*2.2.1. Landsliding in the Serra do Mar*

The Serra do Mar forms the elevated passive margin along the Brazilian Atlantic coast and extends from Rio de Janeiro to Santa Catarina with elevations ranging from 700 to about 2000 m. Most of the area consists of folded and faulted metamorphic and plutonic rocks from the Precambrian age and landscapes range from highly elevated plateaus with steep escarpments to dissected ridge and ravine terrains, and muliconvex hilly terrains [3]. The climate is humid tropical with maximum rainfalls in the summer and without marked dry seasons in the winter. The mean annual rainfall totals range from 1500 to 2500 mm, though annual rainfall may rise locally to 4000mm [68]. About 70 percent of the annual rainfall oc‐ curs in the summer, which is also characterized by high intensity rainfalls [68]. The potential vegetation along the Atlantic coast is pluvial rain forest, which formed a highly diverse as‐ semblage of trees, shrubs, lianas, tree ferns and epiphytes [42, 89]. Settlement and forest clearance have destroyed much of the original rain forest and estimates indicate that the re‐ maining forests merely constitute 5 per cent of the original coverage [20]. Some local meas‐ ures have attempted in recent decades to reverse these trends by the afforestation of pines and other tree species [7]. However, the destruction of forests by increasing rural land use and urbanization remains a major problem [53].

Over the last fifty years, the rapid growth of urban areas has resulted in marked changes in hillslope hydrology and the stability of hillslopes. These changes are also associated with an increasing influence of social and economic factors on risks associated with flooding and landsliding [5, 53]. In several regions, hillslopes, villages and urban areas are affected nearly every year by disastrous landslides, and particularly highly dissected terrains with steep hillslopes and highly weathered, thick regolith mantles are prone to landsliding even under undisturbed conditions [22, 19]. Many important roads cross the Serra do Mar and villages, industrial complexes lying at the foot of mountain slopes and escarpments or in basins and valleys are exposed to serious hazards caused by landsliding [53].

However, in many areas of the Serra do Mar, landslides were presumably the most impor‐ tant formative processes since the Late Quaternary period. Landscape evolution was prob‐ ably non-uniform because of base level changes and climatic changes in the Quaternary, and the intensity of landsliding is likely to have varied as a function of climatic condi‐ tions and periods of river incision [17, 18, 61]. The various controls are often genetically linked with the sensitivity to landsliding and concern several aspects of the long-term de‐ velopment of hillslopes.

#### *2.2.2. Some aspects of the role of long-term process-response systems*

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.

*2.2.1. Landsliding in the Serra do Mar*

8 Environmental Change and Sustainability

and urbanization remains a major problem [53].

valleys are exposed to serious hazards caused by landsliding [53].

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

The Serra do Mar forms the elevated passive margin along the Brazilian Atlantic coast and extends from Rio de Janeiro to Santa Catarina with elevations ranging from 700 to about 2000 m. Most of the area consists of folded and faulted metamorphic and plutonic rocks from the Precambrian age and landscapes range from highly elevated plateaus with steep escarpments to dissected ridge and ravine terrains, and muliconvex hilly terrains [3]. The climate is humid tropical with maximum rainfalls in the summer and without marked dry seasons in the winter. The mean annual rainfall totals range from 1500 to 2500 mm, though annual rainfall may rise locally to 4000mm [68]. About 70 percent of the annual rainfall oc‐ curs in the summer, which is also characterized by high intensity rainfalls [68]. The potential vegetation along the Atlantic coast is pluvial rain forest, which formed a highly diverse as‐ semblage of trees, shrubs, lianas, tree ferns and epiphytes [42, 89]. Settlement and forest clearance have destroyed much of the original rain forest and estimates indicate that the re‐ maining forests merely constitute 5 per cent of the original coverage [20]. Some local meas‐ ures have attempted in recent decades to reverse these trends by the afforestation of pines and other tree species [7]. However, the destruction of forests by increasing rural land use

Over the last fifty years, the rapid growth of urban areas has resulted in marked changes in hillslope hydrology and the stability of hillslopes. These changes are also associated with an increasing influence of social and economic factors on risks associated with flooding and landsliding [5, 53]. In several regions, hillslopes, villages and urban areas are affected nearly every year by disastrous landslides, and particularly highly dissected terrains with steep hillslopes and highly weathered, thick regolith mantles are prone to landsliding even under undisturbed conditions [22, 19]. Many important roads cross the Serra do Mar and villages, industrial complexes lying at the foot of mountain slopes and escarpments or in basins and

