Naoto Takeda

Correction of Missing Riverbed Topography and Hydraulic Calculations for Small and Medium Urban Rivers

Tokuzou Hosoyamada

In recent years, the frequency of heavy rainfall has increased in Japan, causing flooding damage in small and medium-sized rivers. For example, Typhoon No. 19 in 2019 caused backwatering in the Ota River in Nagaoka City, Niigata Prefecture, and flooding was observed in the Jodo River area.
Large rivers (first-class rivers) under the jurisdiction of the national government have relatively well-developed river management systems such as periodic cross-sectional surveying, but small and medium-sized rivers, which account for the majority of river lengths, have inadequate management systems due to their huge river lengths. However, the conventional DEM has a problem that the elevation below the surface of the water is not measured.
In this study, we proposed a method for quickly and inexpensively interpolating missing data for small and medium-sized rivers to produce cross-sectional river profiles for hydraulic analysis. The interpolation algorithm was determined based on a weighted average of adjacent elevation data.
An inundation analysis using the interpolated DEM was also conducted to reproduce water level fluctuations associated with rainfall. As a result, it was confirmed that as rainfall increases, water tends to collect in the low elevation area and the water level reaches its maximum after 36 hours. Thereafter, water levels decreased with decreasing rainfall. On the other hand, some of the areas where the water level rises have problems of consistency with topography and land use, and further improvement is required. The present calculations do not consider building occupancy, underground seepage, urban runoff, and other factors, which would increase the computational burden.
In order to fully verify the usefulness of the interpolated DEM, it is necessary to confirm the applicability of the model to reproduced calculations using actual flood data and to other rivers in the future. The method developed in this study is expected to contribute to future river management, such as flood control, as a quick and inexpensive method for assessing flood risk that reduces the burden of field surveying. The hydrological calculation program we developed is still in the prototype stage, and its application to actual inland flooding analysis has not yet been determined.