Glacial lake outburst floods (GLOF) evolve into debris flows by erosion and sediment entrainment while propagating down a valley, which highly increases peak discharge and volume and causes destructive damage downstream. This study focuses on GLOF hazard assessment in the Bhote Koshi Basin (BKB), where was highly developed glacial lakes and was intensely affected by the Gorkha earthquake. A new 2016 glacial lake inventory was established, and six unreported GLOF events were identified with geomorphic outburst evidence from GaoFen-1 satellite images and Google Earth. A new method was proposed to assess GLOF hazard, in which large numbers of landslides triggered by earthquake were considered to enter into outburst floods enlarge the discharge and volume of debris flow in the downstream. Four GLOF hazard classes were derived according to glacial lake outburst potential and a flow magnitude assessment matrix, in which 11 glacial lakes were identified to have very high hazard and 24 to have high hazard. The GLOF hazard in BKB increased after the earthquake due to landslide deposits, which increased by 216.03 × 106 m3, and provides abundant deposits for outburst floods to evolve into debris flows. We suggest that in regional GLOF hazard assessment, small glacial lakes should not be overlooked for landslide deposit entrainment along a flood route that would increase the peak discharge, especially in earthquake-affected areas where large numbers of landslides were triggered.
Abstract. Meteorological studies have indicated that high alpine environments are strongly affected by climate warming, and periglacial debris flows are frequent in deglaciated regions. The combination of rainfall and air temperature controls the initiation of periglacial debris flows, and the addition of meltwater due to higher air temperatures enhances the complexity of the triggering mechanism compared to that of storm-induced debris flows. On the south-eastern Tibetan Plateau, where temperate glaciers are widely distributed, numerous periglacial debris flows have occurred over the past 100 years, but none occurred in the Tianmo watershed until 2007. In 2007 and 2010, three large-scale debris flows occurred in the Tianmo Valley. In this study, these three debris flow events were chosen to analyse the impacts of the annual meteorological conditions, including the antecedent air temperature and meteorological triggers. The remote sensing images and field measurements of the adjacent glacier suggested that sharp glacier retreats occurred in the 1 to 2 years preceding the events, which coincided with spikes in the mean annual air temperature. Glacial till changes providing enough active sediment driven by a prolonged increase in the air temperature are a prerequisite of periglacial debris flows. Different factors can trigger periglacial debris flows, and they may include high-intensity rainfall, as in the first and third debris flows, or continuous, long-term increases in air temperature, as in the second debris flow event.
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