Experimental Measurement Of Rip Currents With Video-Tracked Drifters

Rip currents are approximately shore-normal, narrow, seaward-flowing currents that originate within surf zone, extend seaward of breaking region. They have significant impact on nearshore water transport and sediment movement. This thesis is on investigating rip currents in physical model test. Rip currents were generated by two intersecting wave trains and measured with video-tracked drifters. Flow field of different wave on two topographies were analyzed.In the first part of the thesis, the experimental set-up and process of using video-tracked drifters are given. Making use of wave reflection on a vertical wall which stands perpendicular to coastal line, the wave become normally incident after the obliquely incident wave train intersecting with the reflecting wave train. Drifters are released to the surface to track the flow field. To obtain mean velocities from CCD images, four steps, including processing images, particle tracking, spatial transformation and calculating the velocities, are taken. Details and error source are also described.In the second part, drifter tracks and velocities are used to analyze the characteristics of the flow field. On the beach of alongshore uniform sandbar, rip currents appear in the nodes of the superimposed wave. Maximum velocities are near the bar crest. Concentrated vortices are also found to accompany with rips. When the incident wave is irregular wave, intermittent rips turn up near the vertical wall. However, these rips are affected by the flow outside the superimposed zone. After inserting a rip channel into the sandbar, rip currents remain in the nodes of the superimposed wave on the incidence of regular wave with longer period, although the alongshore variation of topography would affect the flow field through changing the wave break at the channel. When the period is 1.5s, the rip currents will strength since the rip channel is at the position of node. When the period is 2.0s, the onshore transport between the first node and the wall is reduced, which result the inclination of rip currents. Meanwhile, the rips remain intermittent when irregular wave incident, but concentrated in the rip channel.