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[ACG45-03] Development of sediment yield estimation model in japan by combining RUSLE model and sediment delivery ratio
Keywords:RUSLE model, SDR, Sediment Yield, Sediment Erosion
In Japan, in order to solve the problems caused by changes in the sediment budget such as coastal erosion in river basins, comprehensive sediment management is required to manage and control the sediment system on a long-term basis. As a first step, this study aims to develop a quantitative evaluation method for the amount of sediment that flows into a river channel (sediment yield SY) out of the sediment that collapses in a mountainous area (sediment erosion SE). In order to cover the whole of Japan, we examine the applicability to Japan of a sediment production estimation method developed overseas using the soil erosion model RUSLE and the sediment delivery ratio (SDR) formula.
The sediment delivery ratio (SDR) is defined as the ratio of the sediment yield (SY) to the sediment erosion (SE). In general, the SDR equation is expressed as a power function of the basin area, but the empirical parameters in the equation have not yet been developed for river basins in Japan. In this study, we determine the parameter values for the catchment areas of 60 upstream dams of first-class rivers in Japan. The SDR value was calculated from the ratio of the annual average measured sediment deposition at each upstream dam regarded as the sediment yield (SY), to the sediment erosion (SE), calculated by the soil erosion model, RUSLE. The RUSLE model estimates soil erosion based on rainfall intensity, soil, topography, and land use factors. To calculate the SE, the annual soil erosion rate A obtained by the RUSLE model was multiplied by the soil particle density of 1.8 kg/m3 and converted to volume. Based on the calculated SDR values, we fit them to the model equation expressed as a power function of the watershed area, and propose parameter values in the equation.
As a result of the calculation, the equation SDR = 0.482A-0.502 (coefficient of determination R² = 0.255) was obtained for all catchments with a catchment area of A km2. The SDR equation was also calculated by classifying each catchment into three categories according to the average slope within the catchment: 0-10 degrees, 10-15 degrees, and more than 15 degrees. In the case of 0-10 degrees, the coefficient of determination was almost the same as that for the whole catchment. In the 10-15 degree range, the highest coefficient of determination was R² = 0.475, which was similar to previous studies. On the other hand, little correlation was observed with the watershed area above 15 degrees. Therefore, we can conclude that the SDR is reliable when the slope is small or moderate, but when the slope is very large, it is necessary to consider other factors in addition to topographical factors. In the future, we plan to confirm the validity of the equation by estimating sediment production based on the proposed SDR equation for past rainfall events and applying it to a riverbed fluctuation model.
The sediment delivery ratio (SDR) is defined as the ratio of the sediment yield (SY) to the sediment erosion (SE). In general, the SDR equation is expressed as a power function of the basin area, but the empirical parameters in the equation have not yet been developed for river basins in Japan. In this study, we determine the parameter values for the catchment areas of 60 upstream dams of first-class rivers in Japan. The SDR value was calculated from the ratio of the annual average measured sediment deposition at each upstream dam regarded as the sediment yield (SY), to the sediment erosion (SE), calculated by the soil erosion model, RUSLE. The RUSLE model estimates soil erosion based on rainfall intensity, soil, topography, and land use factors. To calculate the SE, the annual soil erosion rate A obtained by the RUSLE model was multiplied by the soil particle density of 1.8 kg/m3 and converted to volume. Based on the calculated SDR values, we fit them to the model equation expressed as a power function of the watershed area, and propose parameter values in the equation.
As a result of the calculation, the equation SDR = 0.482A-0.502 (coefficient of determination R² = 0.255) was obtained for all catchments with a catchment area of A km2. The SDR equation was also calculated by classifying each catchment into three categories according to the average slope within the catchment: 0-10 degrees, 10-15 degrees, and more than 15 degrees. In the case of 0-10 degrees, the coefficient of determination was almost the same as that for the whole catchment. In the 10-15 degree range, the highest coefficient of determination was R² = 0.475, which was similar to previous studies. On the other hand, little correlation was observed with the watershed area above 15 degrees. Therefore, we can conclude that the SDR is reliable when the slope is small or moderate, but when the slope is very large, it is necessary to consider other factors in addition to topographical factors. In the future, we plan to confirm the validity of the equation by estimating sediment production based on the proposed SDR equation for past rainfall events and applying it to a riverbed fluctuation model.