Reconstruction of the Dynamics of a Catastrophic Crater Lake Outburst Flood, Changbaishan-Tianchi Volcano

Reconstruction of the catastrophic drainage following the Millennium Eruption (ME) of Changbaishan-Tianchi volcano in 946 +/- 20 CE is of great significance, as it contributes to improving the regional maximum flood record and develop rare flood risk analysis. However, limited knowledge exists concerning the failure mode, magnitude, and transport processes of the outburst flooding. In this work, we present a whole system model that describes the paleohydrology of catastrophic drainage using geological records along the downstream valley. The model encompasses the crater lake dynamics, an approximation of the breach erosion process and flood propagation downstream. The boulder competence method was used to constrain by reasonable flow parameters, while mitigating the uncertainty caused by the ambiguous geological paleostage indicators. Paleohydrologic analysis indicates that at least 1 km3 of water was released from the caldera, with the vertical breach erosion rates as high as 34 m/hr. Volcanic activity during the ME II may have directly contributed to triggering of the flood event. The local hydrodynamic response of the downstream riverbed captures the dynamic migration patterns of sediments across spatio-temporal scales, offering a comprehensive interpretation of the specific scouring surfaces observed in the geological profile. The analysis of simulated inundation boundaries reveals that not all recorded inundations can be attributed to the crater lake outburst event. Reconstructions of megafloods based on downstream constraints on flood stage, velocity and discharge can help to infer and constrain the dynamics of dam failure mechanisms, and also contribute to our understanding of these complex paleohydrologic events. Based on the integration of geomorphologic information on megaflood and lahar deposits in the Changbaishan-Tianchi volcanic field, the study discusses the transport processes and triggering conditions of catastrophic hydrologic events. Specifically, we present a whole system model that occurred a catastrophic drainage after the Millennium Eruption (ME). In the system, the dam erosion rate and the paleo-lake level were treated as unknowns. A series of simulations serves to match dynamically simulated critical flow velocity with downstream geomorphic evidence, such as volcanic cone-source boulders and flood deposits. The paleohydrologic analysis indicates that at least 1 km3 of water was released from the caldera, with the erosion rate as close as 34 m/hr. Volcanic activity in ME II may have directly contributed to the largest flood event. Moreover, the velocity constraint of boulder competence method excludes the scenario where the similar inundation area caused by unreasonable parameter combination. The reconstruction of the megaflood not only help to infer the dynamics and mean parameters of the dam failure mechanism, but also to reveal the complex spatial and temporal patterns of paleohydrologic events. Subsequently, the relevant inundation area of the single catastrophic flood event was corrected. A whole system model combining volcanic source boulder and outcrops was proposed to reconstruct catastrophic drainage Considering different outburst scenarios to ascertain the breach incision rate was 30-38 m/hr, and released nearly 1.2 km3 of water Correcting inundation area for the outburst flood event following the Millennium Eruption