Modelling ice-dammed lake outburst floods in the Altai Mountains (Siberia) with HEC-RAS

The ice-dammed lake outburstfloods (jokulhlaups) in the Altai-Mountains are modelled by HEC-RAS 3.0, a superseded version of the well-known HEC 2. The classical procedure of the application of the model is run with the following input data and assumptions: The repeated jokulhlaups were modelled separately with the surfaces of surveyed giant bar surfaces for palaeostage indicators along the uppermost 90 km downstream from the former ice-dam. Depth of flow is limited by overtopped bedrock ridges at tributary mouth as additional minimum level and run-up sediments (thin layers of gravel transported in suspension and deposited in front of local valley slope obstructions) as an indicator of the maximum depth of flow. Previous deposits at the valley bottom are assumed to be eroded completely by the initial flood wave of the outburstfloods. Hence depth of flow is given by the highest elevations of run-up sediments down to the bedrock of the current valley bottom. Deposits of characteristic suspension gravel indicate that during the Pleistocene jokulhlaups the valleys were already incised down to the current level. For dominant subcritical conditions of flow peak discharges for all units can be estimated to be in the order of 10-12*10(6) m(3)/s. This estimation is considerably less than previous estimations for a location within the former ice-dam, but is based on a significant increased number of palaeostage indicators.