Takumi Sugimoto

Study on the Method of Acquiring Topographical Information for Understanding Hydraulic Phenomena of Small-and-Medium-Sized Rivers

Tokuzo Hosoyamada

In recent years, the number of heavy rainfall events has been increasing in Japan, and flood damage has become more pronounced in large rivers and small and medium-sized rivers. In the past, backwatering occurred in the Ota River and the Jodo River in the Shinano River system in Nagaoka City, Niigata Prefecture, when the Shinano River swelled due to Typhoon No. 19 in October 2019, causing flood damage in the Imai area. Understanding the hydraulic phenomena of these small and medium-sized rivers is important in considering future flood countermeasures. However, the length of small and medium-sized rivers in Niigata Prefecture alone is approximately 4,000 km, and it is difficult to manage hydrological data such as water levels and flow rates, as well as cross-sectional survey data for all rivers, which is necessary for numerical analysis, both in terms of budget and time. This study utilizes a numerical elevation model (DEM) for small and medium-sized rivers and riverbed elevation data obtained by separate surveying to supplement riverbed elevation data that is always missing below the surface of the water, to quickly and inexpensively create longitudinal river crossing survey data that can be used in iRIC and our originally developed numerical analysis program. The purpose of this project is to reduce the burden of data preparation, to clarify the problems and issues, and to understand the hydraulic phenomena of small and medium-sized rivers through reproducible calculations.
In this study, longitudinal and cross-sectional survey data of the Ota River in Nagaoka City, Niigata Prefecture, which is the river under study, are prepared based on 2mDEM, 5mDEM, and 10mDEM published by GSI and J-FlwDir published in the Surface Direction Map of Japan. To compensate for the lack of observational data, the flow rate was estimated using iRIC's runoff analysis solver (SRM), and a one-dimensional analysis solver (Nays1D) was used to reproduction calculations of the backwater that occurred in the Ota River when Typhoon No. 19 passed in 2019.As a result, we were able to generate cross-sectional river survey data for numerical analysis based on 2mDEM and 5mDEM. 10mDEM and J-FlwDir are not suitable for generating cross-sectional survey data because of ambiguity in the indication of the bank, high water mark, low channel, and riverbed elevation, which are important in numerical analysis. In particular, J-FlwDir is inadequate for reflecting detailed structures such as levees. The reproduction calculations found that backwater occurred downstream of the Ota River calculation area for approximately 16 hours and 30 minutes, making it difficult for the river water to flow. Therefore, the longitudinal cross-section survey data generated by our original method proved to be useful for the numerical analysis of the actual river.
As a future prospect, although the program was able to automate the generation of cross-sectional survey data to some extent, there are still parts that require human processing, so a fully automated method should be considered in the future. The numerical analysis does not take into account the flow at the confluence of the Ota and Jodo Rivers, so it is necessary to consider the creation of longitudinal survey data and numerical analysis methods that take into account the flow at the confluence of the rivers in the future.