Evaluation of suspended-sediment sources in the Yeongsan River using Cs-137 after major human impacts

摘要

Large direct human impacts on the Yeongsan River resulting from the construction of two large weirs on the main stream, and dredging of almost all sections of the main river has occurred in the past 5 years. Subsequent to these human impacts, some problems related to the sediment budget, such as channel changes, have occurred. To evaluate the dominant source of suspended sediment in the Yeongsan River, three time-integrated suspended-sediment samplers were installed in the upper and middle Yeongsan River (YS-S1, -S2, and -S3), and we obtained suspended-sediment samples every month. Potential sources of the suspended sediment were sampled using a soil scraper on the forest floor and channel bank. Source fingerprinting was conducted using ~(137)Cs, which exhibits distinguishable characteristics of surface and subsurface (bank) materials. Probability distribution functions fit to the detected values of ~(137)Cs at the forest floor and channel bank were derived. The relative contributions of forest floor and bank materials to suspended sediment in the Yeongsan River were calculated using Monte Carlo simulation and a simple mixing model, and the results indicated that the dominant source of suspended sediment was bank materials. Dredging a riverbed and constructing river-crossing facilities can affect the channel in many ways, but it remains unclear from this study how much the human impacts influenced channel erosion due to the absence of pre-interference data. However, this study may contribute to planning prevention measures for sediment problems induced by human impacts in the Yeongsan River catchment. Additionally, to reduce the uncertainty and limitations of the ~(137)Cs methodology, a multi-fingerprinting approach using geochemical analysis, various radionuclides, and so forth is needed for better spatial and temporal evaluation of the sediment sources.