The causes and mechanisms of moraine-dammed lake failures in the Cordillera Blanca, North American Cordillera, and Himalayas

Glacial lake outburst floods (GLOFs) from moraine-dammed lake failures represent a significant threat to inhabitants of high mountain areas across the globe. The first part of this paper summarises the causes and mechanisms of moraine-dammed lake failures through a review of the scientific literature and unpublished reports. There are eight main causes, of which five are characterised as dynamic and three as long-term, and these are associated with around twenty failure mechanisms. The dynamic causes are slope movements into the lake, earthquakes, flood waves from a lake situated upstream, blocking of underground outflow channels, and intensive rainfall or snowmelt. The long-term causes are the melting of buried ice, the impact of hydrostatic pressure, and the effect of time. These causes (triggers) and the consequent mechanisms of dam failure are described in detail. The second part compares the historical moraine-dammed lake failures within three regions between 1900 and 2009: the Cordillera Blanca of Peru, the North American Cordillera, and the Himalayas. It has been found that dynamic causes are around four times more common than long-term causes although significant regional differences have been observed. The most frequent causes in these regions were found to be slope movements in which the displaced material was dominated by solid-state water (ice falls, ice avalanches, and snow avalanches). The other causes tended to show distinct regional patterns while the temporal distribution of events also differs according to region. In the North American Cordillera and Himalayas moraine dam failures occur exclusively during the summer season while in the Cordillera Blanca they are more evenly distributed with the exception of the dry season. This reflects the general climatic setting of each of the study regions. Analýza příčin a mechanismů destrukcí hrází jezer hrazených morénami v pohořích Cordillera Blanca (Peru), Severoamerická Kordillera a Himaláj Přispěvek je členěn do dvou časti. Prvni čast ma rešeršni charakter a shrnuje rozlične přičiny a mechanismy destrukci (protrženi) hrazi jezer hrazenych morenami. Pět dymanickych přičin (různe typy svahovych pohybů, zemětřeseni, povodňova vlna z vyše položeneho jezera, intenzivni dešťove sražky / tani sněhu, ucpani podzemnich odtokovych kanalů) a tři dlouhodobe přičiny (odtavani pohřbeneho ledu, působeni hydrostatickeho tlaku a dlouhodoba degradace tělesa hraze v čase) jsou popsany spolu s mechanismy vedoucimi k destrukcim morenovych hrazi, a to včetně konkretnich přikladů ze zajmovych oblasti. Ve druhe časti přispěvku je provedena srovnavaci analyza těchto udalosti mezi oblastmi pohoři Cordillera Blanca (Peru), Severoamericka Kordillera a pohoři Himalaj. Na zakladě vytvořene databaze protrženych morenovych hrazi mezi lety 1900 a 2009 je zhodnoceno jednak zastoupeni různych přičin, jednak časova distribuce těchto udalosti. Nejfrekventovanějši přičinou ve všech studovanych oblastech byl dynamicky svahovy pohyb do jezera. Zastoupeni a vyskyt dalšich přičin se však mezi jednotlivymi oblastmi vyrazně liši. Časova distribuce udalosti vyrazně odlišuje oblast Cordillera Blanca od zbyvajicich dvou, což do určite miry odraži ročni chod klimatu a na něj vazanych přičin destrukci morenovych hrazi.