Process Understanding of the Togagawa River Flooding 2008 Using a Distributed Rainfall-runoff/flood-inundation Model

A very rapid rise of the river water level occurred in Togagawa River, Hyogo, Japan around 2:40 PM on July 28 2008 due to a localized so-called guerrilla rainfall. The water level rise at Kabutobashi Bridge was 1.3m during 2-3 minutes until 2:50 PM. 5 people including 3 children were dead due to the water level rise in the river. People were drown by the flush flood. The rainfall amount was 24 mm during the 10 minutes. To understand the process of the flooding in urban area on the time, especially to know the contributions of the water from the mountain area and through the drainage network, the authors made rainfall-runoff/flood-inundation models with 5 and 10m resolution using a 2D inundation simulation model based on a shallow water equation with a subsurface flow model of the forest area consisting of an depth-integrated unsaturated flow equation and a drainage network model. As the result, first, the subsurface flow model showed that the simulated volumetric soil water content of the forest area did not increase much due to the short-duration (approx. 10 minutes) of the extreme rainfall, thus the surface flow did not appear in the forest area. Second, the simulated river water level at the Kabutobashi Bridge did not rise rapidly until the level of the observation of the time if the rainwater drainage network was not considered (i.e. the rainwater coming through the drainage network was zero). When the rainwater drainage was implicitly considered in the model, the simulated water level at the Kabutobashi Bridge rapidly rose up to the observation level. By analysing these results, it was inferred that the water level rise was due to the river flow with the effect of the drainage network and the contribution from the rainwater of the mountain area was not significant. The results indicated that the flush flood can be mitigated by diverting the rainwater drainage directly into the sea downstream.