### Current Status of Japan's River Improvements for Flood Prevention  
Many of the river embankments in Japan in the early modern period (until the mid-19 th century) had openings at key points, and when river flow exceeded the channel capacity, river overflow was intentionally generated. While embankment technology was immature, such river planning was effective in predicting and mitigating flood damage, as well as reducing flood peaks [9; 10; 11; 12; 13]. In the latter half of the 19 th century, the modern hydraulic control method of promptly discharging floods to the sea by continuous embankments was introduced by European engineers, and a modern river plan was made in 1911 for 13 nationally controlled rivers, as stipulated by the River Law at the time. However, in many other rivers, embankment systems with overflow openings have survived for a long time. The New River Law, which came into effect in 1964, mandated the formulation of a modern river plan for rivers nationwide. By about 1970, a river improvement master plan was formulated for the 109 nationally controlled water systems, and river channel improvement works were started to safely flow flood discharge downstream as specified in the plan. The annual generation probability of target floods in this plan was set at 1/100 to 1/200 for each river system, based on population concentration and economic importance. However, the master plan did not specify completion deadlines, so necessary expenses are not linked to annual budgetary preparation. The total length of the embankments of Class A rivers, which are directly managed by the national government, is more than 13,000 km. In addition, part of the budget for river improvement works must be spent for restoration after every flood disaster. As a result, the pace of river improvement is still slow. Therefore, in a report submitted in 2000, the Panel on Social Infrastructure Development concluded that if the construction of a continuous embankment is economically or physically difficult, methods other than the continuous embankment, including floodplain countermeasures, should be considered. In addition, to clarify the step-by-step procedure and progress evaluation, the River Law was partially revised in 2005 to prepare a river improvement plan for a period of 30 years for each river system. Currently, construction is underway according to the first-stage river improvement plan, which will be completed around 2035. The blue dots in Figure 1 show the correlation between the river channel flow capacity targeted in the first river improvement plan (hereafter referred to as \"current channel capacity\") and the flood discharge specified in the master plan at the river management reference point. The orange dots are the correlation with the river discharge expected after the flood control by dams and detention basins shown in the master plan (planned high-water flow rate), although many of them are still just planned or under construction. Therefore, even after the completion of the first-stage river improvement, a considerable amount of river channel improvement and flood control facility construction are required. Moreover, in light of the frequent increase in torrential rains in recent years, the MLIT has performed numerical long-term climate simulation and announced that the flood flow rate specified in the master plan must be increased by at least 1.2-fold to meet the annual probability of target floods under future climate change [14]. However, considering the progress of current river improvement projects, even if the target flow rate is increased, it will take a tremendous amount of time to achieve it. In other words, only reforming the master plan does not actually improve protection from flood disasters. Figure 2 shows the relationship between the ratio of extreme flood discharge and that of the annual frequency of occurrence by a sample analysis. The decrease in the return  
Figure 1: Correlation between the current channel capacity and the flood discharge specified in the master plan.