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.
Highlights
Coastal boulders are good evidence of tsunami attack, which is an extremely high energy event capable of displacing, transporting and emplacing huge and heavy loads a few to several hundred meters away from their original locations
Since the 2004 Indian Ocean modern tsunami event, the boulder transport has been further investigated through detailed posttsunami field surveys (Razzhigaeva et al, 2006; Goto et al, 2007; Paris et al, 2010; Etienne et al, 2011)
In this study, a series of dam-break type hydraulic experiments were conducted in an open channel with a concrete block setting on a horizontal bed just above the beach face to simulate the tsunamiinduced boulder overland movement process, including the effect from the boulder orientation, moving type, instantaneous boulder displacement and velocity
Summary
A number of displaced tsunami boulders were observed through the subsequent in-site surveys after the 2011 Tohoku tsunami event. Instantaneous water level and bore current velocity were measured, and the entire boulder moving process was recorded using a video camera. 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). 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
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