. Older sediments of the Saalian glacial peroid
westward of the islands Sylt, Amrum and Eiderstedt temporarily protected the area of the
Wadden Sea against the ocean.
Figure 4: Map of the North Frisian
Wadden Sea. The ten Halligen
(underlined) are islands with only
low or no protecting
embankment. The marsh islands
Pellworm and Nordstrand in
contrast are surrounded by a
dyke.
The contemporary appearance of the North Frisian Wadden Sea and the coastline was
formed by a number of catastrophic storm surges and by land reclamation activities. About
1000 yr BP, the coastal residents started to build up dykes and intensitied peat digging
activities. Two surge events the so called 1 st and 2 nd “Grote Mandränke (drowning of many
people ‘man’) had a major impact to the landscape and happened in 1362 and 1634,. Dozens
of small towns were destroyed and about 100.000 people died during both events. The
today’s appearance of the North Frisian Wadden Sea is a direct consequence of the 2 nd
“Grote Mandränke”. The former island “Strand” was split into “Nordstrand” and “Pellworm”.
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Field guide North Sea – Hallig Hooge 05.09.20121
Major tidal channels formed, cutting into former marshland. Also the development of the
Halligen is related to those events.
Figure 5: Historical map “Frisia Borealis in Ducatus Sleswicensi – Danimarca“, published in
1665. Right side: North Frisian coast in 1240 before the 1 st “Mandränke”. Left side: Coast
line after the 2 nd “Mandränke” in 1651.
2.3 Biological diversity
You can find a large number of organisms that are perfectly adapted to the ever changing
living conditions of the Wadden Sea. These organisms have to cope with a variations of
salinity, water temperature and water depth and numerous predators can easily catch them
during low water. Amazing survival strategies been developed in consequence. A selection of
organisms and adjustment mechanisms are presented during the tidal flat tour. The tidal
flats and adjacent marsh areas act as feeding and resting places for thousands of migrants
that attend the Wadden Sea twice a year during their flyways between the northern
breeding areas and the southern wintering grounds. The huge production of biomass on the
tidal flats enables a time effective recreation phase for the birds. Currently the birds are
preparing themselves for the journey to the wintering grounds, so you may get the chance
to observe some species in large swarms.
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Field guide North Sea – Hallig Hooge 05.09.20121
3 The Halligen
3.1 Overview
The Halligen are small relicts of former marshland, that were not eroded by storm surges.
Finally, 10 Halligen have been preserved against the sea up to now. These small islands are a
natural phenomenon, which is unique worldwide. Although the Halligen are inhabited by
around 300 residents, they have no or only low embankments. Consequently, the Halligen
are inundated up to 50 times a year (so called Landunter, Figure 7). In order to protect
residents and properties the buildings are erected on dwelling mounds (so called Warften).
Due to these environmental conditions a special lifestyle has been developed. Besides their
socio-cultural relevancy, the Halligen are of high ecological importance and are of major
concern in coastal protection issues. Together with extended sand banks (-barriers) located
westward of the Halligen these act as wave-breakers to protect the mainland coast.
Nowadays, the Halligen have to deal with demographic changes and the consequences of
climate change e.g. rising sea level.
Figure 6: Touristic map of Hallig Hooge
Hallig Hooge is the best known of the ten North Frisian Halligen. With an area of 5,5 km 2 , it
offers a lot of opportunities to discover the customs and the culture of the Hallig. About 100
habitants are living on 10 Warften, this makes Hallig Hooge one of the smallest independent
communities in Germany. Nevertheless, it includes church, supermarket, school, youth
camp, five pubs and restaurants and a large number of private guestrooms and holiday flats.
Mudflat hiking, boat trips and bird excursions bring pupils and tourists in touch with the
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Field guide North Sea – Hallig Hooge 05.09.20121
variety and dynamics of the Wadden Sea ecosystem. Nowadays tourism is the major source
of income for most of the inhabitants, but livestock farming and shoreline protection
guarantees employment as well.
3.2 Protection against storm surges
From the end of the 19 th to the beginning of the 20 th century the shorelines of the Halligen
were only protected by block revetments. Without these protections against erosion they
would not have been preserved up to now. Additionally, at the beginning of the 20 th century
Hooge has been equipped with a low dyke in order to protect the Hallig against inundation
during the summer months (summer dyke). The dyke allows a more intensive use of
grassland and meadows. Still, in the stormy season from October to March the marshland is
inundated by higher storm surges.
Figure 7: Picture of Hallig Gröde from 1984. While huge parts are inundated, the Warften are still
above sea level.
