Hills are becoming vulnerable to major land slides in future.
by
Dr. Nitish Priyadarshi
Geologist and Assistant Professor,
Department of Geology,
Ranchi University,
India.
Below are the photographs of the hills around Ranchi
the capital city of Jharkhand state of India. During my recent visit to these
hills in Patratu valley and Angada block I saw some of the areas which are vulnerable to major land slides in
coming future.
Most of the hills are highly weathered. They are steep-sided residual hills made of massive granite-gneiss. Few hills especially in Angada block and Patratu valley near Ranchi are showing the trend of landslides. Many slopes are already in delicate equilibrium and slight increase in rainfall and human interference may trigger the land slides under the influence of gravity. The flow of the Nalkari river, the tributary of Damodar river, can get blocked due to possible land slide in the Patratu valley . Nalkari river contributes water to the Patratu Dam. Excessive precipitation are the frequent cause for landslides which weakens the slope materials. Other factors that weaken slope materials also contribute to the occurrence of landslides. Ranchi receives 1200 to 1600 mm annual rainfall. Most of the hills are having steeper slopes so they are more exposed to rapid erosion than the gentler ones. Many hills of the Ranchi plateau are highly susceptible to slope instability due to the immature and rugged topography, fragile rock conditions, and high rainfall.
The Ranchi Plateau, composed mostly of gneisses and granites, is about 2000 ft. above sea level. The topography is roughly undulating with terraced slopes ending on the one hand on the banks of a local stream and on the other in local upland which may or may not have a low hill, or which may or may not consist of solid rocks exposed bare soils.
The present topographic features of Chotanagpur ( Ranchi plateau) are a clue to the past and geologists think that before the Himalayan movement started in Tertiary times Chotanagpur and adjoining areas were a low peneplain. As a side-effects of the violent Himalayan movements, parts of peninsular upland in general and Chotanagpur peneplain in particular began to be successively uplifted. The Himalayan movements occurred three times during Early and Late Tertiary and Pleistocene times and probably the Chotanagpur peneplain was also concurrently subjected to three successive uplifts.
Landslides occur when the slope (or a portion of it) undergoes some processes that change its condition from stable to unstable. This is essentially due to a decrease in the shear strength of the slope material, to an increase in the shear stress borne by the material, or to a combination of the two. A change in the stability of a slope can be caused by a number of factors, acting together or alone.
Landslides can be initiated in slopes already on the verge of movement by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
Causes of landslides in Ranchi plateau include:
1. Saturation by rain water infiltration.
2. Increase of pore water pressure by rain water infiltration.
3. Increase of hydrostatic pressure in cracks and fractures.
4. Physical and chemical weathering.
5. Deforestation, cultivation and construction.
7. Erosion by river flows.