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[ACG42-P03] Sediment transport analysis from the Abukuma river to the sea by typhoon No.19 in 2019
Keywords:Long-term sediment dynamics, Sediment supply, One-dimensional mixed-grain riverbed fluctuation analysis model, The Abukuma river
A large amount of sediment was run off in the Abukuma river because of the record-breaking heavy rainfall of Typhoon No.19 in October 2019. This kind of heavy flood occurs in the Abukuma river once every few years, and it is important from the viewpoints of the river and coastal management to clarify the influence of sediment transport on the shape of the riverbed and the amount of sediment in the downstream area. To clarify the long-term sediment transport characteristics in the Abukuma river, the purposes of this study are to calculate the long-term sediment dynamics in the lower reaches of the Abukuma river and to estimate the sediment supply to the coast for each particle size with a one-dimensional mixed-grain riverbed fluctuation analysis model.
The Abukuma River, which is the subject of this study, is a first-class river with a drainage area of 5390 km² and a trunk river channel length of 239 km. The source of the water originates in Asahidake, Fukushima Prefecture, it flows into the Kakuda Basin through alternating plains and narrow areas and joins the Shiraishi River, the largest tributary, to flow into the Pacific Ocean. The calculated section is 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma Valley, and the riverbed slope of the calculated section is about 1/3700 to 1/2000. The target runoffs are 47 runoffs with flow rates exceeding 1000 m³/s, when sediment transport generally starts, from 2000 to 2019 when water level and flow data are available. The initial riverbed level was obtained from the measured riverbed level in 1999, The initial riverbed level was obtained from the measured riverbed level in 1999 provided by the Tohoku Regional Development Bureau of the Ministry of Land, Infrastructure, Transport and Tourism, and 101 riverbed levels were given for each transect of 0.2 km in the longitudinal direction. The data from the Marumori Water Level Station was used for the upstream flow rate, and the data from the Arahama Water Level Station was used for the downstream water level. The flow rate of the Shiraishi River was measured using data from the Funaoka Bridge water level station located approximately 6 km upstream from the confluence. The particle size distribution of the riverbed material is based on the measured data of 2002 provided by the Tohoku Regional Development Bureau of the Ministry of Land, Infrastructure, and Transport. The calculation results were validated using the estimated sediment discharge associated with the seven runoff events that recorded 3000 m³/s at the Iwanuma water level observation station from 1999 to 2007.
To validate the one-dimensional riverbed fluctuation model, we compared the results of the analysis and the estimated values of the sediment discharge associated with the seven runoffs from 1999 to 2007. Although there is a tendency to overestimate the amount of fine sand (0.075-0.25mm) discharge when the discharge scale is large, the analysis results for other particle sizes are generally good, and it is judged that this model can be used to study sediment discharge in the Abukuma River. The annual average sediment discharge is about 140,000 m³ of fine sand, about 30,000 m³ of medium sand (0.25-0.85 mm), and about 0.3 m³ of coarse sand (0.85-2 mm), for a total of about 180,000 m³/year. In Typhoon No. 19 in 2019, it was estimated that about 400,000 m³ of sediment was supplied to the coast from rivers, including about 240,000 m³ of fine sand, 140,000 m³ of medium sand, and 20,000 m³ of coarse sand.
The Abukuma River, which is the subject of this study, is a first-class river with a drainage area of 5390 km² and a trunk river channel length of 239 km. The source of the water originates in Asahidake, Fukushima Prefecture, it flows into the Kakuda Basin through alternating plains and narrow areas and joins the Shiraishi River, the largest tributary, to flow into the Pacific Ocean. The calculated section is 36.4 km from Marumori to Arahama in the lower reaches of the Abukuma Valley, and the riverbed slope of the calculated section is about 1/3700 to 1/2000. The target runoffs are 47 runoffs with flow rates exceeding 1000 m³/s, when sediment transport generally starts, from 2000 to 2019 when water level and flow data are available. The initial riverbed level was obtained from the measured riverbed level in 1999, The initial riverbed level was obtained from the measured riverbed level in 1999 provided by the Tohoku Regional Development Bureau of the Ministry of Land, Infrastructure, Transport and Tourism, and 101 riverbed levels were given for each transect of 0.2 km in the longitudinal direction. The data from the Marumori Water Level Station was used for the upstream flow rate, and the data from the Arahama Water Level Station was used for the downstream water level. The flow rate of the Shiraishi River was measured using data from the Funaoka Bridge water level station located approximately 6 km upstream from the confluence. The particle size distribution of the riverbed material is based on the measured data of 2002 provided by the Tohoku Regional Development Bureau of the Ministry of Land, Infrastructure, and Transport. The calculation results were validated using the estimated sediment discharge associated with the seven runoff events that recorded 3000 m³/s at the Iwanuma water level observation station from 1999 to 2007.
To validate the one-dimensional riverbed fluctuation model, we compared the results of the analysis and the estimated values of the sediment discharge associated with the seven runoffs from 1999 to 2007. Although there is a tendency to overestimate the amount of fine sand (0.075-0.25mm) discharge when the discharge scale is large, the analysis results for other particle sizes are generally good, and it is judged that this model can be used to study sediment discharge in the Abukuma River. The annual average sediment discharge is about 140,000 m³ of fine sand, about 30,000 m³ of medium sand (0.25-0.85 mm), and about 0.3 m³ of coarse sand (0.85-2 mm), for a total of about 180,000 m³/year. In Typhoon No. 19 in 2019, it was estimated that about 400,000 m³ of sediment was supplied to the coast from rivers, including about 240,000 m³ of fine sand, 140,000 m³ of medium sand, and 20,000 m³ of coarse sand.