ABSTRACT:
The variation of suspended load resulting from human activities has been extensively investigated in rivers in China, but less is known of the corresponding variation of sediment grain size. Grain-size variation of suspended sediment with respect to regional soil conservation was analyzed using more than 20 years data (1958 to 1985) obtained from hydrologic stations distributed in the mainstem as well as in some tributaries of the Middle Yellow River. Single-factor ANOVA was used in the analysis of grain-size variations in tributaries as well as in the mainstem before and after soil conservation, and the Wilcoxon signed-rank test was used on the entire data set over the whole study area. Comparisons were made at the tributary, the regional, and the mainstem levels. It was found that sediment became finer after soil conservation at most stations, both in the tributaries and the mainstem, though such a trend was not statistically significant at some stations. The regional comparison indicated that sediment over the whole study area became finer after soil conservation at the significance level of 0.05. Additional analyses were performed to evaluate the role of factors that may affect grain size variation such as rainfall, scour and deposition in channels and intrabasinal storage due to soil conservation. It was found that rainfall and scour/deposition in channels have little relation with sediment grain-size reduction in the Middle Yellow River. Intrabasinal storage caused by soil conservation was the primary factor causing grain size decreases in the study area. It was found that F-value in grain size ANOVA before and after soil conservation increased with the decrease of sediment conveyance rate after soil conservation in loess hills. That is to say, the more sediment trapped in watershed by soil conservation measures, the more significantly the grain size became finer in rivers. However, similar relation was not found in the rivers passing both loess hills and sand-gravel hills.
Footnotes
Ni Jinren is a professor and director of the Department of Environmental Engineering at Peking University in Beijing, China. Han Peng is an assistant professor in the Department of Environmental Engineering at Peking University in Beijing, China and a researcher at the Key Laboratory of Water and Sediment Sciences at the Ministry of Education in China.
- Copyright 2005 by the Soil and Water Conservation Society
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