## 7 Considerations for Remodeling the HDP to Improve Flood Control Capability
### Numerical Simulation and Discussion  
Figure 32 shows the flood hydrograph input into a numerical experiment to investigate the hydraulic effect of the modified facility. The maximum discharge and flood duration are the same as those of the 2019 flood (see Figure 25b), but the hydrograph was simplified to have a single peak. The downstream end condition of the calculation reflects the relationship between the flow rate and the water level at the Sukagawa Observatory shown in Figure 18b.  
Figure 33 shows the temporal change in inundation depth (a-e) and the maximum depth distribution (f). The \(T\) in the figures corresponds to the elapse time shown in the hydrograph in Figure 32. The inundation started before \(T\) = 12:00, and just after the flood peak (\(T\) = 16:00), the HDP became almost full. After that, the inundation area expanded upstream until \(T\) = 20:00, and at \(T\) = 24:00, the river overflow had already ended, and the inundation area began shrinking. At \(T\) = 28:00, the inundation was almost over, except in the depression comprising the HDP.  
Because most of the houses are located on hillsides and natural levees, the inundation depth was less than 1 m, as shown in Figure 33f. Therefore, it is thought that the flooding can be dealt with by relatively simple measures, such as raising the house base or surrounding the houses with a tall impermeable fence with a water stop gate that can be closed during flooding. Farmland, however, is flooded over a fairly large area. Therefore, the harvest may be lost once every few decades due to an excessive flood.  
Figure 34 illustrates the flood control effects. Figure 34a compares the downstream end hydrographs obtained by the numerical simulations. The dotted curve is the hydrograph when without any flood control facility, and the difference from this line shows the flood control effect of existing HDP and remodeled HDP, cases (1) and (2), respectively. In case (1), the current HDP quickly filled before the flood peak and did not contribute to reducing the maximum flow rate. This is because, as mentioned in the previous section, the current HDP capacity is insufficient against the excess runoff. In case (2), on the other hand, a peak reduction of about 190 m\({}^{3}\)/s can be seen.  
Figure 31: Expected behavior of floodwater on the floodplain for case (2). (**a**) Flow down phase. (**b**) Storage expansion phase. (**c**) Drainage phase.  
Figure 32: Flood hydrograph for numerical experiment.  
Figure 34b shows the rate of inflow from the overflow weir and the rate of drainage from the downstream gate in case (2). The drainage hydrograph has a time delay of about 8 h with respect to the inflow hydrograph, and the peak value is halved. Figure 34c shows the water levels on both sides of the drainage gate for case (2). Both water levels are about 2 m lower than the height of river embankment, and no overflow occurs.  
Numerical simulation was performed also for a hydrograph having the same profile as that in Figure 32 but with a peak flow rate of 2400 m\({}^{3}\)/s, which corresponds to the maximum flood discharge written in the master plan. The results are shown in Figure 35 in the same way as in Figure 34. Figure 35a shows that the remodeled HDP reduces the flood peak by about 220 m\({}^{3}\)/s. In addition, Figure 35c shows that the water levels on both sides of the drainage gate are about 1 m lower than the height of the river embankment, and no overflow occurs. The results of this calculation suggest that the remodeled HDP will have some flood control effect, even for a runoff event larger than that in 2019.  
Figure 34: Flood control effect for \(Q_{p}=2200\) m\({}^{3}\)/s. (**a**) Flood hydrographs at the downstream end for all three scenarios. (**b**) Hydrographs of inflow and outflow for case (2). (**c**) Water levels at both sides of the second drainage gate for case (2).  
Figure 33: Inundation depth distribution for case (2): (**a–e**) temporal variation (\(T\) is the elapse time shown in Figure 22); (**f**) maximum depth.