Flood control

This paper traces the history of flood control in Japan. Despite centuries of progress, severe flood damage still occurs somewhere almost every year. In planning flood control measures, the degree of protection to be achieved is calculated in terms of the return period of the project storm, which may be 50,100 or 200 years. A hydrograph of the project storm is produced by extrapolation from historical data, and proportions of the project flood are then assigned to various control measures. The most common of these measures are levees, channel improvements, floodways, reservoirs and retarding basins. In addition, sophisticated prediction and warning systems are being developed to allow effective evacuation of threatened areas.