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[ACG42-07] Effects of sediment supply from mountainous area on sediment transport in the lower reaches of the Abukuma River
Keywords:sediment transport, riverbed fluctuation, sediment discharge, sediment boundary conditions, the Abukuma River
Background and Objectives
Appropriate input of sediment characteristics (sediment volume and grain size distribution) from upstream is very important in the analysis of riverbed fluctuations. Although there have been many reports on the analysis of the effects of slope failure in mountainous areas, few studies have evaluated the sediment dynamics in a watershed on a long-term scale and by grain size. In this study, various cases of slope failure in a mountainous area were set up by applying different sediment inflow rates and grain size distributions as upstream boundary conditions, and their long-term effects on sediment transport in the lower reaches of the Abukuma River were evaluated.
Analysis Methods and Conditions
The analysis section was 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma River. 53 flood events with a flow rate exceeding 1,000 m3/s over a 20-year period from 2000 to 2019 were included in the analysis. The initial conditions, i.e., riverbed level and grain size distribution of riverbed materials, were the measured values in 1999. The boundary conditions, i.e., upstream flow and downstream water level, were measured at the Marumori and Arahama water level stations, respectively.
Based on the results of the case with equilibrium sediment supply from upstream (Case 0), sensitivity analysis was conducted for three cases regarding the characteristics of sediment inflow from upstream: Case 1, in which the sediment inflow was set to 0.5, 2, and 5 times that of Case 0; Case 2, in which three patterns of sediment grain size distribution (medium sand 80%, coarse sand 80%, and wide grain size range) were set; and Case 3 was calculated by setting the peak sediment load to be 2, 4 hours before and 2, 4 hours after the peak flow rate. Considering that most of the quicksand is transported in a suspended state, the inflow conditions of suspended load were changed in this study.
Results
In Case 0, the average annual sediment inflow was approximately 135,000 m3, of which approximately 80% was fine sand. The average annual sediment discharge was 159,000 m3, which exceeded the sediment inflow, suggesting that the entire analyzed reach has been eroding for 20 years. The RMSE of the riverbed change was 0.35 m for the entire section.
In Case 1, when the sediment inflow was increased, the riverbed level increased and the riverbed material became finer-grained in the entire section. The sediment discharge rate (defined here as the ratio of sediment discharge to sediment inflow) decreased with increasing the sediment inflow rate, suggesting that a large amount of sediment inflow from upstream may accumulate in the river channel before discharging from the mouth.
In Case 2, most of the medium and coarse sediment inflow from upstream is deposited near the upstream edge of the river where the riverbed gradient is relatively gentle. The amount of sediment discharge was smaller than in Case 0, and fine sand accounted for more than 50% of the sediment discharge in both cases. The amount of sediment discharged from the mouth of the river is considered to vary greatly depending on the amount of fine sand that flows in from upstream and the amount of fine sand deposited in the river channel.
In Case 3, the amount of fine sand discharged slightly increased as the peak sediment volume was accelerated. Compared to Case 0, no significant topographic change was observed.
Conclusion
In this study, sediment transport in the lower reaches of the Abukuma River were evaluated for various cases of slope failure in mountainous areas. The amount of sediment deposited in the river channel increased as the amount of sediment inflow increased, and a trend toward finer grained sediment was observed in the entire section. In particular, most of the medium- and coarse-grained sediments from the upper reaches were deposited near the upstream edge. Regardless of the grain size distribution of the sediment inflow, fine sand accounted for more than 50% of the sediment discharge, indicating that the amount of sediment discharge from the river mouth may be dependent on fine sand.
Appropriate input of sediment characteristics (sediment volume and grain size distribution) from upstream is very important in the analysis of riverbed fluctuations. Although there have been many reports on the analysis of the effects of slope failure in mountainous areas, few studies have evaluated the sediment dynamics in a watershed on a long-term scale and by grain size. In this study, various cases of slope failure in a mountainous area were set up by applying different sediment inflow rates and grain size distributions as upstream boundary conditions, and their long-term effects on sediment transport in the lower reaches of the Abukuma River were evaluated.
Analysis Methods and Conditions
The analysis section was 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma River. 53 flood events with a flow rate exceeding 1,000 m3/s over a 20-year period from 2000 to 2019 were included in the analysis. The initial conditions, i.e., riverbed level and grain size distribution of riverbed materials, were the measured values in 1999. The boundary conditions, i.e., upstream flow and downstream water level, were measured at the Marumori and Arahama water level stations, respectively.
Based on the results of the case with equilibrium sediment supply from upstream (Case 0), sensitivity analysis was conducted for three cases regarding the characteristics of sediment inflow from upstream: Case 1, in which the sediment inflow was set to 0.5, 2, and 5 times that of Case 0; Case 2, in which three patterns of sediment grain size distribution (medium sand 80%, coarse sand 80%, and wide grain size range) were set; and Case 3 was calculated by setting the peak sediment load to be 2, 4 hours before and 2, 4 hours after the peak flow rate. Considering that most of the quicksand is transported in a suspended state, the inflow conditions of suspended load were changed in this study.
Results
In Case 0, the average annual sediment inflow was approximately 135,000 m3, of which approximately 80% was fine sand. The average annual sediment discharge was 159,000 m3, which exceeded the sediment inflow, suggesting that the entire analyzed reach has been eroding for 20 years. The RMSE of the riverbed change was 0.35 m for the entire section.
In Case 1, when the sediment inflow was increased, the riverbed level increased and the riverbed material became finer-grained in the entire section. The sediment discharge rate (defined here as the ratio of sediment discharge to sediment inflow) decreased with increasing the sediment inflow rate, suggesting that a large amount of sediment inflow from upstream may accumulate in the river channel before discharging from the mouth.
In Case 2, most of the medium and coarse sediment inflow from upstream is deposited near the upstream edge of the river where the riverbed gradient is relatively gentle. The amount of sediment discharge was smaller than in Case 0, and fine sand accounted for more than 50% of the sediment discharge in both cases. The amount of sediment discharged from the mouth of the river is considered to vary greatly depending on the amount of fine sand that flows in from upstream and the amount of fine sand deposited in the river channel.
In Case 3, the amount of fine sand discharged slightly increased as the peak sediment volume was accelerated. Compared to Case 0, no significant topographic change was observed.
Conclusion
In this study, sediment transport in the lower reaches of the Abukuma River were evaluated for various cases of slope failure in mountainous areas. The amount of sediment deposited in the river channel increased as the amount of sediment inflow increased, and a trend toward finer grained sediment was observed in the entire section. In particular, most of the medium- and coarse-grained sediments from the upper reaches were deposited near the upstream edge. Regardless of the grain size distribution of the sediment inflow, fine sand accounted for more than 50% of the sediment discharge, indicating that the amount of sediment discharge from the river mouth may be dependent on fine sand.