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In the 2011 Tohoku Earthquake Tsunami, run-up of the tsunami along rivers expanded the damaged area into more inland areas far from the coast. High water level in rivers was kept for more than several times of the wave period in the 2011 event, while more simple periodical oscillations of the water level superposed on the tidal motion was observed in the 2010 Chile Tsunami event. The difference of behaviors of tsunami in rivers was discussed with related to the magnitude of tsunami and river mouth morphology through two numerical experiments in this paper. In the first numerical experiments, the run-up of historical tsunamis was simulated along Kitakami River. It was found that the discharge of tsunami flow across the river mouth was proportional to third power of water depth if overflow of the sand spit occurred, and that caused unbalance of water volume coming into and going out from the river channel. Through the second numerical experiment with simple model bathymetry and sinusoidal waves, it was found that discharge during both tsunami run-up and run-down across river mouths is reduced by sand spits at the river mouths and the reduction effect is more significant during flow rushing into the rivers than that during return flow. [ABSTRACT FROM AUTHOR]

The relations of river morphology and tsunami propagation in rivers were studied at several rivers in the Tohoku region during The Great Chilean Tsunami of 2010 and The Great East Japan Tsunami of 2011. It was found that river mouth morphological features play an important role in the intrusion of low magnitude tsunamis in which the geological and geographical conditions are an important factor. Nevertheless, the effects of these features were not found in the case of an extreme tsunami wave. As the wave enters the river, the propagation depends on other factors. It was found that the intrusion distance correlates well to the riverbed slope. The measurements of water level and riverbed slope were analyzed to propose an empirical method for estimating the damping coefficient for the tsunami propagation in rivers based on the tsunami of 2011. The proposed empirical method was used to approximate the length of the tsunami intrusion into a river by assuming that the furthest distance is given for the ratio of local tsunami wave height to the tsunami wave height at the river entrance of 0.05 (5 %). The estimated intrusion length from the proposed method in this study shows a good comparison with measurement data. [ABSTRACT FROM AUTHOR]