However, in many areas of the Serra do Mar, landslides were presumably the most impor‐ tant formative processes since the Late Quaternary period. Landscape evolution was prob‐ ably non-uniform because of base level changes and climatic changes in the Quaternary, Predictions about the way hillslopes tend to respond to changes in environmental condi‐ tions may be gained from studies of the long-term development of hillslopes. Of particular importance in this respect are the roles of inherited materials and the effects of a differing hillslope-channel coupling strength. Inherited materials may provide information on the processes that have acted during past environmental changes. This enables predictions on the vulnerability of hillslopes with respect to specific slope processes or supports regional surveys on hazards with respect to the geotechnical properties of soils, weathering layers or colluvial deposits. In the Serra do Mar, several lines of evidence suggest that mass move‐ ments have occurred alongside periods of intense colluvial accumulation in the Pleistocene and early Holocene [8, 83, 54].

The accumulation of the colluvium occurred as a result of relatively dry climatic conditions in the Pleistocene and the higher frequency in the magnitude of storm events in the early Holocene. The areal extent of land surfaces currently underlain by colluvial deposits in São Paulo is estimated to be in the range of 50 per cent [30]. Today, the knowledge of the com‐ plex stratigraphy, the geotechnical properties and of the distribution pattern of the colluvial deposits is important as these deposits are often associated with debris flow hazards which often occur after vegetation clearance [46, 19].

The tendency of landscapes to react to environmental changes by landsliding may be also indicated in the hillslope development paths. Many ridge and ravine landscapes in the Serra do Mar encompass steep hillslopes, which are covered by a moderately thick weathering mantle. In southern Sao Paulo, this terrain-type is underlain by mica schists and phyllites and often exhibits summit heights, which are dictated by the steepness of the valley-side slopes and by the spacing of the rivers [61]. These terrains are characterized by v-shaped valleys and straight valley side-slope profiles with a relative relief of 120 to 200m. The valley side slopes exhibit a narrow range of slope angles ranging from 26° to 34° for the mean slope angle and the mean maximum segment slope angle. A consequence of the geometric control of summit height by slope angle and valley spacing is that areas with similar drainage den‐ sity and stream spacings are characterized by accordant summit heights [61, 60]. Such an ad‐ justment is unlikely to result from short-term changes because the incision of the drainage net, the fixation of rivers in valleys and the development of steep valley side slopes with a mean relative relief of 120 to 200m are unlikely to have been accomplished within a period that is shorter than 105 years. Conversely, in order to maintain the geometrical expression, the hillslope processes and the hillslope-channel coupling have had to operate throughout the Holocene period.


**Table 2.** Geotechnical properties of the regolith on mica schistsB- textured B-HorizonT – transitional zone between B-Horizon and SaproliteS – SaproliteShear strength was determined by direct shear tests after consolidation to allow excess of pore pressures

**Figure 1.** Range of plasticity index and liquid limits of B-Horizons and saprolitic weathering products of mica schists (modified after [61]).

Most valley side slopes in the area are covered by numerous landslide scars and landslide deposits of various ages indicating that the important formative hillslope process is shallow landsliding. The valley side slopes are covered by red-yellow podzolic soils, which show marked differences in the geotechnical properties of the soil horizons (Table 2, Figure 1, Fig‐ ure 2). Particularly, at the contact of the B-Horizon to the transitional layer the decline of the cohesion tends to facilitate the development of a subsurface plane of failure. This is also in‐ dicated in the location of slip surfaces of relatively recent landslides, which occurred at a depth of 0.9 to 1.2m below the surface. This depth coincides roughly with the depth of the transitional layer.

Environmental Change and Geomorphic Response in Humid Tropical Mountains http://dx.doi.org/10.5772/ 53395 11

**Horizon clay silt sand cohesion friction angle units weight- per cent kPa degree B** 54.3 26.4 19.3 10.5 31.5 **B** 47.2 23.8 29.0 6.6 30.2 **B** 67.8 15.3 16.9 13.7 29.9 **T** 28.8 19.1 44.9 1.9 30.4 **S** 20.7 28.5 50.8 0.9 38.0

**Table 2.** Geotechnical properties of the regolith on mica schistsB- textured B-HorizonT – transitional zone between B-Horizon and SaproliteS – SaproliteShear strength was determined by direct shear tests after consolidation to allow

**Figure 1.** Range of plasticity index and liquid limits of B-Horizons and saprolitic weathering products of mica schists

Most valley side slopes in the area are covered by numerous landslide scars and landslide deposits of various ages indicating that the important formative hillslope process is shallow landsliding. The valley side slopes are covered by red-yellow podzolic soils, which show marked differences in the geotechnical properties of the soil horizons (Table 2, Figure 1, Fig‐ ure 2). Particularly, at the contact of the B-Horizon to the transitional layer the decline of the cohesion tends to facilitate the development of a subsurface plane of failure. This is also in‐ dicated in the location of slip surfaces of relatively recent landslides, which occurred at a depth of 0.9 to 1.2m below the surface. This depth coincides roughly with the depth of the

excess of pore pressures

10 Environmental Change and Sustainability

(modified after [61]).

transitional layer.

