**2.1. Landslides in humid tropical environments**

Southeast Asia with extraordinary high rates of 5.9 per cent in Sumatra and 4.9 per cent in Madagascar. More recent estimates of gross forest-cover loss in the first decade of the 21st century indicate no reversal of these trends [39]. Recent studies indicate an increase in haz‐ ards in many regions in the tropics. These appear to be linked to changes in global climate, an accelerated and disorderly process of urbanization, deforestation and the associated loss of hydrological storage capacity, particularly in mountainous domains and to the concentra‐

However, the severity of their impact varies spatially, and the intensity and course of the response to environment changes varies in the different physiographic domains depending on the nature and severity of the change and the sensitivity of the landscape. Landscapes can be viewed as systems consisting of various interconnected components or subsystems [15]. As the subsystems tend to interact on different spatial and temporal scales via different feedbacks, they may dampen or reinforce the effects of environmental changes depending on the coupling strength existing between the system components. The crossing of thresh‐ olds, on the other hand, causes a sudden change in the landscape or in the geomorphic proc‐ esses, and the mutual operation of feedbacks and thresholds within the geomorphic system tends to induce a complex response to changes in environmental conditions. A consequence of these interactions is that the rate of change of landscapes as well as the severity of the ge‐

The understanding of developmental patterns in respect of the diverse and complex envi‐ ronmental controls and geomorphic responses in the tropics appears to be an essential pre‐ requisite for the assessment and distinction of climatically-driven and humanly-induced environmental changes as well as for the planning of technological, social and political measures and a sustainable development. The objective of this paper is to demonstrate the role of the geomorphic response to environmental changes on a variety of temporal and spa‐ tial scales. However, a comprehensive and balanced view of the wide range of geomorphic process responses to environmental change, their causes and functional relationships is be‐ yond the scope of this study. Instead, this study attempts to concentrate on the response of hillslope processes and their specific controls in the humid tropics, and, in particular, on the stability of hillslopes, on the role of surface wash processes in accelerating soil erosion, and on the role of weathering processes from the point of view of the availability of nutrients in the soils and the geotechnical properties of the weathered materials. A further objective of this study is to highlight some aspects of the role of the long-term development paths of the landscapes as this factor may provide some indication of susceptibility and of a potential re‐

tion of settlement activity in potential high-risk areas [44].

omorphic response to environmental changes is extremely variable.

sponse to environmental changes on the part of larger-scale landscape units.

The second chapter encompasses a discussion on the various factors which determine rapid mass movements in humid tropical mountains, and provides an overview of the role of ex‐ treme rainfall events in triggering landslides. As the responses of hillslopes are often predis‐ posed by virtue of long-term evolutionary processes, the chapter also includes some case

**1.2. Purpose and objectives**

4 Environmental Change and Sustainability

Rapid mass movements are important processes in mountainous landscapes and include a wide range of types and sizes of landslides and styles of movement (Table 1). Landslides have been documented in nearly all tectonic settings within the tropical area [80, 87]. How‐ ever, large single landslides and landslide events encompassing hundreds to thousands of landslides tend to occur most frequently in tectonically active mountain belts and, although with a somewhat lower frequency, in highly elevated pericratonic areas, whilst landslide events appear to be relatively rare in cratonic areas. [14, 31, 38]. The frequent occurrence of landslides in tectonically active mountains and pericratonic areas can be attributed to a spe‐ cific set of conditions, which include high escarpments, long steep hillslopes in ridge and ravine landscapes, copious rainfalls (high annual rainfall totals and high short-term intensi‐ ty rainfalls), and highly weathered surface materials [59]. Earthquakes, volcanism and rap‐ idly incising streams are further factors acting as trigger mechanisms for rapid mass movements, particularly in tectonically active mountain ranges [55, 80, 87, 12, 56, 38].


**Table 1.** Modified after [84] and [75]A simplified classification of rapid mass movements

In many tropical mountains, landslides are part of a highly dynamic hillslope system, which is characterized by high temporal variability, cyclic changes in stability thresholds and tem‐ poral tendencies of recovery. This system is superimposed by climatic conditions, the effects exerted by the tectonic regime, lithology and structure, weathering processes and the rate of river incision. Where landsliding is the dominant formative process, changes in the environ‐ mental conditions are likely to influence the response of hillslopes by causing changes in the frequency, size and style of landsliding [58, 13, 56]. The off-site effects of large landslide events are the blocking of streams and valleys with landslide debris, and rapid sedimenta‐ tion in the river channels promotes flooding in the downstream parts of the drainage basins. In a study on the impact of the hurricane Hugo in Puerto Rico it has been estimated that about 81per cent of the material transported out of the drainage basin had been supplied by landslides [47]. As events of a similar magnitude tend to occur once in 10 year the total rate of denudation due to landsliding has been suggested to range to about 164 mm/ka [47]. In Papua New Guinea, earthquakes provide an additional trigger mechanism, and estimates of denudation by landsliding indicate rates of 1000mm/ka [72, 34]. However, our understand‐ ing of the long-term contribution of landslides to the total denudation is fragmentary and the extrapolation of denudation rates to larger areas is subject to serious constraints.

