Abstract
Abstract. Run-up processes of the 2011 Tohoku tsunami into the city of Kamaishi, Japan, were simulated numerically using 2-D shallow water equations with a new treatment of building footprints. The model imposes an internal hydraulic condition of permeable and impermeable walls at the building footprint outline on unstructured triangular meshes. Digital data of the building footprint approximated by polygons were overlaid on a 1.0 m resolution terrain model. The hydraulic boundary conditions were ascertained using conventional tsunami propagation calculation from the seismic center to nearshore areas. Run-up flow calculations were conducted under the same hydraulic conditions for several cases having different building permeabilities. Comparison of computation results with field data suggests that the case with a small amount of wall permeability gives better agreement than the case with impermeable condition. Spatial mapping of an indicator for run-up flow intensity (IF = (hU2)max, where h and U respectively denote the inundation depth and flow velocity during the flood, shows fairly good correlation with the distribution of houses destroyed by flooding. As a possible mitigation measure, the influence of the buildings on the flow was assessed using a numerical experiment for solid buildings arrayed alternately in two lines along the coast. Results show that the buildings can prevent seawater from flowing straight to the city center while maintaining access to the sea.
Highlights
Recent urbanization of low-lying coastal areas has increased the potential for property damage, human injury, and death caused by tsunamis
Considering the openings of wooden houses such as doors, windows, or cracks and slits caused by tsunami effects, the shallow water building-hole models (BH) model was improved to express the effects of wall permeability by introducing the “assumption of internal hydraulic conditions” on line segments where the walls were located
The approach presented in this paper demonstrated the possibility of accurate urban flood modeling with an internal hydraulic condition at building side faces, which allows water leakage into buildings, in the context of tsunami run-up in Kamaishi city caused by the 2011 Tohoku earthquake
Summary
Recent urbanization of low-lying coastal areas has increased the potential for property damage, human injury, and death caused by tsunamis. Prediction of swift currents in urban areas by numerical flow simulation is expected to be important for evacuation programs and for city layout planning measures to mitigate tsunami damage. Tsunami simulation models for forecasting wave propagation and deformation from the seismic center to the coast have been developed and improved for decades. These models for high-speed calculations in a wide water body are often based on a set of shallow water equations on a structured rectangular grid system (Imamura et al, 1995). Models with a rectangular grid system were extended to calculate the tsunami run-up on land by formulating the wavefront propagation on a dry bed (Titov and Synolakis, 1995, 1998; Synolakis et al, 2008). The tsunami run-up simulation described above requires more precise flow modeling by introduction of the hydraulic effects of building arrangement
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