Increasing effect of aeolian sediment transport on riverbed evolution under decreased flow conditions in the alluvial desert reach of the Upper Yellow River

摘要

Aim: In this study, an essential role of bed-material load mainly composed of coarse aeolian sand rather than that of wash-load originating from tributaries of the Loess Plateau in the developmentof channel morphology in response to decreased flows in the alluvial desert reach of the Yellow River is outlined.Methodology: Two sediment cores were drilled in the channel beds of the upstream braided strecth (Shizuishan-Sanhuhekou) and the downstream meandering strecth (Sanhuhekou-Toudaoguai), respectively.Results: The following coarse sediment identification of bed material, grain size distributions (GSDs) in suspension and channel geometry measurement indicated that aeolian and fluvial interactions in our study reach trigger and lead to channel aggradation and lateral migration following the operation of the upstream large reservoirs. In response to the decreased flows in recent 30 decades since 1986, the wash-load proportion of both the braided and the meandering channel increased from 82.98% to 91.09 /0 on average, with the bed material-load proportion decreased from 12.26% to 7.22%, showing an evident tendency of getting finer and channel aggradation. The coarse sediment (0.08-0.25 mm), although only a small fraction of 10% on average, is active in suspension as bed material-load or in saltation as bed-load, indicating that the study channel should be transitional channel or transitional to labile, with the calculated river competence indicator t* as 0.92-2.5. In addition, the primary river stability indicator (K) adapted to the two stretches was proposed and calculated by introducing D50 and bank-full discharge instead of maximum discharge. The calculated values showed that the meandering channel (K=0.647) is more stable than the braded channel (K=0.238), which is of primary guiding significance for alluvial river stability prediction.Interpretation: Deterioration in runoff-sediment relations is increasingly threatening the health of the upper Yellow River. Accordingly, with the decline of the sediment trapping ability of existing reservoirs in the future, erosion rates will increasingly affect the fluvial sediment characteristics and even the channel morphology development.