Extreme heavy rainfall events to rise, use bioengineering to check slope instability

The landscape morphology and water resources of the Indian Himalayan region, including Uttarakhand, depend heavily on the monsoon rainfall as the amount and duration of rain affect landslides, soil erosion and sediment transport through rivers and channels. Overall, the region receives around 70% to 80% of the annual rainfall during June to September. On a long-term average, the western Himalayan region, including the whole of Uttarakhand, Himachal Pradesh and Jammu & Kashmir, receives around 275 mm of rainfall. The amount and duration of monsoon rainfall vary significantly with space and time. Due to complex terrain of Uttarakhand, rainfall in a monsoon season can vary substantially even within a few kilometres as it depends on the topography of the area and wind strength. 

Increase in earth’s surface  temperature, climate change

The earth’s surface temperature has increased at the rate of 0.13°C every decade for the last 50 years due to climate change, leading to extremely heavy rainfall-induced devastation such as the Kedarnath disaster of 2013 and the recent tragedies in Pithoragarh and Chamoli districts. Several climate modelling studies have vindicated this fact that the increase in the earth’s surface temperature has increased the frequency and intensity of rainfall, including extreme events. A cloudburst means when an area receives around 100 mm to 150 mm per hour, or greater, of rainfall often associated with hail. An event of prolonged monsoonal rainfall for a few days in a localised region with or without cloudbursts can also have a devastating impact on life and resources. From 1950 to 2015, 37 landslide-related and eight avalanche-related disasters have taken place in India, causing enormous loss of lives and damage to property and natural resources. A significant proportion of this loss can be attributed to extremely heavy rainfall during the monsoon period.

Hard to forecast cloudbursts

Uttarakhand is prone to landslide-related disasters caused by extremely heavy rainfall. In the Uttarakhand Himalayas, two high precipitation belts exist at around 800 m to 1,000 m (parts of Nainital, Almora and Bageshwar valley areas in Kumaon and Srinagar and Rudraprayag valley areas in Garhwal) and 1,800 m to 2,200 m elevation (valleys of high Himalayas such as Nainital and Munsiyari towns in Kumaon and Pauri, Joshimath, Lansdowne and Mussoorie towns in Garhwal). These regions can possibly be categorised as vulnerable to extreme precipitation-induced landslide zones. However, extremely heavy rainfall event such as a cloudburst is difficult to monitor and analyse for short-term ‘forecast’ or even ‘nowcast’. Conventionally, dense networks of rain gauges or radars are required for accurate measurement of such events which could help in developing a forecast model. Till date, very few observation facilities are available throughout the Indian Himalayan region. As indicated by the Intergovernmental Panel on Climate Change, these extremely heavy rainfall events and landslide-related disasters in Uttarakhand are likely to increase in the coming years. Moreover, the state falls in the average global landslide hazard zone category of ‘class 6’ in the one to 9 class range. 

Landslides, floods occur due to excessive rainfall

High intensity precipitation events occur frequently in the middle mountain areas (1,100 m to 2,000 m) and lower plains of Uttarakhand, causing landslides, debris flow and floods. Studies of some recent extreme weather events show that natural disasters associated with exceptionally high intensity rain occur frequently in various parts of the Uttarakhand Himalayas. Earthquakes, rainfall, undercutting of slopes either by alluvial incision or human action (deforestation, road cuts, house foundations, uncontrolled waters, etc.) trigger landslides in active mountains with contrasted climate conditions. Heavy rainfall is one of the main factors of slope instability as it affects surface run-off, infiltration, depth of saturated soil and thus influences soil-moisture condition, cohesion and angle of internal friction. The rock type and structure also play a crucial role in landslide-related disasters. Plutonic or strongly metamorphosed rocks are less likely to cause landslides, whereas the newly formed sedimentary rocks are susceptible to landslips. The Himalayas being situated at the junction of two tectonic plates, Indo-Australian and Eurasian, with numerous active geological faults, a seismic trigger is also a significant contributor to landslide-induced disasters. 

Growing settlements in river valleys cause for concern

Natural disasters associated with extreme weather events are widely distributed in the Uttarakhand Himalayas and are showing an increasing trend. The loss of life and property as well as damage to scarce agricultural land and costly infrastructure due to such disasters are also on the increase. This is due to the recent growth of settlements and physical infrastructure in alluvial plains and river valleys (Srinagar and Bageshwar valleys), which provide opportunities for economic development but are also more vulnerable to disasters. An improved knowledge of the frequency, magnitude, causes and consequences of extreme events is essential to develop short and long-term mitigation measures. However, long-term monitoring of climatic, hydrological and other critical parameters in selected river basins in the Himalayas could help in understanding the underlying processes and developing mitigation strategies. The disaster risk reduction measures must include development of sustainable adaptation policies for vulnerable mountain societies, addressing structural inequalities of buildings in populated areas, alleviating poverty and providing access to resources. For any habitation in a river valley, historical high flood level should be considered. 

Need for hill town planning

The bioengineering technology can be successfully implemented by using vegetation such as bamboo check dam along with civil engineering structures to reduce instability as well as soil erosion and control landslides in Uttarakhand. The bioengineering measures provide mechanical support to soil and control the retention and movement of water. There is an urgent need of hill town planning and architectural norms, regulating footfall of tourists and pilgrims in sensitive areas, promoting ecotourism and regulating commercial tourism, construction of ecologically safer roads and best practices for sustainable development in the Himalayan region.