Estimation of Inundation Damage by Using GIS Data

Yokohama City has routinely conducted benefit-cost (B/C) analysis for inundation control projects in order to implement the projects efficiently with citizens' understanding. In predicting the amount of damages due to inundation, however, a detailed rainfall runoff simulation is required to insure precision and it is considerably costly and takes time. It is worried that the B/C analysis takes time and this may impede the quick decision making on policies. Thus, the City has thought that an inundation prediction model is needed which makes possible a simulation for the whole city area based on a simplified technique briefly.The City has been developing information management based on 250-meter square meshes for the whole jurisdiction. A simplified inundation prediction model with this mesh as a unit by the use of geographical information system (GIS) as well as the result of B/C analysis with this model is presented.A model uses discrimination techniques with reference to Maharanobis distance with two (2) variables, i.e., the degree of inundation risk represented by a ratio of runoff discharge to the recorded runoff draining capacity and that of surface grade representing total discharge capacity of surface and drainage sewers, for each mesh. With this model, whether flood will be occurred and whether flood depth is above or below floor level if so is predicted for each mesh based on two variables thereof. The model, calibrated with the past 10 rainfall events, was found to reproduce the actual inundation fairly well. Although there were some meshes in which inundation was predicted not to occur whereas it actually occurred, there was inundation in one or more out of past 10 rainfall events in most of these meshes. There were few meshes in which the actual inundation was not reproduced.Simulation with the model was carried out for planned rainfalls of various return periods, then B/C analysis was conducted. A B/C ratio reached 8 in drainage districts where drainage system development seemed a pressing need and the one exceed 2 in drainage districts where the priority of inundation control projects seemed relatively low. With the results, the fact that the drainage system development in the City is urgent and important has been realized again.Although the model, a macro model using GIS data, has problems in terms of quantitative evaluation and the accuracy of results, it has been found effective in that it makes possible quick decision making and prioritization of the projects over a wide area relatively easily.