Rapid changes in fluvial morphology in response to the high-energy Yigong outburst flood in 2000: Integrating channel dynamics and flood hydraulics

Abstract

• The flood tended to change the planimetric form of the UR channel while to change the dimension of the DR channel. • There are three channel migration patterns: channel straightening, meander bend migration, and erosion at concave bank while deposition at convex bank. • The peaks of the bank displacement and the shear stress are staggered and the mean staggered distance is 846 m. • The maximum relative migration rate for both UR and DR occurs when (W/R)·U ∼ 1. High-energy outburst floods triggered by failure of landslide- and glacial dams have strongly impacted rivers in eastern Tibet, affecting river geomorphology, human settlements, and infrastructure. However, understanding the impact of these extreme outburst floods on river landscape evolution is hampered due to lack of event-based observations. The second-largest modern outburst flood recorded in the Yigong River in 2000 provides an opportunity to investigate changes in river morphology caused by such extreme floods. By combining remote sensing images from 1984 to 2020 and 2-D numerical flood simulations with the Hydrologic Engineering Center’s River Analysis System (HEC-RAS) software, we show that the riverbank displacement during the Yigong outburst flood (YGOF) is an order of magnitude greater than that in other periods. Channel migration and variation in channel width in the upper reach (UR) straths are greater than those in the down reach (DR) gorges, and the curvature of the UR was greatly reduced by the YGOF while that of the DR increased. Furthermore, the channel morphology after the YGOF has become more dynamic. The relationship among the product of the dimensionless curvature (W/R), simulated flow velocity (U) and relative migration rate (M/W) in both the UR and DR follow similar distribution patterns, in which the maximum relative migration rates in the UR and DR occur where (W/R) · U ∼ 1. The results quantify the impact of the 2000 YGOF on the fluvial systems, which demonstrates that high-energy outburst floods play an extremely important role in mountainous river morphology and promotes our understanding of hydraulic process and geomorphological effects of paleofloods.