Recent extreme weather events in the Nepal Himalayas

Transcription

1 The Extremes ofthe Extremes: Extraordinary Floods (Proceedings of n symposium held al Reykjavik. Iceland. July 2000). lalispubl.no Recent extreme weather events in the Nepal Himalayas SURESH RAJ CHALISE Mountain Natural Resources Division. International Centre for Integrated Mountain Development (ICIMOD). GPO Box Kathmandu. Nepal chalisc@icimod,org.np NARENDRA RAJ KHANAL Central Department of Geography, Trihhnvan University, Kirtipur, Kathmandu. Nepal Abstract Extreme weather events and disasters induced by them seem to be on the increase in the Himalayas in recent years. If glacier lake outburst floods (GLOF) occur more frequently in the high mountains, then floods and landslide events become more frequent in the middle hills and the lower plains. Such disasters associated with extreme weather events cause widespread damage to life and property and destroy valuable infrastructure in the mountains and the adjoining plains every year, severely affecting the pace of development in Nepal. Although, precipitation is the principal triggering factor for such disasters, the fragile and active geology of the Himalaya Mountains, extremely steep slopes and the recent indiscriminate changes in land use and land cover further exacerbate the problem. Key words extreme weather events; natural disasters; glacial lake outburst Hoods; landslide damming; Hoods; Nepal; physiography; Himalayas INTRODUCTION The Nepal Himalaya Mountains are one of the high-energy environments of the world. A combination of active tectonics, rugged topography and highly seasonal and intensive precipitation has made this fragile environment vulnerable to hazards and disasters. Because of steep gradients, the rivers are energetic, producing high runoff and sediment discharge. Intense pressure on natural resources due to increasing population, mass poverty and subsistence agriculture, and recent development of infrastructure such as roads and irrigation canals without proper safety measures, have further exacerbated the problems of surface runoff, erosion, landslides, sedimentation and floods caused by extreme weather events. This paper aims to discuss the types, frequencies and magnitude of disasters associated directly or indirectly with extreme weather events. EXTREME PRECIPITATION EVENTS Extreme weather events associated with heavy rainfalls are the principal cause of natural disasters in Nepal. The average area-weighted annual precipitation for Nepal is about 1630 mm, with half of the country lying within the mm precipitation zone (Chyurlia, 1984). Both temporal and spatial variations in the precipitation are

2 142 Suresh Raj Chalise & Narendra Raj Khanal pronounced. Nearly 80% of the total precipitation occurs during the monsoon season between June and September, followed by 8% during the post-monsoon (October- January), and 12% during the pre-monsoon season (Chyurlia, 1984; Chalise et ah, 1996). Orography causes strong spatial variations in the precipitation. The recorded average annual precipitation ranges from only 163 to 5244 mm. At present there are only 264 pluviométrie stations in the country with more than five years of records, whereas a minimum of stations is required for a good representation of the country as a whole (Chalise et al., 1996). Diurnal rainfall exceeding 200 mm within 24 h producing simultaneous disturbances of the equilibrium of both the slopes and channel at local and regional scale are common in the country. More than 19 daily precipitation events exceeding 400 mm have been recorded from different parts of the country between 1959 and 1993, of which three events had precipitation exceeding 500 mm (Khanal, 1995). An analysis of 24-h rainfall events with rainfall amounts exceeding 100 mm for the months June-October for and has shown an increasing trend in hazardous rainfall events in Nepal (Table 1). This trend in the number of extreme rainfall events during the monsoon in Nepal is in agreement with a recent review on potential impacts of climate change in the Hindu Kush-Himalaya Mountains (Chalise, 1994). Table 1 Comparisons of 24-h rainfall events (June-October) under different categories between and Period 24-h rainfall amount (P in mm) and no. of events (TV) under each category: P,: P 2 : P 3 : P 4 : > AY = 802 AY =130 AY = 22 AY = AY'= 938 AY'= 210 AY' = 29 AY' = 8 DISASTERS INDUCED BY EXTREME WEATHER EVENTS IN NEPAL Disasters induced by extreme weather events occur frequently in the country and can be divided into four types: (a) disasters associated with short duration high intensity precipitation; (b) failure of landslide dams after continuous heavy rain; (c) glacier lake outburst floods (GLOF); and (d) extreme flood events triggered by infrastructure development, and its failure. Landslides, debris flow and all types of floods are directly or indirectly triggered by extreme weather events causing widespread damage. Estimated annual loss from landslides, floods and avalanches in the country as a whole between 1983 and 1998 ranged from 21.2 million Rupees (0.3 million US dollars) in 1991 to 4904 million Rupees (70.1 million US dollars) in 1993 with average annual loss of million Rupees (10.7 million US dollars). Nearly 230 persons are killed by these hazards every year in Nepal (Khanal, 1999). From north to south Nepal can be divided into five longitudinal physiographic regions viz. the High Himal, the high mountains, the middle mountains (hills), the Chure (Siwalik), and the Terai (plain) (Fig. 1). As the geology, topography and climatic conditions in these regions differ, the responses to extreme weather events also differ

