Evaluation of the Evacuation Route Availability Based on an Inundation Flow Simulation

This study analyzes runoff and inundation flow to reproduce the condition of the August 1993 flood that occurred in the upper reaches of Sendai River in the northern part of Kagoshima prefecture, and determines model parameters. Besides, the case of situation more serious than the past floods were assumed, and it is estimated whether the inhabitants could take refuge by traveling on foot on flooded roads to evacuation centers. In the runoff analysis, the distributed runoff model based on the kinematic wave model was used. A Gridded Digital Elevation Model (DEM), 50 m resolution was used to provide terrain data and flood runoff was calculated with the Drainage Direction Matrix (DDM) extracted from DEM. Past discharge hydrographs were fairly well reproduced by the runoff analysis. In the inundation flow analysis, a 2-dimensional unsteady flow model based on the dynamic wave model was used to calculate the expansion of the inundation flow. In the inundation flow analysis, the order of water depth was reproduced well in comparison to actual inundation, however, the estimated inundation area was smaller than the actual one. Next, runoff and inundation flow analyses were carried out with the modeled rainfall, which was more serious than that of past floods, using the model parameters determined above. The simulated flooded areas were almost the same as those caused by torrential rain in August 1993, however, the water was deeper. The current velocity was high just after the area flooded, but it slowed after 1 to 2 hours to about 0.05 m/sec in most of the flooded areas. From a viewpoint of the relations among water depth, height of a person and current velocity, it was quantitatively estimated whether the inhabitants could take refuge or not by traveling on foot on the flooded roads to evacuation centers. Some evacuation centers were located in the flooded areas, so inhabitants could not take refuge there on foot unless they took refuge at early stage of the flood. Whether or not inhabitants could take refuge safely on foot largely depended on their height, and the areas where they could walk safely was limited by their height.