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[HGM03-04] Causes of concentrated revetment damage in the Hatto River in eastern Tottori Prefecture during the heavy rains in western Japan in July 2018
Keywords:short wave-length gravel bars, concentrated revetment damage, the heavy rains in western Japan in July 2018, low water channel construction work, recovery of gravel bars, aerial photo interpretation
Introduction
The Hatto River, which flows through the eastern part of Tottori Prefecture, originates from Mt. Hyonosen (1,509 m.a.s.l.) with a river channel length of 39.1 km and a basin area of 417.3 km2. Due to the heavy rains in western Japan in July 2018, revetments were damaged at five locations 18 to 20 km from the confluence of the Sendai River. The purpose of this study is to clarify the river channel shape in the Hatto River, especially the transition of gravel bars, and to consider the cause of the concentration of revetment damage points in the 2018 flood.
Method
Using aerial photographs of the Geospatial Information Authority of Japan and satellite images of Google Earth Pro, the gravel bars of the Hattou River was investigated. The shooting years were 1947, 1964, 1976 (including some 1975), 1995, 2013, and 2018, and the distributions of water bodies, gravel bars, vegetation, and exposed bedrock were plotted. The front edges of the gravel bars were determined from the distribution pattern of the gravel bars and represented by a red line with dipping direction. In addition, the area ratio of gravel bars and vegetation in the river channel was calculated by dividing it into 0 to 8 km (downstream), 8 to 16 km (middle stream), and 16 to 24 km (upstream) section.
Results and discussions
According to changes in the shape of gravel bars and changes in the area ratio of gravel bars and vegetation, the number of gravel bars that occupied about 50% in 1947 and 1964 decreased to 35% in 1976, and in 1995, as low water channel excavation work and high flood plain construction progressed, most of the artificial flood plain became vegetation areas. After that, the area ratio of the gravel bars decreased to less than 20%. Instead, vegetation increased to about 50%. As for the longitudinal changes, it was clarified that the area ratio of the gravel bars is higher toward the upstream and the area ratio of vegetation is higher toward the downstream. This is a manifestation of the downstream decrease in the amount of bedload per unit width.
The longitudinal distribution of the position of the front edge of the gravel bars and the frequency distribution classified by the length of the front edge of the gravel bars were analyzed by upper, middle, and downstream, and their transitions were investigated. As a result, 56 gravel bars were certified in 1947 over almost the entire area, and the half-wavelength of the gravel bars was about 50 to 1,250 m, and most of them were 50 to 350 m. In the period of high economic growth, the number of gravel bars gradually decreased in 1964 and decreased to 31 in 1976. In 1995, only 5 gravel bars were found. It is thought that the cause is low waterway construction work.
However, the number increased to 18 in 2013 and 33 in 2018, and many short gravel bars with half wavelengths of 100 to 300 m were observed. Especially in the upstream section of 2018, the number of gravel bars recovered to almost the same state as in 1947. Moreover, the short gravel bars with a half wavelength of 50 to 350 m were outstanding. It is thought that this is a reflection of the fact that the number of low-water channel excavation works has decreased and gravel has been transported from the upstream side. The five revetment damage points found at 18 to 20 km are near the downstream end of the gravel bar recovery section, and many short wavelength gravel bars were found. In general, gravel bars have a short wavelength in the early stage of formation, and the wavelength becomes longer with time to reach a steady state. In short-wavelength bar, the angle of collision of the water flow with the riverbank becomes large and local scouring occurs, which is the cause of the concentration of revetment damage.
Conclusion
Due to the river improvement work after the high economic growth period, the gravel bar of the Hatto River was hardly seen around 1995. However, in the 2010s, a short-wavelength gravel bar was restored from the upstream side, increasing the angle of collision of the water stream with the riverbank and inducing damages to the revetment. This is the cause of the concentration of revetment damage observed in the Hatto River in the flood in July 2018. In the future, it is expected that short-wavelength gravel bars will be formed as the recovery section of the gravel bars extends downstream, and there is a high possibility that revetment damage will occur in the vicinity, so it is necessary to pay close attention.
