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Deltares | R&D Highlights 2015 Delta Infrastructure
A common practice to safeguard the stability of coastal structures is the use of rock layers as a filter structure. The standard design philosophy is that any movement in the sand below the filter should be prevented. The application of open filters in coastal structures is a recent design methodology that was investigated by Deltares in a number of Joint Industry Projects. Open filters can have benefits by comparison with traditional filter designs since they reduce bulk material consumption and the number of installed layers. Open filters can therefore result in substantial savings in the final design of a coastal structure without jeopardising the overall stability of the coastal structure.
An open filter is normally installed on a base of sand. The typical characteristic of open filters is that the waves acting on the coastal structure can displace the sand grains inside the rock layer. Accepting small deformations in the base material can lead to considerable savings by comparison with conservative approaches that accept no sand movement.
Until now, the design rules for open filters were based uniquely on laboratory experiments looking at the response of the base material to irregular wave loading. The design rules are therefore limited to the range of parameters studied in the laboratory. To extend applicability, a numerical model has been developed with private partners to predict deformation in the base material. The goal was to develop a numerical model that can assess the effectiveness of open filters without the need to conduct physical model tests. The numerical model has been validated on the basis of existing datasets from the laboratory experiments.
The numerical model has been applied to existing large-scale laboratory datasets from the Delta Flume and it was found that the
model can also reproduce experimental observations on this larger scale. These validation and additional parameter studies have shown that open filters represent a useful design method. It has been confirmed that considerable material savings (a factor of 2 at least) can be obtained by introducing accepting minor deformation levels (in the order of 1 mm at the laboratory scale) in the base material. The model will also be used to study the effect of geotextile on the response of coastal structures and to look at whether local reinforcement with geotextile can lead to even more cost-effective designs for open filters.
Deformations of open filters
in coastal structures
niels.jacobsen@deltares.nl T +31(0)88 335 8359 marcel.vangent@deltares.nl T +31(0)88 335 8246 Further reading: Van Gent et al. (2015). Rock slopes on top of sand: Modelling of open filters under wave loading. Proc. Coastal Structures Conf. , Boston, USA28
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The initial configuration in the numerical model Deformation of the sand core after 9 hours of wave loading
Deformation of the sand core below an open filter in the laboratory tests. The profile of the sand core was initially straight
