Initiation of motion in steep mountain streams: An investigation of how turbulence is altered with channel slope

Current sediment transport equations perform relatively well in low gradient streams, however they often over-predict sediment flux in steep mountain streams. These equations use excess shear stress to predict sediment flux, which is a function of the difference between the mean applied and critical shear stresses. Steep mountain streams are characterized by wide distributions of bed material, which affects sediment flux in two ways. The many available pocket geometry configurations result in a wide range of bed critical shear stresses, while increased flow turbulence over boulders causes a wide distribution of applied shear stresses. Therefore, mean values of these stresses are oversimplifications and result in poor sediment flux estimations. This study aims to investigate the role of turbulence in the initiation of sediment motion in steep mountain streams through a set of flume experiments. For a range of channel slopes, an instrumented mobile test grain is used to measure the local turbulence at the initiation of motion. Turbulence around the grain is measured using 3D particle imaging velocimetry, pressure is measured using sensors inside the mobile grain, and forces on the grain are measured using load cells placed within the bed material. The critical shear stress of the mobile grain is held constant through all experiments by fixing its pocket geometry. These data will aid in understanding the relationship between slope, turbulence and the initiation of motion in steep mountain streams. Such information can also be used to develop an equation to predict the initiation of motion in these systems