Landslide!

This year too, late monsoon rains brought news of devastation as mountain-sides succumbed to the pull of gravity – from the Indian north-east westward, through Bhutan, Nepal, Himachal Pradesh and beyond. The mountains are "dynamic" and up to an extent landslides are a part of the natural process, but their numbers are on the increase, say experts.

While there have been much generalisation and hyperbole about the causes and consequences of landslides, there is more study of the phenomena today, than ever before. Experts remain divided about deforestation´s role in causing landslides and on how to promote "slope stability." Although trees are widely believed to stabilise slopes, some believe that grasses provide more protection. In some cases, says one geologist, the weight of trees on a slope could actually trigger a landslide.

The hill farmer has been held responsible for much erosion leading to landslides, but, increasingly, the peasant is being regarded as victim rather than perpetrator. Simplistic theories are being discarded and more detailed studies are carried out to understand the complexities of landslides. For example, the combination of roads and irrigation canals on unstable slopes often aggravate the instability, and increases the likelihood of landslides.

This much is clear: that the variety and individual nature of landslides demand more consideration from geologists, social-scientists, engineers, planners and policy-makers. This photo essay takes a look at the several landslides that occurred in Nepal, in order to introduce some of their variety and complexity. Dates indicate when the pictures were taken.

Tagaring, Lamjung

 Courtesy – Harka Bahadur Gurung (October 1964)

This landslide was initiated by the great earthquake of January 1939 and has grown since. The slide buried the salt brine of Nun-Khani on the banks of the Marsyangdi river and swept away some of the fields of Tagaring village. Above the slide is a temperate oak forest. Hundreds of large and small landslides were triggered in east Nepal by the quake of August 1988. The Bajhang region in west Nepal is another area where earthquakes trigger many landslides.

Bajhang

 Courtesy – Amod Mani Dikshit (January 1987)

Rock strata that jut outward and downward on this mountain flank in Bajhang, along with limestone rock and the underlying clay, present an ideal condition for what geologists call "structural failure." The heavier limestone presses down upon a layer of clay, and when water seeps into the weaker clay, movement of the whole mass is imminent. Landslides are much rarer where the strata are at right angles to the slope.

Lele, Lalitpur

Courtesy – Kunda Dixit (September 1982)

Sometimes, not deforestation, nor weak soil structure, but the sheer impact of heavy rainfall, can tip the balance. Landslides tend to occur towards the latter half of the monsoon, because by then, the soil is waterlogged and much heavier. Sometimes a cloudburst – torrential rainfall concentrated in a small area – can leave behind a hillside scarred by scores of landslides, as was the case in Lele.

Bhakunday Chaur, Kabhre Palanchok

Courtesy – Dipak Gyawali (December 1988)

On this site, below the hill of Dapcha in Mathura Pati village, was once a field, flat enough to be called Bhakunday Chaur (ball field). Due to excessive grazing by livestock, the grass cover of the field disappeared about two generations ago. First, there was "sheet erosion," or topsoil runoff, followed by "rill formation" – a series of shallow depressions on the surface. With successive monsoons and continued grazing, these rills expanded to form gullies, crevasses and finally, the canyons that we see today.

Langmoche Glacier, Khumbu Himal

Courtesy – AMD (March 1986)

Landslides and avalanches are common in the high Himal, but only occasionally do they affect the lives of those in the valleys, such as when the collapse of a glacial lake releases vast quantities of water. The picture shows where the Dig Tso moraine, which held back a lake at the mouth of Langmoche Glacier, was breached. The wall of water that was released into the Dudh Kosi wreaked havoc up to 40 km downstream, taking four lives, destroying 30 houses and a hydro-power plant, below Namche Bazaar. It is thought that an avalanche of ice and rock falling into the lake led to a surge wave, which destroyed the moraine dam.

Sunkosi Valley

Courtesy – T.B. Shrestha

The numerous land slips seen in this photograph are largely due to deforestation. Some scientists believe that the loss of tree covering and binding accelerates the rate of soil erosion. When no preventive measures are taken, the most effective stabiliser of the soil seems to be the much maligned banmara weed (adenoforum euphatorium), otherwise notorious, for its proliferation and ecological uselessness. The vegetative covering of banmara softens the impact of rainfall and minmises surface erosion.

Kapurkot, Salyan Highway

Courtesy – AMD (April 1985)

The slide in this photograph was the result of road construction. The "incision" made by construction crews using earth movers and dynamite, in an otherwise stable slope, can create landslides. This happens, either due to removal of the vegetative covering, or by weakening of the rock structure. Furthermore, when the debris from small slides is cleared for the road, their natural function as a retaining wall for the slope is removed. This then induces larger slides. The economic losses caused by ill-considered road alignments, have been phenomenal.