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[ACG43-P07] Effect of upstream sediment inflow conditions on sediment dynamics by grain size in the lower reaches of the Abukuma River
Keywords:sediment transport, sediment loads, grain size distribution, one-dimensional model, boundary conditions, Abukuma river
Background and Objectives
Appropriate input of the amount of sediment inflow from the upstream edge and its grain size distribution is very important. There have been many reports on the analysis of sediment inflow from upstream, assuming the effects of slope failure in mountainous areas and human activities such as dams and land use changes, but insufficient knowledge has been obtained to evaluate sediment dynamics in a watershed on a long-term scale and by grain size. In this study, a one-dimensional riverbed fluctuation analysis is conducted for the period 2000-2019 in the lower reaches of the Abukuma River to evaluate the long-term effects of various upstream sediment conditions on the sediment dynamics in the basin.
Analysis Methods and Conditions
The analysis section was 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma River and included 53 discharges exceeding 1,000 m3/s over 20 years. The initial conditions of riverbed level and grain size distribution of riverbed materials were the measured values in 2000. Nine-grain size classes were used: 0.062 mm (silt), 0.089 mm, 0.16 mm (fine sand), 0.33 mm, 0.60 mm (medium sand), 1.3 mm (coarse sand), 3.1 mm (fine gravel), 9.5 mm (medium gravel) and 38 mm (coarse gravel).
A sensitivity analysis was conducted on the sediment inflow from the upstream based on the results of the equilibrium sediment supply from the upstream edge. The sediment inflow was set to 0.1, 0.2, 0.5, 2, 5, and 10 times the equilibrium suspended sediment inflow, respectively.
Results
The average annual sediment transport from the river to the coast was 319,000 m3/year when the equilibrium sediment inflow from the upstream edge was applied, and silt accounted for about 70% of the sediment outflow.
The relationship between the total sediment inflow from the upstream edge and the total sediment transport to the coast was found to be linear. The slope of the regression line was positive, and the amount of sediment outflow increased as the amount of sediment inflow from the upstream edge increased. Since the slope is less than 1, it can be inferred that the amount of sediment outflow is smaller than the amount of sediment inflow, which in turn increases the amount of sediment deposited in the river channel.
The relationship between the slope of the regression line and the distance from the upstream edge decreased as the distance from the upstream edge increased. Sediment inflow from the upstream edge was considered to have been deposited within the section up to a certain point. The change in slope value was especially large near the upstream edge, suggesting that a relatively large amount of sediment was deposited at this section. The intercept of the regression line tended to increase with increasing distance from the upstream edge. Since the value of the intercept is considered to be the amount of sediment deposited in the river channel, it can be inferred that the value of the intercept changed depending on the riverbed material in the section up to a certain point.
The slope of the regression line was close to 1 for sediments with grain sizes of 0.062 and 0.089 mm, suggesting that most of the sediment inflow was transported directly to the coast. The slope of the regression line for the sediment with a grain size of 0.16 mm was 0.65. 65% of the sediment inflow was transported to the coast and 35% was deposited in the river channel. For medium and coarse sand, the change in sediment outflow was small to the change in sediment inflow. The intercept value of medium sand was larger than that of other grain sizes, suggesting that the sediment transport potential to the coast changed depending on the amount of sediment that was originally deposited in the river channel.
Conclusion
In this study, we evaluated the long-term effects of various upstream sediment conditions on sediment dynamics in the basin. Sediment transport to the coast was linearly related to sediment inflow from the upstream edge, and as sediment inflow increased, sediment outflow increased and sediment deposition in the river channel also increased. The slope of the regression line tended to decrease and the intercept to increase with increasing distance from the upstream edge. The relationship between the two was different for each grain size, with silt and fine sand showing an increase in sediment transport to the coast as inflow increased. On the other hand, the sediment outflow of medium and coarse sand was almost constant regardless of the sediment inflow, suggesting that the sediment outflow potential varies depending on the amount of sediment deposited in the channel.
Appropriate input of the amount of sediment inflow from the upstream edge and its grain size distribution is very important. There have been many reports on the analysis of sediment inflow from upstream, assuming the effects of slope failure in mountainous areas and human activities such as dams and land use changes, but insufficient knowledge has been obtained to evaluate sediment dynamics in a watershed on a long-term scale and by grain size. In this study, a one-dimensional riverbed fluctuation analysis is conducted for the period 2000-2019 in the lower reaches of the Abukuma River to evaluate the long-term effects of various upstream sediment conditions on the sediment dynamics in the basin.
Analysis Methods and Conditions
The analysis section was 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma River and included 53 discharges exceeding 1,000 m3/s over 20 years. The initial conditions of riverbed level and grain size distribution of riverbed materials were the measured values in 2000. Nine-grain size classes were used: 0.062 mm (silt), 0.089 mm, 0.16 mm (fine sand), 0.33 mm, 0.60 mm (medium sand), 1.3 mm (coarse sand), 3.1 mm (fine gravel), 9.5 mm (medium gravel) and 38 mm (coarse gravel).
A sensitivity analysis was conducted on the sediment inflow from the upstream based on the results of the equilibrium sediment supply from the upstream edge. The sediment inflow was set to 0.1, 0.2, 0.5, 2, 5, and 10 times the equilibrium suspended sediment inflow, respectively.
Results
The average annual sediment transport from the river to the coast was 319,000 m3/year when the equilibrium sediment inflow from the upstream edge was applied, and silt accounted for about 70% of the sediment outflow.
The relationship between the total sediment inflow from the upstream edge and the total sediment transport to the coast was found to be linear. The slope of the regression line was positive, and the amount of sediment outflow increased as the amount of sediment inflow from the upstream edge increased. Since the slope is less than 1, it can be inferred that the amount of sediment outflow is smaller than the amount of sediment inflow, which in turn increases the amount of sediment deposited in the river channel.
The relationship between the slope of the regression line and the distance from the upstream edge decreased as the distance from the upstream edge increased. Sediment inflow from the upstream edge was considered to have been deposited within the section up to a certain point. The change in slope value was especially large near the upstream edge, suggesting that a relatively large amount of sediment was deposited at this section. The intercept of the regression line tended to increase with increasing distance from the upstream edge. Since the value of the intercept is considered to be the amount of sediment deposited in the river channel, it can be inferred that the value of the intercept changed depending on the riverbed material in the section up to a certain point.
The slope of the regression line was close to 1 for sediments with grain sizes of 0.062 and 0.089 mm, suggesting that most of the sediment inflow was transported directly to the coast. The slope of the regression line for the sediment with a grain size of 0.16 mm was 0.65. 65% of the sediment inflow was transported to the coast and 35% was deposited in the river channel. For medium and coarse sand, the change in sediment outflow was small to the change in sediment inflow. The intercept value of medium sand was larger than that of other grain sizes, suggesting that the sediment transport potential to the coast changed depending on the amount of sediment that was originally deposited in the river channel.
Conclusion
In this study, we evaluated the long-term effects of various upstream sediment conditions on sediment dynamics in the basin. Sediment transport to the coast was linearly related to sediment inflow from the upstream edge, and as sediment inflow increased, sediment outflow increased and sediment deposition in the river channel also increased. The slope of the regression line tended to decrease and the intercept to increase with increasing distance from the upstream edge. The relationship between the two was different for each grain size, with silt and fine sand showing an increase in sediment transport to the coast as inflow increased. On the other hand, the sediment outflow of medium and coarse sand was almost constant regardless of the sediment inflow, suggesting that the sediment outflow potential varies depending on the amount of sediment deposited in the channel.