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close this bookThe Himalayan Dilemma: Reconciling Development and Conservation (UNU, 1989, 295 pages)
close this folder5. Mountain slope instability: natural processes or human intervention?
View the documentIntroduction
View the documentThe nature of mountain geomorphology: what is known about slope process in densely populated mountain terrain?
View the documentEstimation of denudation rates in the Himalaya
View the documentGeomorphic processes and human interventions: summing up

Introduction

The previous two chapters dealt with questions and assumptions about processes of change in the area of forest cover, the causes and effects of deforestation, and the relationship between soil erosion and the transformation of forested hillslopes to other forms of land use. The conventional assumptions (myths) concerning the linkages between these dynamic mountain processes were heavily challenged and the argument was made that a much longer-term perspective and much more reliable data are needed.

Another group of linkages that hold together several components of the Theory of Himalayan Environmental Degradation relate deforestation on steep slopes and construction of agricultural terraces to a rapid acceleration in gullying and landslide incidence and increased soil erosion. It is claimed that these in turn produce serious deleterious downstream impacts. We ourselves (Ives, 1981; Ives and Messerli, 1981) held this assumption on the initiation of the Nepal Mountain Hazards Mapping Project in 1978; we have subsequently reversed our position.

As somewhat representative of'expatriate experts" making short visits, we were able to observe the Middle Mountains of Nepal from brief road traverses out from Kathmandu in 1978 and 1979. Like many other visitors, we timed our presence to coincide with good weather - March/April and October - and were duly impressed with the large number of landslide scars (strictly - debris flows) and gullies that obviously had engulfed significant amounts of agricultural terrace land during the preceding summer monsoon periods. Within this context the 'Kakani Phase' of the Nepal Mountain Hazards Mapping Project was set in motion. This included studies of stream channel morphology and landslide and gully dynamics (Caine and Mool, 1981, 1982), investigations concerning hazard perception of indigenous subsistence farmers and their coping strategies (Johnson et al., 1982; Gurung, 1988), and systematic mapping of land use, geomorphic features, and mountain hazards on a scale of 1:10,000 (Kienholz et al., 1983, 1984). Since the project necessitated repeated visits of considerable duration to the field area and fieldwork throughout the agricultural cycle over a five-year period, we were afforded the hitherto uncommon perspective of time. We were also able to develop communication with the subsistence farmers. This gave us a greater understanding of the landscape changes that have been occurring over several generations and an appreciation of local attitudes to dynamic slope processes and local responses to them.

It became apparent that many landslide scars are eventually re-terraced and stabilized and that irrigation systems are repaired: this is the most important stabilizing process (Figure 5.1). In certain instances the local people perceive a landslide to be a beneficial occurrence because the more easily worked earth of the landslide scar actually facilitates terrace construction. In other instances landslides are deliberately triggered by water diversions in order to facilitate new terrace construction (Kienholz et al., 1984; Sumitra M. Gurung, personal communiction, May 1985). We began to appreciate the complicated balance between slope stability, the particular stage of the agricultural cycle, rainfall incidence, availability of emergency labour, and type of terrace that is threatened. Thus our early predictions on rates of progressive land loss and estimated population growth (Caine and Mool, 1981, 1982; Ives and Messerli, 1981) had to be adjusted. We also observed that the indigenous farmers had evolved an intricate set of coping strategies that, in addition to subsequent reterracing of collapsed slopes, included changes in land use to match changes in slope stability; Johnson et al. (1982) introduced the concept of agricultural deintensification, as an adjustment to the threat of slope instability. Moreover, in strictly physical terms, it has been demonstrated that many of the bedrock types in the Middle Mountains, and specifically in the Kakani test area, undergo very rapid weathering and a high incidence of soil formation; thus they can withstand a relatively high rate of soil loss (Peters and Mool, 1983).

Over the slightly longer period (1978 87) frequent visits to Kathmandu facilitated repeat photography of original landslide scars and slope segments at different times of the year. Figures 5.2 and 5.3 illustrate only a single example. Nevertheless, it is thought-provoking to see that an inherently unstable landslide scar photographed in 1978 is almost invisible in 1987.

Furthermore, the new terraces that have been cut into the original landslide scar were supporting vigorous crops of maize and rice in August 1986 (Ives, 1987). The kind of data contained in Figures 5.2 and 5.3 are not adequate for regional extrapolation. They are introduced here to explain how experts, who depend on short-term visits, can be led to believe that the Middle Mountains are on the point of collapsing into the Ganges River when their observations are confined to a single tourist season (which is frequently the case) and lack a longer-term perspective and a close communication with the local people, particularly in the local language.

The experiences illustrated above are not considered as proof that there are no problems of landsliding, gullying and soil erosion in the Nepal Middle Mountains; most emphatically, there are. Rather, our intent is to argue that the worst scenarios that have been used to permeate the conservationist, development/ aid, and scientific literature may be exaggerations and, possibly, gross exaggerations that are emotionally or intuitively based. As human population numbers have increased over the past hundred years or so, leading to conversion of forested land to agricultural terraces on steeper and more marginal slopes, presumably more energy per unit of land is required to maintain a balance between stability and instability. The indigenous population may be losing ground' but not nearly so rapidly as has been assumed (Eckholm, 1976). Furthermore, it would be inappropriate to create the impression that subsistence farmers enjoy the landslide activity. Houses, human lives, and livestock are lost to landslides, and the terror of sleepless nights in small houses on steep slopes during periods of torrential rain is not to be dismissed lightly. Our aim here is to establish a better sense of proportion. It is proposed, for instance, that some of the most densely populated and extensively terraced land in the Middle Mountains probably experiences some of the lowest rates of soil erosion and land loss. A very real danger of soil erosion and slope collapse would arise if such areas were abandoned, a situation well known in the European Alps. Moreover, poor location, design, and maintenance of roads in this type of terrain has led to landslide regimes that are an intermittent or continual problem and source of stream sediment. If one looks closely at most photographs that show landslides, purportedly the result of deforestation and poor farming practices, one can almost invariably see that they have been initiated by a road or heavily used trail (Hamilton, personal communication, April 1987).

These more recent observations prompted a modified approach to geomorphic process studies during the 'Khumbu Phase' of the Mountain Hazards Mapping Project. Thus, in association with the production of hazards maps at a scale of I :50,000 (Zimmermann et al., 1986), Byers, Thorn, and Ives (1985), and Byers (1986, 1987c) planned for detailed soil erosion process studies at more than thirty plots through a range of altitude of over 1,000 m during the entire 1984 summer monsoon. The field experience in the Kakani area also prompted our close questioning of all the linkages relating deforestation and increased agricultural terracing to soil erosion, gullying and landslide activity.