AN EXPERIMENTAL STUDY ON THE TSUNAMI BOULDER MOVEMENT

Abstract

A number of displaced tsunami boulders were observed through the subsequent in-site surveys after the 2011 Tohoku tsunami event. In this study, a series of dam-break type hydraulic experiments were carried out in an open channel with a concrete block setting on a horizontal bed just above the beach face to simulate the tsunami-induced boulder overland movement process. Three different initial water head settings were applied with three different tsunami- boulder interaction angles. Instantaneous water level and bore current velocity were measured, and the entire boulder moving process was recorded using a video camera. Sliding boulder movement was observed in the experiments. Comparing to the normal incident tsunami, oblique incoming tsunami wave resulted in a longer boulder displacement owing to the larger current-projected area. It was confirmed that the block dislodgement is not triggered immediately by the arriving of the water bore, but there exists a measurable time lag between the tsunami arrival and the inception of boulder movement. A small difference in the initial water head settings could lead to a significant difference on the total boulder displacement. As for the boulder moving process, three phases were identified from the experiment, i.e., the acceleration phase (at the beginning stage), the deceleration phase (in the middle stage), and the relatively steady moving phase (in the end). From the experiment, it was confirmed that the difference in the total boulder displacement is mainly ascribed to the boulder moving velocity difference during the deceleration and steady moving phases.