The Qinghai-Tibetan Plateau, known as the world’s “third pole”, is home to several large rivers in Asia. Its geomorphology is exceptionally vulnerable to climate change, which has had a significant impact on historical riverbed development through runoff and sedimentation processes. However, there is limited research combining climate change, sedimentology, and chronology with river dynamics to investigate riverbed evolution patterns in geological-historical time scales and their changes in overland flow capacity. In the current study, the evolution of a representative portion of the river channel in the Nangqian basin in the Lancang River headwaters was investigated to explore the reaction of the riverbed to climatic change during the geological period via field surveys, riverbed drilling, optically stimulated luminescence (OSL) dating and bankfull channel geometry parameters. The generalized channel section of the historical period was obtained by linking sedimentary layers of the same age on the distribution map of borehole sections, and the bankfull area of the river was computed accordingly. The restored bankfull areas can effectively reflect the ability of historical river channels to transport water and sediment, thus reflecting the climate change at that time. The findings showed that river morphology in the mounded river section could be successfully reconstructed using OSL dating and sedimentary records and that the conceptual sections of the historical warm periods at 2000 years (2 ka) and 0.7 ka can be recovered. Based on the reconstruction, the calculated bankfull areas during the two warm events were larger than present by factors of 1.28 and 1.9, respectively, indicating a stronger capacity for transporting water and sediments. This is the first trial in the Lancang headwaters to investigate the response of river morphology to climate change on a geological time scale.

After the construction of the Three Gorges Dam (TGD) in China, the downstream has been affected by the reduction in sediment discharge and regulation of flow processes, which have resulted in severe scouring and changes hydrological regime. Consequently, the spawning ground of Chinese sturgeon distributed along the downstream Yichang reach could be affected. This study examined the effects of TGD on the streamflow, sediment load and channel morphology downstream based on in situ measured data. Results showed that, after the impoundment of the TGD, sediment load at the downstream Yichang hydrological station decreased significantly, and the Yichang reach continued to be scoured. The distribution of erosion was uneven, and the scouring mainly occurred in the branching channels. The channel gradient and riverbed roughness increased with the erosion of the river cross section. After more than 10 years of erosion, the riverbed scouring and armouring in the Yichang reach was basically completed, thus we expected that the spawning grounds of Chinese sturgeon could be retain as the riverbed tends to be stable. The findings in this work have implications in the protection of the critically endangered Chinese sturgeon.

The impacts of backwater due to large dam construction on flow may lead to navigation or flood control problems in curved rivers. This study conducted flume experiments to investigate the effects of backwater on the velocity distribution characteristics of a 90-degree bend. The experimental results show that the backwater degree (η, defined as the ratio of flow depth under backwater to that under non-backwater conditions) has significant impacts on the three-dimensional velocity distribution in the bend. The depth-averaged velocities decrease with increasing backwater degree, and the deflection degrees of depth-averaged velocities are found to be highly related to the backwater degree and cross-sectional position. In this experimental setup, the mean cross-sectional velocity decreases by 67.2% as η increases from 1.00 to 3.64 for Q = 35 L/s; 63.7% as η increases from 1.00 to 3.26 for Q = 52 L/s; and 60.1% as η increases from 1.00 to 2.80 for Q = 52 L/s. The maximum values of transversal and vertical velocities near the riverbed gradually shift to the inner bank as the backwater degree increases at the 45° cross section. The center of the high transversal velocity area shifts about 0.1 m toward the inner bank as the backwater degree increases from 1.00 to 3.26 for Q = 52 L/s, which can reduce the erosion of the riverbed near the outer bank. In the current study, we also demonstrate that the growth and decay processes of secondary flow cells under backwater conditions are similar to those under non-backwater conditions. However, the scales and positions of the secondary flow cells change continuously with different backwater degrees. From the entrance to the exit of the bend, the secondary flow intensity first increases, and then decreases, with its maximum values occurring at the 45° cross section. The findings detailed in this manuscript provide insights for navigation channel design in reservoir backwater zones.

Laboratory experiment is an effective method to reveal the mechanism of bedrock erosion. In this study, a model device for bedrock erosion under high velocity flow conditions is presented. The model device is mainly composed of a high water head tank, a tilting flume, and a water circulating system. It is able to continuously operate at constant flow conditions for extended periods of time, and it has the advantages of simple structure and convenient for model testing.

Laboratory flume experiments were conducted in order to identify the temporal evolution of bed surface grain size properties by unsteady flows under zero sediment feeding conditions. Purposely designed sediment beds were composed by well-sorted gravel and sand, for each case, detailed data of the bed surface composition were collected. It is found that for all cases, the final sediment bed in the whole reach always shows a coarsening feature, and the coarsening process is much more developed near the inlet boundary. Further, the time variation of the percents of gravel and sand are observed increase or decrease alternately during the experiment and finally close to their initial percents. Besides, the experimental results show that there is no direct correlation between the flow unsteadiness and the coarsening degree of the final bed.

High-speed water flow conditions can cause erosion of the bedrock in engineering areas. Due to the lack of accurate evaluation of bedrock scour and erosion rates, there has been a consumption of manpower and resources without achieving satisfactory engineering outcomes. Therefore, studying the scouring and erosion effects of water flow on bedrock is of significant importance for maintaining the sustainable development and safety of engineering projects. Using the bedrock prototype from the Xiaonanhai site in the upper reaches of the Yangtze River, a model test device was developed to conduct anti-scour tests on the bedrock. The study quantitatively examined the basic physical properties, incipient erosion velocity, and erosion rates of different types of bedrock. The study found that the prototype bedrock under natural exposure, submerged immersion, and alternating wet and dry conditions showed a trend of decreased tensile strength, with the alternating wet and dry conditions being the most detrimental to maintaining the physical properties of the rock mass. The anti-scour velocity of silty claystone and clayey siltstone samples increased with the increase in tensile strength, and the erosion rate increased with the increase in shear stress. If the shear stress is kept constant, the erosion rate decreases with the increase in tensile strength. The erosion rate is inversely proportional to the ratio of the bedrock’s tensile strength to the riverbed shear stress, with the fitting relationship showing a piecewise linear distribution. The research results can provide guidance for the safe production of engineering involving bedrock erosion in engineering reservoir areas that are conducive to sustainable development.