Keywords:
barrier island
coastal protection
beach nourishment
sediment distribution
Contact:
Location: Germany, North Sea, Lower Saxony
The island of Norderney
Norderney belongs to the East Frisian Islands which together with
the Dutch West Frisian Islands form a classical system of protecting
sandy barrier-islands (dune islands) and back-barrier tidal flats,
developed by the coincidence of tides, currents, surf and wind-born
accretion. The island has a length of 14 km, a maximal width of
2 km and an area of approximatly 25 km². In the classification
based on tidal range, Norderney with a mean tidal range of almost
2.4 m, is mesotidal. The beaches of the island can be classified
according to hydro- and morphodynamic conditions, depending on
the influence of tidal induced currents or wave induced currents.
Management
On the western spit, the beach which was studied is situated within
the ebb delta of the tidal inlet of Norderney, where the wave
energy is reduced by ebb delta shoals. The transport processes
changed by constructions of groins, revetments and sea walls.
Continous erosion of the western spit has made beach restoration
by artificial sand nourishment necessary, carried out at irregular
intervals since 1951. The main aim was to ensure the stability
of the solid coastal protection structures. In May 1989 and in
June 1992, the beach was refilled again with well sorted fine
sand, last time the shoreface was additionally filled. The filling
material was dredged from an ebb shoal across the tidal inlet
of Norderney with a mean grain size much finer than the original
beach sediments. For almost six years, starting in 1989, nourishment
samples were taken to measure the grain-size distribution, the
carbonate content and the heavy mineral content.
Results
Two years after the first nourishment it was noticed, that because
of selective erosion of finer sediment particles and relative
enrichment of coarser grains, the grain size spectrum of the beach
refilling was the same as before the replenishment. The natural
grain size spectrum is in an equilibrium with the hydrodynamic
forces, whereas the refilled fine grained sand was handled as
‘foreign’ material and sorted out. After the second nourishment
it was discovered, that the influence of grain-size is limited,
the beach fill will be eroded because it is not in a natural equilibrium
with its boundary conditions, but grain-density has a significant
influence on beach fill longevity. Heavy minerals have the required
higher critical threshold stress without changing the structural
properties of the sand beaches. Because in the Northern Sea there
are no locations with sufficient heavy mineral sands, this method
may be neither practical nor economical, but it is a possible
answer for other regions with suitable resources.
However, although artificial beach nourishment is a common tool
for active coastal protection measures, beach nourishment only
compensates erosional symptoms instead of eliminating causes.
References
- Eitner, V. (1996): The effect of Sedimentary Texture on Beach
Fill Longevity. Journal of Coastal Research 12 (2). S. 447-461.
Fort Lauderdale. Florida. (KJr96b).
- Eitner, V. & G. Ragutzki (1994): Effects of Artificial
Beach Nourishment on Nearshore Sediment Distribution (Island
of Norderney, Southern North Sea. Journal of Coastal Research
10 (3). S. 673-650. Fort Lauderdale. Florida. (KJr94c).
- Kunz, H. (1993): Sand losses from an artificially nourished
beach stabilized by groynes. In: Stauble & Kraus (eds.).
Beach Nourishment Engineering and Management Considerations:
191-205. American Society of Civil Engineers. New York. (DV13)
