Rugosity and blocking indices of artificial reefs and their correlations with wake volume

Abstract

Like aquatic habitats in nature, artificial reefs (ARs) have been investigated to determine whether their complexity affects species diversity. Moreover, studies have been carried out to investigate wake regions of ARs, which provide a shelter, feeding ground, rest area, and temporary stopover for marine species; however, few have examined the correlation between the complexity of the AR and characteristics of the wake region. In this study, we first modelled three ideal scenarios and six real ARs, all designed with different degrees of rugosity and/or water-blocking effects. Second, we obtained the wake volumes of the target structures using the element-based finite volume method and investigated their correlations with the rugosity index (RI), blocking index (BI), and rugosity-blocking index (RBI). Our results showed that for securing a large wake region, it is necessary to implement not only a large upper surface with a certain degree of rugosity (e.g. 2.0 for a tunnel- or arch-type AR) but also a large front surface area with sufficient water-blocking effects (e.g. 0.5 for a cube- or box-type AR). Overall, the RI, BI, and RBI were useful for distinguishing the characteristics of the upper and front surfaces, and accordingly to estimate wake volumes.