A recent geological study has shed some light on the cause of a major, yet elusive destructive natural hazard triggered by failed natural dams holding back glacial lakes. The findings show how previously unrecognized factors like thinning glacier ice and moisture levels in the ground surrounding a lake can determine the size and frequency of Glacier Lake Outburst Floods, or GLOFs.
The risks of these glacial floods are generally considered increasingly acute across the world, as warming atmospheric temperatures prompt ice and snow on mountain ranges to retreat and to swell glacial lakes.
Landslides in moraines as triggers of glacial lake outburst floods: example from Palcacocha Lake (Cordillera Blanca, Peru), published in Landslides in July 2016, centers its study on Lake Palcacocha in the Cordillera Blanca mountain region of central Peru. Since Palcacocha is one of almost 600 lakes in the Cordillera Blanca mountain range dammed by glacial moraines, the population of the region lives under serious threat of GLOFs.
The Landslides article is a step in understanding a previously understudied geological phenomenon. As little as five years ago scientists acknowledged the lack of research on the subject.
“We don’t really have the scientific evidence of these slopes breaking off and moraine stability… but personal observations are suggesting there are a lot of those…” said Ph.D. environmental historian Mark Carey in a 2011 video where he describes GOLFs.
Glacial Lake Outburst Flood risks do not always emanate from mountain glacier meltwater that flows downstream. As this study shows, in some instances, trillions of gallons of water can be trapped by a moraine, a formation of mixed rock, which forms a natural dam. A weakening over time, or a sudden event, such as a landslide, could then result in the moraine dam’s collapse.
The massive amount of water is suddenly then released, and a wall of debris-filled liquid speeds down the mountainside with a destructive force capable of leveling entire city blocks.
GLOFs have presented an ongoing risk to people and their homes dating back to 1703, especially in the Cordillera Blanca region, according to United States Geological Survey records. In December of 1941, a breach in the glacial moraine restraining Palcacocha Lake led to the destruction of a significant portion of the city of Huaraz and killed approximately 5,000 people.
Scientists and government agencies, like the Control Commission of Cordillera Blanca Lakes created by the Peruvian government following the 1941 GLOF, have recognized the need to better understand and control GLOFs. The study found that as global temperatures rise and glaciers retreat, greater amounts of glacier melt water will continue to fill up mountain lakes, chucks of ice will fall off glaciers, and wetter moraines will become more prone to landslides.
The team of mostly Czech geologists and hydrologists (J. Klimeš; J. Novotný; I. Novotná; V. Vilímek; A. Emmer; M. Kusák; F. Hartvich) along with Spanish, Peruvian and Swiss scientists (B. Jordán de Urries; A. Cochachin Rapre; H. Frey and T. Strozzi) investigated the ability of a glacial moraine’s slope to stay intact, called shear strength, and modeled the potential of landslides and falling ice to cause GLOFs.
After extensive field investigations, calculations and research into historical events, the study found several causal factors that can determine the severity of a GLOF. These include size and angle of entry of a landslide, shape and depth of the glacial lake, glacier thickness and human preventative engineering such as canals and supporting dams. Frequency and size of a landslide is determined by the stability of surface material, a characteristic called shear strength, which can be influenced by something as subtle as the crystalline shape of the predominant mineral in the rock.
The scientists determined that waves caused by moraine landslides and falling ice would most likely lead to over-toppings of the natural dam. An example would be the 2003 Palcacocha Lake GLOF, which was caused by falling ice. No one died in this flood, but sediment from the floodwaters blocked the Huaraz’s main water treatment facility, leaving 60 percent of the population without drinking water for six days. Additionally, small events like the one in 2003 weaken the natural and manmade dams, which without monitoring could eventually give out and result in a more catastrophic occurrence.
Most recent measurements estimate Palcacocha Lake holds 4.5 trillion gallons of glacier meltwater, which is enough to fill approximately 6,800 olympic size pools. The potential of a catastrophic flood following the collapse of the moraine dam is a serious threat to the growing city that lies beneath it.
“Climate-driven environmental changes may critically affect stabilities of slopes above glacial lakes, possibly triggering large moraine landslides,” write the authors in the article. They call for continued monitoring of glacial lakes.