The Hatto River, which flows through the eastern part of Tottori Prefecture, originates from Mt. Hyonosen (1,509 m.a.s.l.) with a river channel length of 39.1 km and a basin area of 417.3 km2. Due to the heavy rains in western Japan in July 2018, revetments were damaged at five locations 18 to 20 km from the confluence of the Sendai River. The purpose of this study is to clarify the river channel shape in the Hatto River, especially the transition of gravel bars, and to consider the cause of the concentration of revetment damage points in the 2018 flood.
Method
Using aerial photographs of the Geospatial Information Authority of Japan and satellite images of Google Earth Pro, the gravel bars of the Hattou River was investigated. The shooting years were 1947, 1964, 1976 (including some 1975), 1995, 2013, and 2018, and the distributions of water bodies, gravel bars, vegetation, and exposed bedrock were plotted. The front edges of the gravel bars were determined from the distribution pattern of the gravel bars and represented by a red line with dipping direction. In addition, the area ratio of gravel bars and vegetation in the river channel was calculated by dividing it into 0 to 8 km (downstream), 8 to 16 km (middle stream), and 16 to 24 km (upstream) section.
Results and discussions
According to changes in the shape of gravel bars and changes in the area ratio of gravel bars and vegetation, the number of gravel bars that occupied about 50% in 1947 and 1964 decreased to 35% in 1976, and in 1995, as low water channel excavation work and high flood plain construction progressed, most of the artificial flood plain became vegetation areas. After that, the area ratio of the gravel bars decreased to less than 20%. Instead, vegetation increased to about 50%. As for the longitudinal changes, it was clarified that the area ratio of the gravel bars is higher toward the upstream and the area ratio of vegetation is higher toward the downstream. This is a manifestation of the downstream decrease in the amount of bedload per unit width.
The longitudinal distribution of the position of the front edge of the gravel bars and the frequency distribution classified by the length of the front edge of the gravel bars were analyzed by upper, middle, and downstream, and their transitions were investigated. As a result, 56 gravel bars were certified in 1947 over almost the entire area, and the half-wavelength of the gravel bars was about 50 to 1,250 m, and most of them were 50 to 350 m. In the period of high economic growth, the number of gravel bars gradually decreased in 1964 and decreased to 31 in 1976. In 1995, only 5 gravel bars were found. It is thought that the cause is low waterway construction work.
However, the number increased to 18 in 2013 and 33 in 2018, and many short gravel bars with half wavelengths of 100 to 300 m were observed. Especially in the upstream section of 2018, the number of gravel bars recovered to almost the same state as in 1947. Moreover, the short gravel bars with a half wavelength of 50 to 350 m were outstanding. It is thought that this is a reflection of the fact that the number of low-water channel excavation works has decreased and gravel has been transported from the upstream side. The five revetment damage points found at 18 to 20 km are near the downstream end of the gravel bar recovery section, and many short wavelength gravel bars were found. In general, gravel bars have a short wavelength in the early stage of formation, and the wavelength becomes longer with time to reach a steady state. In short-wavelength bar, the angle of collision of the water flow with the riverbank becomes large and local scouring occurs, which is the cause of the concentration of revetment damage.
Conclusion
Due to the river improvement work after the high economic growth period, the gravel bar of the Hatto River was hardly seen around 1995. However, in the 2010s, a short-wavelength gravel bar was restored from the upstream side, increasing the angle of collision of the water stream with the riverbank and inducing damages to the revetment. This is the cause of the concentration of revetment damage observed in the Hatto River in the flood in July 2018. In the future, it is expected that short-wavelength gravel bars will be formed as the recovery section of the gravel bars extends downstream, and there is a high possibility that revetment damage will occur in the vicinity, so it is necessary to pay close attention.