Home > HAZARDS IN NEPAL
Flood is a recurrent problem in the Tarai as well as in the mountain regions.
In the mountainous regions, rivers are in spates during the monsoon season. Bank undercutting, inundation of the flood plains are the results. But more disastrous are the floods in the high gradient tributary streams due to cloud bursts or high intensity rainfall concentrated usually in a small catchment. Such flash floods cause triggering of landslide, deep scouring of the stream bed and the side slopes and they rapidly develop into debris flows capable of transporting several cubic meter sized boulders.
The problem of flooding in the Tarai is also high due to the high bed load, in addition to the suspended load, carried by the rivers. In the plains, almost all the rivers are widening and cutting their banks each year.
Landslide and Debris Flow
The causes of landslides in Nepal can be assigned to a complex interaction of several factors which are natural as well as human activity related. High relief, concentrated monsoon rainfall, withdraw of underlying as well as lateral supports by toe cutting and bank erosion, presence of weaker rocks, active neotectonic movements and a complex geological history, which has resulted in very intense faulting, folding and fracturation of the rocks, are the natural factors causing landslides in Nepal. But human activities are also responsible for the very high extent, and they add to the density of landslides in the country. Overgrazing of protective grassy cover, mass felling of trees leading to an unprecedented deforestation, disturbance of the hill slopes by road/canal construction, non consideration of the geologic conditions in the corresponding location, planning or designs of infrastructures etc. are some of the important anthropogenic factors leading to landslides.
There are other social causes for the greater extent of damage. Unawareness on the part of the population and the decision makers may be cited as the most important of all the anthropogenic activity related indirect causes of landslides.
All these factors set the stage for the landslide to take place. But the stability balance is usually tipped by one of the two triggers, namely; a) rainfall/cloud burst, and b) earthquake, excavation and transitory stresses.
High intensity rainfalls, which occur frequently during the monsoon season are found to have triggered many highly destructive debris slides and debris flow along the high gradient hill slope channels. Majority of the landslides in Nepal occur in the late monsoon periods. Incessant rainfall during the period, when the antecedent moisture content of the land surface reaches a certain critical stage, is found to be accompanied by landslides. Earthquake is another important landslide triggerer. Apart from developing fissures both along and parallel to the hill slopes, and thus generating the potentials for debris slides, the earthquakes are found to trigger a variety of landslide types including huge rock slide, rock fall and slumps. Some of the very big landslides have been reported to have been initiated by the Nepal-Bihar earthquake of 1934 AD.
Debris flows are frequent in the mountainous parts of the country. They are caused by deep scouring of the stream bed and side slope by a high gradient stream. Materials are the deeply weathered rock and colluvium in the middle mountains and the thick glacial deposits in the high mountains. Damming of rivers and tributaries due to landslides and debris and subsequent sudden breaching of the dam is another important phenomena for the generation of debris flows. In the higher mountains, debris flows are frequently generated by Glacial Lakes Outburst Floods (GLOFs) due to the breaching of the moraines or glacier ice damming the lakes. The debris flow travels usually to greater distances along the river valley and destroy terraces, infrastructures and settlements along its course. Destruction of bridges, hydropower facilities and other infrastructures along the Sunkoshi and Bhote Koshi rivers in 1967 and 1996 were due to debris flows.
Epidemics are in fact the number one killer in Nepal, with an average of 410 deaths per year. During 1983-2010, 22,306 people lost their lives (MoHA,2004;DWIDP, 2010). Fifty-two per cent of those deaths were caused by epidemics. The pattern is not much changed during later time as well. Lack of treated drinking water supply and poor hygiene conditions, not only in the rural areas but also in the crowded, unplanned urban settlements are the cause for the potentially high risk from epidemic. gastro-enteritis, cholera, encephalitis, meningitis, dysentery and diarrhea account for more than 50% of the total deaths due to epidemics. Break out of epidemics after a major disaster such as flood and earthquake etc. is quite frequent.
Epidemics of contagious diseases have two peaks: during the months of May and June, before the rainy season begins and in August, the height of the monsoon. Unsafe drinking water and poor sanitation are the main causes of water-borne diseases in Nepal. Waterborne diseases continue to take lives in Nepal. In fact, over 80 per cent of all illness is attributed to inadequate access to clean water supplies, poor sanitation and poor hygiene practices.