Even in the case of shorter time scales, the triggering of slope failures depends on several interconnected and interacting factors. Site-specific factors such as slope, the relative relief, the degree of dissection of the landscape, the density of the vegetation cover, the geotechni‐ cal properties, the thickness of the material on the hillslopes and the intensity of land use determine the susceptibility of hillslopes to landsliding. Earthquakes and rainfall amounts are commonly the decisive triggers of landslides [76, 72, 38]. The incidence of landslides is closely associated with the timing, intensity and duration of the rainfall and the antecedent rainfall amounts [78, 1, 33]. These factors control the accumulation of moisture in the rego‐ lith and, hence, are associated with the likelihood of high pore water pressures. Hillslope steepness, on the other hand, controls the downslope directed forces and the rate of down‐ slope subsurface water flow whilst the planform of the hillslopes determines the conver‐ gence and divergence of the surface and subsurface water flow lines and controls the size of the moisture-supplying area. The thickness of the weathering cover, its weathering degree, layering and textural characteristics, on the other hand, controls the hydrologic behaviour of the slopes and the type of movement. The accumulation of water by subsurface flow and infiltration in the regolith may also control the position of landslides on the hillslopes. In Pu‐ erto Rico most hillslopes failed at an elevation range of 600 to 800m because of the supply of water from higher elevated hillslope units [71]. Similar inferences concerning the position of landslides are indicated in studies of [48]. These indicate that slope failures resulting from extreme rainstorms are triggered on the middle or lower slope units of the hillslopes whilst landslides triggered by earthquakes tend to occur on the upper slopes.

Although landslide events are closely associated with high intensity rainfall events or peri‐ ods of prolonged rainfall, there is no direct link between rainfall amount, rainfall intensity and the number and volume of landslides. Several studies have shown that rainfall events of similar order are capable of triggering different landslide volumes and of producing different landslide occurrences and landslide types [49, 37, 24]. This indicates that differ‐ ent thresholds are involved in the occurrence of landslide episodes. These thresholds are often interconnected by various feedbacks, resulting in complex relationships between the threshold of slope failure and the accumulation of moisture during the rainfall season and antecedent seasons, and the intensity of the rainfall event triggering the landslide [33, 62]. Instability thresholds of this type often depend on a number of site-specific factors. These may be associated with the impact of previous landslides on hillslope form, hillslope hydrol‐ ogy, regolith thickness, and with materials which are inherited from former landslide events.

river incision. Where landsliding is the dominant formative process, changes in the environ‐ mental conditions are likely to influence the response of hillslopes by causing changes in the frequency, size and style of landsliding [58, 13, 56]. The off-site effects of large landslide events are the blocking of streams and valleys with landslide debris, and rapid sedimenta‐ tion in the river channels promotes flooding in the downstream parts of the drainage basins. In a study on the impact of the hurricane Hugo in Puerto Rico it has been estimated that about 81per cent of the material transported out of the drainage basin had been supplied by landslides [47]. As events of a similar magnitude tend to occur once in 10 year the total rate of denudation due to landsliding has been suggested to range to about 164 mm/ka [47]. In Papua New Guinea, earthquakes provide an additional trigger mechanism, and estimates of denudation by landsliding indicate rates of 1000mm/ka [72, 34]. However, our understand‐ ing of the long-term contribution of landslides to the total denudation is fragmentary and

6 Environmental Change and Sustainability

the extrapolation of denudation rates to larger areas is subject to serious constraints.

landslides triggered by earthquakes tend to occur on the upper slopes.

Although landslide events are closely associated with high intensity rainfall events or peri‐ ods of prolonged rainfall, there is no direct link between rainfall amount, rainfall intensity and the number and volume of landslides. Several studies have shown that rainfall events of similar order are capable of triggering different landslide volumes and of producing different landslide occurrences and landslide types [49, 37, 24]. This indicates that differ‐ ent thresholds are involved in the occurrence of landslide episodes. These thresholds are often interconnected by various feedbacks, resulting in complex relationships between the threshold of slope failure and the accumulation of moisture during the rainfall season and antecedent seasons, and the intensity of the rainfall event triggering the landslide [33, 62].

