PARAMETERS

LANDUSE
An overall assessment of the area in terms of vegetation cover suggests that the highest incidence of landslides are seen in degraded forest areas in the upper slopes. The discontinuous soil covered zones among the rock out crops forming the plateau edges with natural degraded forests and grass lands are zones of high susceptibility. The undisturbed forest, natural grass lands and forest plantations have not indicated incidence of slides. Other plantations like coffee, cardamom, rubber etc. show incidence of slides mainly due to improper land management practices and cultivation of seasonal crops among them in vulnerable zones.
SOIL THICKNESS
Most of the slides in the study area are confined to the overburden. The soil thickness of the region varies from 0.5 to 2.5 metres. Maximum variation in soil thickness is noted in the middle zone between the eastern plateau region and western foothills, coinciding with the plateau margins. Slide intensity is maximum in this zone. This variation in soil thickness is also well reflected in other controlling factors like slope, landform, relative relief, drainage etc. The soil horizon shows dominance of sand and gravel sized particles. There is an increase in sand content towards depth providing a granular zone which aids good underground seepage.
SLOPE
Among the slope categories mapped in the area, maximum slide population is seen in the S4 (25o to 32o) category followed by the S5 (>32o) and S3 (16o to 25o). This indicates that slopes above 16o are to be evaluated with caution especially when other factors are also not disposed in a manner contributing to long term stability. The slope length also is an important factor. The overall assessment indicates that maximum slide occurrence is associated with 150 to 300m length category followed by slopes of length over 300 m which have limited occurrence in the area. Taking these two factors any slope of magnitude above 16o and over 150 m length is critically disposed as far as mass movements in this area are concerned. Further, an increase in landslide possibility is seen in such slopes which are oriented towards the south west direction.
LANDFORM
All controlling factors like slope, relative relief, thickness of overburden etc. also varies in tune with the landform and its stage of development. The middle zone which shows maximum aerial extent of side slopes with 'V' shaped youthful valleys also shows maximum variation in slope magnitude, relative relief and overburden thickness. This zone being the most active denudation zone shows maximum incidence of landslides
RELATIVE RELIEF
The relative relief distribution pattern clearly brings out the three terrain set ups viz., the plateau region, midlands and the plateau margin along with the dissected marginal zone. Predominant relative relief ranges are 100-200 m to 200-300 m in the plateau region, 0-100 m to 100-200 m in the midland region and 200-300 m to 500-600 m in the plateau margin with its adjoining dissected marginal zone respectively. The third zone indicated above shows maximum variation of relative relief. As a corollary to this it is presumed that the plateau region shows a matured state of geomorphic development compared to the midland zone. The plateau margin and associated dissected hills represents the youthful zone in this region undergoing active denudation and hence are characteristic sites for mass movements. Further, within this region the most favourable relative relief range for mass movements are 200-300 m and 300-400 m depending on other controlling factors which can modify the stability conditions
DRAINAGE
Drainage characterisation is an important factor which reflects the slope evolution of an area and an indicator of the mass wasting and related erosional aspects. Zones with the parallel pattern of drainage associated with strong slope control is the most probable situation for mass movements/landslides. Density wise, as far as the study area is concerned the range between 3-4 seems to be probable sites of slope failure depending upon the effect of other controlling factors.
RAINFALL
The study area receives a higher rainfall of + 4000 mm which is higher than the annual average computed for the state. Landslides occur when a high intensity rain follows a prolonged steady rainy season i.e. when the over burden is already saturated. This normally occurs during the middle phase of south-west monsoon or if the pre-monsoon is high in the initial phase. The north-east monsoon shows high variation especially in the high land region. The high intensity rainfall in this season associated with cyclonic storms in the east coast can trigger landslides especially if there is no dry period between south-west monsoon and north-east monsoon. It is surmised that a two days rain in excess of 300 mm during a continuous steady rainy period may induce landslides in critical areas.
GEOTECHNICAL STUDIES
Geotechnical analyses were carried out on the samples from the study area to characterise the soil types with reference to the landslide occurrence. The grain size analysis indicate the dominance of coarse particles which is typical of a hilly area. The soils are mostly immature in nature. The stress-strain characteristics indicate the uncemented nature of soil. Low cohesion and high angle of shearing resistance under natural condition promotes stability of the region. However, under wet conditions the angle of shearing resistance is drastically reduced lowering the factor of safety to less than unity.
GEOLOGY
The Predominating rock type of the area is Charnockite and its variants. Conformable bands of garnet biotite gneiss having dissemination of graphite are noted in a few localities. Late proterozoic igneous activity is represented in the area in the form of a few granite plutons. Dolerite and gabbro dykes of mesozoic-tertiary age traverse the country rocks. The geological set up of the Precambrians which cover the majority of the terrain has little influence on the debris flows except to its role in the formation, changes and disposition of the overburden which develops on it.

