Numerical modelling of the flow about artificial reefs

Abstract

Artificial reefs have been serving the world in many fields, such as protecting beach erosion, enhancing recreational fishing, surfing and fostering biotic diversity, for many years. One major use for an artificial reef is to deploy it effectively to attract and aggregate fishes and marine life organisms for commercial and scientific purposes. The global hydrodynamic conditions in the vicinity of an artificial reef dictate its structural design (reef stability etc) and the nature of its large scale environmental impact (sediment transport and erosion etc), as well as its operational characteristics. On the West Coast of Scotland an artificial reef was deployed under the direction of Dunstaffnage Marine Laboratory in 2001. It was designed by the fishing industry to promote the economic potential of reef-based fisheries such as the European Lobster (Homarus gammarus (L.)). The size of the reef site equates to about 50 football pitches. There are forty two artificial reefs, comprising two different types of reef module, and 25,000 tons of concrete blocks have been used to create each one of them. However, the investigation of global hydrodynamics requires solving the tidal flow in Loch Linnhe. A numerical model has been adopted to solve the 2D shallow water equations using a Finite Volume Godunov-type scheme. The scheme has the ability to deal with complicated topography such as Loch Linnhe. The initial and boundary conditions of the two-dimensional numerical model were imposed using tidal records obtained from the UK Hydrographical Office for the inlet and outlet of the Loch. A tidal field measurement was carried out using ADCP (Acoustic Doppler Current Profiler) instruments for the purposes of input to the model as well as its validation. These instruments were positioned on location near a group of artificial reefs to gather tidal wave elevations and currents profiles. The results of a two-dimensional numerical model were compared with the data given by the ADCP instruments and tides gauges station in the Loch. A good agreement was observed between the numerical model and data measurement taken from ADCP. The result of the two-dimensional numerical model indicates the ability of the model to represent the complex tidal conditions in the region convincingly. The local hydrodynamic conditions dictate flow separation and the production of turbulence generating eddies and vortices over a range of scales. This, in turn, determines the water quality characteristics, such as the oxygen content, and settlement patterns over the reef and regions favourable, or otherwise, to fish activity (predation, evasion, congregation etc). For these reasons, a reliable procedure for determining the hydrodynamics of local and global of flows about artificial reefs is an essential prerequisite to their satisfactory design. The ADCP instruments were placed in three locations over a complete spring-neap tides cycle around a selected reef to investigate the three-dimensional hydrodynamics affecting it. The RNG k-e turbulent model based on Fluent CFD (ANAYSIS 13) was matched to the global flows output from the shallow water flow model and used to simulate the hydrodynamic forces and flow fields with different flow velocity profiles of the tidal currents. The tidal current dynamics profile over a complete daily spring tidal cycle was investigated to identify the flow regimes on the reef. The results of the numerical model were compared with the data gained from the ADCP beside the chosen artificial reef.