Experimental study and analysis of local scouring of artificial reef on silty sand bed under steady currents

Artificial reefs (ARs) are an important part of marine ranching, which are installed in seabed to protect and cultivate marine life systems. After ARs are deployed, changes in seabed conditions may intensify local scouring of seabed nearby ARs and influence the stability of ARs. Therefore, it is necessary to investigate the scouring characteristics. In this study, a physical model experiment was conducted in an experimental sink to investigate the local scouring of an artificial reef at different water flow velocities, which was placed on silty sand bed. The experiment results show that the scouring response of the silty bed to the 0.11 m/s flow velocity is not significant. The depth of the scouring hole behind the reef model is 0.5 mm, and there is no obvious scouring hole on both sides. When the flow velocity increases to 0.22 and 0.27 m/s, sand scales are uniformly distributed on the surface of sand bed. The depth of the scouring hole behind is 1.0 and 1.5 mm respectively. The maximum depth of the scouring holes on both sides is 4.5 mm and the width is 4.0 cm and 7.5 cm respectively. Combined with the experiment results, the numerical simulation method was introduced to analyse the mechanism of scouring phenomenon nearby ARs. According to the simulation, the flow field, shear force and vortex distribution are similar to the scouring pattern of silty sand bed in the experiments. The numerical simulation results show that the holes in the surface and multi-legs of reef increase the flow and scouring strength. The high value area of above three parameters corresponds to the area with stronger scouring. It is concluded that the scouring strength of ARs in the silty sand bed increases with flow velocity, but it is not strong enough to influence the stability of reef model. The structure of ARs does have effects on the local scouring. Flow field, stability and scouring pattern should be taken into consideration during the design of ARs.