[M-IS20] Tsunami deposit

[MIS20-P09] Reproduction of tsunami deposit structures and analysis of hydraulic characteristics using a quasi-three-dimensional simulation model

*Sota Nakano¹, Daisuke Sugawara² (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.International Research Institute of Disaster Science, Tohoku University)

In tsunami deposits, specific sedimentary structures such as normal grading and reverse grading, unit structures, landward thinning and fining can be observed. Formation factor of landward thinning and fining is given by ideas such as based on the concept of a decrease in "flow capacity," which is the total volume of particles that the flow can support (Fujiwara, 2007). Landward fining is also explained by the concept of the preferential deposition of coarse sand due to differences in settling velocity (Fujino, 2006). However, it is difficult to directly determine hydraulic parameters at the time of formation of actual sedimentary structures. Therefore, in this study, aiming to quantitatively clarify the relationship between sedimentary structures and hydraulic parameters and to reexamine interpretations of tsunami deposits, simulations of the 2011 Tohoku-Oki tsunami in the Sendai Plain were conducted, and the hydraulic conditions at the time of tsunami deposit formation were analyzed based on the results. In the Sendai Plain, sediment data along transects have been obtained by such as Abe et al. (2012) and Goto et al. (2012). In this study, numerical simulations targeting these transects were performed and compared with field data.
The numerical model used for the simulation is Delft3D, a quasi-three-dimensional simulation model developed by Deltares, Netherlands. This model can resolve not only horizontal but also vertical distributions of physical quantities, enabling detailed analysis of the relationship between sedimentary structures and hydraulic parameters. Due to the high computational load in case of considering grain size distribution in large-scale three-dimensional calculations, we conducted separately the two-dimensional horizontal and two-dimensional vertical calculations. Firstly, tsunami generation, propagation, inundation, and sediment transport were reproduced using two-dimensional horizontal calculations. Then, using the water level time series obtained from these calculations as input data for the two-dimensional vertical calculations, and detailed computations of tsunami inundation and sediment transport along transects were conducted. Simulated horizontal and vertical distributions of sediment grain composition of tsunami deposits and hydraulic parameters were obtained by this approach.
As a result of two-dimensional horizontal calculations, macroscopic landward thinning was reproduced, but at a microscopic scale, layer thicknesses varied significantly in front and behind local topographic highs. In two-dimensional vertical calculations, landward fining, normal grading and reverse grading of tsunami deposits were reproduced. The average grain size of the deposits showed minimal influence from local topographic highs compared to layer thicknesses, generally decreasing monotonically towards the inland. The formation factors of these sedimentary structures can be explained by differences in the rate of change of suspended sediment concentration of each grain-size class. During tsunami inundation, flow velocity and inundation depth decreased almost monotonically in space and time, indicating that the decrease in flow capacity corresponds to reductions in suspended sediment concentration and average grain size.
In this presentation, furthermore, simulations are conducted by altering the boundary conditions of vertical two-dimensional calculations to clarify whether the decrease in flow capacity or settling velocity affects changes in suspended sediment concentration and average grain size. Based on the results of these simulations, relationship between tsunami deposits and hydraulic parameters is examined.