Fractal characterization of sediment particle size distribution in the water-level fluctuation zone of the Three Gorges Reservoir, China

Abstract

The combined effect of periodic water impoundment and seasonal natural flood events has created a 30 m high water-level fluctuation zone (WLFZ) around the Three Gorges Reservoir (TGR), China, forming a unique eco-landscape. Siltation, eutrophication, enrichment of heavy metals, and methane emissions in the WLFZ have been widely associated with sediment and soil particles generated from the upstream catchment or upland slopes. However, little attention has been paid to the complexity of sediment particle-size distributions in the WLFZ. In the present study, core samples (from a 345 cm thick sediment core from the base of the WLFZ), slope transect surface samples (across/up a WLFZ slope), and along-river/longitudinal surface samples (from the reservoir reaches) were collected. Laser granulometry and a volume-based fractal model were used to reveal the characteristics of sediment particle-size distributions. Results indicate that the alternation of coarse and fine particles in the sedimentary core profile is represented as a fluctuation of low and high values of fractal dimension (D), ranging from 2.59 to 2.77. On the WLFZ slope transect, surface sediment particles coarsen with increasing elevation, sand content increases from 3.3% to 78.5%, and D decreases from 2.76 to 2.53. Longitudinally, surface sediments demonstrate a downstream-fining trend, and D increases gradually downstream. D is significantly positively correlated with the fine particle content. We conclude that D is a useful measure for evaluating sediment particle-size distribution.