Influence of boulder submergence ratio on local flow field: Implications for sediment and fish instream habitat

Abstract

Boulder submergence ratio may influence surrounding flow structures and as a result, local sediment transport as well as instream habitat preference. In this work, particle image velocimetry (PIV) measurements were used to study the effects of varying submergence ratios in Regimes 2 (1.3 < submergence ratio < 4) and 3 (1 < submergence ratio < 1.3) on the local flow field downstream of a boulder within a rock-ramp boulder array. It was found that in the wake of a boulder, the mean and turbulent flow characteristics for the lowest submergence ratio (Regime 3) varied substantially from the higher ones in Regime 2. From the variations of principal Reynolds shear stress and turbulent bursting events, a higher likelihood of intense downstream erosion was predicted for the lowest submergence ratio. Increased levels of mean velocity and turbulence also indicated that the local flow in the lowest submergence ratio may provide a less preferred habitat for fish. The findings suggest that based on the most common flow stages using a boulder size that generates higher submergence ratios in Regime 2 may improve the performance of stream restoration projects.