In the whole Pacific Ocean, the probability, which a super-great earthquake of Magnitude 9 will occur, is by no means low, like the 1960 Chile earthquake, the 1964 Alaska earthquake, the 2004 Sumatra earthquake, and the 20" off Pacific Ocean of Tohoku earthquake. Moreover, since the scale of the topographical change by the supergreat earthquake will become large, the prediction of the topographical change is important. Therefore, in this research, by proposing a method for determining a bed load coefficient rationally, the working efficiency of topographical change prediction by a tsunami is raised.
As numerical simulation models for predicting topographical change by a tsunami, there are models of Fujii et al. (1998), Takahashi et al. (2000), Nishihata et al. (2006), Kihara and Matsuyama (2007), Nakamura and Mizutani (2008), and Ca et al. (2010). The calculation process of these models is as follows:
The distribution of fluid velocity is gotten using nonlinear long wave theory. Then, the bottom distribution of non-dimensional shear stress (i.e. Shields parameter) is gotten using the fluid velocity.
The distribution of bed load rates is gotten using an empirical equation with the non-dimensional shear stress. Moreover, the distribution of deposition rates of a suspended load and entrainment rates from the bottom are gotten using the kind of a diffusion equation or empirical equations.
Ground surface elevation is gotten by substituting bed load rates, deposition rates and entrainment rates to a continuity equation of sediment.
(4) The calculations (1)∼(3) are repeated.
In the above-mentioned process, a suitable coefficient of the empirical equation for the bed load must be selected in order to obtain correct bed load rates. For the purpose, we need some verification simulations for every coast. However, supposing we can produce diagrams which can determine the suitable coefficient by setting basic parameters instead of the verification simulations, laborsaving of the topographical change prediction can be attained. Therefore, in this research, to a numerical simulation model using Ribberink's formula (1998) as the empirical equation for getting a bed load rate in an oscillating flow, the diagrams were produced by inverse analysis using scour data from many hydraulic experiments in our lab with a scale of 1/20. Especially, since we could not find existing experiments conducted by changing uniformity coefficients and dry densities, our experiments were implemented by changing median grain sizes, uniformity coefficients, and dry densities. Moreover, we confirmed that numerical simulations using bed load coefficients taken from the diagrams can reproduce topographical changes by the huge tsunami due to the 2011 off Pacific Ocean of Tohoku earthquake.
