Artificial reefs

Introduction

An artificial reef (AR) can be defined as any solid man-made structure which has been submerged in the natural environment (Bohnsack, 1989). They may be purposely placed to alter local hydrodynamics (coastal defense ARs) or this effect may be incidental (ship wrecks, offshore windmills, oil rigs etc.). Regardless of their construction or purpose, artificial reefs generally involve the introduction of a hard substrate to a soft bottom environment, altering both the abiotic as the biotic properties of the environment. Coastal defense artificial reefs are therefore often constructed as multipurpose structures, i.e. besides the main function to defend the coast from erosion they also maximise secondary objectives such as to improve spot surfability, to stabilize nourishment material and to create marine parks of increased biodiversity. Since different aims, materials and configurations as well as site dependent factors are to be considered, design criteria for these structures are not easily defined a priori.

Materials

Stones

Stones are especially used for rubble mound structures whose hydraulic stability is verified through the formulae available in literature for low-crested structures (Burcharth et al., 2007 ). Un-corrected design of the structure by a wrong selection of stone size causes a low efficiency of the AR. Structure reshaping induced by wave breaking over the reef may occur and produce instability, with stones rolling down from the structure. Stones are relatively cheaper than other materials but can be more dangerous for surfing safety.

Geobags, geotextile sand containers and geotubes

Geotubes consist of sacks made by geotextile that are filled with sand or gravel. They are characterized by a length and diameter of 20 and 3 m respectively and are particularly recommended for deep bottoms (Matteotti et al., 2003). Geotextile sand containers (GSCs) and geobags are very similar to geotubes, although their dimensions are smaller than that and thus are usually adopted at lower depths. The cost of a reef made of GSCs is usually greater than in case of other materials because of the difficult placement. GSCs and geobags are often employed as revetments or as seaward slopes of nearshore structures. Unfortunately, no exhaustive analyses regarding element stability are available the need to combine numerical and physical models with practice and experience in the field to establish the real stability of the GSC remains.

Reef units

Figure 1: Examples of different reef units

Reef units of different shapes and constructive characteristics can be used to produce ARs. Their macro-roughness induces the dumping of the incident wave agitation above the reef, creating local turbulence and vortices. Usually they are made by steel, reinforced or pre-stressed concrete, fibreglass or a variety of composite materials. These units are produced individually on shore and transported to the staging area. Here they could be combined in a variety of configurations thus allowing adjustment to local conditions and needs. Most of the units are designed to provide nursery areas for fish, to permit a good water recirculation, promote current deflections and marine colonization (O’Leary, 2001). Some examples are shown in Figure 1.

Figure 1.e shows the so-called tecnoreef modules. Constructed from concrete-based natural products, they pose little impact on the ecosystem. Each module consists of reinforced octagonal plates, which are characterized by holes (usually four) having a quarter of a circle shape. The modules are assembled together to create a sort of pyramid that usually consists of three octagonal plates, the so called “base system”. The structure basis is made larger than the crest thus ensuring the resistance and stability to cross-shore currents and drag forces. They globally reduce cross-shore currents due to friction and turbulence The consequent deposition of transported sediments within and around the pyramids is increased by the differently inclined facets on the external and internal surfaces of the modules. These irregularities facilitate localized microcurrents and create continuous circular currents (spheres of water) which release their energy upwards within each element. The constant circulation and exchange of water allows the influx of nutrients and permanent occupation by flora and fauna.

Equally note-worthy are the reef ball elements shown in Figure 1.f. These have an hemispherical shape (Harris, 1995) and are typically 1-2 m high and 2 m wide. A reef ball is made by polyester with glass reinforcement and is thus very resistant to corrosion. The reef ball contains a buoy that allows it to float during construction phase and that is deflated when the element is definitively placed. They are characterised by high porosity (40-50%), roughness and many cavities or holes which have a double action. First off, the roughness and holes increase the structure complexity which assures a quick colonisations by marine species and aquatic plants (Pilarczyk, 2003). Secondly, these structures are specifically designed to generate turbulence, vortices and vertical jets which dissipates wave energy. Under storm attacks, the geometry of reef balls reduces its stability whereas its porosity, through the reduction of lifting forces, causes a beneficial compensating effect.

