Image of the Week – Yes, you’re looking at one of Peru’s most dangerous glacial lakes!

David Rounce September 9, 2016 Image of the Week, Regular Editor 43 Comments

Lake Palcacocha is one of Peru’s most dangerous glacial lakes due to the threat of an avalanche entering the lake and causing a glacial lake outburst flood. The siphons installed in 2011 lowered the level of the lake by 3-5 m in an attempt to reduce the flood hazard of the lake. [Credit: David Rounce]

As mountain glaciers melt and recede, they often leave behind large glacial lake that are contained by the glaciers’ old terminal moraines. These glacial lakes are found throughout the world and can pose a significant flood hazard to downstream communities and infrastructure.

The image of this week focuses on Lake Palcacocha, a large glacial lake located in Peru’s Cordillera Blanca at an elevation of 4,562 m. It is one of Peru’s most dangerous glacial lakes because it threats to flood the inhabited valley downstream.

More than 75 years of flood hazard

In 1941, an avalanche entered the glacial lake, causing a tsunami-like wave that overtopped and eroded its terminal moraine, and ultimately triggered a glacial lake outburst flood. The flood traveled down the valley and killed an estimated 1,800-6,000 people in the city of Huaraz (see map below). The flood drained the volume of the lake from 10-12 million m³ to just 0.5 million m³.

A map showing the location of Lake Palcacocha and the city of Huaraz below [Credit: fig 1 from Somos-Valenzuela et al., (2016)] LINK: http://www.hydrol-earth-syst-sci.net/20/2519/2016/

A map showing the location of Lake Palcacocha and the city of Huaraz [Credit: fig 1 from Somos-Valenzuela et al., (2016)]

The 1941 flood drained the volume of the lake from 10-12 million m³ to just 0.5 million m³

In 1974, to prevent this from happening again, a drainage structure was built that lowered the level of the lake by 8 m. However, over the years as the glacier continued to recede, the glacial lake continued to grow. In 2011, siphons were also installed (see our image this week) within the drainage structure to lower the lake by an additional 3-5 m.

Visiting the hazardous lake

Despite these major efforts to reduce the flood hazard, the lake is now over 73 m deep with a volume exceeding 17 million m³. As part of the Foro Internacional de Glaciares y Ecosistemas de Montaña, a weeklong conference bringing together international experts on topics related to the social, ecological, hydrological, and hazard studies associated with mountain ecosystems, INAIGEM (Instituto Nacional de Investigación en Glaciares y Ecosistemas de Montaña) organized a field expedition that brought ~30 participants to visit Lake Palcacocha. The view of the lake, with its two 6,000+ m peaks behind it, was absolutely stunning. Unfortunately, the glaciers on these peaks are the very things that threaten the lake’s safety, as an avalanche entering the lake could cause another glacial lake outburst flood like the 1941 event – a possibility Peru is very well aware of.

Modelling the glacial lake outburst flood

In response to this threat, a team from the University of Texas at Austin (Daene McKinney, Marcelo Somos-Valenzuela, Rachel Chisolm, and Denny Rivas) has been working closely with Peruvian organizations (INAIGEM and the Glaciology Unit) to model the potential flood from Lake Palcacocha. Hydraulic models were used to investigate the downstream impact, produce preliminary hazard maps, and quantify the amount that the lake should be lowered in order to reduce the hazard of the lake to a safe level. The study of Somos-Valenzuela et al. (2016) found that lowering the level of the lake by 30 m would reduce the total affected area by 30% and, more importantly, would reduce the intensity of the flood (a combination of the water depth and velocity) for most of the city from high to low thereby making the lake much safer.

Overview of the avalanche, lake, and terminal moraine (left) and the potential inundation downstream based on the current level of the lake (right) performed by the University of Texas at Austin. [Credit: fig 2 (left) and 10 (right) from Somos-Valenzuela et al., (2016)]

A hopeful future

The good news for Peru is they have extensive experience lowering the level of their lakes and safely reducing the hazards. Since the Government of Peru established a Glaciology Unit in 1951, Peru has successfully lowered the level over 30 glacial lakes that were considered to be hazardous. Additionally, the citizens of Huaraz are well aware of the hazardous lakes situated above their city and want them to be lowered as well. Given Peru’s track record, hopefully their concerns will be alleviated soon.