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[ACG45-P05] Evaluation on the Effects of Gravel Mining and Dam Construction on Sediment Dynamics in the Yoshino River
Keywords:gravel mining, dam construction, sediment dynamics, riverbed fluctuation, sediment load, Yoshino River
Gravel mining from river channels and the construction of dams affect the sediment dynamics of rivers, causing problems such as coastal erosion and riverbed degradation. It is important to understand the changes in sediment dynamics caused by gravel mining and dams and to manage sediment comprehensively to deal with these issues. In Japan, during rapid economic growth after World War II, gravel mining from river channels and construction of dams were actively conducted. In the Yoshino River, gravel mining was conducted on a large scale, mainly in the 1960s and 1970s. Furthermore, the Ikeda Dam was completed in 1975. In this study, we evaluated the effects of gravel mining and dam construction on the sediment dynamics of the Yoshino River by estimating the riverbed fluctuations and sediment load over 35 years from 1965 to 1999 using a one-dimensional riverbed deformation analysis.
The study section was 77.8 km from Ikeda Dam to the mouth of the river in the Yoshino River. The discharge at the upstream end was the daily average flow rate of Ikeda (unbanked) for 1965-1974 and the hourly discharge of Ikeda Dam for 1975-1999. The initial riverbed elevation was the measured riverbed elevation in 1965. The grain size distribution of the riverbed material was measurement in 1997. We converted the riverbed elevation at these weirs to the elevation of each weir and set that the riverbed elevation at that location would not decrease. We analyzed two cases: the first assumes no gravel mining or dam construction, and the second assumes gravel mining and dam construction as in the actual situation. Riverbed degradation due to gravel mining was calculated by dividing the annual gravel mining volume by the channel area and applied at the beginning of each year. The amount of gravel mining was assumed to be six times the permitted amount before 1974 and the same as the permitted amount after 1975. In addition, we assumed that the Ikeda Dam supplied 30% of the equilibrium sediment load upstream of the dam.
The changes in the riverbed elevation in 1974, based on the measured riverbed elevation in 1965, were calculated to have decreased by an average of 0.8 m when gravel mining was carried out. The riverbed elevation seems to have declined mainly during large-scale gravel mining. On the other hand, after the gravel mining volume was decreased and the Ikeda Dam was completed in 1975, the riverbed fluctuation showed a stable trend. The analysis results showed that the mean grain size of the riverbed in case of gravel mining and dam construction after 1975 increased compared to the case without implementation. Our results suggest that the stabilization of the riverbed after the completion of the dam is related to the reduction of sediment supplied by the dam and the granulation of the riverbed.
Changes in sediment load depending on gravel mining during 1965-1974 were observed mainly in a limited area from Kakihara weir to the estuary. The sediment load to the sea area decreased by about 10% because of the implementation of gravel mining. Sediment load in 1975-1999 was halved in the entire section due to gravel mining and dam construction. This suggested that the decrease in sediment supplied by Ikeda Dam affected even the estuary.
The study section was 77.8 km from Ikeda Dam to the mouth of the river in the Yoshino River. The discharge at the upstream end was the daily average flow rate of Ikeda (unbanked) for 1965-1974 and the hourly discharge of Ikeda Dam for 1975-1999. The initial riverbed elevation was the measured riverbed elevation in 1965. The grain size distribution of the riverbed material was measurement in 1997. We converted the riverbed elevation at these weirs to the elevation of each weir and set that the riverbed elevation at that location would not decrease. We analyzed two cases: the first assumes no gravel mining or dam construction, and the second assumes gravel mining and dam construction as in the actual situation. Riverbed degradation due to gravel mining was calculated by dividing the annual gravel mining volume by the channel area and applied at the beginning of each year. The amount of gravel mining was assumed to be six times the permitted amount before 1974 and the same as the permitted amount after 1975. In addition, we assumed that the Ikeda Dam supplied 30% of the equilibrium sediment load upstream of the dam.
The changes in the riverbed elevation in 1974, based on the measured riverbed elevation in 1965, were calculated to have decreased by an average of 0.8 m when gravel mining was carried out. The riverbed elevation seems to have declined mainly during large-scale gravel mining. On the other hand, after the gravel mining volume was decreased and the Ikeda Dam was completed in 1975, the riverbed fluctuation showed a stable trend. The analysis results showed that the mean grain size of the riverbed in case of gravel mining and dam construction after 1975 increased compared to the case without implementation. Our results suggest that the stabilization of the riverbed after the completion of the dam is related to the reduction of sediment supplied by the dam and the granulation of the riverbed.
Changes in sediment load depending on gravel mining during 1965-1974 were observed mainly in a limited area from Kakihara weir to the estuary. The sediment load to the sea area decreased by about 10% because of the implementation of gravel mining. Sediment load in 1975-1999 was halved in the entire section due to gravel mining and dam construction. This suggested that the decrease in sediment supplied by Ikeda Dam affected even the estuary.