Complex morphodynamic processes of a large braided channel in response to different flood flow discharges on the Qinghai-Tibet Plateau

The frequency and intensity of river floods in the Qinghai-Tibet Plateau (QTP) are increasing under the background of climate warming. In particular, the braided channel in the wide valley section in the Source Region of the Yangtze River (SRYR) has undergone drastic adjustments due to recent summer floods and further adjustments are expected in the future as climate change continues. Nonetheless, a lack of a quantitative description of the phenomenon persists. In order to unravel morphodynamic processes of a braided river under different flood flow discharges, a two-dimensional morphodynamic model was established using Delft3D to simulate the local braided channel in the Tuotuo River based on field observations and remote sensing images in the SRYR. The data of 1, 2, 5, and 10-yr flood hydrographs from 1985 to 2020 were obtained, using the hydrological data at the Tuotuohe Gauging Station. Specifically, two groups of actual flood hydrographs were used to simulate the hydrodynamic and riverbed adjustment of the braided channel reach. The following findings were revealed: (i) increases in discharge cause increases of the water level, and thus the flood flow erode and submerge the sandbars. During a flood, the previous branches are replaced by new branches and the braided channel shows a significantly different configuration after the flood. (ii) With the increase of the flood discharge, the amount of sediment erosion and deposition in the braided channels escalates. Specifically, the sediment erosion and deposition of the 10-yr flood will increase by 3.1 times and 4.7 times, respectively, in comparison with the 1-yr flood. (iii) When the discharge is greater than 180 m3/s, the braiding intensity in the wide valley section (>1.2 km) has a significant decreasing trend with the increase in the flood flow. When the discharge is less than 180 m3/s, the correlation between the braiding intensity and flood flow is not significant in the wide reach. (iv) The braiding intensity of the wide reach as flood frequency increases from the 1- to 2-yr events increases by 14.3%, 11.6%, respectively, and decreases for the 5- to 10-yr events by 3.2% and 10.8%, respectively. In the narrow reach, the increases are 25.5% and 11.2%, for the 1- and 2-yr events, respectively, and the decreases are 25.0% and 53.3% for the 5- and 10-yr events, respectively. The parabolic shape of the correlation between the braided intensity and flood discharge increases first, reaches a peak, and then decreases. Overall, the current study contributes to the understanding of the impact of gradually increasing flood discharge on the morphodynamics of braided rivers in the QTP.