Geometry

Figure 2: Potential layouts for artificial reefs

Layout

There are many schemes adopted for ARs, especially for those made by geotextile, depending on their principal function. After defining the objectives to be achieved, an appropriate geometry can be selected. Here, the case of submerged structures that are used to defend the coast from erosion are primarily considered. The layout in Figure 2.a represents the traditional configuration of gap separated rubble mound barriers parallel to the littoral zone. The waves that break over the reef induce an inshore increase of sea level and a simultaneous decrease of wave height due to energy dissipation over the crest (Pilarczyk, 2003). Figure 2.b - 2.f shows some sample layouts for artificial reefs and artificial surfing reefs (ASRs). Overall, the structure is composed by two convergent wedges and a focus where the waves break (Ranasinghe et al., 2006), defining a-frame waves, i.e waves that break in a central point while breakpoint evolves in symmetrical way from the case of submerged structures that are used to defend the coast from erosion. The geometries in figures 2.d and 2.f show a large front of the structure parallel to the shore and thus are characterized by an intermediate behaviour between a simple AR and an ASR. Many modifications exist where a central channel (also referred to as a rip channel or paddling channel) is introduced to the scheme (Van Ettinger, 2005). Fig. 2.f represents such an ASR with a central gap. The focus gap can be at the same depth of the seabed or raised from it. The rip channel reduces the intensity of rip currents and thus increase safety of surfers.

Figure 3: Some typical cross sections of artificial reefs

Cross-section

The cross-section of ARs varies according to the used materials and the main function of the structure. Figure 3.a shows the cross section of a submerged barrier made of GSCs. The crest width of such a reef is much greater than that of traditional low crested structures and rubble mound breakwaters. The crest width must in fact be sufficiently large to dissipate wave energy (Pilarczyk, 2003). Moreover, both seaward and landward slopes are steeper than in traditional breakwaters, reducing the volumes of material together with construction costs. In the body of the barrier, GSCs are longitudinally disposed whereas on the landward and seaward slopes GSCs are transversely disposed (Koerner et al., 2006). This reduces the number of gaps which are exposed to breaking waves, increasing AR stability (Recio et al., 2009). In figure 3.b, the body of the reef is composed by differently sized GSCs in order to fill all the gaps between the GSCs (Recio et al., 2009), while the seaward and landward slopes are covered by geotubes. Clearly, for geotubes stability the angle at seabed must be lower than in the previous scheme. Figure 3.c shows an Aquareef that is composed by an internal core made of rubble stones and one upper layer of special blocks that induce localized turbulence, dissipating incident wave energy. This scheme is the most common on Japanese coasts (Pilarczyk, 2003). In Figure 3.d the core of the reef is made of geotubes characterized by different dimensions while armour and filter layers are composed by stones (Pilarczyk, 2003). This ASR profile is characterized by a submerged crest over which the waves peel (Figure 3). A linear or convex profile for wedges can be adopted (Van Ettinger, 2005). The convex profile is better than the linear one because it can compensate the change of the breaker intensity but it is more difficult to be constructed. The linear profile is easier to be constructed, meanwhile the wave peels, we can observe the phenomenon of “wave pinching” that is a reduction of the pipe under plunging waves (Mead, 2003). At the back of the ASR, wedges can be cut or uncut to modulate the velocity of rip and longshore currents. If a cut profile for wedges is adopted, the velocity of rip currents is generally lower than with the un-cut profile (Van Ettinger, 2005 ).

See also

Theseus Official Deliverable 2.1 - Integrated inventory of data and prototype experience on coastal defences and technologies

References

The main author of this article is De Rijcke, Maarten Please note that others may also have edited the contents of this article. Citation: De Rijcke, Maarten (2011): Artificial reefs. Available from http://www.coastalwiki.org/wiki/Artificial_reefs [accessed on 24-11-2024] For other articles by this author see Category:Articles by De Rijcke, Maarten For an overview of contributions by this author see Special:Contributions/MaartenDeRijcke