Hydrodynamic modelling of outburst flood hazard in the Pokhara Valley, Nepal

In May 2012, a sudden outburst flood along the Seti Khola river caused 72 fatalities and damaged infrastructure in the northern Pokhara Valley, Nepal. This destructive event raised concerns about possible future landslide- or glacier-surge-related outburst floods from the Higher Himalayas. The Seti Khola runs along one of the steepest topographic gradients in this mountain belt. The river is fed by the debris-covered Sabche glacier, Nepal’s only observed surging glacier, below the flanks of Annapurna III (c. 7500 m asl) and reaches Pokhara, the country’s second largest city, at about 850 m asl. Over a course of some 40 km, the Seti Khola shaped the Pokhara Valley’s distinctive landscape of unpaired, several tens of meters to >100-m high alluvial terraces that alternate with deep slot gorges of <1 km length, all mostly cut into deposits of medieval and earlier outburst and outwash deposits. These abrupt changes in channel cross section provide many potential locations of hydraulic ponding during floods. We present a reanalysis of the 2012 Seti Khola outburst flood, and combine field-based surveys of valley geometry, flood markers, and surface roughness (i.e. Manning’s n value estimates) with landform mapping from high-resolution satellite images and digital elevation models. These components form the input for a one-dimensional steady flow simulation in HEC-RAS that allows us to reconstruct the dynamics, stage height, and runout from the 2012 Seti Khola flood. Validated by both this recent and the catastrophic historic events, we use our model to simulate future scenarios of inundation by these infrequent but potentially highly destructive outburst floods and compare them to the Pokhara Valley’s recurring monsoonal floods.