Quantification of Swash-Zone Velocities in the Sheet Flow Layer Using Particle Image Velocimetry

Two swash-zone experiments were conducted in the laboratory to investigate the flow structure in the sheet flow layer. Swash forcing was generated using a dam-break mechanism over a flat section of a wave flume located at the Center for Applied Coastal Research (University of Delaware). Swash flows occurred over a planar sloping mobile bed (1:7) with different grain sizes (D₅₀ = 0.59 mm and 1.03 mm) for the 2 experiments. Imagery of a vertical plane in the lower few centimeters of the water column was collected at 301 Hz using a Teledyne Dalsa Genie HM640 camera. The velocity profile in the same region was collected at 100 Hz with a co-located Vectrino II Profiling Velocimeter. Water depth was measured concurrently at the same cross-shore location by an ultrasonic sensor. Particle image velocimetry (PIV) was used between sequential image frames to estimate the velocity profile near the bed and in the sheet flow layer. The goal was to validate PIV with Vectrino II data higher in the water column and provide a method to determine the velocities into the sheet flow layer where the Vectrino II and other current meters cannot measure. Preliminary results show that PIV is capable of determining velocity earlier in the uprush phase, later in the backwash phase, and into the sheet flow layer. Results comparing the sheet flow velocities for the 2 grain sizes and between PIV and in situ velocities will be presented.