THE GEOMORPHIC IMPACT OF OUTBURST FLOODS: INTEGRATING FIELD OBSERVATIONS AND NUMERICAL SIMULATIONS OF AN EXTREME FLOOD EVENT, EASTERN HIMALAYA

Advancements in computing and numerical modeling have made it possible to efficiently simulate flow characteristics of high-magnitude outburst floods through rugged topography in order to quantify their impact on landscapes, human lives, and infrastructure. By coupling these models with field observations we can advance our understanding of the geomorphic impact of repeated floods through time. Here, we reconcile geomorphic observations of the second largest historical outburst flood on record with simulations using GeoClaw open source software for modeling geophysical flows. This landslide-dam outburst flood was sourced in Tibet on the Yigong River in June 2000, scouring vegetation, triggering landslides and depositing flood sands in hydraulically sheltered areas downstream. We mapped these features in the field and remotely using Google Earth and Landsat-7 imagery, and simulated the flood with a reconstructed 2 Gm³ impounded lake using instantaneous dam failure. Simulated flow speeds are sensitive to the chosen roughness (Manning’s n) value, but hydrographs and the spatial patterns of peak flow speeds are robust over the full range of values tested (n=0.02 to 0.1). Inundation patterns show excellent correspondence to observations within the resolution of the topographic data. Flood simulations are within 28% of the reported peak discharge 18 km downstream from the source and produce flow depths that meet or exceed 73% of high-water indicators mapped remotely along this well-characterized segment of the river valley. Our best-fit model flow depths meet or exceed 6 of 8 high-water markers surveyed in the field at locations >175 km downstream, and inundate 9 of 12 observed slackwater deposits. Finally, GeoClaw simulations (1) produce peak flow speeds and momentum fluxes in locations that correlate with landslides that were observed directly after the event and (2) produce inundation patterns and flow depths consistent with the style of deposition observed far downstream, displaying a clear link between flood hydraulics and geomorphic change. Results suggest that GeoClaw can accurately simulate high-magnitude outburst floods through mountainous topography, showing the potential of this modeling approach to improve hazard predictions and our understanding of the geomorphic impact of outburst floods.