Proper irrigation methods can reduce landslide risk: Study

  • Bhawani Negi, Hindustan Times, Shimla
  • Updated: May 06, 2015 20:05 IST

Considering the area's topography, scientific agricultural or horticultural activities and suitable irrigation methods could help address the landslide problem in Urni village in Kinnaur district, according to a recent study conducted by the Geological Survey of India. The study suggested this would not only help in preventing landslides in the village but also in the entire Sutlej river valley.

The study observed drainage systems originating from the agricultural or horticulture area tend to converge to a single point and this single point is just above the crown of the Urni landslide where the seepage water acts as lubricating agent in this structurally fragile slope with thin soil cover thereby triggering the landslide action.

Aided by satellite imagery interpretation studies and geological filed observations, it has been observed that the present area of landslide is a result of reactivation of the old Urni landslide, due to change in the rainfall rains and snowfall pattern in the last two years.

Satellite images as well as the ground truth surveys shows traces of the old landslides for about 100 m on the left bank of the Sutlej river, where as there is only a few landslide on the right bank and major one being the present Urni landslide.

However, in comparison to the left bank, the right bank, near where National Highway 5 runs, is more stable and compact at the bottom of the valley along the river. Besides, the highway has been widened at the bottom of this location having steep slope which has destabilized the old landslide due to the cutting of the slope.

The study therefore suggested the landslide area could be avoided by constructing a small diversion tunnel in the hard compact and massive granitic gneissic complex rock. This would reduce the present make shift diversion road length from 25 km to 6.5 km between Tapri and Choling.

The study further maintained agricultural fields required regular irrigation, which in turn saturated the thin soil cover in the steep sloping surface and as the agricultural slope is steep with thin soil cover the soil gets saturated with water which increases the weight of the soil mass, thereby triggering the landslide action due to weak angle repose.

By reducing irrigation in the horticulture or agricultural fields would certainly reduce the phenomenon of landslide immediately below such agricultural fields in the steep sloping surfaces especially in those areas where there are traces of old landslides.

It was observed that along the entire length of the road from Tapri to Choling about 6.5 km in length on the left bank of the Sutlej the present landslide location is the only one having largest area under agriculture or horticulture activity in steep gradient slope at about 45 degree slope immediately above Urni landslide.

Satellite imageries revealed this phenomena had been widely observed in the entire Kinnaur valley, where agricultural or horticultural activities are practiced on steep slopes having this soil cover and where unregulated irrigation was practiced using high-density polyethylene irrigation pipes.

Kinnaur district, which is located in the higher Himalayan region, comprises of metamorphic rock sequence, geographically it lies in the rain shadow region even during monsoon region. One of the factors causing instability in the rocks is fluctuation of soil moisture contents in the steep slopes.

This may be caused by interference with natural drainage conditions like increase in soil moisture due to excessive irrigation in sloping terraced fields or by unseasonal rainfall or water flow from melting snow.

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