### Topographical Features of the Upper Reaches of Rivers in Japan  
To specifically plan river overflow in accordance with the new policy, attention must be paid to the topographical features of the floodplain. Here, the characteristics typical of a Japanese river floodplain are explained using the Abukuma River, which is the site of this study, as an example. As shown in Figure 3, 70% of Japan's land area is mountainous, with rivers flowing through the narrow valleys between them. Figure 4 is the \"landform classification map for flood control [15]\" of the upper reaches of the Abukuma River, which is surrounded by a red frame in Figure 3. This type of map shows the classification of the microtopography formed by river floods, such as floodplains, natural levees, and old river tracks. In the valley bottom plain, small basins and constrictions are connected in series, and it can be seen that the meandering river channel often touches the hill edges (hereafter referred to as \"hill corners\"). Due to these topographical features, until recently, floodplains (yellowish green) in the basins were temporarily flooded during deluges and had the flood control effect of reducing the peak discharge. However, by the continuous levees (black lines with scales) built monotonously according to the conventional river improvement policy, the flood retarding effect was lost, and the peak of the downstream flood increased.  
Figure 3: Elevation topographic map of Japan’s land. The red rectangle shows the area for Figure 4.  
Figure 2: Variation in return period due versus variation in design flood flow rate.  
Figure 5 is an enlarged view of the area outlined by the red rectangle in Figure 4. The pink dots indicate the distribution of houses. Most of the houses are on plateaus that are unaffected by river floods and on natural levees that are less frequently flooded. The floodplains mostly comprise paddy fields, with almost no houses. In other words, for a long time, land use has been based on the expectation of river flooding.  
There are three basin terrains in the figure; for convenience, they are called the first, second, and third basins from the upstream direction hereafter. The Hamao Detention Pond (HDP), which is described in detail in Section 3.2, is located in the third basin. The river section directly managed by the national government extends up to the second basin, where continuous levees have been constructed. In contrast, the river channel of the first basin is managed by the Fukushima prefecture, and the embankment remains unfinished (brown in the figure) due to the restrictions of the prefecture's budget.  
A closer look at the shape of these unfinished embankments reveals that they are open upstream of the hill corners, as marked by the three red open arrows. Until the 1970s, such levee openings existed in the upper reaches of many rivers in Japan, but they gradually disappeared as continuous levees were extended from downstream [16]. Old river tracks and drainage channels were connected to such levee openings and were used for drainage after the paddy fields on floodplains were inundated. In addition, as shown in Figure 6, it is probable that floodwaters from the opening gradually spread upstream during the large flood, which contributed to the reduction of the flood peak.  
Figure 4: Microtopography of the valley bottom plain (the upper Abukuma River). The red rectangle shows the area for Figure 5.  
Figure 5: Three basin terrains in the upstream of the Abukuma River.  
Figure 6: Schematic view of the function of levee opening located at hill corner.