Habitat hydraulic complexity metrics around boulders in a rock-ramp arrangement in support of river restoration

Instream boulder placement has been commonly used to restore degraded streams by enhancing instream complexity. Boulder-induced variations in turbulent flow characteristics affect the quality and availability of aquatic habitats in riverine systems. This experimental research aims to evaluate the influence of boulder placement in rock-ramp arrangement on turbulent-based habitat complexity metrics. Acoustic Doppler velocimeter (ADV) measurements were performed in an open channel flume (Ecohydraulics Flume) to obtain velocity fields in the vicinity of boulders. To evaluate the effects of boulder arrangement and hydraulic conditions on the habitat complexity metrics, multiple boulder concentrations and flow events were tested. The studied turbulent-based habitat complexity metrics were turbulent kinetic energy, turbulent energy dissipation rate, integral length scale, and Reynolds shear stress. The implications of these metrics for fish habitat availability were discussed. Additionally, the projected near-bed Reynolds shear stress was also assessed in connection with geomorphologic changes around boulders to shed light on local sediment transport patterns (e.g., erosion and deposition) in a rock-ramp arrangement. The results of this work may help stream restoration projects that employ boulder placement techniques by providing a better understanding of interactions between boulders, turbulent flow, and instream habitat.