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A devastating wall of water gushed down the Bhotekoshi/Sunkoshi River in Nepal on July 5, 2016. It came from a lake that had been dammed by a glacial moraine, but the dam broke and discharged more than 100,000 tons of water all at once. An international team of GFZ and Nepali scientists was able to record the sudden outburst with seismometers deployed the year before in the wake of the catastrophic Ghorka earthquake in April 2015. In a study to be published in Science, the authors argue that such major flood events have a greater impact on erosion rates than the annual monsoon rainfalls.
"We could identify two distinct pulses," says Kristen Cook. The first came from the wall of water, the second one only seconds later from rocks and coarse sediment within the water. The latter caused the biggest damage. Bridges were destroyed as well as hydro-power stations and roads. In the aftermath, a number of landslides came down as the river banks were destabilized by the erosive force of the water.
These floods originate from lakes that have formed behind a landslide dam or in association with a glacier, dammed by a frontal moraine or glacial ice. Such lakes can drain catastrophically for several reasons, including mass movements or avalanches into the lake, seismic activity, piping within the dam, overtopping of the dam, or degradation of blocking ice (4, 6). The resulting floods can have short-lived discharges up to several orders of magnitude higher than background discharges in the receiving rivers (7).
The cause of the breach is unknown, but fresh deposits above the lake suggest that it may have been associated with a debris flow event, possibly increasing the volume of the flood (Fig. 1B). ... The zone of damage sits within the area affected by strong ground motion and landsliding induced by the 2015 moment magnitude 7.8 Gorkha earthquake, which had an estimated return time of a few hundred years (15, 16).
The flood impact extended into the adjacent hillslopes through undercutting and destabilization of the river banks, leading to bank collapses, slumps, and landslides. The extensive flood-induced damage to local infrastructure was almost exclusively the result of bank erosion and mass wasting, rather than inundation (fig. S4).
Analysis of 5-m-resolution RapidEye imagery (table S3) indicates that the 2016 GLOF caused the mean width of the active channel between the Zhangzangbo confluence and Barabise to increase from 29.5 ± 3 m in 2015 to 41.3 ± 3 m in 2016, with highly variable widening throughout the mapped area (Fig. 3)
The frequency of GLOFs in the central Himalaya is difficult to establish, because records are incomplete and recorded floods may not be correctly identified as GLOFs (3, 23). Nevertheless, GLOFs are relatively common in the Himalaya, with a major flood occurring at least once every 2 years on average (4, 24–26). The Bhotekoshi River has experienced GLOFs in 1935, 1964, 1981, and 2016, suggesting a return period of about 30 years (2).
A warming climate is thought to promote glacial lake formation in some areas as retreating glaciers create space for lakes behind abandoned end moraines and increased melting rates supply more water to potential lakes. This, in turn, may increase GLOF frequency and/or magnitude (25, 31–33). The potential for increased GLOF activity in response to climate change therefore not only represents increased risk to communities in these regions but may also strongly affect the pace of landscape change in a way that is not reflected in precipitation-dependent erosion models.