Flash floods in the Himalayan Region: Causes, concerns and mitigation

Background:

Recent flash flood incidents in Uttarakhand is another warning of the dangers that Himalayan states like Uttarakhand, Himachal and Jammu & Kashmir and Sikkim face from natural processes like landslides, snow avalanches cloudbursts or lake bursts.

Increasing glacial lakes

Glaciers are the largest source of freshwater outside of the Polar Regions. Glaciers and snow melt in the Himalayan ecosystem are the source of water for several rivers across the subcontinent, and are responsible for maintaining the perennial supply of water in the river systems like the Indus, Ganges, and Brahmaputra to over a billion people.

But these glaciers have reduced considerably in mass and surface area since the little ice age period. Global temperatures began to rise after 1850. They have climbed more rapidly in the 20th century as greenhouse gas levels soared. The rise has been even steeper since the early 1980s. Some models predict that an increase in global temperatures by 2°C from 1850 by 2070 would result in 45% of the medium and large glaciers (10 sq km or more) disappearing completely. Nearly 70% smaller glaciers are likely to melt away. Shrinking glaciers have led to the formation of a large number of glacial lakes all across the Himalayas. Many of these high-altitude lakes are potentially dangerous, because of their potential to cause flash floods in the event of a breach.

There has been an increase in the number of such lakes in the last few decades because of an acceleration in the glacial melt. A 2005 study by Kathmandu-based ICIMOD (International Centre for Integrated Mountain Development) had listed 127 such lakes in Uttarakhand. More recent studies put the number of such lakes in the state at around 400. To glaciologists, this does not come as a surprise, but it is certainly a cause for concern.

How big is the threat of such incidents continuing?

There are over 2,000 glaciers in the Himalayan region. Almost all of them are receding. Most of the glaciers also have debris cover. When glaciers retreat due to rising temperatures, the snow melts but the debris remains. This debris aids in the formation of lakes.

Over the years, the frequency of formation of these lakes has increased. But despite that, there are not many GLOF (glacial lake outburst flood) events happening in Uttarakhand. Not as many as in Sikkim, for example. This is because Uttarakhand has very steep slopes, and the water manages to find a way out.

Monitoring

The first step in tackling the threat from these glacial lakes is to start monitoring them and the glaciers more actively and regularly. We do not need to monitor every glacier. Glaciers in one basin do not have remarkably different properties. If we identify one or two benchmark glaciers in every basin, those that are more easily accessible, and do detailed studies, then the results can be extrapolated to the rest of the glaciers in the basin or the state.

But relying only on satellites and remote sensing is not going to be enough. It is important to get people and measuring instruments on the ground. That is why accessibility becomes an important factor. But we need to very closely measure the bathymetric changes, the mechanisms of expansion, and changes in water levels, discharge balance, mass balance, and other attributes. It requires a lot of manpower and money, but we must be able to invest these resources.

A dataset that tracks the progressive changes is crucial for any decision making. Some amount of monitoring is already happening but it is dispersed. What is required is a consolidated state of glaciers in India, with the ability to zoom in on any of them and track the changes happening year by year.

Planning

Construction-related activities in the state might not have a direct link to such incidents, but these are not entirely benign. The Himalayas are very young mountain systems, and extremely fragile. A minor change in orientation of the rocks can be enough to trigger landslides. It is important to include glaciers in any environment impact assessment for major projects such as construction of dams. The entire catchment areas should be made part of the impact assessment. In fact, project owners must be asked to invest in such studies. After all, their own assets are also at stake.

Mitigation

If we monitor the glaciers regularly, it would enable us to identify the lakes that need mitigation solutions. Several structural and geotechnical measures can be applied, and there are successful examples where the threat from these lakes have been reduced.

It is possible to construct channels for gradual and regulated discharge of water from these lakes, which will reduce the pressure on them, and minimise the chances of a breach. At the same time, it also reduces the volume of water that goes into the flash flood. Also, alarm systems can be set up at the lakes that will warn the community downstream whenever an overflow happens. A preparatory response drill will also have to be worked out, like we have done for cyclones and tsunami.