ReviewScour around coastal structures: a summary of recent research☆
Introduction
When a structure is placed in a marine environment, the presence of the structure will change the flow pattern in its immediate neighbourhood, resulting in one or more of the following phenomena: the contraction of flow; the formation of a horseshoe vortex in front of the structure; the formation of lee-wake vortices (with or without vortex shedding) behind the structure; the generation of turbulence; the occurrence of reflection and diffraction of waves; the occurrence of wave breaking; and the pressure differentials in the soil that may produce “quick” condition/liquefaction allowing material to be carried off by currents. These changes usually cause an increase in the local sediment transport capacity and thus lead to scour.
The scour is a threat to the stability of coastal structures such as piles, breakwaters, seawalls, etc. Such structures are usually exposed to currents, waves, and combined waves and currents. Clearly, scour processes in the marine environment (with waves being the dominating flow effect) are more complex than in steady-current flows such as in rivers. In river hydraulics, a long tradition exists for studying scour around hydraulic structures. Scour at a bridge pier, for example, has been studied most extensively Breusers and Raudkivi, 1991, Melville and Coleman, 2000, simply because it has been realized that this is an important cause of bridge failure. The scour problems in coastal and offshore engineering have not received the same kind of attention. Analysis of failures due to scour has proven that more basic knowledge on scouring needs to be accommodated in the design exercise. To study the potential risk for scour in the vicinity of coastal structures, and to prepare and disseminate practical guidelines, a 3-year research program (1997–2000), Scour Around Coastal Structures (SCARCOST), has been undertaken by a consortium within the framework of MAST (Marine Science and Technology) program of the European Union (EU).
The consortium consisted of nine research institutions from six European countries, namely Department of Hydrodynamics and Water Resources (ISVA), Technical University of Denmark; HR Wallingford (HR), Great Britain; The Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology (SINTEF), Trondheim, Norway; Laboratoire des Ecoulements Geophysiques et Industriels (LEGI) Université Joseph Fourier, Grenoble, France; Department of Civil Engineering, The University of Liverpool (UL), Great Britain; Instituto do Mar (IMAR), University of Coimbra, Portugal; Centre International D'Investigació dels Recursos Costaners (CIIRC-UPC), Barcelona, Spain; Department of Engineering Science, University of Oxford (UOX), Great Britain; and Coastal and Ocean Engineering, Norges Teknisk-Naturvitenskapelige Universitet (NTNU), Trondheim, Norway.
The results of this research program have been published, or are to be published, in various journal and conference papers, in research reports, and in conference abstracts. The purpose of the present paper is to summarize the results of this research program, thereby providing a synthesis of the work, an overview account of the project, combining the separately reported elements of the project into a whole.
The project is divided into two sub-projects. In the first sub-project, attention has been primarily concentrated on the water column with flow processes and the resulting scour. In the second sub-project, processes associated with the sediment have been the major concern. The results from these two sub-projects will be given in the present paper under the following two headings:
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Flow and scour processes (Section 2), and
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Sediment behaviour close to the structure and scour (Section 3).
Section snippets
Flow and scour processes
As mentioned in the preceding paragraphs, the focus in this sub-project has been the water column with flow processes and the resulting scour.
An overview of the state-of-the-art on flow and scour processes has been presented in the book by Whitehouse (1998) and in a review paper by Sumer and Fredsøe (1999)*.1 The book by Whitehouse (1998) provides methods and guidelines for the engineer to adopt when assessing
Sediment behaviour close to the structure and scour
The main objective here has been to investigate the effect of wave-induced pore pressures in the bottom sand on the scour around coastal structures, and sand transport in general.
As mentioned in Section 1, the research themes investigated in this sub-project of the research program are: (1) field measurement and analysis of wave-induced pore pressures and effective stresses around a bottom seated cylinder; (2) non-linear soil modelling with respect to wave-induced pore pressures and gradients;
Conclusions regarding flow and scour processes
The main findings from the laboratory work are listed below. (In all cases, the experiments were designed with the most appropriate scaling to study the relevant scour processes. This means that the results will become less relevant to situations where other processes are acting).
1. Scour-depth development around a single pile in linear waves can be predicted as a function of the KC number. The predictions can be extended to non-linear waves provided an appropriate definition of the wave
Acknowledgements
The results reported here represent the work of the entire consortium who have undertaken the European Community funded MAST III research program Scour Around Coastal Structures (SCARCOST), and not just the authors. The other members of the research program undertaken by the consortium include: M. Mory, B. O'Connor, F.J. Seabra-Santos, A. Sanchez-Arcilla, G. Sills, and Ø. Arntsen, and their co-workers A. Roulund, J. Sutherland, J.S. Damgaard, G. Svanø, B. Brørs, P. Larroude, C. Rose, S. Pan, J.
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