3 Recent extreme weather events in the Nepal Himalayas 143 Fig. 1 Physiographic regions of Nepal. from region to region. High fluxes of sediment are produced particularly in the High Mountain in the north in glacier and periglacier areas by snow, ice and frost. Periodic fluctuations in climatic conditions and advance and retreat of glaciers have produced several hazardous glacier lakes in the High Himal. Landslides, debris flow and floods associated with extreme weather events are more frequent in the middle mountains and Chure Hills whereas flash floods are common in the Terai. Extreme events in the High Himal: glacier lake outburst floods (GLOF) Glacier lakes are common in the High Himal of Nepal. The general retreat of glaciers in this century in the Himalaya Mountains has resulted in many glacier lakes (Yamada, 1998). Both moraine dammed and ice dammed lakes are commonly found in Nepal. Floods from ice dammed lakes are characterized by a more gentle rise in the discharge and moderate peak discharge whereas a moraine dammed lake outburst discharge reaches extremely high peaks with flood fronts sometimes reaching m in height and consisting of water, debris, boulders and trees moving at a very high speed. As both ablation and accumulation of snow and ice occur during the monsoon months in the Himalayan glaciers, unlike elsewhere, the risk of GLOF is very high during a monsoon in Nepal. So far 15 GLOF events have been reported which either occurred or extended into Nepalese territory (Khanal, 1997; Yamada, 1998). Of all the high-risk glacier lakes, Lake Tsho Rolpa located at an elevation of 4580 m a.s.l. in the Rolwaling Himal has been identified to be in imminent danger of bursting. It has grown from only cm" in to 1.65 km 2 in The estimated volume of water in the lake at present is 100 million m 3. A worst-case scenario estimate of the peak discharge is more than 7000 m 3 s" 1 and the downstream impact can be up to 100 km (Department of Hydrology and Meteorology, 1996). Nearly people in 17 villages and the 60 MW Kliimti hydropower station located about 65 km downstream from the lake and other

