## 8 Conclusions  
The rapid increase of excess runoff due to global climate change will be unavoidable. However, at this time, flood data are limited due to the infrequency of excess runoff events in each basin and further, large floods often damage measuring equipment, accidentally creating periods without data. Therefore, it is not easy to establish new design standards for flood control facilities against excess runoff at this moment. In addition, considering budget constraints for flood control projects, it is desirable to remodel existing facilities by the simplest method possible.  
Therefore, in the first part of this study, the flood control effect of the existing detention basin (HDP) during the excess runoff event that occurred on the Abukuma River in 2019 was analyzed by combining the fragmentary field observation data with numerical simulation. In the second part, in accordance with the MLIT's new flood control policy to allow excess water to flow onto floodplains, a simple and low-cost remodeling method for the HDP was proposed and examined for its flood control effect.  
In this remodeling plan, the surrounding levee on the upstream floodplain slope is removed, and the top of the overflow levee is raised to the HWL. As a result, the excess water volume exceeding the HDP's capacity flows out to the upstream floodplain to expand the water surface area, and the flood control effect is greatly improved. Although the village in the upstream floodplain is flooded to some extent, it was confirmed that inundation damage could be sufficiently mitigated by simple improvements to house structures for waterproofing.  
The numerical model used in this study is a simple one commonly used to estimate the inundation extent in river management works in Japan, which combines one-dimensional unsteady flow equations for a river channel and two-dimensional shallow water equations with regular rectangular grids for floodplains. Despite being a very simple model, the calculation results were in good overall agreement with observation data from the 2019 flood. This calculation is very fast, and the computation cost is low, so it is suitable for the rough design of flood control facilities.  
For specific and detailed facility planning, numerical simulation using a more precise model will be required. For example, a numerical model using an unstructured triangular grid system [20] can express the effects of the existence of structures, such as houses and roads, in floodplains and the effects of trees in river channels on the flow field, but the calculation load is very high. Therefore, preliminary studies to readily compare alternatives possible under the given topographic and land use conditions are required. The approach using the simple model will be useful as demonstrated in this study.  
Figure 35: Flood control effect for \(Q_{p}=2400\ m^{3}/s\). (**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).  
**Author Contributions:** Conceptualization, T.I.; methodology, S.H. and T.I.; software, S.H.; validation, S.H. and T.I.; formal analysis, S.H. and T.I.; investigation, S.H. and T.I.; resources, S.H.; data curation, S.H. and T.I.; writing--original draft preparation, T.I.; writing--review and editing, T.I.; visualization, S.H. and T.I.; supervision, T.I.; project administration, T.I. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Not applicable.  
**Acknowledgments:** We would like to thank Ministry of Land, Infrastructure, Transport and Tourism, Tohoku Regional Development Bureau, Fukushima Office of Rivers and National Highway for providing field survey data for this study.  
**Conflicts of Interest:** The authors declare no conflict of interest.