2019 Volume 75 Issue 2 Pages I_707-I_712
Undertow is a significant offshore-ward current that is induced by a shoreward water mass flux under wave breaking and Eulerian drift in non-wave-breaking conditions. The undertow generates offshore sediment transport and also affects swimming safety. To increase the understanding of the undertow, field observations were conducted on a barred beach, Hasaki coast, Japan, for 21 days from May 13 to June 2, 2016. A Horizontal Acoustic Doppler Current Profiler was used to measure cross-shore currents, i.e., undertow, and water levels in the surf zone. Observed data was analyzed to investigate the characteristics of the spatial and wave-energetic distributions of the undertow exceedance probability (hereafter as PE). When the wave energy flux level was high, the undertow became larger, and its PE increased. By contrast, the PE during a low wave energy level decreased with larger water depths, and its lowest value was in the trough region. Applying a Weibull distribution to PE curves can develop a statistical model of the undertow, and it can be considered that the Weibull parameters are affected by the combination of normalized values of the wave energy flux, relative surf zone location, and normalized water depth. The two parameters of Weibull distribution was able to estimate using the normalized parameter, and the model well predicted the PE. Furthermore, to demonstrate the effect of tidal ranges on the undertow, the PE distributions for each wave energy level and location were distinguished by the water level, i.e. tidal elevation, as high-tide or low-tide durations. The undertow PE was greater for low-tide data than those for high-tide and the full data at a low wave energy level.