4 144 Suresh Raj Chalise & Narendra Raj Khanal constructions along the river are at risk. Mitigation works for immediate, short- and longterm protection are being undertaken by the Department of Hydrology and Meteorology of His Majesty's Government of Nepal, with support from The Netherlands Government. As an immediate measure the Meteor burst early warning system has been installed at 19 places in 17 villages. As a short-term measure, the hydraulic siphoning technique is used to lower the water level. In addition a 6.4-m-wide and 3-m-deep open channel is under construction, which is considered a safer and better option than the siphoning system. In the long tenn the lake level will be lowered by 20 m to avoid the risk of GLOF more or less permanently. The engineering work, which is the first of its kind in the entire Himalaya Mountains, is not only costly but also extremely challenging, as the project has to be carried out at a very high altitude in an area without roads. An early warning system has also been proposed for Khumbu (east Nepal) by using VHF radio to protect the people from the risk of GLOF from Lake Imja glacier lake. However, such an early warning system, which was adopted in Bhutan, did not appear effective in reducing the risk at the time of the outburst in 1994 (Kattelmann & Watanabe, 1998). As the risks of GLOF are likely to increase due to increasing occurrence of extreme weather events as well as due to increasing investment in physical infrastructure in areas downstream of glacier lakes, a combination of various options of mitigation measures needs to be adopted. Disasters associated with extreme events in the middle mountains and lower plains High intensity precipitation events occur frequently in the middle mountains and lower plains causing landslides, debris flows and floods, destroying life and property. Studies of some recent extreme weather events and the damage caused by them are summarized in Table 2. These studies show that natural disasters associated with exceptionally high intensity precipitation at a local scale occur frequently in various parts of the country. Landslide damming floods due to extreme weather events are also common in the middle hills and the high mountains of Nepal. Narrow river channels are temporarily blocked by landslides, debris flows and also by the rivers themselves at confluence points. Breaching of dams formed by landslides and debris flows causes damage along the river channel not only in downstream areas but also in some pounded areas upstream. More than 11 events of such landslide damming floods have been reported from different parts of the country between 1970 and 1987 (Khanal, 1997). Floods triggered by infrastructure development are also on the increase due to rapid development of infrastructure without an adequate understanding of extreme events, their recurrence and potential impacts. For example, check dams and embankments in the Butwal area (west Nepal) which were constructed after the flood of 1970 collapsed in 1981 and 41 people, 120 houses, two mills and one bridge were swept away. Similarly, the collapse of check dams in the Rapti River in 1993 caused the loss of life of 24 people and damage to 2206 houses in Chitwan (central Nepal). Gaur, the headquarters of Rautahat district in the Terai has been suffering from floods triggered by the construction of a high ring dam in India. Almost every year thousands of families in Saptari and Sunsari (southern districts bordering India) districts are affected by the flood triggered by the Kosi barrage.

5 Recent extreme weather events in the Nepal Himalayas 145 Table 2 Recent disasters due to extreme ofthe extreme events in Nepal. River Date Precipitation event and basin(s) spatial coverage Lele: 30 Sepsouth of tember Kathmandu 1981 Bagmati and Narayani: south of Kathmandu July 1993 Larcha: 22 July northeast of 1996 Kathmandu Andhikhola: 30 south of August Pokhara 1998 (west Nepal) Precipitation of exceptionally high intensity occurred, but it was not recorded. 540 mm rainfall in 24 h on 19 July with maximum intensity of 70 mm h" 1 and recurrence interval of less than 100 years. Such high intensity precipitation occurred over 530 km 2. Similarly, on 20 July, the 24-h precipitation was 483 mm covering about km 2. Continuous rainfall for three days. Volume not recorded. More than 238 mm of rain in 24 h with a spatial coverage of 55 km". Damage and losses Nearly 47 shallow landslides per km" were initiated. Twenty-seven people, more than 48 houses, eight shops and seven water turbines were swept away. Debris flows damaged virtually all the agricultural land within the basin. Many shallow and deep landslides (more than 47 landslides per km 2 ) were initiated on the mountain slopes causing flash floods in the lowland area. Nearly families in mountain areas and families in lowland areas were affected, 160 persons in the highland and 815 persons in the lowland areas were killed. Structures such as the Kulekhani hydroelectricity power plant, Bagmati barrage, roads, bridges, irrigation canals and check dams were severely damaged. Debris flows originating about 7 km upstream destroyed the settlement at Larcha. Eighteen houses and 54 persons, roads, bridges, transmission lines and water mills were swept away. Many landslides were initiated. In some subdrainage basins up to 28% ofthe hillslope was affected by landslides. Flood height increased up to 6.2 m and channel width increased from 10 m to 120 m destroying houses and agricultural land along the rivers. Fifty-five persons were killed and 640 houses were destroyed. The estimated total loss was million Rupees (4.8 million US dollars). Sources: Manandhar & Khanal (1988); Dhital et al. (1993); International Centre for Integrated Mountain Development (1996); Khanal (1999). DISCUSSION AND CONCLUSIONS Natural disasters associated with extreme weather events are widely distributed in Nepal and show an increasing trend. The losses of life and property as well as the damage to scarce agricultural land and costly infrastructure due to such disasters are also on the increase. This is due to recent growth in settlements and physical infrastructure in alluvial fans and river valleys, which provide opportunities for economic development but are also more vulnerable to disasters. Risk assessment for different types of hazards, introduction of safety measures, guidelines for proper land use and construction standards and regulations are urgently needed for better preparedness and reducing the losses. An improved knowledge of the frequency, magnitude, causes and consequences of extreme events is also essential to develop short- and long-term mitigation measures. In the absence of long-term hydrometeorological data, it is difficult to introduce early warning systems and develop standards for infrastructure to reduce risks and losses. The main critical parameter that needs to be observed in the finest details is the

