Morphodynamic processes in a large gravel-bed braided channel in response to runoff change: a case study in the Source Region of Yangtze River

In recent decades, climate warming has accelerated the melting of glaciers on the Qinghai-Tibet Plateau (QTP) so as to cause significant changes in water and sediment flux in the Source Region of Yangtze River (SRYR). The braided channel in the glacier piedmont in the SRYR may be adjusted due to the change of glacier runoff, but there is still a lack of clear understanding on this problem. This study applied Delft3D to set up a morphodynamic numerical model in the local braided channel of the Tuotuo River, a main tributary in the SRYR in conjunction with the field observation and remote sensing images. We divided different discharge levels according to the daily discharge data at the Tuotuohe gauging station, so 37 simulation scenarios are designed in consideration of sediment median size and Manning’s coefficient. The results are presented that (i) the increasing discharge significantly alters the characteristics of water and sediment movement. When the discharge is higher than the multi-year average peak discharge, we found an obvious confluence adjustment of bedload transport and flow velocity in the confluence area, moreover the former lagging behind the latter. However, when the discharge is lower than the average discharge in August, the confluence adjustment is not obvious. (ii) After discharge exceeds the average discharge in August, most of sandbars are gradually submerged because overflow occurs at the top of the sandbars. After the increase of runoff in the downstream of glaciers caused by climate warming, the braiding index reaches the maximum in advance, the submerged time of sandbars prolonging, and the morphodynamic process of river channels will be intensified. (iii) The fluvial erosion capacity after the flood peak is higher than before the flood peak (about 14.5% higher). The specific manifestation is that the sandbar changes little before the water level rises. After the water level declining leads to the sandbar exposed again, the shape of the sandbar changes drastically. This research has shed light on understanding the morphological adjustment of braided rivers on the QTP under the runoff change which mainly induced by climate change.