The scour-deposition characteristics of sediment fractions in desert aggrading rivers—taking the upper reaches of the Yellow River as an example

There are large differences in the grain sizes of incoming sediments in different source areas of aggrading rivers in desert areas. The scour-deposition characteristics of sediment fractions affect river governance decision-making. Based on a large number of prototype observation datasets, this study has systematically sorted and analysed the data of water and sediment (particularly, the sediment fractions) in the Ningxia-Inner Mongolia Reach (Ning-Meng Reach) of the Yellow River since the 1960s, specifically to determine the sources of coarse sediment. It has been clarified that the channel sediments are mainly composed of incoming sediments from the main stream and tributaries and that the corresponding coarse sediments mainly come from seasonal tributaries and aeolian sand that is transported into the Yellow River in the reach. It has been considered that the sediment in the reach has a low incipient velocity—when the discharge exceeds 1500 m³/s, the sediment in the riverbed generally meets the incipient velocity and transportation conditions. Meanwhile, the study has revealed the characteristics of sediment scour deposition during different floods in the Ning-Meng Reach. The results have shown that the sediments of aggrading rivers in desert areas come from the main stream and seasonal tributaries, along with aeolian sand that is transported into the Yellow River. The grain sizes of the majority of the sediments are smaller than 0.05 mm, but there is still a certain proportion of coarse sediment with grain sizes larger than 0.05 mm. Both the median and mean grain sizes of the riverbed sediment vary between approximately 0.12 and 0.22 mm, and almost all of the sediment grains have low incipient velocities. Furthermore, the scour deposition process of the channel mainly depends on the river sediment transportation capacity. The entire reach can evolve into scour deposition, or the deposition efficiency can decrease due to the increasing discharge during non-floodplain floods, while the conducive discharges are 2500–3000 m³/s for the transportation of each sediment fraction.