Keywords:
island barrier dune
shoreface nourishment
NOURTEC project
RIACON project
food web
zoobenthos
Location: The Netherlands, Province of Friesland,
Terschelling
The Wadden Sea island Terschelling
Terschelling is a barrier island situated in the Dutch Wadden
Sea. It is about 30 km long and 3 to 4 km wide. Dune forming processes
of this island began some 2000 years ago. The island is rich in
contrasts and compared to the mainland it is influenced by a ‘specific’
climate with features from the North as well as from the South.
The high biodiversity with both species normally occuring more
northern and southern is due to this phenomenon. The island has
national as well as international significance.
The NOURTEC and the RIACON project
The project NOURTEC (Innovative Nourishment Techniques Evaluation)
is part of the MAST-II program of the EU. The effectiveness of
shoreface nourishment as an alternative for beach nourishment
and its relevant physical processes have been studied between
May 1993 and December 1996 on different sites of the Wadden Sea
area in Denmark (Torsminde, 1993), Germany (Norderney, 1992) and
The Netherlands (Terschelling, 1993).
To evaluate the risk of shoreface nourishment and subaqueous sand
extraction for the marine benthic community and its effect on
its consumers higher up in the foodweb, an additional project
was launched in 1994, the RIACON (Risk Assessment of Coastal Nourishment
Techniques). Five sites along the European coastline in Belgium,
Denmark, Germany, Spain and The Netherlands (Terschelling) were
investigated for two years.
Nourishment on Terschelling
The objective of the Terschelling nourishment was to prevent for
a period of 8 years the retreat in a 4.5 km long stretch of the
coastline in the center of the island. Most of the 2 million m³
of sand (taken from 21-22 m depth approx. 7 km NW of the nourishment
site) was dumped in the trough between two breaker bars. The dumped
sand had similar grain sizes as the native sand, it was only slightly
coarser (D50 = 200 µm and 180 µm
respectively). Coastal profiles have been measured each year since
1965 with echosounding and beach levelling.
Results
It is necessary to define beforehand clear objectives and design
parameters of a shoreface nourishment which can be evaluated quantitatively
later. From 1965 till the nourishment the mean coastline position
has shown a gradual retreat of a few meters per year. The nourishment
was supposed to compensate the natural losses of 8 years (approximately
20 m). However, the result shows a much greater gain, already
30 m at the end of 1996, which still increases. The nourishment
acts as a submerged breakwater trapping sand. Further advantages
that may offer the shoreface nourishment are reduced costs (since
some handling and the installation of, e.g. pipelines, are avoided),
while any ongoing recreation is no longer hindered. So far, the
experiment is declared a great success.
With respect to the ecological effects, shoreface nourishment
initially caused a reduction in species diversity, abundance and
biomass of zoobenthic species on the nourishment site. Also the
structure of the zoobenthic community differed but approximately
one year after the nourishment recovery was already in progress.
After two years densities, biomass and diversity had largely regained
pre-nourishment values. However, this is not due for long-living
bivalve and sea urchin species. After 2 years the population structure
was still unbalanced due to the absence of older year classes.
The risks for fishes in food webs are expected to be relatively
small. But populations of diving ducks are likely to be affected
during their winter stay in the North Sea, when nourishment takes
place on beds of their favorite bivalve food species.
On the extraction site, the measure significantly reduced species
abundance and biomass. The sediment disturbance was followed by
the development of spionid polychaetes (Spio filicornis
and Spiophanes bombyx). The recovery of the original benthic
fauna proceeded within one year. As on the nourishment site, this
is not due to long-living bivalve and sea urchin species. The
danger with respect to food webs is considered to be small because
of the fast recovery of the benthic fauna and the depth of the
sites, which makes them to less preferred food areas for diving
ducks.
References:
- Essink, K. (1996): Risk Assessment of Coastal Nourishment
Techniques – RIACON. Wadden Sea Newsletter 1: 10-12. (18.1.3.1).
- Essink, K. & J.A. van Dalfsen (1998): The RIACON Project.
Wadden Sea Newsletter 1 (18.8).
- Hillen, R. & J. de Ruig (1995): Dynamic Preservation
on the Wadden Islands of The Netherlands. Wadden Sea Newsletter
1: 15-18. (18.1.3.1).
- Spanhoff, R. (1998): Success of the Shoreface Nourishment
at Terschelling. Wadden Sea Newsletter 1 (18.8).
