Reproducing Log-Law Velocity Profiles In An Ex-Situ Scour Testing Device

Erosion in cohesive soils is a complex subject. Many parameters pertaining to flow and soil conditions influence the erosion process. Separating the influence of flow parameters from that of the soil parameters in the study is essential. An ex-situ scour testing device (ESTD) was built to reproduce the effect of an open channel flow to the stream bed in a well-controlled condition. This device ensured that hydrodynamic forces on the soils were similar to that in the field. The testing channel was 58 cm long, 12 cm wide and 2 cm deep. Flow was propelled by a moving belt on top of the channel and by a pump in the system. Using particle image velocimetry (PIV), it was found that the belt itself produced an S-shape velocity profile. Higher belt speeds generated more curved velocity profiles. The top part of the profile had a higher velocity gradient compared to the bottom part. By a coordinated combination of the belt speed and pump power, the velocity profile could be made close to that of an open channel flow. The flow condition was controlled by varying belt speed and keeping flow rate constant in the system. Flow velocity profiles on channel beds with 6 different roughnesses were measured. The ESTD also featured a direct measurement of hydrodynamic forces acting on cohesive soil specimens using a direct force gauge. Soil specimens were mounted on the force sensor so that instantaneous shear and normal forces on specimens could be recorded. Mock-up specimens with various roughnesses were used measuring bed shear stress under a variety of flow conditions and bed conditions. The bed shear stress also provided information for the validation of log-law flow profile.