Hyper-concentrated flow response to aeolian and fluvial interactions from a desert watershed upstream of the Yellow River

Abstract

Aeolian and fluvial interactions (AFIs) are critical earth-surface processes in arid zones, especially in desert watersheds. The response of hyper-concentrated flows to seasonal alternate AFIs shows very high rates of sediment transport and has important environmental and ecological consequences from local to global scales. Here, we present the aeolian processes-induced hyper-concentrated (AHC) flows that occurred in the Sudalaer desert watershed upstream of the Yellow River, which primarily transport non-cohesive coarse aeolian sand (>0.08mm) and show a peak suspended sediment concentration (SSC) of 1.1 to 1.4×106mgl−3. Our field study and theoretical analysis indicate that non-cohesive coarse aeolian sand downstream in the channel can be entrained from the bed and can be suspended in the turbulent flow by the significant runoff generated upstream with a SSC γ0 value of 0.5×106mgl−3. Severe aeolian processes can provide an abundant coarse sediment supply in the channel, which, once entrained, can also trigger and promote the development of AHC flows. We define, for the first time, the ratio of the weight percentage of coarse sediment to fine sediment C>0.08mm/C<0.05mm as the optimal grain size indicator (OGI) in suspended sediment, indicating that, as the fraction of coarse sediment increases, the significant runoff gradually changes to hyper-concentrated flows and reaches the peak SSC when OGI=3.25. Due to the high frequency of sandstorms and the infrequency of rainstorms, most of the significant rainfall-induced runoff with a certain SSC γ0 can develop into AHC flows and can substantially contribute to the total sediment yield, even leading to deleterious effects on the downstream river system and ecology. Compared with other desert watersheds in semiarid regions, we propose that a SSC γ0 of 500kgm−3 is the threshold criteria for the occurrence of AHC flows in the arid desert watersheds. Comprehensive governing of soil erosion in the upstream gully-dissected slopes is an essential and effective measure for controlling AHC flows.