Natural Hazards Management
Coastal Erosion		Landslide		Earthquake	Lightning	Radioactivity	Flood	Drought

Introduction	EVALUATION STUDY IN TERMS OF LANDSLIDE MITIGATION IN PARTS OF WESTERN GHATS KERALA (A project Sponsored by Ministry of Agriculture, Govt. of India)
Study area
Methodology
Parameters
Landslide Hazard Zonation
Socio-Economic Evaluation
Conclusions
Recommendations
LANDUSE An overall assessment of the area in terms of vegetation cover suggests that the highest incidence of landslides are seen in degraded forest areas in the upper slopes. The discontinuous soil covered zones among the rock out crops forming the plateau edges with natural degraded forests and grass lands are zones of high susceptibility. The undisturbed forest, natural grass lands and forest plantations have not indicated incidence of slides. Other plantations like coffee, cardamom, rubber etc. show incidence of slides mainly due to improper land management practices and cultivation of seasonal crops among them in vulnerable zones. SOIL THICKNESS Most of the slides in the study area are confined to the overburden. The soil thickness of the region varies from 0.5 to 2.5 metres. Maximum variation in soil thickness is noted in the middle zone between the eastern plateau region and western foothills, coinciding with the plateau margins. Slide intensity is maximum in this zone. This variation in soil thickness is also well reflected in other controlling factors like slope, landform, relative relief, drainage etc. The soil horizon shows dominance of sand and gravel sized particles. There is an increase in sand content towards depth providing a granular zone which aids good underground seepage. SLOPE Among the slope categories mapped in the area, maximum slide population is seen in the S4 (25o to 32o) category followed by the S5 (>32o) and S3 (16o to 25o). This indicates that slopes above 16o are to be evaluated with caution especially when other factors are also not disposed in a manner contributing to long term stability. The slope length also is an important factor. The overall assessment indicates that maximum slide occurrence is associated with 150 to 300m length category followed by slopes of length over 300 m which have limited occurrence in the area. Taking these two factors any slope of magnitude above 16o and over 150 m length is critically disposed as far as mass movements in this area are concerned. Further, an increase in landslide possibility is seen in such slopes which are oriented towards the south west direction. LANDFORM All controlling factors like slope, relative relief, thickness of overburden etc. also varies in tune with the landform and its stage of development. The middle zone which shows maximum aerial extent of side slopes with 'V' shaped youthful valleys also shows maximum variation in slope magnitude, relative relief and overburden thickness. This zone being the most active denudation zone shows maximum incidence of landslides RELATIVE RELIEF The relative relief distribution pattern clearly brings out the three terrain set ups viz., the plateau region, midlands and the plateau margin along with the dissected marginal zone. Predominant relative relief ranges are 100-200 m to 200-300 m in the plateau region, 0-100 m to 100-200 m in the midland region and 200-300 m to 500-600 m in the plateau margin with its adjoining dissected marginal zone respectively. The third zone indicated above shows maximum variation of relative relief. As a corollary to this it is presumed that the plateau region shows a matured state of geomorphic development compared to the midland zone. The plateau margin and associated dissected hills represents the youthful zone in this region undergoing active denudation and hence are characteristic sites for mass movements. Further, within this region the most favourable relative relief range for mass movements are 200-300 m and 300-400 m depending on other controlling factors which can modify the stability conditions DRAINAGE Drainage characterisation is an important factor which reflects the slope evolution of an area and an indicator of the mass wasting and related erosional aspects. Zones with the parallel pattern of drainage associated with strong slope control is the most probable situation for mass movements/landslides. Density wise, as far as the study area is concerned the range between 3-4 seems to be probable sites of slope failure depending upon the effect of other controlling factors. RAINFALL The study area receives a higher rainfall of + 4000 mm which is higher than the annual average computed for the state. Landslides occur when a high intensity rain follows a prolonged steady rainy season i.e. when the over burden is already saturated. This normally occurs during the middle phase of south-west monsoon or if the pre-monsoon is high in the initial phase. The north-east monsoon shows high variation especially in the high land region. The high intensity rainfall in this season associated with cyclonic storms in the east coast can trigger landslides especially if there is no dry period between south-west monsoon and north-east monsoon. It is surmised that a two days rain in excess of 300 mm during a continuous steady rainy period may induce landslides in critical areas. GEOTECHNICAL STUDIES Geotechnical analyses were carried out on the samples from the study area to characterise the soil types with reference to the landslide occurrence. The grain size analysis indicate the dominance of coarse particles which is typical of a hilly area. The soils are mostly immature in nature. The stress-strain characteristics indicate the uncemented nature of soil. Low cohesion and high angle of shearing resistance under natural condition promotes stability of the region. However, under wet conditions the angle of shearing resistance is drastically reduced lowering the factor of safety to less than unity. GEOLOGY The Predominating rock type of the area is Charnockite and its variants. Conformable bands of garnet biotite gneiss having dissemination of graphite are noted in a few localities. Late proterozoic igneous activity is represented in the area in the form of a few granite plutons. Dolerite and gabbro dykes of mesozoic-tertiary age traverse the country rocks. The geological set up of the Precambrians which cover the majority of the terrain has little influence on the debris flows except to its role in the formation, changes and disposition of the overburden which develops on it.