Non-seismic landslide hazards along the Himalayan Arc

Asia is the most landslide-prone continent of the terrestrial surface of the Earth, and the southern edge of the Himalayan Arc represents a particular hotspot within this wider area. In this paper an analysis is made of the occurrence and impact of non-seismic landslides within the Himalayan Arc, using the Durham Fatal landslide database as the primary source. It is shown that the occurrence of fatal landslides in time and space is heavily influenced by the confluence of high rates of tectonic processes, the occurrence of monsoon rainfall and the presence of a vulnerable population. In the second part of the paper an example of an acute landslide hazard in the Himalayan region is presented. The 4 th January 2010 landslide at Attabad in GilgitBaltistan, N. Pakistan, blocked the Hunza river to a height of about 120 m. Over the next five months a lake with a total volume of over 450 million m 3 developed, drowning farmland and houses, blocking the Karakoram Highway and isolating about 25,000 people on the north side of the blockage. The paper presents data on the development of both the lake and of seepage through the dam, and on the way in which flow developed during the overtopping event. At the time of writing the lake was still present in the landscape, generating a substantive hazard to downstream communities and representing a huge management problem for the National Disaster Management Agency. crisis-inducing 4 th January 2010 valley-blocking landslide at Attabad in Hunza, N. Pakistan. 1.2 The Durham Fatal Landslide Database The DFLD represents an attempt to collate reliable data on the occurrence of landslides that cause loss of life. Global data are collected on a daily basis using a combination of online searches, analyses of government data, reviews of the technical and scientific literature and through direct reports from correspondents. Data are only collected for landslides that cause loss of life. For each landslide information is recorded on the date of occurrence, the location, the landslide type, the trigger, the number of fatalities and the number of people injured. In addition the database includes a free-text section that allows other details of the slide to be recorded where they are available. All mass movements are included, but floods and snow / ice avalanches are excluded. Strenuous efforts are made to validate the resulting dataset, and records are corrected where more data become available, even where this is a long time after the event. The reliability of the database is discussed in several previous publications (Petley et al. 2005; Petley et al. 2007; Petley et al. 2008) but in brief such databases can be shown to provide a good overview of the hazards, though with quite large error bars (Downton and Pielke 2005 for example). Most such databases underestimate the true hazard as result of under-reporting of smaller events (i.e. in this case those with a small number of fatalities) and those that happen in very remote locations. The concept behind such a data collection exercise requires recognition of two key aspects. First, fatalities represent the only indicator of loss that is both universal and practicable. Other indicators, such as economic loss, have a variable value around the world, or even within countries, rendering comparison between areas essentially impossible. Attempts to correct economic value according to an economic indicator such as per capita GDP have proven to be unsatisfactory (Downton and Pielke 2005). On the other hand, an indicator such as the number of landslides occurring is universal, but essentially impossible to collate across even small areas of terrain. Fatalities on the other hand are a “universal currency” in that life holds the same essential value in every environment. Thus, fatalities allow comparison of trends and distributions in both space and time (Dao and Peduzzi 2003). The second factor has been the extraordinary growth of global connectivity associated with the development of internet-based communication. In particular, the availability of web-based news coverage has meant that reports of local events are now available globally, especially when searched using news aggregator tools. Landslides that result in even a single fatality are generally reported in the local press at least, and these reports are then widely available via news aggregators. Thus, a systematic analysis allows a reasonable estimate of the impact of landslides in terms of fatalities to be gained. 2 FATAL LANDSLIDES ALONG THE HIMALAYAN ARC 2.1 Context – the Global landslide distribution As of 28 th May 2010, the DFLD has records for a total of 2,836 fatal landslide events, causing 78,354 Figure 1: Recorded NFLs worldwide between 1 st January 2006 and 31 st December 2009. Each dot represents a single NFL. The background image is a digital elevation model for which dark tones represent terrain with a high elevation.