5:15 PM - 7:15 PM
[ACG48-P10] TIME SERIES ANALYSIS OF SEDIMENT PRODUCTION USING RUSLE
Keywords:RUSLE, Sediment production, SEDD model, Sediment transport time
Background and Objectives
Many flood control problems are currently occurring in Japanese rivers due to sediment deposition and erosion. For example, sediment from mountainous areas accumulates at bends and near weirs, where the flow velocity is slower, and this impedes the flow of water, resulting in overflow during floods and the deterioration of river habitats due to the decline of the riverbed. One of the countermeasures is to control the amount of sediment that passes through dams and weirs by operating the sluice gates in accordance with the increase or decrease in sediment production in mountainous areas. For this purpose, it is necessary to predict the time-series changes in the amount of sediment that will pass through the dam or weir several hours prior to the operation of the sluice gate. However, no model has yet been established that can perfectly represent the sediment production in mountainous areas, especially the time-series changes in sediment volume on a detailed timescale, such as daily. This is due to the difficulty of using a physical model to track the sequence of events in which sediment produced in mountainous areas flows into rivers and is repeatedly deposited and re-moved in the river channel and transported downstream.
Therefore, in this study, RUSLE, an empirical model was adopted as a sediment production model, and a method to calculate sediment production in mountainous areas on an hourly basis was investigated.
Target Watershed
The target watershed is the Higashikarasugawa River located in Fukushima City, Fukushima Prefecture. The target runoff was the 2049 Boso Peninsula typhoon that occurred in September 2019, and analysis was conducted for rainfall from 0:00 to 24:00 on September 9. Suspended load was observed at the Higashikarasagawa No.1 weir located downstream of the Higashikarasagawa River from 9:00 to 23:00 on September 9, and was compared with the calculated values.
Analysis Procedure
In order to calculate the time series of sediment transport at the observation points, it is necessary to consider the amount of sediment produced within the watershed and the transport time. First, the watershed was divided into eight subwatersheds, and the amount of sediment erosion was calculated for each subwatershed using RUSLE. The SEDD model was then used to calculate the proportion of eroded sediment that reaches the monitoring station (SDR), the time required for transport from each sub-watershed to the monitoring station, and the time-series change in sediment transport at the monitoring station. The time of sediment transport was calculated for the time when the sediment flowed down the slope and the time when it flowed down the river channel, respectively, and the parameter α was adjusted to match the time-series change in the observed values.
Results and Conclusions
This study examined a method to represent time series changes in sediment production in mountainous areas by combining the RUSLE and SEDD models, which are empirical models. The results showed that the NS coefficient was 0.6 when the value of the transport time parameter α was 12.5, and that the observed and calculated time-series changes were generally consistent. In the future, it is necessary to examine the applicability of the parameters estimated in this study to other regions.
Many flood control problems are currently occurring in Japanese rivers due to sediment deposition and erosion. For example, sediment from mountainous areas accumulates at bends and near weirs, where the flow velocity is slower, and this impedes the flow of water, resulting in overflow during floods and the deterioration of river habitats due to the decline of the riverbed. One of the countermeasures is to control the amount of sediment that passes through dams and weirs by operating the sluice gates in accordance with the increase or decrease in sediment production in mountainous areas. For this purpose, it is necessary to predict the time-series changes in the amount of sediment that will pass through the dam or weir several hours prior to the operation of the sluice gate. However, no model has yet been established that can perfectly represent the sediment production in mountainous areas, especially the time-series changes in sediment volume on a detailed timescale, such as daily. This is due to the difficulty of using a physical model to track the sequence of events in which sediment produced in mountainous areas flows into rivers and is repeatedly deposited and re-moved in the river channel and transported downstream.
Therefore, in this study, RUSLE, an empirical model was adopted as a sediment production model, and a method to calculate sediment production in mountainous areas on an hourly basis was investigated.
Target Watershed
The target watershed is the Higashikarasugawa River located in Fukushima City, Fukushima Prefecture. The target runoff was the 2049 Boso Peninsula typhoon that occurred in September 2019, and analysis was conducted for rainfall from 0:00 to 24:00 on September 9. Suspended load was observed at the Higashikarasagawa No.1 weir located downstream of the Higashikarasagawa River from 9:00 to 23:00 on September 9, and was compared with the calculated values.
Analysis Procedure
In order to calculate the time series of sediment transport at the observation points, it is necessary to consider the amount of sediment produced within the watershed and the transport time. First, the watershed was divided into eight subwatersheds, and the amount of sediment erosion was calculated for each subwatershed using RUSLE. The SEDD model was then used to calculate the proportion of eroded sediment that reaches the monitoring station (SDR), the time required for transport from each sub-watershed to the monitoring station, and the time-series change in sediment transport at the monitoring station. The time of sediment transport was calculated for the time when the sediment flowed down the slope and the time when it flowed down the river channel, respectively, and the parameter α was adjusted to match the time-series change in the observed values.
Results and Conclusions
This study examined a method to represent time series changes in sediment production in mountainous areas by combining the RUSLE and SEDD models, which are empirical models. The results showed that the NS coefficient was 0.6 when the value of the transport time parameter α was 12.5, and that the observed and calculated time-series changes were generally consistent. In the future, it is necessary to examine the applicability of the parameters estimated in this study to other regions.