Silent Tsunamis

Any environmental feature or process that adversely affects humans and their activities can be regarded as a hazard (Richardson and Reynolds, 2000). Glacial lake outburst floods (GLOFs) are one of these features, releasing up to 30,000m³s^-1 of water into the valley below having substantial social and economic impacts on the communities in the affected area. Although GLOF events rarely affect as many communities as many other hazards such as earthquakes, Richardson and Reynolds (2000) state that the damage in down valley areas for the local communities is of equal significance. The formation and growth of glacial lakes is directly related to glacial mass and therefore climate change and will be the focus of this post.

TYPES OF GLOF EVENTS

GLOFs are mainly triggered by two mechanisms: an outburst from a lake dammed by a terminal moraine (moraine-dammed lakes), or ice-dammed lakes, formed when an advancing tributary glacier blocks the river or discharge of a melting glacier in the main valley causing an outburst when it retreats in the summer months. Identifying how the glacial lake has been formed is highly significant, as each lake will respond differently to changes in glacial mass. Glacial retreat may cause a reduction in ice-dammed lakes in a particular region, however, the increased discharge may result in the formation of more moraine-dammed glacial lakes. Therefore acknowledging the contribution of each process to the glacial lakes present in a region is essential in order to assess how the risk from these lakes may alter with climate change in the future.

Figure 1: Lake Imja Tsho, a moriane-dammed lake in Nepal classified as 'hazardous' on risk assessment inventories (Source: Bajrachraraya et al., 2007: 35).

 

DISTRIBUTION AND RISK OF GLOFs

At some point, every country in the Himalayan region has suffered a GLOF event with 25 in Nepal since the 1930s (Hewitt and Liu, 2010). Qingha (1991) study of 17 GLOF events in the Karakoram Mountains noted that all of the events occurred between June to November coinciding with the monsoon and greatest snowmelt rates. A study by Kattelman (2003) also supports this, with all of the recorded GLOF events in Nepal occurring in July and August. As noted in a previous post, the strength and extent of the Asian monsoon has decreased in some regions of the Himalaya since the 1920s. Following the notion that precipitation contributes to the volume of water in glacial lakes, it could be hypothesised that the risk from GLOFs could decrease, supported by a decline in the number of erosion glacial lakes identified by the ICIMOD between 1960 and 2001 (Bajrachrarya et al., (2007). However, contrary to this, the majority of studies indicate a rise in the overall number of hazardous glacial lakes since the 1950s (Qingha, 1991).

Bajrachrarya et al., (2007) summarise a baseline study conducted by the ICIMOD of over 9,000 glacial lakes in the Himalayas between 1960 and 2001. Studying topographic maps of the region, the ICIMOD monitored the formation and development of glacial lakes that were larger than 0.003km² (the smallest resolution lakes could be identified using the maps available). Major glacial lakes were stated as any lake larger than 0.02 km² which contain at least 6 x 10⁵m³ of water. Focusing on the Dudh Koshi sub-basin in Nepal and the Pho Chu sub-basin in Bhutan, the study noted that 37% and 32% of the glacial lakes identified in 1960 had disappeared respectively (Bajrachrarya et al., 2007). Most of these were either minor supraglacial ponds that had merged to form a single lake, or lakes that were not glacier-fed. However, although the number of lakes had decreased, the overall area of the remaining lake, the majority of which were moraine-dammed, had grown by 21% in Nepal and 8% in Bhutan (Bajrachrarya et al., 2007). In moraine-dammed lakes, as the volume of water grows the hydrostatic pressure acting on the moraine increases and overtopping becomes more frequent. Therefore, as the area increases, the potential of an outburst event also grows (Richardson and Reynolds, 2000).

Table 1: Summary of the types of glacial lakes studies by the ICIMOD in the Dudh Koshi sub-basin between 1960-2001) (Bajrachrarya et al., 2007: 24)

Compared to Nepal where most of the glacial lakes are moraine-dammed, ice-dammed lakes contribute to a substantial proportion of the GLOF events in the Karakoram region (Quingha, 1991; Hewitt and Liu, 2011). Contrasting to other regions of the Himalayas, some of the glaciers in the Karakoram mountain range have been advancing since the 1950s. However, although hazards from moraine-dammed glacial lakes may be lower than in other regions, over 90 outbursts have occurred due to the formation of lakes impounded behind advancing and surging glaciers (Hewitt and Liu, 2011). Of these, 10 have occurred in the Yarkland basin which supports 1.8 million people, 38 million hectares of irrigated land and six hydropower stations (Hewitt and Liu, 2011). Direct costs from an outburst in 1999 were estimated at US$ 25 million, affecting over 18,700 hectares of agricultural land and the risks and potential damage will continue to increase as the Kashgar District, imitating many other populated valleys in the Himalayas, continues to be developed (Hewitt and Liu, 2011).

Hewitt and Liu (2011) state that the future risk of ice-dammed outbursts in the Karakoram region is unclear. Whilst some glaciers have been slowly retreating or have remained static, an increasing number, mainly in the highest part of the range have been advancing, including at least nine glaciers that have been recorded with ice-dammed lakes in the past. Therefore, the study predicted that the overall risk from GLOF events for communities in the Karakoram region, like most of the other regions in the Himalaya is likely to increase with climate change