Boulder placement is a common method to restore degraded instream habitats by enhancing habitat complexity. This experimental study is the foremost attempt to systematically investigate the influence of rock-ramp boulder placement with varying boulder concentration and flow rate on habitat hydraulic complexity metrics, including the kinetic energy gradient and modified recirculation metrics. By adding boulders to a reach, the modified recirculation metric increased by one order of magnitude for all boulder concentrations. Based on the studied metrics, boulder placement with the highest boulder concentration (λ = 8.3%) resulted in the greatest habitat hydraulic complexity. A set of relationships of moderate strength were proposed to predict the metrics in reaches with boulders by having information about only boulder concentration, habitat characteristic size, and reach-averaged flow characteristics. Based on the available data from the literature, boulder placement especially at higher concentrations may provide suitable habitats for several riverine fish species. Further studies are needed to establish a reliable linkage between the metrics and instream species, to test a wider variety of parameters for verifying and improving the range of applicability of the proposed relationships, and to find the structural configuration at which the habitat complexity is maximized or optimized for a certain species.
© 2022. The Author(s).