Glacier outburst floods originating from glacial water pockets: what do we know?

The term "water pocket" is often used as an umbrella term to describe the unknown origin of glacial outburst floods. There is currently no consensus on its definition and the formation and rupture mechanisms of water pockets remain poorly understood. Here, we define a glacial water pocket as an englacial or subglacial water-filled cavity with a volume larger than 1000 m3. Glacier outburst floods originating from the rupture of a water pocket are called water pocket outburst floods (WPOFs). WPOFs are in contrast to glacier lake outburst floods (GLOFs), for which the water giving rise to a flood stems from a detectable reservoir located either in the glacier forefield, at the surface of the glacier, at the glacier margin, or at the glacier base.Here, we aim to understand the mechanisms behind WPOFs from alpine glaciers by analyzing their spatial and temporal distribution, pre-event meteorological conditions, and the glacio-geomorphic features of the glaciers from which the floods originate. We updated an inventory of known WPOFs in the Swiss Alps to 91 events from 37 individual glaciers. Among all the recorded events, 64 events have direct observations of the flood at the glacier tongue, while 27 events are characterized as speculative because of the lack of direct observations. Infrastructure damage was reported for 43 events, and two WPOFs caused the death of three people. Most WPOFs occurred between June and September, linked to meltwater input. Meteorological data indicate anomalously high temperatures during the days preceding most events and heavy precipitation on 25 % of days for which WPOFs occur, indicating that water pockets typically rupture during periods of high water input.Based on the collected information, we propose four mechanisms of water pocket formation: temporary subglacial channel blockage, hydraulic barriers, water-filled crevasses, and accumulation of liquid water behind barriers of cold ice (thermal barriers). Overall, our analysis highlights the challenge of understanding WPOFs due to the sub-surface nature of water pockets, emphasizing the need for field-based research to improve their detection and monitoring.