During the 1980s local authorities started to construct higher water permeable revetments
additional to the former non permeable block revetments seaward of the embankment.
These constructions were established to decrease the erosive force of waves and thus to
protect the dyke (Fig. 8). After a Landunter the Hallig is drained via ditches, channels and
tidal gates.
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Field guide North Sea – Hallig Hooge 05.09.20121
Figure 8: Elevated revetments at a beginning storm surge on the south shore of Hallig Hooge
Figure 9: A distant view on Hallig Hooge.
The first Warften (Figure 1, 7 and 9) were constructed more than 2000 yr BP. Recently, their
heights had to be raised due to the rising sea level. Lots of adjustments were conducted
after the catastrophic storm sure event in 1962 and most of the houses had to be rebuilt.
Additional adjustments were made after the heavy storm surges in 1976 and in 1981. The
Warften were widened and Warft slopes flattened. Further on, the top of the Warften were
surrounded by a small dyke. A shelter room was installed in the first floor of each home. Solid
concrete pillars supporting this shelter room were deep seated in the ground soil.
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Field guide North Sea – Hallig Hooge 05.09.20121
3.3 Water supply on Hallig Hooge
Over centuries the supply with drinking water was one of the major challenges for the
inhabitants. Rainwater was collected into reservoirs (Sood) via the thatched roofs and
passed to artificial fresh water pools (Fething) beside the houses. Water out of the “Sood”
was lifted via the "Soodschwengel" and used primarily as drinking water, whereas cattle was
supplied out of the “Fething”. After storm surges, when sea water has contaminated the
“Fething” or during the dry summer months, water shortages occured menancing man and
cattle. Fortunately, since 1965 Hooge receives its fresh water via pipeline from the mainland.
Figure 10: Water supply system on the Warften: A) Fething, B) Sood, C) Soodschwengel
3.4 Sights on the Warften
3.4.1 Hanswarft
The Hanswarft is somehow the “capital” of Hallig Hooge. There you will find the town hall,
the community center, the volunteer fire department, a selection of shops, restaurants and
the tourist office. An insightful view of the life on a Hallig can be provided by a couple of
special museums: The storm surge cinema was made up for showing only documentary films
about the Landunters. Various extraordinary and unique Hallig items have been assembled
in the museum of the former postman ("Postschiffer") Hans von Holdt that he set up in his
former cowshed after his retirement. The Königspesel, an old Frisian living room of the 18th
Century, is a touristic attraction since the Danish King Frederick VI. visited the Hallig that
then belonged to the Danish territory on 2. July 1825. While a storm surge made a departure
impossible people offered their finest living room to the King to spend the night. The room
has been preserved with almost all original inventory up to now. Finally, the environmental
organisation "Schutzstation Wattenmeer" is located on Hanswarft, too. It has been founded
1963 on Hooge and since then expanded to the entire North Frisian coast line as well as to
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Field guide North Sea – Hallig Hooge 05.09.20121
the islands and Halligen. In their Wadden Sea House school classes, members and supporters
are given the opportunity to get to know the North Sea, nature conservation work and, of
course, Hooge.
3.4.2 Kirchwarft
The church is certainly the greatest touristic attraction on Hallig Hooge. It is a small brick
building with a thatched roof, that has been constructed between 1637 and 1642 after the
great flood of 1634 what makes it the oldest building on Hooge. The furniture is in many
parts made up of stranded debris of other churches which perished in the great flood of
1634. As the surface of the Kirchwarft was not elevated, since its year of construction it was
often submerged during high storm surges. That is why the floor of the church consists of
shell sand, so water can seep into the ground after higher storm surges. In the end
Kirchwarft was enclosed by a 2m high dike in the 1980s. The church is surrounded by the
cemetery where a simple wooden cross catches your eye. Here not identified corpses that
have been washed up by the sea are buried. Interesting is also the bell tower that stands
isolated next to the church and is not structurally linked to it like it is common in Schleswig-
Holstein marshlands, in neighboring Denmark and other parts of Scandinavia. The storm
surge pale in front of the Kirchwarft illustrates the height of prominent storm surges in the
past.
Figure 11: View of Kirchwarft during a “Landunter”.
4 Project SAHall (Sediment Accumulation Halligen)
4.1 Rising sea level threatens the Halligen
Currently the mean high water reaches 1.37 m a.s.l. (m ü. NN) around Hallig Hooge. Since
1950 it increased approximately 4mm/a. The mean spring high water is about 1.65m a.s.l.
while the average surface area height of Hallog Hooge is approx. 1.50m a.s.l. Obviously,
Hallig Hooge would be submerged every two weeks if there were no coastal protection like
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Field guide North Sea – Hallig Hooge 05.09.20121
embankments and floodgates. Due to these facilities Hooge is flooded only four to five times
a year, mostly in the months between October and March.
