Video-derived near bed and sheet flow sediment particle velocities in dam-break-driven swash

This short communication considers a video-based approach to quantify near bed and sheet flow swash zone sediment particle velocities over a mobile bed in a laboratory setting and relate the profile shape to sheet thickness and a velocity that can be measured outside the sheet layer (by for instance a current meter) Near bed high speed imagery was recorded during a dam-break driven swash event in a flume with optically clear walls. Repeated swash events for two different median sediment sizes were tested. Optical Current Meter (OCM) analysis was applied to bed parallel image time stacks extracted at elevations from below the at rest bed to within the lower water column. OCM results were compared to in situ velocity measurements, where possible, obtained with an acoustic Doppler profiling velocimeter (ADPV). OCM results compared well with ADPV measurements for moderate suspended sediment concentrations (based on visual observation; sediment concentrations were not measured). Too high of a sediment concentration over saturated the image and did not provide distinct sediment particle trajectories in a consistent direction for OCM analysis. This saturation occurred during uprush for both sediment sizes. Too little of a sediment concentration provided an inadequate number of sediment particle trajectories to track, such as during flow reversal for the coarser sediment. For coarser sediment, backwash velocities were well resolved in OCM analysis with velocities comparing well (correlation coefficient > 0.8) to ADPV estimates. The dimensionless backwash sheet flow sediment particle velocity profile (normalized by the velocity at the top of the sheet) scaled with the dimensionless elevation (normalized by the sheet layer thickness) to the 0.62 power with 95% confidence intervals for the exponent ranging from 0.47 to 0.76.