Unsteady 1D and 2D hydraulic models with ice dam break for Quaternary megaflood, Altai Mountains, southern Siberia

One of the largest known floods occurred during the Late Quaternary, emanating from an ice-dammed lake in Asia. Glacial lake Kuray–Chuja was formed by a 600-m-high ice dam converging in the Chuja River valley of the Altai Mountains in southern Siberia. The dam impounded up to 594 km3 of water in the Kuray and Chuja basins. At least three floods from lake Kuray–Chuja occurred, but only the largest, or the most recent, is modelled herein. The discharge, through an ice dam breach by tunnelling or over-topping, is analysed using dam breach equations including one specifically developed for ice dam failures. From these calculations it is concluded that the ice dam need not have failed when the water was at a maximum depth (i.e. 600 m deep) but, in consideration with flood routing models, it is probable that the lake emptied by over-topping under conditions of maximum water level. Although an over-topping model is favoured, a collapse of the ice dam due to initial tunnel development in the ice body cannot be precluded. The resultant flood wave ran down the Chuja River valley to the confluence with the Katun River and beyond. One-dimensional and two-dimensional unsteady and non-uniform flow modelling of the flood wave routed down the river valleys is presented that includes modelling a channel bifurcation at the confluence and backwater effects. The depth of the flood model is constrained by the altitudes of the tops of giant bars deposited by the palaeoflood, which indicate maximum flood stage. The results of the ice dam failure calculations and the flow modelling are independent of each other and are consistent, indicating in each case a flood of the order of 10 M m3 s? 1, with best-fit solutions providing estimated peak flood discharges of 9 to 11 M m3 s? 1. A breach, 1 km wide and 250 m deep, developed in the ice dam in as little as 11.6 h whereas the flood duration required to evacuate the total lake volume was around 1 day.

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