Keywords:
barrier dune
fjord
coastal protection
beach nourishment
bottom-fauna
Contact: Jens Tyge Møller. Institute of Geology.
University of Åarhus. DK 8000 Åarhus. Denmark
Location: Denmark, North Sea, West coast of Jutland
The Ringkøbing Fjord
The west coast of Jutland is influenced by strong wind and wave
actions, the pattern of immediate sealevel variations in the North
Sea and the tides. The latter is of decreasing importance from
south to north. Most sandy regions are subject to erosion. Continuing
erosion will result in penetration by the sea during westerly
gales, which would be disastrous for populated areas.
On the west coast between Horns rev and Lodbjerg several embayments
(danish fjord) are open to the lowlying ancient sea floor. In
prehistoric times the Ringkøbing Fjord was cut off from
the North Sea by the barrier Holmslands Klit (klit = danish dune
or dune ridge) and turned into a lagoon with rather brackish water.
The fresh water from streams flowing into the fjord breached the
barrier and maintained a natural outlet, which moved slowly southwards
to the southern end of Ringkøbing Fjord.
The channel on Hvide Sande and its consequences
Flooding became increasingly a nuisance in the town of Ringkøbing
and to farmers. Therfore and for the wish to have a navigable
channel to the North Sea, in 1910 the natural outlet for water
near Nymindegab was closed and a new one was excavated through
the barrier of Hvide Sande. Erosion increased the width of this
artificial channel to 230 m in only a few years which had consequently
to be closed. But the new excavated channel near the old natural
outlet at Nymindegab was too shallow for fishing vessels. In 1931
the old channel was re-opened but had consequently to be controlled
by sluices (including a ship lock). The entrance is poorly protected
by two jetties. To prevent sediment filling up the entrance, another
jetty was constructed. But the construction of jetties caused
increased erosi-on through vortices of the beach south of Hvide
Sande, in particular at Årgab. However, the barrier has
to be preserved because of the protection of the town of Ringkøbing
and the population along the fjord.
Management
The Danish Hydrographic Authority (Kystinspektoratet) was charged
with the launch of laboratory models, investigating several possibilities
for protecting the barrier coast. As a result, the most important
measure against coastal erosion along the beach south of Hvide
sand, and in particular along the dune cliff of Årgab, appears
to be continuous and regular beach nourishment throughout the
year. The sand should be dredged in deep water and dumped in shallow
water, approximatly 4-5 m depth. This material can be transported
by waves towards the beach, then moved by bulldozers towards the
dune cliff, partly to protect the dune foot and partly as buffer
during winter storms. In addition the dune foot at Årgab
is re-inforced with nylon and steel nets fixed with cobbles. The
Kystinspektoratet and the Vandkvalitetsinstituttet did an environmental
assessment along the central part of the Danish North Sea coast,
how the gaining of sand and the nourishment affect the bottom-fauna.
The conclusion is, that in the areas of sandgaining, the fauna
re-establishes within one year and in the nourished areas within
2-3 years. However, the re-established individuals after one year
are smaller in size.
An outlet in such a vulnerable barrier coast as that in Ringkøbing
Fjord will always disturb the coastline, especially if the outlet
has to be dredged and maintained as an entrance to an important
fishing harbour. Any measure for keeping a fixed outlet with depths
adequate for navigation will disturb sand transportation along
the barrier coast.
Location map of Ringkøbing Fjord
References
- Andersen, J. O. (1997): Coast Protection at the Danish North
Sea coast. In: Coastal Dunes: management, protection and research.
EUCC Dune seminar. Skagen. Denmark.(32Nbd97).
- Laustrup, C. & H. T. Madsen (1997?): Evaluation of the
Effect of 20 Years of Nourishment. Available in the EUCC library
(32PC).
- Møller, J. T. (1990): Artificial Beach Nourishment
on the Danish North Sea Coast. Journal of Coastal Research 6:
1-9. (KJr90s).