6 146 Suresh Raj Chalise & Narendra Raj Khanal precipitation, which is the principal triggering factor. However, measurement of precipitation in the mountain areas is difficult and its validity for generalization is limited. A denser observation network will surely help but has implications of costs and physical accessibility. Long-term monitoring of climatic, hydrological, and other critical parameters in selected river basins in different physiographic regions of the country could help in understanding the underlying processes and develop mitigation strategies. Data generated from such monitoring works will also help in understanding the impacts of global warming on the country. REFERENCES Chalise, S. R. (1994) Mountain environments and climate change in the Hindu Kush-Himalayas. In: Mountain Environments in Changing Climate (ed. by M. Beniston), Routledge, London. Chalise, S. R., Shrestha, M. L, Thapa, K. B., Shrestha, B. R. & Bajracharya. B. (1996) Climatic and Hydrological Alias of Nepal. International Centre for Integrated Mountain Development, ICathmandti. Chyurlia, J. I'. (1984) Water Resources' Report. Land Resource Mapping Project, HMG/Nepal and Government of Canada. Kenting Earth Sciences Ltd, Kathmandu. Department of Hydrology and Meteorology (1996) Glacier lake outburst Hood study of the Tamakoshi basin. Unpublished report. His Majesty's Government of Nepal, Kathmandu. Dhital, M., Khanal, N. & Thapa, K. B. ( 1993) The role of extreme weather events, mass movements, and land use changes in increasing natural hazards: A report of the preliminary field assessment and workshop on causes of the recent damage incurred in south-central Nepal (19-20 July 1993). International Centre for Integrated Mountain Development, Kathmandu. International Centre for Integrated Mountain Development (1CIMOD) (1996) A preliminary field assessment of the debris flow disaster of July 22, 1996 at Larcha. Unpublished report. 1C1MOD. Kathmandu. Kattelmann, R. & Watanabe, T. ( 1998) Approaches to reducing the hazards of an outburst flood of Imja glacier lake, Khumbu Himal. In: Ecohydrology of High Mountain Areas, Proceedings of the International Conference (ed. by S. R. Chalise, A. Herrmann, N. R. Khanal, II. Lang, L. Molnar & A. P. Pokhrel ) (Proc. Int. Conf. Kathmandu, March 1996), Mountain Natural Resources Division, International Centre for Integrated Mountain Development, Kathmandu. Khanal, N. R. (1995) The 1993 extreme event in Nepal and its consequences. The Geographers Point, vol. IV (1 and 2), Centre for Nepalese Geography, Kathmandu. Khanal, N. R. (1997) floods in Nepal. In: River Elood Disaster (Proc. 1CSU SC/IDNDR Workshop, 1996), 33^16. German IHP/OIIP National Committee, Koblenz. Khanal, N. R. (1999) Study of landslides and flood affected area in Syangja and Rupandehi districts of Nepal. Unpublished report submitted to Mountain Natural Resources Division, International Centre for Integrated Mountain Development, Kathmandu. Manandhar, I. N. & Khanal, N. R. (1988) Study on landscape processes with special reference to landslides in Lele watershed, central Nepal. Unpublished report submitted to Research Division, Tribhuvan University, Kathmandu. Yamada, T. ( 1998) Glacier Lake and its Outburst Elood in the Nepal Himalaya. Monograph no 1, Data Center for Glacier Research, Japanese Society of Snow and Ice, Tokyo.