4.3 Types of reef structures

The type of structures employed for the construction of an artificial reef is a key element for its success both in terms of stability over time and achievement of the expected ecological results. Therefore, it is important to take into account both the engineering aspects and the scope of the artificial reef when planning the reef units and/or the reef sets.

Reef units can range from very simple modules (e.g. rocks or manmade cubes placed singly on the seabed) to sophisticated, intricately designed structures made of several different materials (e.g. steel and concrete, steel and fiberglass) designed to provide optimum surface area through three?dimensional complexity while manipulating water flow and creating upwelling. Simple reef units can be assembled in reef sets to increase the three?dimensional complexity of the reef, hence enhancing its potential in the recruitment of larvae of benthic organisms and fish species. For the same scope, different typologies of reef units and/or reef sets can be used to create an artificial reef.

Shape, height and weight of the reef units and reef sets are crucial for their stability and durability. It often happens that structures completely sink in muddy bottoms because they do not have a base adequate to support their weight. Complex modules may collapse due to the forces of currents and waves. Hence, the ratio of weight to surface area is crucial for the stability of the artificial reef units.
Nevertheless, structures of opportunity such as waste material are still largely employed. These structures include, for example, old ships, aircrafts, old vehicles such as cars, bus, train carriages, tracks, car tires, debris from demolition projects and parts of obsolete offshore platforms. Among the countries of the Mediterranean and the Black Sea, the use of these materials is strictly regulated by national laws according to the international conventions and protocols to avoid the dumping of waste at sea. It is important to underline the need to clean up these structures prior deployment in order to avoid the release of hydrocarbons, anti?fouling and heavy metal pollutants in the surrounding environment and the costs related to these operations (more specific information on the procedures to be followed are reported in UNEP MAP, 2005). In fact, from an economic viewpoint, although these “waste” materials may be initially cheaper to obtain (such as a “gifted” decommissioned naval vessel), the cost of preparation and clean?up prior to scuttling may be prohibitively expensive.
Lighter gauge metal, fiberglass and ferro?cement vessels tend to collapse. Moreover, fiberglass hulls have a low density and need to be appropriately weighted with denser materials to avoid movement to the sea surface. Car tires are highly unstable and may contribute to degradation of the marine environment. The sinking of car bodies causes both dispersion of harmful substances in the environment and disintegration of the metal parts with consequent loss of fouling organisms settled on them (Relini and Orsi Relini, 1971). It has been estimated that car bodies may have about three years of useful life as an artificial reef (Atlantic and Gulf States Marine Fisheries Commission, 2004).
Different technical project approaches are required when using modules specifically designed for artificial reefs and constructed with new or pristine materials. In the former, particular attention should be addressed to design and spatial arrangement of the structures, while in the latter, especially in the case of old ships and similar structures of opportunity, cleaning and siting the structures should be the primary issues to be taken into account. As a precautionary approach, structures of opportunity should not be placed close to sensitive natural habitats (Gobierno de Espana, Ministerio de Medio Ambiente, 2008).