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Collapse and recovery process of the sand spit at the Tenryu River mouth on the Pacific Coast of Japan

ABSTRACT This paper investigates the collapse and recovery processes of the sand spit at the Tenryu River mouth on the Pacific Coast of Japan, when two characteristic typhoons, Man-Yi and Fitow, passed over the study site in the year 2007. Although these two typhoons caused equally high storm waves, these two events were different in principal wave directions and in the amount of river discharges. As a result, Man-Yi collapsed the sand spit, while Fitow rather enhanced the recovery of the sand spit. Successive still images recorded by six field cameras were analyzed to investigate the dynamic morphology change of the river mouth for 2 months during which these two events had occurred. Comparisons of obtained topography changes and various hydrodynamic characteristics yielded several findings: (i) the sand spit was breached approximately 6 h after the peak of flow velocity and 2 h after the peak of the water head difference across the sand spit; (ii) the breached part of the sand spit was refilled by wave-induced shoreward sediment transport; and (iii) a core sample showed three clear layers of graded sedimentary structures of gravel, which correspond to the number of observed high waves overtopping the sand spit.