The Impending Himalayan OutburstPrinter-friendly version |
The Himalayan glacial lakes are about to burst. There are at least 42 in Nepal and Bhutan that are potentially dangerous. And those in the Dudh Koshi and Pho Chu basins are the most precarious.
A Nepal Monitor Report
For more than a decade, the fear that the Himalayan glaciers may be melting have attracted the attention of geologists and policymakers. During this time, conferences and papers have been written about the topic and media coverage has also increased on this issue.
And finally, here comes the most comprehensive study on the topic-- Impact of Climate Change on Himalayan Glaciers and Glacial Lakes: Case Studies on GLOF and Associated Hazards in Nepal and Bhutan. This 119-page scientific study focuses on the impact of warming temperatures on glaciers and glacial lakes in the Himalayan region. It warns of impending glacial lake outburst floods (GLOFs) – when rising waters from glacial melt breach dams in glacial lakes – and calls for early warning and mitigation measures to avert disaster.
The document, released on June 5 in Kathmandu and Bangkok on the World Environment Day, is jointly produced by the International Centre for Integrated Mountain Development (ICIMOD) with the United Nations Environment Programme (UNEP) and authored by B. Shrestha, P. Mool and S.R. Bajracharaya.
The report says the Himalayan glaciers are rapidly shrinking and this process is accelerating at alarming rates in past decades as a result of global warming, and that this phenomenon will have catastrophic consequences for communities living downstream and millions who rely on glacial melt water.
The global mean temperature is expected to increase between 1.4 to 5.8ºC over the next hundred years. The consequences of this change in global climate are already being witnessed in the Himalayas where glaciers and glacial lakes are changing at alarming rates. Himalayan glaciers are retreating at rates ranging from 10 to 60m per year and many small glaciers (<0.2 sq.km) have already disappeared, says the report.
Nearly 15000 glaciers and 9000 glacial lakes are found in the Himalayan mountain chain which stretches 2500 km across five countries – Bhutan, Nepal, Pakistan, India and China. The mountain range feeds nine perennial river systems in the region and constitutes a lifeline for nearly 1.3 billion people downstream.
Himalayan glaciers are shrinking at an average of 10 to 60 m annually, with some retreating by 74 m a year. In China, glaciers have been retreating at a rate of 5.5 per cent in the last three decades. With current climate change projections two-thirds of China’s glaciers are likely to disappear by 2050, and almost all would be gone by 2100.
On the Indian subcontinent, temperatures are predicted to increase between 3.5 and 5.5ºC by 2100 and an even greater increase is predicted for the Tibetan Plateau. It is estimated that a 1ºC rise in temperature will cause alpine glaciers worldwide to shrink as much as 40 per cent in area and more than 50 per cent in volume as compared to 1850.
Significant changes were also seen in the Indian Himalaya, with the highest rate of glacial retreat found in the Bada Shigri Glacier and lowest in the Chhota Shigri Glacier in the Chenab River Basin, where glaciers are retreating by 6.8 to 29.8 m each year.
In Bhutan, the Luggye Glacier retreated by 160 m yearly from 1988 to 1993 resulting in rapid growth of the Luggye Tso Lake. The Raphstreng Glacier retreated 35 m every year on average from 1984 to 1998 but from 1988 to 1993 the retreat rate almost doubled to 60 m per year.
Glacier retreat has been accelerating in Nepal since the 1990s, with dramatic retreats recorded between 1994 and 1998 especially in the Dudh Koshi sub-basin where all of the glaciers studied have retreat by 10 to 59 m yearly. The Dudh Koshi sub-basin is the largest basin and most densely glaciated region in Nepal.
Melting glaciers are also leading to some of the fastest-growing glacial lakes in the region. Some glacial lakes have grown by almost 800 per cent since the 1970s.
Nepal and Bhutan have the highest concentration of glacial lakes with 20 potentially dangerous lakes in Nepal and 24 in Bhutan. Of this, Lake Imja Tso in the Dudh Koshi Basin in Nepal, home to Mount Everest and one of the most popular tourist destinations and trekking route is among the most hazardous. In fact, Lake Imja Tso was virtually nonexistent in 1960, it now covers nearly 1 sq.km and the Imja glacier which feeds it is retreating at an unprecedented 74m per year (between 2001 and 2006).
The study shows that the terminus of most of the high altitude valley glaciers in Bhutan, China, and Nepal are retreating very fast; vertical shifts as great as 100m have been recorded during the last fifty years and retreat rates of 30m per year are common. As glaciers retreat, glacial lakes grow, and many Himalayan basins are reporting very fast growing lakes.
The report reviews (see chapter 2) the status of Himalayan glaciers in China, India, Bhutan, and Nepal. Recent studies by ICIMOD show that glaciers in the Dhud-Koshi sub-basin of Nepal are retreating at unpredicted rates; glacier retreat rates of 10 to 60m per year and, in exceptional cases, as fast as 74m per year, have been recorded.
It notes that regular monitoring of potentially dangerous glacial lakes at high risk for GLOF events is essential. Among the documented 24 potentially dangerous glacial lakes in Bhutan and 20 in Nepal, eight lakes are located in the Pho Chu basin of Bhutan and twelve in the Dudh Koshi sub-basin of Nepal. These two basins have the highest concentration of glacial lakes in their respective countries.
The study focused on these glacial lakes (See Chapter 3) in these two sub-basins; with the highest concentration of glacial lakes they are also ‘hot spots’ for potential GLOF activity. Lake Imja Tsho in Nepal and Lake Raphstreng Tso in Bhutan are discussed in detail.
