Suspended sediment transport response to upstream wash-load supply in the sand-bed reach of the Upper Yellow River, China

- Wash sediment supply regulates SSC levels in the Upper Yellow River.
- SSC/discharge ratios show a definite relation between input and output flows.
- We establish a modified rating curve in a water-sediment mismatched river.

SummaryWash load is a major component of suspended sediment transport in the sand-bed reach of the Upper Yellow River, China. This wash load sediment originates from the Loess region, with the high runoff mainly originating from the rock mountains of its upstream basin. These characteristics result in a mismatch between water and sediment sources and a low probability of high runoffs meeting high suspended sediment concentration (SSC) flows. As a result, higher runoff with lower SSC levels (HR-LS) and lower runoff with higher SSC values (HS) occur, whose SSCs do not follow the typical power form for flow discharges, Ci = αQβ, where Ci and Q are SSC and flow discharge, respectively. Here, we modify the traditional power form with an upstream wash-load supply function C1−β to satisfy the relation between the water and wash load sediment concentrations in water-sediment mismatched cases, Ci = αQβC1−β, where C is an input flow's SSC. Using the daily flow discharges and SSCs of nine typical HR-LS flows and 18 HS flows in our study reach from 1960 to 2012, we find that β changes in response to input flow conditions and downstream transport distances. When the downstream transport distance is between 360 and 663.5 km, β varies between 0.3 and 0.6 in a HS input flow condition, while in the HR-LS input flow case, β tends to be greater than 0.6 (between 0.74 and 0.65). The entrainment rate of an HR-LS flow and the deposition rate of an HS flow appear to be asymmetrically balanced, establishing a primary mechanism for channel aggradation and upward fining of floodplains in our study reach.