Wave and Structure Interaction Porous Coastal Structures

Poblo Higuera

Introduction

Most coastal structures rely on an energy-dissipation function to fulfil their defence role, therefore it is not surprising that they are tightly linked with porous materials. Energy dissipation, which is enhanced by the porous materials, aids in several important tasks such as reducing the reflection coefficient, reducing runup and overtopping, and protecting the structure from direct wave impacts or scour. Furthermore, the porous layers or aprons may constitute a dynamic part of the structure, which depending on the typology, can help to identify structural failure at early stages and allow to repair it in time.

Although a wide range of coastal structure typologies comprise porous media elements but in this introduction only the most relevant breakwater types will be discussed. The reader is referred to Oumeraci and Kortenhaus (1997) for a more thorough classification of traditional coastal defence structures. Other types of structures that will not be covered in this chapter but are relevant are, for example fish cages or mussel rafts, which are being widely applied in aquaculture and can be assimilated to highly porous structures.

In rubble mound breakwaters (emerged or submerged), layers of different rock materials and/or pre-cast concrete units are arranged forming slopes. In some cases a crown wall is placed on top of the porous mantles to offer additional protection against overtopping. In vertical and composite breakwaters, a concrete caisson is supported by a foundation made of porous materials which may also be protected with external porous mantles. Furthermore, aprons made of porous materials can be placed around the lower part of other types of structures (e.g., pile foundations, seawalls), to protect them against scour. In all these cases, porous media need to be accounted for in order to, for example, obtain accurate flow patterns in the vicinity of the structure (for functionality calculations) and accurate pressure distributions around the monolithic elements (for stability calculations).

In the last decades Reynolds-Averaged Navier-Stokes-based (RANS) modelling, also called Computational Fluid Dynamics (CFD), has become widespread boosted, in some sense, by the release of open source models validated by the coastal community (Jacobsen et al., 2012; Higuera et ah, 2013a) and the increase in computational capacity. In view of the large number of structural typologies that include porous media and the significant role that the porous materials play in the structural function, it can be concluded that it is of utmost importance to have the capability to simulate porous media numerically. As a matter of fact,

Department of Civil and Envnonmental Engineering, Faculty of Engineering, University of Auckland, Auckland, 1010, New Zealand. Email: This email address is being protected from spam bots, you need Javascript enabled to view it there are numerous references in the literature that deal with wave interaction with porous structures using different approaches, from which the most relevant will be reviewed in the next chapter. Furthermore, the importance of numerical modelling in the design process of coastal structures is also increasing. For example, CFD is already a standard design tool, complementary to physical modelling, in the Spanish design guidelines for coastal structures (ROM^[1]).

The numerical simulation of flow through porous media is complex and presents several challenges. The most obvious complexity is how to represent the porous materials, which often present an intricate (random) internal geometry. All the factors required need to be correctly defined, in a unique and physical way, as they will affect the flow rate that can go through the materials. Another challenging task is two-phase flow advection, because as it will be discussed later in this chapter, not only the basic flow motion equations need to be modified, but also the surface tracking techniques to ensure mass conservation when the flow interface crosses these materials.

This chapter is organized as follows. A brief literature review is included in the next section. Afterwards, the mathematical formulation for multiphase flow through porous media is introduced. Next, a CFD numerical model is described and two applications are presented. The first application demonstrates the advantages of using numerical modelling to pre-design coastal structure modifications, while the second shows the capability of simulating sediment by coupling a Discrete Element Method model with CFD via a set of equations with time- varying porosity. Finally, the concluding remarks are enunciated.

• [1] http://www.puertos.es/es-es/ROM, in Spanish. Some documents are available in English.