Suspended sediment dynamics at different time scales in the Loushui River, south-central China

Abstract

Suspended sediment dynamics were investigated in the Loushui River basin, a secondary tributary of the Yangtze River in southern China. The variations of suspended sediment transport were analyzed at different time scales (among different years, monthly and seasonal, and within-flood event) based on 25-year hydrological data from the outlet hydrometric station, and explained by the contribution of flood events, precipitation and human activities. Four stages were classified by the non-parametric statistical method of Pettitt change-point test and Cumulative anomaly test. At the annual scale, the relationship between suspended sediment concentration (SSC) and water discharge (Q) varied at four stages. The suspended sediment load (SSL) showed parabolic relationship with annual precipitation and significantly influenced by floods (contribution averaged at 78% before 1985; at 55% after 2007). SSL drastically decreased after 2007, mainly induced by the construction of the Jiangya reservoir. Sediment 'store-release' phenomenon was used to explain the seasonal variations of suspended sediment load. Monthly SSC-Q hysteresis loops indicated that the sediment storage seasons swung between winter/spring and late autumn at different stages. At the within-flood scale, three types of SSC-Q hysteresis loops were observed for the maximum flood events, including clockwise pattern (44%), counter-clockwise pattern (24%) and figure-eight pattern (32%). The variations of SSC-Q hysteresis patterns at four stages implicated the changing suspended sediment source materials with time. During the years 1966-1978, the suspended sediment mainly came from channels for the maximum flood events, while, materials from the upper reaches or slopes dominated the later period 1979-1985 and 2007-2011.

Suspended sediment transport dynamics in rivers: Multi-scale drivers of temporal variation

“…Furthermore, other studies subdivide calibration data into groups related to seasonality, hydrology or flood limbs to improve the outputs (Eder et al, 2010;Fang et al, 2015). As a result, studies are highly inconsistent (and difficult to interpret) in terms of the relationship between discharge and sediment dynamics (Sun et al, 2015).…”

“…Changes in vegetation cover (e.g. due to crop rotation or natural seasonal variations) can cause a shift in the dominant sediment source to the river (Belmont et al, 2011;Rovira et al, 2015;Sun et al, 2015). Furthermore, erosion hotspots such as gullies can form on fields during storm events, causing an increased contribution of sediment from from a specific source.…”

“…Increase in sediment loads can result from a greater sediment supply, e.g. as a result of soil erosion due to intensification of land use, mining and mineral exploitation or construction works (Fan et al, 2012;Fuchs et al, 2011;Gao et al, 2015;Mohr et al, 2014;Sun et al, 2015;Zhang et al, 2009). …”

“…However, research has increasingly highlighted the stochastic and variable nature of each stage of this basic process (Phillips, 2003). In fact, it is this complexity in the field-catchment-river sediment transfer system that makes estimation of the provenance, transport and deposition of sediment in rivers so challenging (Gao, 2008;Poulenard et al, 2009;Rickson, 2014;Sun et al, 2015;Zhang et al, 2013;Zheng et al, 2012). Currently, we are unable to accurately predict SS concentrations in rivers over multiple timescales because we lack comprehensive understanding of how different drivers of SS transport interact over space and time.…”

“…First, the choice of timescale in many sediment studies limits a priori our understanding of the potential explanatory factors driving SS transport (Cao et al, 2007;Dean et al, 2016;Harvey, 2002;Sun et al, 2015;Zhang et al, 2013). In general, studies of SS transport dynamics have focused predominantly at a specific temporal scale, e.g.…”