Coupled FEMDEM/Fluids for coastal engineers with special reference to armour stability and breakage

Abstract

Sea-level rise and increased storminess present huge challenges to coastal engineers worldwide. The seaward slope of many breakwaters and shoreline defence structures consists of thousands of interlocking units of concrete or rock making up a massive granular defence against wave attack. The units are placed freely to form an armour layer which is intended to both dissipate wave energy and remain structurally stable. Design guidance on the mass and shape of these units is based on empirical equations derived from Froude scale physical model tests. The two main failure modes for concrete armour layers are displacement (hydraulic instability) and breakage (structural instability) which are strongly coupled. Breakage mechanisms cannot all be faithfully reproduced under scaled physical models. Fundamental understanding of the forces governing such wave-structure interaction remains poor and unit breakages continue to baffle the designers of concrete armour units. This paper illustrates a range of DEM and FEMDEM methods being developed to model the granular solid skeleton of freely packed brittle units. Such discrete element methods are increasingly being used by engineers for solids modelling. They are especially powerful when coupled with a CFD model which can resolve ocean wave dynamics. The aim is to describe a framework for coupled modelling technologies applicable to coastal engineering problems. Preliminary simulation test cases, still at proof of concept stage, but based on a wealth of validation studies are presented. Thus, we report a snap-shot of progress towards a future where designers combine multi-physics numerical technology with knowledge from scaled physical models for a better understanding of wave energy turbulence, block movement, and internal stresses within armour units.

Keywords:

• wave–structure interaction
• discrete element method
• finite element method
• fluid coupling
• breakwaters
• concrete armour units

Acknowledgements

The authors are grateful for funding from EPSRC under grant GR/S42699/01 GR/S42705/01 and industrial support from Sogreah, CLI and Baird Associates. We wish to thank ICT-HPC at Imperial College for computer resources and support.

Notes

¹Information on the various units can be found via websites of the inventors and agents for the units.

²e.g. see http://amcg.ese.ic.ac.uk/FLUIDITY (accessed 30.03.08).