5:15 PM - 7:15 PM
[ACG48-P09] Grain Size-Specific Assessment of the Impact of River Channel Characteristics on Long-Term Sediment Supply from Rivers to the Coast
Keywords:One-dimensional riverbed fluctuation analysis, Mixed grain size, Sediment dynamics, River channel characteristics, Japan, Sediment supply
Introduction
Coastal erosion is accelerating globally due to increasing coastal populations and human activities. Understanding sediment budgets is crucial for addressing this issue, as it informs sediment transport dynamics, impacts on water quality, ecosystem health, and changes in river morphology. Rivers contribute most of the sediment supplied to the ocean. To estimate sediment supply from rivers, using a riverbed fluctuation analysis model aids in simulating sediment transport, particularly in movable bed rivers with sand and gravel. There are many reports of long-term analyses of sediment supply from rivers to the coast, but there is not enough knowledge to evaluate each grain size. This study conducts a one-dimensional riverbed fluctuation analysis for three Japanese rivers: Abukuma, Abe, and Yoshino Rivers. The rivers targeted had different channel characteristics regarding discharge, riverbed slope, and riverbed material. The impact of these differences in channel characteristics on sediment dynamics in the downstream area was evaluated for each grain size. It examined whether the Rouse number, a dimensionless number, could be used to evaluate these characteristics.
Methods
This study used a one-dimensional, mixed grain-size riverbed fluctuation model developed by Takebayashi and Fujita to evaluate the long-term sediment dynamics from silt to cobbles. The bedload was calculated using the Ashida, Egashira, and Liu formula. The settling velocity was calculated using the Rubey formula. The equilibrium near-bed sediment concentration was calculated using the Lane and Kalinske formula. The analysis sections extend 37.2 km from the river mouth to Marumori for the Abukuma River, 22.0 km from the river mouth to Tamahata Bridge for the Abe River, and 77.8 km from the river mouth to the Ikeda Dam for the Yoshino River. The analysis period covers the following: from 2000 to 2019 for the Abukuma River with discharge of 200 m3/s or more; from 2012 to 2022 for the Abe River with discharge of 100 m3/s or more; and from 1965 to 1999 for the Yoshino River with discharge of 1,000 m3/s or more. This study referred to the results of previous research on the Yoshino River to compare rivers with different characteristics.
Results
The RMSE for riverbed fluctuations was 0.45 m for the Abukuma River and 0.30 m for the Abe River. The annual average sediment supply to the ocean from the Abukuma River was 46.5x104 m3/yr for silt and very fine sand, 12.8x104 m3/yr for fine sand, 4.4x104 m3/yr for medium sand, 1.6x104 m3/yr for coarse sand, and 0.4x104 m3/yr for very coarse sand. The annual average sediment supply to the ocean from the Abe River was 226.1x104 m3/yr for silt and very fine sand, 54.4x104 m3/yr for fine sand, 6.7x104 m3/yr for medium sand, 0.6x104 m3/yr for coarse sand, and 0.3x104 m3/yr for very coarse sand.
The relationship between the sediment inflow of fine sediment, ranging from silt to fine sand, from the upstream end and the sediment outflow to the ocean was one-to-one for all rivers; the sediment outflow increased as the inflow increased. In contrast, the slope of the regression line for coarse sediment, ranging from medium sand to very coarse sand, differed among rivers with mild slopes and those with steep slopes.
Analyzing the sediment transport pattern for coarse sediment in the river channel using the Rouse number revealed that the sediment transport pattern in the mild-slope rivers of the Abukuma and Yoshino Rivers shifted from bedload to suspended load as the discharge increased. Therefore, coarse sediment may be deposited in the river channel during low discharge, supplying the sea during flood events. On the steep-slope rivers of the Abe River, coarse sediment is transported as a suspended load even at low discharge, regardless of the increase in discharge. This means the quantity of coarse sediment (medium to very coarse sand) deposited in the river channel may be small.
Conclusion
This study conducted a riverbed fluctuation analysis on Japanese rivers with different channel characteristics and assessed the long-term characteristics of sediment supply to the coast. The results revealed that sediment dynamics differ between fine sediment, ranging from silt to fine sand, and coarse sediment, ranging from medium sand to very coarse sand. The study also suggested that the Rouse number could evaluate river sediment dynamics.
