Geoscience Reference
In-Depth Information
Dynamic coastal protection
In accordance with recommendation (4) of the Delta program, the 50 million
euro pilot project Sand Motor started in 2010. At a strategic location along the
North Sea coast 20 million m 3 of sands dredged from 20 km in sea is positioned
before the coast in the form of a hook (erosion island, Fig 17.8). With time, this
sand will be spread along the coast by wind, waves and sea currents, and provides
effective beach nourishment and more. With this innovative method, using natural
forces (building with nature) extra space for nature and recreation is created and it
contributes to safety against flooding, erosion and sea level rise. The expectation is
that the sand motor will disappear in a period of 20 years and supports a shoreline
progression of 100 metre over 20 kilometres coast, to the north.
Overtopping and erosion
An important dike failure mechanism is inner slope sliding due to overtopping.
Due to alternate dehydration and saturation, the top layer loses strength. In fact,
this is how most of the dikes in the southwest of The Netherlands failed during the
1953 flood disaster. The amount and the frequency of overtopping a dike is a topic
of research. Clearly, it determines the height of the dike crest above the design
water level (the freeboard or excess height), and hence influences the cost level of
the flood defence system. As this freeboard can be different in different countries,
it may lead to complications in case of trans-national rivers. e.g. the debate about
the maximum possible discharge that can reach the Netherlands via the Rhine.
If a dike crest is too low and there is no space for a higher dike, a superstructure
is built. Some examples are concrete Muralt superstructures, T-walls (on levees in
New Orleans) and sheet piling (Pettemer Sea Dike). Their purpose is to reduce
overtopping. Recent events in New Orleans have shown that these superstructures
may fail under extreme conditions and destabilise the dike. The probable causes, as
understood from tests, are a combination of geotechnical and hydraulic processes,
such as heavy overtopping, local scour, wall rotation, hydraulic shortcuts, piping
and extreme soft soil deformation (peat layers). A proper design of such structures
requires an intense interaction of expertise in surface water hydraulics,
geohydrology and geotechnics.
In the Netherlands, the crest height of a dike is traditionally designed to allow a
small amount of overtopping, for instance 2% of the waves may overtop during a
storm, or a small average overtopping discharge of 0.1 to 1 [l/sm] (litre per second
per metre width) is allowed. The method to determine the overtopping discharge is
described in the Eur0top wave-overtopping manual. For example, for a significant
wave height of 1 metre, a free board is required of 3 metre for limiting overtopping
to 0.1 l/sm, 2.2 metre for 1 l/sm and 1.4 metre for 10 l/sm. For higher waves, the
differences in the required free board for various overtopping levels become larger.
Dike grass cover stability under overtopping conditions is based on constant
overflow tests. In case of wave overtopping, however, individual overtopping
volumes can be large, generating flow velocities up to 9 m/s for short periods of
time. Since 2007, a wave-overtopping simulator (Fig 17.9a) is used to test the
strength of grass covers against erosion. It has a 4 m wide water container, during
testing being filled with a constant discharge to provide the random overtopping
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