Keywords:
island barrier dune
rolling foredune
digital model
reshape
Contact: Mrs. D. van der Wal. Royal Holloway. University
of London. Dept. of Geology. Egham. Surrey TW20 0EX. United Kingdom.
Location: The Netherlands, Province of Friesland,
Terschelling
The Wadden Sea island Terschelling
Terschelling is a barrier island in the Dutch Wadden Sea and is
about 30 km long and 3 to 4 km wide. Dune forming processes of
this island began some 2000 years ago. Since 1910 the island has
been managed through the National Forest Service, which aimed
in the first half of the 20th century to fix the drifting sand
as well as support the afforestation and cultivation. Large areas
even in the central part of today’s nature areas were drained.
Nowadays the management is changed to a more natural orientated
manner; dunes, forests, salt marshes and mud flats are the main
habitats. 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 from the North and the South is due to this
phenomenon. The nature reserve ‘Boschplaat’ with a surface area
of approximately 4400 ha accomodates about 400 plant species,
nearly a quarter of all plant species in the Netherlands. The
abundance of plant species has made the environment of West Terschelling
to one of the three richest butterfly areas in the Netherlands.
The island has also national as well as international significance
as a resting and breeding habitat.
The ‘rolling’ foredune of Terschelling
A 5 km long artificial foredune is situated on the north-east
coast of Terschelling and comprises former foredunes and sand
dykes, connecting isolated dunes and dune complexes. For many
years, the fo-redune has been managed as a so-called ‘rolling’
foredune. This means that a policy of controlled retreat of the
entire foredune body was applied to prevent marine erosion as
well as to maximise the amount of sand available in times of emergence.
Following a decision of the Dutch Government to maintain the coastline
of 1990, this foredune has now to be maintained at its current
position. Future coastal recession will be counteracted by artificial
shore-face nourishment.
Methods
A plan has been made to reshape the morphology of the foredune
according to a geomorphological design and to integrate the dune
into the natural development of this part of the island. To help
dune managers to attain this goal, a simulation model has been
developed to produce a Digital Terrain Mo-del (DTM) with the necessary
geometrical information. Boundary conditions for the DTM are:
(1) the safety of the coast; (2) a natural transition to the existing
foredunes at the western and eastern ends; (3) a limited supply
of sand which is available for reshaping. The DTM is generated
by the PARALLEL model designed by van der Wal by using data of
the JARKUS data base of Rijkswaterstaat of the Ministry of Transport
and Public Works as a source for computing. The data base comprises
yearly beach and foredune profiles. Three reference sites upshore
and downshore of the planning area were selected to obtain form
parameters for a natural foredune.
Results
Aeolian dune-forming processes are influenced both in space and
time by various factors, e.g. the availability of sand, the rate
of sand movement (which is influenced by wind velocity, direction
and fetch, charachteristics of the sand, humidity etc.) and vegetation.
These factors and their interactions have to be taken into account
when considering management measures in foredunes as well as geo-morphological
engineering. The DTM has been developed to assist the dune manager
to reshape the foredune with respect to the creation of a ‘natural’
landscape. To convert the present dyke into the idealized DTM,
about 0.22 * 106 m3 of sand will be needed. This can be realised
within the envisaged medium-scale planning period of 5 years by
trapping sand with fences or marram grass (Ammophila arenaria)
and other sand-binding vegetation. The second option would be
the use of heavy earth-moving machines.
Terschelling
References
- Staatsbosbeheer (1995): Terschelling, naturally!. National
Forest Service. Driebergen. (33.9.1).
- Van der Wal, D. (1996): The development of a digital terrain
model for the geomorphological engineering of the ‘rolling’
foredune of Terschelling, The Netherlands. Journal of Costal
Conservation 2: 55-62. EUCC.. Sweden. (KJc96a).
- Van der Wal, D., J.H.B.W. Elgershuizen & P.D. Jungerius
(1996): The development of a digital terrain model for the geo-morphological
engineering of the rolling foredune of Terschelling. In: Salman,
Berends & Bonazountas (eds.). Coastal Management and Habitat
Conservation: 427-438. EUCC. Leiden. The Netherlands. (PCB96).
