Detection of Groundwater Flow Velocity Field in the Swash Zone of the Coral Gravel Beach Using Particle Tracking Velocimetry

The swash zone of coarse sediment beaches, due to its high porosity, involves strong surface water and groundwater exchange. This leads to the need for more groundwater data in the swash zone for understanding and modeling of its dynamic behaviors. This study focuses on the development of a groundwater velocity detection method in the swash zone of the coarse coral gravel bed. The coral gravel beach was prepared in a wave flume with tracer particles injected inside the bed. The movement of particles during wave runup and rundown was captured by a high-speed camera. Each image frame was then subtracted from an initial frame, to mask out the background of non-moving gravels under the bed surface. A particle tracking velocimetry (PTV) technique was then applied on these modified images which highlight only the moving tracers inside the coral gravel bed. As validation, eye-detected particle movement was manually collected during the runup and rundown at multiple locations inside the bed of the swash zone. Comparison of these data showed good velocity detection skill (magnitude and timing) of the present PTV-based method, particularly when there was high contrast between gravels and tracer particles, and supported the applicability of the proposed method for quick and effective detection of spatiotemporal distribution of velocities in the porous gravel bed where direct velocity measurement using conventional velocity meter instrument is not possible. Results show that magnitudes of vertical velocity components are comparable to those of horizontal velocity components by a factor of around 0.4 and have a time lead of around 0.1 of the wave period.