Secrets of dirty ice
- Sumit Basu
- Oct 22, 2025
- 6 min read
Updated: Jan 5
Shruthi Pandey, P Venkitanarayanan and Ishan Sharma are trying to understand the failure mechanism of ice laden with dirt when subjected to sudden, rapid and very large loads. Dirty ice is often the cement binding boulders that make up glacial dams holding huge volumes of water. Understanding how they fail is crucial to modelling ice dam bursts that are known to cause devastation in the high mountains.
In mid-June 2013, the village of Kedarnath and the nearby settlements of Rambara and Gaurikund were in the middle of a normal, busy pilgrimage season. The villagers found the prolonged and heavy rainy spells to be a bit unusual for the time of year, but being busy making the most of the short tourist season, they were not unduly bothered. But even for the local population used to mudslides and flash floods, what happened next was beyond their wildest imagination. Around 7 pm on 16th and again 12 hours later, two massive flows of rocky debris hurtled down the mountain slopes, wiping Rambara off the map and devastating Kedarnath beyond recognition. By some estimates, more than 6000 people were buried under or carried away by the rocky flow. Almost all vital infrastructure in the region, including several hydropower plants, was destroyed.
We now have a nearly complete picture of the sequence of events that led to this disaster. Above Kedarnath lies the Chorabari Tal, a small glacial lake. The most catastrophic debris flow on the morning of the 17th resulted when the relatively low points in the natural dam of unstratified glacial drift, (made of loosely packed boulders, gravel, sand, and clay, called a moraine, see Box 1) was breached due to huge amount of water and debris flowing into the lake from surrounding areas (see, Allen, et al., 2016 https://doi.org/10.1007/s10346-015-0584-3 for the exact sequence of events that led to the breach, also Box 2). This is a particularly high-fatality example of what geologists call a glacial lake outburst flood (GLOF).
Box 1:
Glaciers are moving masses of ice that have formed by turning accumulated snow into ice, layer by layer and under appropriate conditions. As the glacier moves, it carries with it a huge amount of debris -- mostly products of erosion between the molten glacier bottom and the terrain beneath and some amount of stuff deposited on the top. The debris accumulates on the edges of the glacier, both on its sides and at the end, creating ridges of loosely consolidated natural dams called moraines. If subsequent climate changes cause the glacier to retreat, it leaves behind a glacial lake dammed by the moraines. Moraine dams can be unstable and pose a serious threat, especially in the Himalayan region where close to 2000 such lakes have been identified. The most serious threat comes from sudden surges in pressure on the moraine wall, which can quickly give away.
`Dirty ice’ which is the scientific jargon for normal ice with trapped sediments and dust, often plays a major role in such events. All GLOF events involve catastrophic moraine breaches triggered by sudden impact events caused by avalanches or rapid inflow of debris (as in the case of the Kedarnath event) into the glacial lake that the moraine is holding in place. In case of the 1985 Dig Tsho GLOF event in Nepal, a massive avalanche first hit the lakes, set off a surge wave which eventually destroyed the moraine (Vuichard and Zimmerman, 1987, 10.2307/3673305). Unlike the Chorabari Tal, this was an ice-core moraine made up of `dirty ice’ cementing the rubble together. The impact of the surge wave and the subsequent overtopping of the moraine can potentially cause the ice to melt, loosen the boulders and destabilise the natural dam. Given the interest in global climate change, understanding the mechanical behaviour of sediment-laden dirty ice, especially when it is subjected to loads varying rapidly in time, has become an absolute necessity.Even without the embedded `dirt’, pristine ice is a notoriously difficult material to model (Cole, 2001, https://doi.org/10.1016/S0013-7944(01)00031-5). The challenges are numerous. Firstly, processes that lead to deformation of ice operate at micro-scales while applications in glaciers and icebergs are of geological dimensions. Add to that the different kinds of ice polycrystalline microstructures that can form under conditions as different as those deep inside glaciers, on top thin floating sheets, in ice consolidated from loose snow or from sea water. The ice can be subject to loading regimes that cover many orders of magnitude in strain rate, as well as a considerable range in temperature. Cracks propagate very slowly through ice, finding the path of least resistance under the influence of global loads and microstructural fluctuations. It is little wonder, therefore, that the mechanical behaviour of even pristine ice exhibits wide scatter. Using very high speed imaging, Shruti Pandey, P Venkitanarayanan and Ishan Sharma had addressed the problem of high strain rate failure of pristine polycrystalline and granular ice in an earlier work (Pandey et al., 2024, https://doi.org/10.1016/j.coldregions.2024.104295). The ice was crushed under compression in a special set-up called the Split Hopkinson bar and imaged with a very high speed camera.
Now, in a new paper, the same authors have extended their work to the mechanics of ice--silica particle mixtures, addressing how particle content and morphology change the compressive strength of dirty ice. To assess how the amount of embedded dirt influences properties, ice--silica mixtures containing various amounts of silica particles were prepared in the laboratory. To probe how particle shape influences strength, the study compares mixtures prepared using irregular, naturally-shaped river sand particles and perfectly spherical glass silica beads.
Box 2:
Companion glacier on 16th June. The flow entrains more debris as it flows down b and impacts the village at c and continues to Rambara. On 17th the western lateral moraine of the Chorabari lake at d is breached. The points e-g are the Chorabari glacier tongue, Mandakini river and Saraswati-Mandakini meeting point. The debris followed the paths 1, 2 and 3. From Allen et al, 2016.
The findings reveal a clear trend: while both particle types enhance strength as their volume fraction increases, mixtures containing irregular river sand particles show significantly higher strength enhancement than smooth glass beads. Ultra high speed imaging then reveals the exact mechanism by which damage accumulates inside the impacted dirty ice. The more the number of silica particles, more are the isolated web of cracks formed and faster they join to create an interconnected network, which causes pieces of the sample to break off. Based on these systematic experiments, the authors propose a new empirical model that predicts the dynamic compressive strength of ice--silica mixtures as a fraction of that of pristine ice, by accounting for both silica volume fraction and particle morphology. These results plug a key gap: rather than guessing how dirty ice holds up under stress, modelers now have systematic data to calibrate simulations for avalanches, frozen-ground foundations, or icy planetary surfaces alike.
The saga of dirty ice composites does not end here. Fracture of ice has more facets to it. The brittle crushing seen in these experiments are different from what happens at slower rates where cracks grow in a more ductile manner. Thus, failure of sediment-laden ice depends on how it is loaded. Since the spectrum of loading rates that accompany a GLOF-causing trigger is wide, the entire gamut of damage progression with rate needs to be mapped out. Further, utilising these insights to predict and prevent disasters will require larger-scale modelling scenarios where the dirty ice acts as an interstitial, confined glue holding together massive pieces of rocks. There may be more twists to the tale.
Horrible article