Coupled models for waves, a porous seabed and breakwater interactions

• Dr. Dong Jeng
• Professor Phillip Liu (Cornell University, USA)

Australia’s coastal zone is a unique geological, physical, and biological area of vital economic and environmental value; its protection is of utmost importance. Marine structures such as caissons-type breakwaters are commonly used to provide such protection. The impact of ocean waves on coastal environments around a coastal structure is an important issue concerning many research areas. Wave-seabed-structure interactions that result in the instability of a structure have been recognised as the dominant factor in numerous examples of failure of coastal structures.

The lack of knowledge of the mechanism of wave-seabed-structure interactions has created significant uncertainties in caisson-type breakwater design and management. This lack of surety is the reason that most design guidelines recommend conservative protection of structures regarding seabed stability. Thus, a more reliable approach for stability analysis for a breakwater is desired.

The processes of wave, seabed and structure interaction are very complicated in part because they involve numerous processes. The first is the wave breaking process, initiated by wave shoaling over a varying bathymetry and/or an inclined surface of the structure. The second is the turbulence generation inside the boundary layer near the sea bottom or the structure. The third mechanism is attributed to the turbulence flow in the porous media. The fourth mechanism is the wave-induced pore pressure within the seabed and the stability of the foundation around the breakwater. The fifth process comes about with the coupling of the above mechanisms. The OI (Professor Liu) and his group have carried out a series of intensive studies of the first two mechanisms, while ARCIF Jeng and his group have intensively studied the third and fourth mechanisms. To date, there is no model available for the fifth process that integrates all processes into one package. In this project, two research groups (USyd & Cornell) will collaborate to link what both have and couple them together to have a full understanding of the whole phenomenon.

The major objective of this project is to improve the understanding of the mechanism of wave-seabed-structure interactions, and develop a method that is capable of predicting the seabed instability around a caisson-type structure. Specifically, we will (1) develop a coupled model for the process of non-linear ocean wave, seabed and caisson-type breakwater interaction; (2) Investigate fundamental mechanisms of the stability of coastal structures – eg foundations under wave loading and rocking of the structure; and (3) investigate the possible protection of the seabed and provide a guideline for practical engineering applications.

The major innovation of this project is the integration of wave and soil models. Our numerical models will be the first attempt in the world to couple the wave model and soil model to investigate the wave-seabed-structure interaction including the rocking of a breakwater. Existing models have de-coupled the problem due to the complexity of wave and soil characteristics. The research outcomes will provide an optimum design guideline for coastal engineers in the design of protection for a foundation in the vicinity of a breakwater. These guidelines will have immediate benefits for all of the above applications. In the long term, our work will assist in designing sound strategies for protecting coastal structures and the environment around them.

This project is supported by Australian Research Council Linkage-International Fellowship (2008).