Ignorance of traditional practices under the lure of economic opportunities, comfort, and convenience have aggravated the menace of landslides in the Himalayan region.
The Himalayan region has always been prone to a number of hazards that are generally owed to its unique tectonic and geomorphic setup as also evolutionary history, and meteorological conditions. Stabilisation of southwest monsoon over the subcontinent with the rise of the Himalayan mountain range ensured bountiful rains in this region but these were not uniformly distributed throughout the year. So the Himalayan region has been prone to drought, together with flash flood, landslide, and earthquake.
The communities settling in this region were repeatedly affected by these hazards. Sometimes they managed to escape the impact but these hazards often took heavy toll of their population, and assets so painstakingly created.
Except for earthquake that remained a big mystery until recently or till the later half of the previous century, the reasons of other hazards were not hard to understand for these inquisitive, and observant people who had mastered the art of recharge zone management, and water harvesting quite early. These people understood that water played a major role in the occurrence of these hazards.
Though looked upon as being a curse for the Himalayan region even today, particularly because of recurring huge losses incurred by these, landslides constitute an important slope modifying process in the hills, and there is nothing abnormal in their occurrence. Landslides promote soil formation, and agriculture in the mountains is often done over old stabilized landslides.
Technically, landslide is a downslope movement of rock, soil, and debris under the impact of gravity. The Law of Inertia however mandates all objects to remain where these are, and it applies as well to the loose, and unstable rock mass on the hill slope. A landslide therefore cannot initiate on its own without an external trigger. In case of earthquake induced landslides this trigger is provided by ground shaking, but most landslides do take place without ground shaking.
Majority of landslides however occur during the rainy season, and the presence of water changes the dynamics of forces acting on the debris, and rock mass resting on the hill slope; increased weight and enhanced pore water pressure together with reduced frictional forces tilt the balance in favour of forces acting in the downslope direction or forces trying to destabilise the debris, and rock mass. Presence of water thus provides the trigger for landslides to initiate.
Water is often present in the landslide debris in varying proportions but the movement could also take place under dry conditions. So for landslides to occur under dry conditions there has to be some other trigger. Toe erosion by streams and rivers as also toe cutting by anthropogenic action could well be such a trigger.
The term landslide encompasses five modes of slope movement; fall, topple, slide, spread, and flow. These are further subdivided according to the type of geologic material (bedrock, debris, or earth). Debris flows (commonly referred to as mudflows or mudslides), and rock falls are the examples of common landslide types.
The people residing in the Himalayan region paid particular attention on enhancing water availability, and thereby understood the intricacies of the recharge–discharge relationship of water. This became possible only with keen observation of nature, and their surroundings where landslides took place on routine basis, particularly during the monsoon period.
Landslides and the indigenous people
Landslides have always been occurring in the hills, these mostly accompanied rains, and often took toll of the resources of the hill people. Landslides were thus a cause of concern, and therefore people started to pay particular attention on factors that could trigger slope instability.
When do landslides take place? Which areas are affected more and why? Why are some areas free of landslides? Why do landslides accompany rains? Besides attempting answers to these queries, these people also maintained record of landslide occurrences in their proximity.
Observing, recording, analysing, and correlating; these people understood the relationship between excessive rains, ensuing saturation of soil mass, and the occurrence of landslides. With their experience, and accumulated knowledge of generations these people thus deduced that landslides can be prevented by avoiding presence of excess water on the hill slopes, or by quickly draining off the same.
Landslides often take place at a certain place repeatedly; landslides at Kaliasaur, Banswara, Pagal Nala, and Lambagar have been active for decades. Taking clue from this, and analysing the records of previous landslide incidences so meticulously maintained for generations, and correlating these with slope conditions, these people developed an algorithm to comment on the susceptibility of slope failure or they developed the art of identifying slopes vulnerable to landslides. Armed with this knowledge, and understanding these people developed ways of averting landslides, and minimising losses caused by these.
Landslide mitigation measures
Settlement over higher ground
The economy of the Himalayan region has always been dependent upon agriculture, and pastoralism.
People of the region reared large herd of animals that provided milk, meat, wool, and manure besides draught, haulage, and transport power. These animals were generally left for grazing in the community pastures, and forests surrounding the villages. Importance of pastoralism in the economy of the region can be better appreciated from the fact that some communities of the region engaged in cross border trade with Tibet wherein sheep and yak (Bos grunniens) reared by these people were utilised for carrying various commodities across the Higher Himalayan passes. During the winter months these communities descended to the plains of Terai – Bhabhar region, and commenced their cross border journey in the summers engaging in barter with the local communities on the way. Regular migration of these communities in pursuit of trade from Terai – Bhabhar to Tibet thus rejuvinated the economy of the region.
The agriculture however has been the mainstay of the economy for most population of the region, and was mainly practiced on middle, and lower valley slopes; over alluvial terraces along the streams and rivers, and on colluvial terraces developed on the middle slopes generally over slabilised old landslides.
The sources of water in the Himalayan region are also present mostly on middle, and lower slopes. The springs and seepages generally appear in the proximity of the slope break, and these people developed dharas around these. Shallow groundwater reserves are also easy to locate and exploit in middle and lower slopes, and therefore not many naulas are located in the higher reaches.