4.2 Supporting the natural growth of the Hallig surface
During storm surges fine sediments are suspended in the water which settle on the lands
surface. This process anually raises the surface area height of the Halligen. Research
conducted scince 2005 by the Geoscience Centre Göttingen shows, that this natural process
of sedimentation cannot fully compensate the current rate of sea level rise on Hallig Hooge
nad Hallig Langeness.
As a consequence test fields were set up on Hallig Hooge and Langeneß to study four
different methods in order to enhance the natural sedimentation process. All methods aim
to derease the current velocity off the drainage water and thus to increase the settling time
for suspended material. On Hallig Hooge the following three methods are tested:
Ockelützwarft
Brache
A
Referenzfläche
Umwallung
Referenzfläche
Brachestreifen
Referenzfläche
Umwallung
0 m 500 m
B
Hanswarft
Figure 12: Position of the test fields A and B on Hallig Hooge. Test areas in field A are highlighted in
yellow and are neighbored by reference areas.
4.2.1 Fallow area
The western part of Test field A is taken out of any use. It is separated from the reference
area lying on the eastern side next to it by a barbed wire fence. In the fallow area the current
velocity is slowed down by the high growing vegetation. This measure should support a
higher sedimentation rate compared to the grazed reference areas.
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Field guide North Sea – Hallig Hooge 05.09.20121
4.2.2 Fallow strip
The 2-3 m wide rim of a meadow is protected from grazing cattle by a barbed wire fence.
(Figure 13a). The basic concept is similar to the idea of the fallow land: The high growing
vegetation slowdown the drainage water that flows off into the adjacent ditches after a
“Landunter”. At the border to the adjacent reference areas the fallow strip is about 20 m
rectangular extended into the reference areas (Figure 13b) to slowdown a potential
drainage of the test area via the reference area.
a
Figure 13: Fenced area where the method "fallow strip" is studied on Test field A on Hooge
b
4.2.3 Surrounding embankment
It took much more effort to implement the method "surrounding embankment", when the
test field was enclosed by an dyke of 2 m width and 40-60 cm height. Its overall length is
340 m. Already in November 2009 the building material was taken from the two ditches
bordering Test field A to North and to the South. For this purpose the ditch has been
excavated at a length of more than 1300m. In April 2010 the dredged material was carried
together, the embankment was build (Figure 14) and finally grass was seeded.
Figure 14: Building of the embankment in test field A (Hooge) in April 2010
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Field guide North Sea – Hallig Hooge 05.09.20121
The embanked area in Test field A is drained via four outlets. The outlets, each 4m wide, are
equipped with timber pegs and fascines (Figure 15). These arrangement decreases the
current velocity of the runoff water and protects the embankment against erosion.
Figure 15: Above-ground through-let used for drainage of the rampart area in September 2010
4.2.4 Test field B
The large test field B (4000m 2 ) is completely surrounded by an lower embankment
compared to field A. It is neighboring Hanswarft (Figure 12 and 16) at its eastern rim. The
northern, western and southern sides of the field are surrounded by a ditch. The dyke is
showing an irregular shape due to the dimension of the field: The embankment is 20 to
60cm high and 1.5m to 3.0m wide at the basement.
Figure 16: View of test field B (Hooge) in July 2010 close to Hanswarft.
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Field guide North Sea – Hallig Hooge 05.09.20121
Figure 17: Construction of the dyke and
drainage pipes on test field B in May 2010.
Figure 18: Drainage
pipe in a ditch.
Figure 19: Ditch
filled with shell
detritus.
In contrast to the dyked area in Test field A, test field B is drained via drainage pipes. The
pipes are placed in 30-60cm deep ditches (Figure 18) and covered with shell detritus (Figure
19). Finally the filled ditch was covered with grass sods again. The ditch excavation was used
to build up the dyke. Figure 20 illustrates that there is still water on testing field B two days
after the beginning of a storm surge “Landunter”. The other parts of the Hallig are already
dry again.
Figure 20: View from Hanswarft of test field B two days after a storm surge.
4.3 Results
The test fields are currently in charge for two winter seasons and only a limited number of
storm surge happened since. A full evaluation of the tested methods will be possible in a
couple of years after a significant number of inundations.
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