Detection of rip current using camera monitoring techniques

Rip currents are approximately shore normal seaward flows which are strong, localized and rather narrow. They are known that stacked water by longshore currents suddenly flow back out to sea as rip currents. They are transient phenomena and their generation time and location are unpredictable. They are also doing significant roles for offshore sediment transport and beach erosion. Rip currents can be very hazardous to swimmers or floaters because of their strong seaward flows and sudden depth changes by narrow and strong flows. Because of its importance in terms of safety, shoreline evolution and pollutant transport, a number of studies have been attempted to find out their mechanisms. However, understanding of rip currents is still not enough to make warning to people in the water by predicting their location and time.

This paper investigates the development of rip currents using camera images. Since rip currents are developed by longshore currents, the observed longshore current variations in space and time can be used to detect rip current generation. Most of the time convergence of two longshore currents in the opposite direction is the outbreak of rip current. In order to observe longshore currents, an optical current meter(OCM) technique proposed by Chickadel et al.(2003) is used. The relationship between rip current generation time and longshore current velocity variation observed by OCM is analyzed from the images taken on the shore. The direct measurement of rip current velocity is also tested using image analysis techniques. Quantitative estimation of rip current strength is also conducted by using average and variance image of rip current area. These efforts will contribute to reduce the hazards of swimmers by prediction and warning of rip current generation.