Even in the case of shorter time scales, the triggering of slope failures depends on several interconnected and interacting factors. Site-specific factors such as slope, the relative relief, the degree of dissection of the landscape, the density of the vegetation cover, the geotechni‐ cal properties, the thickness of the material on the hillslopes and the intensity of land use determine the susceptibility of hillslopes to landsliding. Earthquakes and rainfall amounts are commonly the decisive triggers of landslides [76, 72, 38]. The incidence of landslides is closely associated with the timing, intensity and duration of the rainfall and the antecedent rainfall amounts [78, 1, 33]. These factors control the accumulation of moisture in the rego‐ lith and, hence, are associated with the likelihood of high pore water pressures. Hillslope steepness, on the other hand, controls the downslope directed forces and the rate of down‐ slope subsurface water flow whilst the planform of the hillslopes determines the conver‐ gence and divergence of the surface and subsurface water flow lines and controls the size of the moisture-supplying area. The thickness of the weathering cover, its weathering degree, layering and textural characteristics, on the other hand, controls the hydrologic behaviour of the slopes and the type of movement. The accumulation of water by subsurface flow and infiltration in the regolith may also control the position of landslides on the hillslopes. In Pu‐ erto Rico most hillslopes failed at an elevation range of 600 to 800m because of the supply of water from higher elevated hillslope units [71]. Similar inferences concerning the position of landslides are indicated in studies of [48]. These indicate that slope failures resulting from extreme rainstorms are triggered on the middle or lower slope units of the hillslopes whilst

However, even in tropical mountains with a high relief, steep hillslopes, high rainfalls and a high likelihood of high-pore water pressure, high-intensity landslide events may be rare [27]. Several factors have to work synergistically in order to trigger large-scale landslide events. Apart from bioclimatic conditions and specific structural and tectonic settings, the state of the landscape controls the response of hillslopes to environmental changes as the magnitude-frequency relationship of landslide events depends on the long-term association between overall denudation rates and the renewal of regolith by weathering. Studies on shallow landsliding in Borneo have demonstrated that under a given set of climatic, geologi‐ cal/structural conditions, landsliding is only possible where weathering processes are able to maintain a regolith thickness that is equal to or thicker than the threshold of critical sliding depth [27]. This indicates that on hillslopes where the regolith remains below the thickness necessary to trigger landslides, as the rate of regolith renewal is unable to keep pace with the gross denudation rate and the rate of river incision is too slow to steepen hillslopes to‐ wards a new threshold angle for landsliding, at a lower regolith thickness, large landslide events will be rare. However, this type of "regolith-supply limited" or "weathering limited" conditions appears to occur more often in tectonically active mountain belts or in terrains underlain by highly resistant rocks. As high weathering rates are characteristic features in many hot and humid tropical regions, the weathering processes and the geotechnical prop‐ erties of the weathering mantles are of prime importance for an understanding of the land‐ slide dynamics in tropical mountains.

FThe important role of chemical weathering in the development of impermeable layers in the regolith, and the importance of the highly variable geotechnical properties of the sapro‐ lite, soil and colluvium on hillslopes of the Serra do Mar (Brazil) has been emphasized by several authors [32, 46]. Another set of factors is associated with the coupling strength of hillslopes and rivers, the imprints of formerly different climatic conditions including hill‐ slope deposits with variable geotechnical properties as well as the delayed response of hill‐ slopes to the change from dry to humid conditions in the transitional periods from the Pleistocene to Holocene and their influence on the developmental paths of hillslopes [81, 82, 61]. The interaction of these different factors may result in hillslopes which are highly prone to landsliding, though the trigger mechanisms often depend on a site-specific combination of factors as different landscape components of the mountainous terrain are affected.

Human interference in the form of deforestation and urbanization and increased rural land use coupled with infrastructural measures and construction resulting in an oversteepening or undercutting of hillslopes and changes in hillslope hydrology frequently exacerbate the susceptibility of hillslopes to landsliding. The combined sum of the effects of human modifi‐ cations and alterations in the mountainous domains has increased the socioeconomic impact of landsliding and also the risks in areas with a much lower natural susceptibility to land‐ sliding [41]. Although the contemporary landscape setting, the geotechnical properties of the material, climate and the impact of human alterations determine to a large degree the incidence of mass movements, the triggering of slope failures may be also associated with processes that occurred in the past. The landscapes in which mass movements occur are of‐ ten a composite of forms and deposits that are genetically linked with actual process dy‐ namics on the hillslopes. The long-term component in studies of mass movements has often been neglected because of the underlying assumption that the current state of a hillslope or landscape is ascertainable from an analysis of the contemporary process-response system. In many cases, this assumption appears to be justified. However, the knowledge of the longterm developmental paths of landscapes may lead to predictions of the susceptibility or sen‐ sitivity to react to environmental changes or may lead to predictions on the consequences and impacts of past events which were caused by environmental change.

#### **2.2. Form-process relationships and geomorphic response in south-eastern Brazil**
