Keywords

1 Introduction

Rockfall is a sub-type of the phenomena related to as landslide in which a single rock or boulder or a small groups of independently moving rocks become dislodged from an exposed face or slope and move downward by means of some combination of sliding, rolling, bouncing or free-fall under the action of gravity (Varnes 1978). In some cases rockfall refers to quantitative measure, describing small phenomena ranging from few m3 up to 10,000 m3 events, whereas rockslide is used to describe more than 1,00,000 m3 and rock-avalanches can reach several millions m3 (Dussauge-Peisser et al. 2002).

Keefer (2002) studied 40 worldwide historical landslides induced by earthquakes and found rockfalls to be the most abundant of all type of landslides, along with disrupted soil slides and rockslides. According to World Bank data, 95 % of the deaths occurred in developing and under-developing countries due to natural disasters i.e. earthquakes, landslides, floods and rock-falls etc. (Kremier and Arnould 2000).

2 Causes of Rockfalls in India

There are 14 causes of rockfall. Out of 14 causes, 6 are directly related to water, namely rain, freeze-thaw, snow-melt, channeled runoff, differential erosion, and springs and seeps. Also growth of tree roots in cracks is indirectly related to rainfall that can open the cracks and loosen the rock blocks. These seven causes of rock-fall together account for 68 % of the total falls (Hoek and Bray 1981) (Fig. 87.1).

Fig. 87.1
figure 1

Pie chart showing causes of rock-falls (modified after Hoek and Bray 1981)

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India, due to its unique climatic conditions and its closeness to geodynamical active areas, has always been vulnerable to large number of landslides due to rainfall. The Himalayas, including Jammu and Kashmir, Himachal Pradesh, Gharwal, Kumaon, Sikkim, the Darjeeling Hills, Arunachal, and the Northeast hill, are where the rate of incidence of landslides/rock-falls range from high to very high whereas the Western Ghat, Nilgris hills have high to moderate landslides/rock-falls incidences and the Eastern Ghats, Vindhyan ranges show low landslides/rock-falls incidences (Kumar et al. 2008).

About 90 % of the landslides/rock-falls takes place during monsoon or winter rains in the Northwestern Himalayas. Moreover rains play a vital role in triggering most of the landslides in the Himalayan region. From south to north, the Indian Himalayas can be divided into different climatic zones, i.e. the tropical, sub-tropical, sub-temperate, semi-arid and arid zones. Precipitation also varies across these climatic zones. Monsoon precipitation is largely received south of the Higher Himalayas as compare to north Himalayas. Snow brought by winter rains at higher altitudes is more pronounced in the western region as compare to eastern region of Himalayas.

The rainfall data plotted against the landslides/rock-falls events since 1901–2010 indicates that most of the landslides/rock-falls are associated to monsoon time during June-September (Fig. 87.2). However some of the landslides/rock-falls do occurs in the month of Dec–March (Table 87.1). Also Table 87.1 demonstrates some notable landslides/rock-falls events happened in the past in India.

Fig. 87.2
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Rainfall distributions for different states of India since 1901–2010. Also showing events of landslides/rock-falls (Source www.imd.gov.in)

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Table 87.1 Summary of landslide and rock-fall events occurred in India (Sources GSI reports and newspaper)
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3 Rainfall and Landslides in India

Indian highways, roadways and railways in mountainous and hilly areas have been experiencing horrific landslide incidences that caused rockfalls. According to Sharda (2007), 15 % of the land area is affected by landslides. Every year during the monsoon period, several region of India is affected by landslides/rock-falls. Landslide along Jammu-Srinagar National Highway in July, 1967, caused by heavy rainfall, triggered rock-fall at Bailey bridge across Panthal Nala and completely destroyed the bridge. The Malpa rock-fall tragedy of 18th August 1998 instantly killed 220 people and wiped-out the entire village of Malpa on the right bank of river Kali with the tracking route, in the Kumaun Himalaya of the state of Uttarakhand.

A few notable examples of the recent rock-falls caused by rainfall are as follows;

The recent most casualty caused by landslide, induced by heavy rainfall, occurred on 16th and 17th June 2013 at Uttarakhand. According to Indian Meteorological Department (IMD), Uttarakhand received rainfall greater than 400 % from normal rainfall. These heavy rainfalls triggered the landslide and the town of Rambara was totally washed away. Also thousands of pilgrims and local at Kedarnath town were buried under debris or flowed away by the flood caused by heavy rain.

On 25th July 2013, in the western part of India on NH-222 near Malshej Ghat, Maharashtra, rockfall triggered after the preceding day one week heavy rainfall. Giant boulders almost of two truck size, fell on two men, killing them and blocked the highway for almost a week. In a similar situation, rockfall occurred at Wadala area, Mumbai, Maharashtra. The rock-fall destroyed the house and buried it. However no causality was caused. This event too happened after very heavy rainfall.

Heavy rainfall on August 2012 in Uttarkashi was the source of flash flood and landslide that took the life of 31 peoples. Also thousands of pilgrims were trapped there due to wiped out roads. Not only has the Himalayan region suffered from casualties due to rainfall induced landslide but also northeastern part has been the witness of such type of casualties. In June 2012, the landslide triggered by heavy monsoon in Ri-Bhoi district, Meghalaya, blocked the NH-40. Also at the same district, on 26th July 2012, landslide killed two members of the family when the house was buried in the landslide. Rock-fall occurred after 18th September 2011 earthquake in Sikkim caused death to person and damage a car at Baluwakhani area.

Highways and roadways were not only affected by landslides and rockfalls in India, but railways too have history of landslides and rock-falls that caused loss of life’s and property as well as economy. The most recent devastating accident related to rock-fall along the railway occurred on 19th July 2012, when Chhatrapati Shivaji Terminus bound Kasara local, derailed from the track due to boulder on the track after the minor landslide and was rammed by speeding Vaidarbha Express causing one death and 14 others injured. This is not only the case of railway accidents due to falling blocks/boulders on the railway tracks but also we have few more in the past viz. Vaibhavwadi train crash in June, 2003 (ExpressIndia 2003) and Karanjadi train crash in June, 2004 (People’s Daily 2004).

4 Conclusion

The primary concern is to save the lives of the people living and/or travelling in rockfall prone areas. The number of death due to rock-falls is much smaller than the number of people died in stream and river and certainly much smaller than the number of people died in traffic accidents. However one big rockfall in wrong place at a wrong time, could cause dramatically increase in the number of death as describe in this article. At present, rockfall is a new filed in India and it needs more research for handling the problem related to rockfall. Due to this reason we are focusing on learning everything we can about rockfall using laser scanning, computer modeling, and monitoring of slope/cliff faces. Also efforts are being made to develop a relationship between rainfall and triggering mechanisms for rock-falls.