Landslides are simply defined as the mass movement of rock, debris or earth down a slope and have come to include a broad range of motions whereby falling, sliding and flowing under the influence of gravity dislodges earth material. They often take place in conjunction with earthquakes, floods and volcanoes. At times, prolonged rainfall causing heavy block the flow or river for quite some time. The formation of river blocks can cause havoc to the settlements downstream on it's bursting. In the hilly terrain of India including the Himalayas, landslides have been a major and widely spread natural disaster the often strike life and property and occupy a position of major concern
The two regions most vulnerable to landslides are the Himalayas and the Western Ghats. The Himalayas mountain belt comprise of tectonically unstable younger geological formations subjected to severe seismic activity. The Western Ghats and nilgiris are geologically stable but have uplifted plateau margins influenced by neo- tectonic activity. Compared to Western Ghats region, the slides in the Himalayas region are huge and massive and in most cases the overburden along with the underlying litho logy is displaced during sliding particularly due to the seismic factor.
Incidences of Landslides in India
Incidences of Landslides
High to very high
Western Ghats and the Nilgiris
Modern to high
Landslides Zonation Mopping is a modern method to identify landslides prone areas and has been in use in India since 1980s The major parameters that call for evaluation are as follows:
- Slope-Magnitude, length and Direction
- Soil thickness
- Relative relief
- Land use
- Drainage- pattern and density
- Landslide affected population
Causes of Landslides
Landslides can be caused by
1. Poor ground conditions
2. Geomorphic phenomena
3. Natural physical forces
4. Quite often due to heavy spells of rainfall coupled with impeded drainage.
A Checklist of Causes of Landslides Ground Causes
- Weak, sensitivity, or weathered materials
- Adverse ground structure (joints, fissures etc.)
- Physical property variation (permeability, plasticity etc)
Ground uplift (volcanic, tectonic etc)
Erosion (wind, water)
Scour4. Deposition loading in the slope crest.
Vegetation removal (by forest fire, drought etc)
Rapid draw- down
Shrink and swell
Man- made Causes
Excavation (particularly at the toe of slope
Loading of slope crest
Draw -down (of reservoir)
Water impoundment and leakage from utilities
An overall evaluation of the pattern and nature of landslide2 occurrences in the Kerala part of Western Ghats and its corresponding eastern flank falling within Tamil Nadu reveals the following main features:
Almost all mass movements occur during monsoons (SW and NE monsoon) in the western flank of western Ghats and during occasional cyclonic events in the eastern flank indicating that main triggering mechanism is the over- saturation of overburden caused by heavy rains
There seems to be a relation between intensity of rainfall and slope failures.
Majority of the catastrophic mass movements is confined to the overburden without affecting the underlying bedrock.
Improper land use practices such as heavy tilling, agricultural practices and settlement patterns have contributed to creep and withdrawal of toe support in many cases
A common factor noticed in most of these vulnerable slopes deforestation in the recent past, cultivation of seasonal crops and increase in settlements.
In all the vulnerable slopes terracing/ contour bounding is adopted mainly to prevent soil erosion and to enhance percolation during dry season for cultivation of cash crops as well as seasonal crops. Invariably, in all these cases, natural drainage lines on slopes are blocked or modified without adequate provision for surface drainage of excess storm water during high intensity rains prevalent in the area.
In some areas developmental activities like construction of buildings, road cutting, embankments, cut and fill structures causes modification of natural slopes, blocking of surface drainage, loading of critical slopes and withdrawal to toe support promoting vulnerability of critical slopes.
In general the chief mitigatory measures to be adopted for lanslide areas are
- Drainage correction,
- Proper land use measures,
- Reforestation for the areas occupied by degraded vegetation
- Creation of awareness among local population.
The most important triggering mechanism for mass movements is the water infiltrating into the overburden during heavy rains and consequent increase in pore pressure within the overburden. When this happens in steep slopes the safety factor of the slope material gets considerably reduced causing it to move down. Hence the natural way of preventing this situation is by reducing infiltration and allowing excess water to move down without hindrance. As such, the first and foremost mitigation measure is drainage correction. This involves maintenance of natural drainage channels both micro and macro in vulnerable slopes.
The universal use of contour bunding for all types of terrain without consideration of the slope, overburden thickness and texture or drainage set- up needs to be controlled especially in the plateau edge regions. It is time to think about alternative and innovations, which are suitable for the terrain, to be set up. It need not be over-emphasized the governmental agencies have a lot to contribute in this field.
Leaving aside the 'critical zones' with settlements could be avoided altogether and which could be preferably used for permanent vegetation, the 'highly unstable zones' generally lie in the upper regions, which are occupied by highly degraded vegetation. These areas warrant immediate afforestation measures with suitable plant species. The afforestation programme should be properly planned so the little slope modification is done in the process. Bounding of any sort using boulders etc. has to be avoided. The selection of suitable plant species should be such that can with stand the existing stress conditions in this terrain.
CONTOUR TRENCHES AND STONE WALLS
- Adopted for hill slopes >20%
- Normal size: 1000 cm2 to 2500 cm2
- Continuous or interrupted
- Stone terraces and walls are adopted whether stones are available
It helps to bring sloping land into different level strips to enable cultivation.
It consists of construction of step like fields along contours by half cutting and half filling. Original slope is converted into level fields. The vertical and horizontal intervals are decided based land slope.
Approximate cost for laying the terrace is Rs. 8000/- per ha.