Characteristics of Sediment Entrainment and Transport Depending on Initial Water Depth under Tsunami-like Bore in Laboratory Experiments

In identification of (paleo)tsunami deposit, which is a possible key to reconstruct the history and behavior of paleotsunami, existence of marine sediments within it has been considered to be an important evidence. However, the recent studies of the 2011 Tohoku-oki tsunami deposits and flume experiments reported that onshore tsunami deposits did not necessarily contain a considerable rate of marine sediments. Little is known about control factors to affect content rate of marine sediments in onshore tsunami deposits. On this point we performed a series of flume experiments to examine effects of water depth above a sand bed on sediment entrainment and transport by tsunami. The present experiment directed tsunami-like bores with similar near-bed flow velocity (= 47-97 cm/s) onto a sand bed with different initial water depth above the bed (= 1-5 cm). The observation showed that, although near-bed flow velocity rapidly increased at the same moment with rising water level (i.e., bore head arriving), the timing of sediment entrainment under a passing bore varied depending on initial water depth. For runs with shallower water depth, the bed sediment was entrained at the same time as the bore head arrived. In contrast, for runs with deeper water depth, entrainment of the bed sediment lagged behind the bore head arriving. This result suggested that, depending on initial water depth above sea bed, timing of sediment entrainment can not necessarily coincide with increasing of near-bed flow velocity and bed shear stress. Although further studies to investigate relationship between water depth, characteristics of tsunami and the time lag in natural are required, water depth can have effects on volume and timing of sediment entrainment (i.e. erosion) from sea bottom due to tsunami.