Coastal erosion is accelerating globally due to increasing coastal populations and human activities. Understanding sediment budgets is crucial for addressing this issue, as it informs sediment transport dynamics, impacts on water quality, ecosystem health, and changes in river morphology. Rivers contribute most of the sediment supplied to the ocean. To estimate sediment supply from rivers, using a riverbed fluctuation analysis model aids in simulating sediment transport, particularly in movable bed rivers with sand and gravel. There are many reports of long-term analyses of sediment supply from rivers to the coast, but there is not enough knowledge to evaluate each grain size. This study conducts a one-dimensional riverbed fluctuation analysis for three Japanese rivers: Abukuma, Abe, and Yoshino Rivers. The rivers targeted had different channel characteristics regarding discharge, riverbed slope, and riverbed material. The impact of these differences in channel characteristics on sediment dynamics in the downstream area was evaluated for each grain size. It examined whether the Rouse number, a dimensionless number, could be used to evaluate these characteristics.
Methods
This study used a one-dimensional, mixed grain-size riverbed fluctuation model developed by Takebayashi and Fujita to evaluate the long-term sediment dynamics from silt to cobbles. The bedload was calculated using the Ashida, Egashira, and Liu formula. The settling velocity was calculated using the Rubey formula. The equilibrium near-bed sediment concentration was calculated using the Lane and Kalinske formula. The analysis sections extend 37.2 km from the river mouth to Marumori for the Abukuma River, 22.0 km from the river mouth to Tamahata Bridge for the Abe River, and 77.8 km from the river mouth to the Ikeda Dam for the Yoshino River. The analysis period covers the following: from 2000 to 2019 for the Abukuma River with discharge of 200 m3/s or more; from 2012 to 2022 for the Abe River with discharge of 100 m3/s or more; and from 1965 to 1999 for the Yoshino River with discharge of 1,000 m3/s or more. This study referred to the results of previous research on the Yoshino River to compare rivers with different characteristics.
Results
The RMSE for riverbed fluctuations was 0.45 m for the Abukuma River and 0.30 m for the Abe River. The annual average sediment supply to the ocean from the Abukuma River was 46.5x104 m3/yr for silt and very fine sand, 12.8x104 m3/yr for fine sand, 4.4x104 m3/yr for medium sand, 1.6x104 m3/yr for coarse sand, and 0.4x104 m3/yr for very coarse sand. The annual average sediment supply to the ocean from the Abe River was 226.1x104 m3/yr for silt and very fine sand, 54.4x104 m3/yr for fine sand, 6.7x104 m3/yr for medium sand, 0.6x104 m3/yr for coarse sand, and 0.3x104 m3/yr for very coarse sand.
The relationship between the sediment inflow of fine sediment, ranging from silt to fine sand, from the upstream end and the sediment outflow to the ocean was one-to-one for all rivers; the sediment outflow increased as the inflow increased. In contrast, the slope of the regression line for coarse sediment, ranging from medium sand to very coarse sand, differed among rivers with mild slopes and those with steep slopes.
Analyzing the sediment transport pattern for coarse sediment in the river channel using the Rouse number revealed that the sediment transport pattern in the mild-slope rivers of the Abukuma and Yoshino Rivers shifted from bedload to suspended load as the discharge increased. Therefore, coarse sediment may be deposited in the river channel during low discharge, supplying the sea during flood events. On the steep-slope rivers of the Abe River, coarse sediment is transported as a suspended load even at low discharge, regardless of the increase in discharge. This means the quantity of coarse sediment (medium to very coarse sand) deposited in the river channel may be small.
Conclusion
This study conducted a riverbed fluctuation analysis on Japanese rivers with different channel characteristics and assessed the long-term characteristics of sediment supply to the coast. The results revealed that sediment dynamics differ between fine sediment, ranging from silt to fine sand, and coarse sediment, ranging from medium sand to very coarse sand. The study also suggested that the Rouse number could evaluate river sediment dynamics.