Keywords:
foredunes
nourishment
aeolian sand transport
beach
grain size
Contact: Mrs. D. van der Wal. Royal Holloway. University
of London. Dept. of Geology. Egham. TW20 0EX. Surrey. UK.
Location: The Netherlands. Ameland, Den Helder
Beach and dune nourishment in The Netherlands
In 1990, the Dutch government decided to stop further long-term
landward retreat of the coastline. Nourishment has been chosen
as the principal method to counteract structural erosion in this
dynamic preservation policy. Nourishment implies a direct supply
of sediment. Currently, the fill is mainly placed on the beach
or shoreface. In this way, impoverished coastal sections can be
supported in a flexible way. In The Netherlands, nourishment sand
is derived from the bottom of the North Sea, generally seawards
of the contour line of 20 m of depth. Usually, the sand is extracted
by a dredge and brought to a discharge location. The material
is then brought ashore and spread out over the beach, where it
is remodelled by bulldozers and cranes. The nourishments have
a planned life time of three to ten years. Periodic replenishment
is often carried out, as the basic cause of erosion and the negative
sand budget often continue to exist.
The increased amount of sand on the beach acts as a direct buffer
against wave energy. Part of the sand will be blown into the foredunes,
where it is stored to be available in times of very high floods
or erosion. However, an extreme rate of aeolian sand transport
may diminish the efficiency of the nourishment. In addition, a
change in the sediment exchange rate between the beach and adjacent
dunes may have geomorphological and ecological consequences.
Study methods
A project was carried out by the University of Amsterdam between
1994 and 1999, to assess the impact of beach nourishment on the
beach-dune system. The study was conducted on several nourishment
sites along the sandy coast of the Netherlands. In addition, several
adjacent unnourished sites were studied for reference. On the
sites, sand was sampled, both for sedimentological analysis and
for wind tunnel experiments. The long-term development was studied
using data obtained from Rijkswaterstaat. On two beaches (on Ameland
and near Den Helder), field measurements of aeolian sand transport
and related factors were conducted, both prior to nourishment
and after nourishment. The topography and foredune vegetation
have been monitored for several years. Data from the two sites
have also been used to apply aeolian sand transport and air flow
models to evaluate the impact of beach nourishment.
Results
The study has shown that nourishment mainly promotes dune growth
by forming a buffer against wave energy, thus preventing dune
toe erosion (which was, on average, effective for the first four
years after nourishment for the Dutch situation), and by temporally
enlarging the aeolian sediment rate to the dunes. This significant
increase in aeolian sand transport is apparent one year after
nourishment. Nevertheless, nourishment may reduce aeolian sand
transport in individual cases. Several factors, relating to the
characteristics of the fill material and nourishment design determine
the erodibility of the sand, the availability of sand and the
erosivity of the wind. They may, therefore, restrict or amplify
the rate of sand transport on a nourished beach, as compared to
an unnourished beach.
The wind tunnel experiments reveal that nourishment beach sand
is less susceptible to wind erosion than native beach sand. This
is because the grain size distribution (rather than the grain
size) of nourishment sand differs from the distribution of native
sand. Nourishment sand is less sorted, is more suitable for compaction,
and often contains shell fragments, that act to reduce aeolian
sand transport due to the formation of a lag surface after initial
sand transport. On the beach, a source of sediment for transport
is then found on the intertidal beach, where the shells are reworked
by the sea. Although the shell fragments selectively lag behind
by aeolian processes, an increase in shell fragments, and therefore
calcium carbonate content, is found in the foredunes, which may
have ecological consequences. Another factor that is changed by
nourishment is the availability of sand. The enlarged beach enhances
the rate of sand transport to the dunes. The model study shows
that the shape of a beach nourishment is a less important factor
for aeolian sand transport.
Nourishment projects should address these geomorphological and
ecological aspects, and should be site specific, taking local
conditions into account.
Cross section fill placing a. at the leeward side of the foredune,
b. on top of the foredune, c. against the sea side front of
the dunes, d. on (the upper part of) the beach, often combined
with e, a high buffer (a so-called 'banquet'), f. on the foreshore,
and g. on the shore face.
References:
- de Ruig, J.H.M. (1995): The Dutch experience: four years
of dynamic preservation of the coastline. In: Healy & M.
Doody (eds). Directions in European Coastal Management. Samara
Publishing Limited. Cardigan (PC95).
- de Ruig, J.H.M. & R. Hillen (1997): Developments in Dutch
coastline management: Conclusions from the second governmental
coastal report. Journal of Coastal Conservation 3: 203-210 (KJc97b).
- van der Wal, D. (1998): The impact of the grain-size distribution
of nourishment sand on aeolian sand transport. Journal of Coastal
Research 14 (2): 620-631 (KJr98b).
- van der Wal, D. (1999): Aeolian transport of nourishment
sand in beach-dune environments. Ph.D. thesis University of
Amsterdam, 157 p.
