CR
Cryospheric Sciences

Moraine

Image of the Week – Supraglacial debris variations in space and time!

Image of the Week – Supraglacial debris variations in space and time!

There is still a huge amount we don’t know about how glaciers respond to climate change. One of the most challenging areas is determining the response of debris-covered glaciers. Previously, we have reported on a number of fieldwork expeditions to debris-covered glaciers but with this Image of The Week we want to show you another way to investigate these complex glaciers – numerical modelling!


Debris-covered glaciers

Debris-covered glaciers occur globally, with a great many being found in the Himalaya-Karakoram mountain range. For example, in the Everest Region of Nepal 33% of glacier area is debris covered (Thakuri et al., 2014). The response of debris-covered glaciers to future climate change in such regions has huge implications for water resources, with one fifth of the world’s population relying on water from the Himalayan region for their survival (Immerzeel et al., 2010).

Debris-covered glaciers respond to climate change differently to debris-free glaciers as the supraglacial debris layer acts as a barrier between the atmosphere and glacier (Reznichenko et al., 2010). The supraglacial debris layer has several key influences on the glacier dynamics:

  • Glacier ablation (loss of mass from the ice surface) is enhanced or inhibited depending on debris layer thickness and properties – see our previous post.
  • Supraglacial debris causes glaciers to reduce in volume through surface lowering rather than terminus retreat (typical of debris free mountain glaciers).

Understanding the influence of a supraglacial debris layer on mass loss or gain is, therefore, key in determining the future of these glaciers. The properties of supraglacial debris layers can vary in time and space both in debris layer thickness and distribution, as well as properties of the rocks which make up the debris (e.g. albedo, surface roughness, porosity, size and moisture content). It is these characteristics of the debris-cover which control the heat transfer through the debris and therefore the amount of thermal energy that reaches the underlying ice causing melting (Nicholson and Benn, 2006). In order to better predict the future of debris-covered glaciers we needs to be able to numerically model their behaviour. This means we need a better understanding of the variations in debris cover and how this affects the ice dynamics.

How does a supraglacial debris layer vary in time and space?

Our Image of the Week (Fig. 1) shows a schematic of how debris distribution can vary spatially across a glacier surface and also this can change through time. The main inputs of debris are:

  • Upper regions: snow and ice avalanches in the upper reaches of the glacier.
  • Mid and Lower regions: rock avalanches and rock falls (Mihalcea et al., 2006).

These irregular mass movement events vary in frequency and magnitude, and therefore affect debris distribution across the glacier surface but also through time. The irregularity of them makes it really hard to predict and simulate! Luckily, debris transport is a little more predictable.

Figure 2: An ice cliff emerging out of the supraglacial debris layer on Khumbu Glacier, Nepal, with Nuptse in the background. [Credit: M. Gibson]

Debris is initially transported along medial moraines (glacially transported debris)  in the upper and mid-sections of the glacier, this is known as entrained debris. The various sources of entrained debris combine to form a continuous debris cover in the lower reaches of the glacier (Fig. 1). As a supraglacial debris layer is forming, such as for Baltoro glacier (Fig. 1), the boundary between the continuous debris layer and entrained debris sections progresses further upglacier over time.

Eventually transported debris will reach the terminus of the glacier and be deposited (Fig. 1), mainly due to a decrease in surface velocity of the glacier towards the terminus. However, once debris is deposited it doesn’t just sit there; debris is constantly being shifted around as ablation (surface melting) occurs. As ablation occurs the debris surface ablates unevenly, as the thickness of the debris layer is spatially variable. Uneven ablation, otherwise known as differential surface lowering, causes the glacier surface to be made up of topographic highs and lows, the latter of which sometimes become filled with water, forming supraglacial ponds (Fig. 1) . Another product of debris shifting is that ice cliffs, such as the one seen in Fig. 2, are exposed. These features are initially formed when englacial channels collapse  or debris layers slide (Kirkbride, 1993). All this movement and shifting means that not only do glacier models have to consider variation in debris layers across the glacier and through time, but also the presence of ice cliffs and supraglacial ponds. They are important as they have a very different surface energy balance to debris-covered ice. To complicate things further the frequency and area of ice cliffs and supraglacial ponds also vary through time! You see the complexity of the problem…

