## 7 Considerations for Remodeling the HDP to Improve Flood Control Capability  
### Remodeling Policy and Trial Design  
The increased frequency of floods exceeding the discharge capacity of river channels due to global climate change is concerning, especially because it will take a long time and substantial funding to improve long river channels and build new flood control dams and detention basins. To alleviate this dilemma, we may consider two possible methods that are relatively simple, have low cost, and possibly could be implemented in a short time. One is to resize existing detention basins to increase flood control capabilities. The other is to accept some amount of flood damage by releasing the water volume that exceeds the capacity of the river channel into the floodplain as safely as possible. By combining these  
Figure 28: Calculated water level at the second drainage gate during the 2019 flood.  
Figure 29: Comparison of the inflow to the HDP and the overflow to the left side of the floodplain during the 2019 flood.  
two ideas, a simple remodeling of the HDP is considered, and its effect is evaluated by a numerical experiment.  
As explained in Section 2.2, in many small basins in the upper reaches of the Abukuma River, inundation was more frequent before the construction of levees than it is at present, and the basins functioned as natural water-detention basins. In fact, the houses are located on the hills around the basins and on the natural levees, even after the recent levee construction, and the parts of the floodplain with low elevation are used as paddy fields. Therefore, the settlements in these areas are considered to be able to accept some amount of flooding. Therefore, according to the concept explained in Section 2.3, we examined the improvement of the flood control capacity of the HDP by remodeling it into a borderless detention pond that gradually floods from the downstream side without hav-ing a surrounding embankment.  
Figure 30 shows a schematic diagram of the facility improvement. Panel (a) shows the current HDP, the area of which is limited to about 1/5 of the total area of the basin by the surrounding levees. Houses and farmlands in this area were demolished, and the ground was excavated to increase the water storage capacity. However, as described in the previous section, the HDP quickly filled during the 2019 flood and did not contribute to reducing its peak.  
Therefore, remodeling was considered as shown in panel (b). In the remodeled HDP, the surrounding levee on the land side of the HDP is removed and the top of the overflow weir is raised to the HWL. Villages and farmlands remain in the floodplain. Note that it is assumed that the levees with overtopping in 2019 are increased to a height sufficient to prevent recurrence. The thick arrows marked with (a), (b), or (c) indicate the expected flow pattern on the floodplain at each temporal stage, the side views of which is depicted in Figure 31 in the next page:  
1. When the water level in the river channel exceeds HWL, some of the river water starts flowing onto the floodplain from the overflow weir and flows downstream along the floodplain slope.
2. After the flood flow reaches the downstream end of the basin, it forms a pond that gradually expands upstream due to the backwater.
3. When the river water level outside the drainage gate becomes lower than the land side water level, the gate is opened, and the stored water is discharged to the river.  
As the result of surrounding levee removing, the relationship between the water level and the storage capacity of the HDP, as shown in Figure 15, is improved especially above the elevation of 239 T.P.m, which will become effective against excess runoff.  
Figure 30: Schematic diagram of remodeling the HDP (upper: plan view, lower: side view). (**A**) Case (1), existing HDP. (**B**) Case (2), remodeled HDP.