## 3 Description of the Study Site
### Terrain and Hydraulic Facilities of the HDP  
Figure 12a,b shows an enlarged map of the basin terrain and structure of the HDP, respectively, in which KP indicates the river channel distance from the Miyagi-Fukushima prefectural border. The HDP is located near the exit of the basin of about 300 ha, and it has an area of 62 ha and storage capacity of 1,800,000 m\({}^{3}\)/s. The intricate part of the river trace near the HDP corresponds to the old river channels straightened in the early 2000s. Natural levees are widely developed on the left bank upstream of the HDP, and the central part of the basin is classified as a floodplain. This means that the river often overflowed onto the floodplains before the levee was constructed for channel improvement. Figure 13 shows the elevation of the floodplain along the survey line (C-C) indicated in Figure 12a. The HDP was partially excavated from the original ground level to create the required storage capacity. The basin terrain has a slope of about 1/500. Figure 14 shows a longitudinal view of the left and right banks, as well as the planned HWL. The levee design height (green line) is the HWL plus the freeboard stipulated by the structural ordinance for river facilities [9]. The levee further has an extra height of about 50 cm in anticipation of future land subsidence. The top of the left levee from 98.8 to 100.2 KP is horizontal because it also serves as an enclosure levee for the detention pond. At 98.6 KP, there is a gate for draining local runoff (hereafter referred to as the second drainage gate), and the levee does not have additional height at this location. The top of the left levee around 101.4 KP and 103.4 KP is lower because road bridges that predate the levee construction are present. The right levee from  
Figure 11: Aerial photographs Hamao Basin terrain: (**a**) before HDP construction, (**b**) after HDP construction. Figure 12: Landform classification map of study site: (**a**) basin terrain; (**b**) Hamao Detention Basin. 101.6 to 102.4 KP was not completed at the time of the 2019 flood and was lower than the design height. In this area, as indicated by colored triangles in Figure 12a, observation facilities are located at many positions. Two water level observatories (indicated by red triangles), named Sukagawa and Kosakuda, are located near the exit and the entrance of the basin terrain. At the time of the flood, in addition, the flow rate was estimated from the velocity measurement using a float thrown into the river from the bridge. However, for the 2019 flood, data are not available from the Kosakuda Observatory because the water gauge was damaged by submergence. Water gauges are also installed on the land side of two drainage gates (indicated by green triangles), which were built inside and outside of the HDP. Because flooding was anticipated from the locations of the two road bridges upstream of the HDP (indicated by blue triangles), temporary water gauges, named 3 L water gauges [17] have been installed on the bridges to measure the water levels only during floods. The solid blue line in Figure 15 shows the relationship between the water level and the storage capacity of the HDP. As currently configured, the inflow begins when the river channel water level exceeds 238.35 T.P.m. Here, the T.P.m is the elevation in meter measured from the average tide level in Tokyo Bay. The top of the surrounding embankment is at 241.7 T.P.m. In contrast, the solid red line shows the relationship where the surrounding embankment of the detention pond is removed. This is explained in Section 7. Figure 16 shows the estimated results of the flood control effect on the 1998 flood. A peak reduction of about 90 m\({}^{3}\)/s is shown. The peak flow rate of this flood is about 70% of the high-water flow rate written in the master plan, and the probability of occurrence is estimated from Figure 2 to be about 45 years.