In the lower reaches of the valley the water of streams and rivers has also been available, and these people had been utilising the water of spring fed streams for drinking purposes from the very beginning.
Despite water and agricultural lands being located in middle and lower slopes, the people of the region never settled down on the lower reaches of the valley, and preferred habitations over firm ground at higher elevations that were safe from both landslides, flood and flash floods.
Decision to settle down at higher reaches was at the same time strategically advantageous. During times when internal strife was high, one could keep track of the movement of adversaries over a large geographical area from these higher locations, and thus ensure safety of the community.
Proximity to workplace, and market still drive our choice of residence because proximity to these provides convenience, and comfort. These people however willingly accepted the challenge of traversing long distances on daily basis for various agricultural pursuits, as also for fetching water. Unlike us they thus accorded high priority to safety and security of their community over comfort, and convenience.
Settling down at higher reaches, that are generally at significant distance from streams and rivers, could however not been possible without mastering the art of groundwater exploitation, that was a breakthrough in growth and development of habitations in this rugged terrain.
Moreover, based on knowledge of seismic wave behaviour we know that the impact of earthquakes is relatively more over soft surface rather than that over rocky ground. This fact was revealed by Greek philosopher Aristotle in 350 BC, and the people living in the region might also be aware of this fact.
It is important to note out here that lately the region is witnessing a trend whereby people are leaving their traditional habitations and settling down at lower elevations, often in close proximity or rivers and streams as also over alluvial terraces that were traditionally utilised only for agriculture. This trend has been fuelled by the construction of roads that are generally aligned in the proximity of the streams and rivers. Both economic opportunities, and convenience thus lure the people to settle down in the proximity of roads. Moreover, tourists also prefer to stay close to the road head and a river front hotel or guest house thus commands premium rates.
It is added that the roads generally represent an anthropogenic trigger for landslides to initiate; adequate provisions for draining out water and supporting the uphill mass are generally not provided. These make areas in the proximity of roads generally prone to landslides.
In the previous decade large proportion of landslide as also flash flood induced losses have therefore been associated with habitations that have come up in the recent past. This warrants review of settlement pattern in the Himalayan region.
Seasonal migration
People in many areas of the Himalayan region, particularly in the higher reaches have two habitations, and the people practice migration to higher ground during the monsoon period together with their animals. This ensures safety of the people during monsoons, from both flash floods and landslides.
Terrace farming
Terrace farming is so common in the Himalayan region that we often fail to appreciate that these are actually the result of the hard labour put in by the people of the region. These terraces are often developed over old stabilised landslides.
Though done primarily for augmenting area under agriculture, and perhaps with scant regard to landslide mitigation, terracing has improved the stability of the hill slopes. Even today, terracing is the most widely used technique for stabilising landslide prone slopes, and one can only argue if the same is not motivated by the tradition of the hill people.
With high rate of migration from the region large tracts of hitherto cultivated lands presently lay barren. No one is really around to care for these barren terraces that were once routinely maintained. Wear and tear of the retaining walls often initiates mudflow from these terraces wherein the soil of the terraces starts to flow downslope. This high density fast moving fluid causes heavy erosion, and often takes the shape of a landslide at a short distance. Such incidences could have been avoided if the terraces were maintained as before.
Unbunded far flung fields
During the rainy reason people observed washing away of the top soil of the agricultural terraces making the downslope flow muddy, and turbid. The productivity of fields exposed to prolonged soil erosion was at the same time experienced to be depleted. The people therefore paid special attention towards conserving the top soil. Earthen bunds were accordingly built along the outer edges of the terraces, and grasses and shrubs were often grown over these.
Despite this these people clearly recognised that prolonged stagnation of water in the terraces due to these bunds could destabilise these. Provision of draining out water from the terraces during spells of continued rains was therefore resorted to.
It was for this reason that the people often left the far flung, and hard to manage agricultural terraces without bunds. This was part of a planned strategy of the people of this region to rule out possibility of stagnation of water in these terraces, and thus to avoid chances of their getting destabilised. For ease of draining out rainwater these far flung fields were often maintained as being outward sloping.
Jungle guls
In order to restrict the built up of pore water pressure, particularly around identified vulnerable slopes these people devised ways of disposing rainwater into the drainage channels located in close proximity through a network of horizontally disposed jungle guls (stone lines canals) constructed, and maintained for this very purpose in the upper reaches of the identified vulnerable location.
These structures prevented entry of large proportion of rainwater into the identified vulnerable slope, and thus helped in maintaining the pore water pressure within the threshold limits, and thereby reduced chances of slope instability.
Remnants of these stone lined jungle guls can be seen around Ransi and other places in Madhyamaheshwar and Kali Ganga valleys. The construction, repair and upkeep of these structures was the responsibility of the community and special efforts were put in before the onset of monsoon period to ensure their efficient functioning.
Landslide occurrence of 1998 in Madhyamaheshwar and Kali Ganga valleys can therefore be studied in the light of breakdown of the tradition of constructing and maintenance of jungle guls.
excellent way of preventing landslide.