Hydrodynamic controls and sedimentary responses of eddy bars during outburst floods

Catastrophic outburst floods are among the most powerful geomorphic agents on Earth, capable of rapidly reshaping mountain river systems and leaving long-lasting sedimentary records. Along the Yarlung Tsangpo River at the southern margin of the Tibetan Plateau, repeated floods triggered by landslide dam or glacial lake failures have produced large eddy bars that preserve information on flood dynamics. However, the hydrodynamic processes linking unsteady flow behavior, vortex activity, and sedimentary responses within flow separation zones remain poorly understood. Here, we reconstructed two Holocene outburst flood events by integrating two-dimensional hydrodynamic simulations with field observations of eddy bars and high-water marks, allowing comparison with the observed geomorphic evidence. Model results indicate that bed shear stresses within flow separation zones commonly exceed entrainment thresholds for coarse sand and fine gravel, allowing sustained near-bed suspension and efficient sediment transport. Under such energetic conditions, the power spectral densities of velocity at the flow separation points, along the separated shear layer, and within the interior of the flow separation zones exhibit broadband and aperiodic behavior with approximate inertial-subrange scaling. These spectral features indicate turbulence-like, multi-scale flow fluctuations associated with unsteady vortex activity during outburst flooding. These fluctuations reflect dynamically evolving vortex configurations, including relatively stable single-vortex systems and more transient multi-vortex systems characterized by splitting, merging, and decay. The field evidence combining the rhythmic horizontal lamination in the eddy bar and the variation of grain size indicates that the eddy bar formation may have recorded continuous erosion-sedimentary cycles driven by intermittent vortex activities. Overall, this study provides an integrated hydrodynamic and sedimentological perspective on eddy-bar formation during catastrophic outburst floods and supports their interpretation as geomorphic archives that retain information about unsteady flood dynamics, highlighting their value for paleoflood reconstruction in high-relief valley river systems.