Modelling spatially and temporally varying debris layers

Numerical modelling is key to understanding how supraglacial debris layers affect glacier mass balance. However, current numerical modelling often either omits the presence of a supraglacial debris layer entirely, or a debris layer that is static in time and/or space (e.g. Collier et al., 2013; Rowan et al., 2015; Shea et al., 2014). However, as outlined earlier, these supraglacial debris layers are not static in time or space. Understanding the extent to which spatiotemporal variations in supraglacial debris distribution occur could aid identification of when glaciers became debris-covered, glaciers that will become debris-covered glaciers in the future, and the timescales over which supraglacial debris layers vary. The latter is particularly relevant to numerical modelling as it would result in total glacier ablation being calculated more precisely throughout the modelling time period. Understanding the interaction between glacier dynamics and debris distribution is therefore key to reconstructing debris-covered glacier systems as accurately as possible.

Edited by Emma Smith


Morgan Gibson is a PhD student at Aberystwyth University, UK, and is researching the role of supraglacial debris in ablation of Himalaya-Karakoram debris-covered glaciers. Morgan’s work focuses on: the extent to which supraglacial debris properties vary spatially; how glacier dynamics control supraglacial debris distribution; and the importance of spatial and temporal variations in debris properties on ablation of Himalaya-Karakoram debris-covered glaciers. Morgan tweets at @morgan_gibson, contact email address: mog2@aber.ac.uk.

Image of the Week – It’s all a bit erratic in Yosemite!

Image of the Week – It’s all a bit erratic in Yosemite!

When you think of California, with its sun-soaked beaches and Hollywood glamour, glaciers may not be the first thing that spring to mind – even for ice nerds like us. However, Yosemite National Park in California’s Sierra Nevada is famous for its dramatic landscape, which was created by glacial action. With our latest image of the week we show you some of the features that were left behind by ancient glaciers.


What do we see here?

Although Yosemite is now largely glacier-free the imprint of large-scale glaciation is evident everywhere you look. During the last glacial maximum (LGM), around 26,000 to 18,000 years ago, much of North America was covered in ice. Evidence of this can be seen in the strange landscape, shown in our image of the week. The bedrock surface in this area is polished and smoothed due to a huge ice mass that was moving over it, crushing anything in it’s path. When this ice mass melted rocks and stones it transported were released from the ice and left strewn on the smoothed bedrock surface. These abandoned rocks and stones are know as glacial erratics. Some of these erratics will have travelled from far-away regions to their resting place today.

During the last glacial maximum (LGM), around 26,000 to 18,000 years ago, much of North America was covered in ice.

Glaciers that still remain!

There are still two glaciers in Yosemite, Lyell and Maclure, residing in the highest peaks of the National Park. Park rangers have been monitoring them since the 1930s (Fig. 2), so there is a comprehensive record of how they have changed over this period. Sadly, as with many other glaciers around the world this means a huge amount mass has been lost – read more about it here!

Figure 2: Survey on Maclure Glacier by park rangers in the 1930s [Credit: National Parks Service]

On a more cheerful note – Here at the EGU Cryosphere Blog we think it is rather fantastic that the park rangers of the 1930s conducted fieldwork in a suit, tie and wide-brimmed hat and we are hoping some of you might be encouraged to bring this fashion back! 😀

If you do please make sure to let us know, posting it on social media an tagging us @EGU_CR! Here are a few more ideas of historical “fieldwork fashion” to wet your appetite: Danish explorers in polar bear suits, 1864-65 Belgian-Dutch Antarctic Expedition and of course Shackleton’s Endurance expedition!


Imaggeo, what is it?

You like this image of the week? Good news, you are free to re-use it in your presentation and publication because it comes from Imaggeo, the EGU open access image repository.

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

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

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 and 10 from Somos-Valenzuela et al., (2016)] LINK: http://www.hydrol-earth-syst-sci.net/20/2519/2016/ .

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.

Further Reading

Edited by Sophie Berger and Emma Smith