Forest fires are prevalent in the Tarai forests as well as in the forests belonging to the Siwaliks and middle mountains. Although damages by such fires have been severe in terms of environmental degradation, these have not been followed as disasters to communities.
On the other hand, disasters due to fires in settlements are common in the Tarai where most of the houses are thatched roof and clustered and there is little fire safety measures implemented. Fires are sporadic also in the mountain settlements. Most fire disasters occur during summer (April - July) when temperatures are high and strong winds occur. The large number of small fire incidences has a cumulative effect throughout the country
Storms (line-squalls) with heavy rainfall and hail are common during the summer months in the tropical and sub-tropical parts of the country. Major damage from storms has been recorded in eastern Nepal (1980) and mid-western Nepal (1983). Storm winds of even moderate velocities have major effects in the Tarai, where most of the houses and structures are lightly roofed.
The most occurring disaster in March/April and October are hailstorms that have a disastrous effect especially on agriculture. While most of the hail that precipitates from the clouds is fairly small and virtually harmless, there have been cases of golf ball sized hail that causes much damage especially to the standing crops and inflict injuries. Thunderstorms dominate the weather during May to September. A thunderstorm is normally accompanied by heavy precipitation. During wintertime, especially in December and January, the northern areas of Nepal suffer harsh snow storms.
Glacial lake outburst flood events
Apart from landslides and river erosion, the high mountains or Himalayas of Nepal, covering about 15 prcent of the country, are quite susceptible to land degradation caused by glacial lake outburst floods (GLOF). These mountains, with an average elevation of 4 500 m. on, are mostly covered with snow and ice throughout the year. Since the second half of the twentieth century, the large glaciers of the high mountains have been experiencing rapid melting, resulting in the formation of a large number of glacial lakes. This may well be a result of global warming. Almost all the glacial lakes of the Himalayas are formed by a glacier terminus dammed by moraine. These moraine dams are not geologically consolidated enough and a slight disturbance can break the balance of the structure, resulting in an abrupt release of a great amount of water and generating floods. These floods can cause serious damage to infrastructure, houses, and the environment along the flood path downstream. This phenomenon is called a glacial lake outburst flood (GLOF).
In Nepal, GLOF events have been occurring for many decades, but this catastrophic glacier phenomenon came into the limelight only after 1985, when the Dig Tsho glacier outburst took place. Investigations into the nature of glacial lakes began in the country. In 1996, the Water and Energy Commission Secretariat (WECS) of Nepal reported that five lakes were potentially dangerous, namely, Dig Tsho, Imja, Lower Barun, Tsho Rolpa, and Thulagi, all lying above 4100m. Their extent ranges from 0.6 to 1.39 sq. km. The maximum depth ranges from 81 to 131m, with ages above 30 years. A study done by ICIMOD and UNEP (UNEP, 2001) reported 27 potentially dangerous lakes in Nepal. In ten of them GLOF events have occurred in the past few years and some have been regenerating after the event. This means activities have to be planned carefully in order to avoid human-instigated triggering factors creating an outburst. A monitoring system for lakes with outburst risk should be established to avoid floodhazards.
As the northern part of the country is covered with snow peaks, avalanche is very common and sometimes it claims the life of human being as well. The avalanche of November, 1995 killed 43 people including some foreign trekkers at Khumbu and Kanchanjungha areas. In 2 January 1999 A.D. 5 people were swept away by the avalanche that occurred in Chunchet Village Development Committee Ward No. 8 of Gorkha district.
Cold Wave and Heat Wave
In Nepal, cold waves –related casualties have taken place not much in the very cold places in the high Himalayas, but in the sub-tropical Tarai. They have also caused death and injury to livestock and wildlife. If a cold wave is accompanied by heavy and persistent snow, grazing animals may be unable to reach needed food and die of hypothermia or starvation. Such cold waves can cause famines and result in numerous fatalities. November to February are the months for the cold waves.
Heat waves with rise in atmospheric average temperature well above the average of a region have been reported to induce adverse effects on human populations, crops, properties and services. This is common in Tarai region in southern Nepal.