Effect of Tidal Range on Undertow Distributions Based on Exceedance Probability Analysis

In a surf zone, undertow is classified as one of significant seaward currents that influences on the suspended sediment transport and its cumulative effect finally brings about beach erosion. When undertow is being generated by wave mass flux under high energetic wave conditions, it is not only able to carry a large amount of sediment seaward but also the existing science dogma describes as a danger for swimming. Although the undertow models have been developed for suitably managing nearshore zones, many investigations of undertow were based on laboratory data that there is a space for discussing the effect of tidal range.

In this study, field observations were performed at the Hasaki Oceanographical Research Station in Japan. The undertow and water level were observed by using a nominal Horizontal Acoustic Doppler Current Profiler (H-ADCP) under both of storm and calm conditions. The observed data were arranged following the rank of wave energy flux from high to low energy level and then analyzed by using exceedance probability to investigate the characteristics of undertow distributions. To emphasize the effect of tidal range, the distribution of undertow exceedance probability was divided into high-tide and low-tide durations, which were defined by using the mean water level and standard deviation. The results show that the undertow exceedance probability increased with an increase in wave energy flux, whereas the differences between undertow exceedance probability of high-tide and low-tide data enlarged with a decrease in wave energy flux. At a low energy wave condition, the undertow was subject to low-tide and its exceedance probability was higher than those of full-data and high-tide data. Therefore, even in a non-wave-breaking condition, the undertow can be periodically induced by the changes in water level and this effect should be accounted for the undertow prediction.