Records show 15 GLOF events recorded in Nepal, 6 in the Tibet Autonomous Region of China (with consequences for Nepal) and 5 in Bhutan since the early 1970s. Examples in Nepal include: Nare (1977), Nagma Pokhari (1980), Dig Tsho (1985), Chhubung (1991), and Tam Pokhari (1998). The Tibetan cases include: the Zhangzhangbo (1981) and Jinco (1982). Some Bhutanese GLOF include the Luggye Tso GLOF (1994), the Zhangzhangbo (1981).
The Zhangzhangbo GLOF of 1981 caused damage in the Zhangzangbo and Sun Koshi valleys. It destroyed the Sun Koshi Power Station and the Friendship Bridge at the Nepal-China border, as well as two other bridges and devastated extensive sections of the Arniko Highway; losses reportedly totalled more than US $3 million.
Observations were also made in the Pho Chu basin of the Bhutan Himalaya, where the change in size of some glacial lakes has been as high as 800 per cent over the past 40 years. At present, several supraglacial ponds on the Thorthormi glacier are growing quickly and merging. These lakes pose a threat because of their proximity to other large glacial lakes in the Pho Chu sub-basin where, in a worst-case glacial lake outburst flood (GLOF) scenario, they could cascade on to these other lakes with catastrophic consequences.
Hazard assessment, especially in those river valleys that are known to be potentially at risk for GLOF events, is essential in developing the most appropriate responses and mitigation measures, says the study. Hydrodynamic dam breach modeling can help in understanding flood height, flood routing, and potential discharge from a likely GLOF event. The hydrological modeling of glacial lakes, terrain classification, and vulnerability assessment help in devising mitigation measures and early warning systems. A dam-breach model developed by the National Weather Services (NWS-BREACH) was used to simulate the outburst hydrographs of Lakes Imja Tsho in Nepal and Raphstreng Tso in Bhutan. The model provides information on discharge and flood arrival time in downstream areas (See Chapter 4).
Based on observations of damage caused by the Dig Tsho GLOF of 1985, the vulnerability of various terrain units in the vicinity of a possible Imja Tsho GLOF was assessed. This terrain classification scheme provided valuable information on the possible extent of the damage to be expected in the event of an Imja Tsho GLOF. The vulnerability analysis in the Imja and Dudh Koshi valleys indicated that the upper terrace of the Syomare village as well as lower terraces identified in Ghat, Chutawa, Chermading, Phakding, Benkar, Tawa, and Jorsalle villages could be severely damaged by a GLOF event at Lake Imja Tsho.
While GLOF modeling can give some useful insights it is helpful to supplement these both with data gathered from previous GLOF events and with field data. The study of past GLOF affected areas allows classification of various terrain units. This type of classification assesses the extent of damage sustained by a particular terrain unit after a GLOF event and uses that information to predict what damage subsequent GLOFs might cause, information that can then be incorporated into hazard maps.
GLOF mitigation measures and early warning systems applied in the Nepal and Bhutan Himalayas are also discussed. Such techniques are quite expensive and require much detailed field-work and maintenance, an alternative, which is being considered in a feasibility study, is regular temporal monitoring of glacial lakes by RADAR satellite-based techniques to detect any changes and provide an early warning.
The study also discusses the terrain classification of the Langmoche valley where the Dig Tsho GLOF occurred in 1985 and shows that a similar classification scheme can be applied in the Imja valley (See Chapter 5)
Mitigation measures may help to prevent a GLOF event and/or reduce the severity of its impact. Early warning systems, including satellite-based and other techniques, are helpful in reducing the threat that GLOFs pose to people in the downstream areas. The study also identifies examples of both the mitigation measures and early warning systems that are already in place in Bhutan and Nepal (See Chapter 6). In addition, it discusses preliminary data from a new regular temporal monitoring system using RADAR datasets to monitor the growth of glacial lakes in Nepal.
“Communities and businesses are still reeling from the devastating 1985 Dig Sho GLOF which is also found in the same basin. We anticipate that the impact of a GLOF in Imja Sho will be six times greater, and will extract a heavy toll on heavily populated settlements downstream, not to mention the devastation it would bring to infrastructure and agricultural lands. There will be a high human and economic cost,” said Surendra Shrestha, Regional Director of the UNEP Regional Office for Asia and the Pacific.
Glacial floods are a regular occurrence in the Himalaya region with varying degrees of socio-economic impact. Their impact can be quite extensive since they destroy villages, agricultural lands, roads, bridges, hydropower, trekking trails as well human lives and property. Again, as mentioned above, the big example is the Tibetan Zhangzhangbo GLOF in 1981 that caused extensive infrastructural damage and nearly US$3 million in losses. The Dig Tsho GLOF in Nepal in 1985 destroyed a power plant (with a loss of US$1.3 million), destroyed homes and lands and many losses of lives. The Luggye Tso GLOF in Bhutan in 1994 damaged sacred areas, cultivated land and lives.
The Hindu Kush-Himalaya glaciers are also an important source of freshwater for hundreds of millions of people living downstream. Glacial retreat is also causing long term loss of natural water storage of fresh water.
“The significance of these glaciers to downstream communities is of particular importance. Changes in glacier ice or snowmelt affects water yield to downstream regions heightening the risk of water shortages, impacts irrigation water for crops and may disrupt industry and power generation,” said Dr. Andreas Schild, Director General of the International Centre for Integrated Mountain Development.
“We have to continue monitoring glaciers and glacial lakes to ensure sound management of these valuable water resources. In addition, the use of early warning systems like satellite-based techniques and mitigation measures such as dam breach and hydrodynamic modeling are important to reduce risks to vulnerable mountain populations,’ Shrestha added.