Local Sediment Erosion and Deposition Patterns around Instream Boulders

Instream boulders placement in a rock-ramp boulder arrangement have been commonly used in river restoration projects. However, local erosion and deposition around boulders may completely or partially deteriorate boulders’ functionality under varying flow events. It may also influence instream habitat quality by creating scour holes downstream of boulders and limiting spawning grounds. This study focuses on what arrangements of boulders and bed slopes yield minimal disturbance of local bed and sediment transport rates. The Ecohydraulics flume was used to evaluate the local sediment pattern with the presence of boulders at unsteady flows. A total of 12 experimental scenarios were completed to examine the scour patterns and sediment movement in four boulder arrangements (S1, S2, S3, and S4), covering boulder concentrations λ = 0 (no boulder), 3.4, 5.4, and 8.3% (respectively) having three different bed slopes (1.5, 1.0, and 0.5%). Photos of the bed were taken before and after the experiment to examine the bathymetry and scour patterns. The photos were then processed in SfM (Structure from Motion) software package to estimate the local erosion and deposition around boulders, which creates a 3D reconstruction from the photos of the flume bed. Our data suggest that S2, a boulder concentration of λ = 3.4% yields minimal disturbance on local erosion and deposition and sediment transportation. The result of this research may help river restoration projects to select boulder arrangements that ensure the instream boulders’ functionality.