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1 November 2015 Glacial Lake Outburst Flood Risk in the Poiqu/Bhote Koshi/Sun Koshi River Basin in the Central Himalayas
Narendra Raj Khanal, Jin-Ming Hu, Pradeep Mool
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Abstract

The Himalayas have experienced several glacial lake outburst floods (GLOFs), and the risk of GLOFs is now increasing in the context of global warming. Poiqu watershed in the Tibet Autonomous Region, China, also known as the Bhote Koshi and Sun Koshi downstream in Nepal, has been identified as highly prone to GLOFs. This study explored the distribution of and changes in glacial lakes, past GLOFs and the resulting losses, risk from potential future GLOFs, and risk reduction initiatives within the watershed. A relationship was established between lake area and volume of lake water based on data from 33 lakes surveyed within the Hindu Kush Himalayan region, and the maximum possible discharge was estimated using this and other previously developed empirical equations. We recommend different strategies to reduce GLOF risk and highlight the need for a glacial lake monitoring and early-warning system. We also recommend strong regional cooperation, especially on issues related to transboundary rivers.

Introduction

Glacial lake outburst floods (GLOFs), resulting from the sudden release of water from lakes impounded by moraine or ice dams, can be a major hazard in high mountain areas. Many moraine and ice dams are comparatively weak and can fail suddenly, resulting in the release of a debris-filled flood wave. Such flooding often results in injury and loss of life as well as serious damage to property and livelihoods far downstream (Hewitt 1982; Haeberli 1983; Ives 1986; Vuichard and Zimmermann 1987; Xu 1988; Ding and Liu 1992; Watanabe and Rothacher 1996; Dwivedi et al 2000; Richardson and Reynolds 2000; Carey et al 2012; NEC 2012; Khanal et al 2013; Liu et al 2013). The Hindu Kush–Himalayan (HKH) region has experienced many GLOF events over the years. At least 14 GLOF events have been reported that originated within Nepal (ICIMOD 2011), 30 in the Tibet Autonomous Region (TAR) in China (Liu et al 2013), and 21 in Bhutan and adjacent areas in the TAR (Komori et al 2012). GLOFs are becoming more frequent in the Pakistan Himalayas (Rehman et al 2013); there were 5 GLOFs in the Hunza basin of the Karakoram during 2007 and 2008, which severely affected nearby communities and pose a threat for the future (Ashraf et al 2012).

A small (statistically insignificant) increase in GLOF events in the Himalayas between 1940 and 2000 has been reported (Richardson and Reynolds 2000), but it is still not possible to determine whether there has been an increase in such events in recent years (Komori 2012). Observations in the Himalayas show a warming trend, increasing temperature extremes, and recession of mountain glaciers (Field et al 2012), and all projections for the 21st century show glaciers continuing to lose mass. Expansion or formation of lakes as a result of ice melt at the margins of receding glaciers increases the likelihood of GLOFs (Barros et al 2014; Field et al 2014). Climate change played a major role in the substantial increase in glacial lake area in the eastern Himalayas (Bhutan and Nepal) between 1990 and 2009 (Gardelle et al 2011), where the hazard from moraine- and ice-dammed lakes continues to increase (Field et al 2014).

The Poiqu watershed in TAR, China, is highly prone to GLOFs. The Poiqu River extends into Nepal, where it is known as the Bhote Koshi as far south as Barhabise and then as the Sun Koshi. Five GLOF events have been reported in this watershed since 1935. Wheat fields were damaged and several yaks were swept away by a GLOF from Taraco glacial lake on 28 August 1935 (LIGG et al 1988). Two GLOF events have been reported from Zhangzambu (Ci-Ren-Ma-Co) glacial lake, 1 in 1964 and 1 in 1981 (LIGG et al 1988; Xu 1988). The latter resulted in the destruction of Quxiang village and a highway bridge in China and damage to 47 houses, 12 bridges, 27 km of road, 1 of the gates in the Sun Koshi hydropower dam, and a transmission line in Nepal. In Nepal, 5 people were swept away, the power supply was cut for 31 days, traffic was blocked and trade disrupted for 36 days, and transport services were affected for 3 years; total losses were estimated to be close to US$ 4 million (Khanal and Acharya 2008). The maximum discharge in the 1981 GLOF was estimated to be 15,920 m3/s 23 minutes after bursting. The flood lasted for 60 minutes, and the total outflow was 19 million m3 (Xu 1988). The peak discharge of 2316 m3/s (gauge height of 6.99 m) at Barhabise about 50 km downstream was 16 times greater than the average annual flood (maximum flow). The flood carried nearly 4 million m3 of mixed debris; the debris flow dammed the Poiqu River, and the water level rose to 30 m, destroying Quxiang village, located upstream of the confluence between the Zhangzambu tributary and Poiqu River (Xu 1988). Two GLOF events were also reported from Jia-Long-Co glacial lake in 2002. The event on 29 June 2002 destroyed bridges and resulted in an estimated economic loss of 3.05 million yuan renminbi (approximately US$ 370,000) (Chen et al 2013).

The glaciers in the Poiqu watershed are shrinking, and their retreat has been accelerating since 2000 (Xiang et al 2014). The rapid expansion of glacier-fed lakes has increased the risk of GLOFs (Chen, Cui, Li, et al 2007; Xiang et al 2014; Wang et al 2015). In