**Figure 2.** Shallow translational landslide and earthflow which resulted from a single rainstorm and the high moisture content in the regolith. (Photo Römer).

**Figure 3.** Limiting regolith thickness for a safety factor of slope stability (F =1.0) as a function of pore pressure ratio (ru) and the distribution of maximum segment slope angles on hillslopes underlain by mica schists. The bulk unit weight of the regolith ( γ = ρ g) = 17.4 kPa, the cohesion (c ) = 1.9 kPa, and the friction angle (φ) = 30.4°. The limiting regolith thickness has been calculated by using the infinite model for translational landslides [11]. The factor of safety (F) has been calculated by F = c + (γd cos<sup>2</sup>α - ru γd) tanφ/ (γd sinα cosα) with d = regolith depth (m), α = slope angle (°), ru = ρwg dw cos2α /γd; ρ<sup>w</sup> = density of the water (kg m-3), g = gravitational acceleration (9.81 m s-2), d<sup>w</sup> = vertical height of the water table above the slide plane (modified after [61]).

A back calculation of the slope failures indicates that most valley side slopes are stable in a dry state, but tend to become instable at pore pressure ratios of 0.1 to 0.5 (Fig. 3). The close coincidences between the slope angle of the maximum segments, the threshold slope angle for failure and the threshold regolith depth indicates that the long-term formative process on the hillslopes is landsliding. This implies that as long as river incision enables the mainte‐ nance of steep slope angles, all hillslopes are likely to be affected by reoccurring landslides in the same places as long as weathering processes supply enough material to cross the threshold regolith thickness for slope failure with respect to the slope angle and the geotech‐ nical properties again. However, the study also indicates that the form-process relationship is associated with events that are characterized by a low frequency and high magnitude re‐ occurring at temporal scales of several decades to centuries rather than being the result of continuously acting formative processes. It is easy to suppose that landscapes originating from such a process-response system where hillslope evolution resulted in the development of slopes close to the threshold of slope failure tend to respond violently to environmental changes and human interferences.

#### *2.2.3. Extreme rainfall events and landsliding*

Apart from human interferences, the high relative relief, steep hillslopes and the thick weathering layers, the most decisive factor contributing to landsliding is high rainfall. In the Serra do Mar landslide events are likely to occur independently of antecedent rainfalls and regardless of the vegetation cover and human interferences where rainfall exceeds 250 mm/24h [37]. Furthermore, the occurrence of landslides is promoted on most hillslopes which are steeper than 40% [32].

Since 1928, the Serra do Mar has been affected by about 25 to 30 extreme landslide disasters due to intense rainfall events, which have caused thousands of deaths and extensive dam‐ age to the infrastructure and various structures, though many smaller landslide events re‐ sulting in various degrees of damage tend to occur every year [23, 32, 19]. In the period from 1988 to 2000, the number of landslide fatalities in Santa Catarina, São Paulo, Rio de Janeiro, Minas Gerais, Bahia and Pernambuco averaged between 13 to 50 and locally, in coastal areas, between 51 to 364 [5]. About 85 percent of the landslide disasters occurred during the summer season, and most of the larger events that are documented in the scientific literature concentrate on the period between December and March [46, 53, 19, 68]. However, an extra‐ ordinary rainfall event was recorded in the winter of 2004. The event was caused by a cold frontal passage which became stationary in the coastal area of south-eastern Brazil [68]. Once the initially cold post-frontal anticyclone had acquired barotropic equivalent charac‐ teristics, a persistent southerly and south-easterly flow of winds became established which was impeded along the rise of the Serra do Mar causing advection and high rainfall. The event caused serious flooding and landslides along the coastal region of São Paulo [68].

Although any generalization of the functional relationships between the incidence, type and rate of movements may be overridden by local site-specific factors, the results of studies on landsliding in south-eastern Brazil suggest that most landslides occur in the late rainy sea‐ son when the accumulation of moisture in the regolith has attained a temporal maximum [1, 19]. The increase in moisture in the regolith causes a rise of the pore water pressure and hence, results in a lowering of the threshold rainfall intensity necessary to trigger landslides. However, the triggering of landslides is also a function of slope angle, slope form and of the material on the hillslopes. On steep hillslopes with a relatively thin weathering cover, shal‐ low landslides appear to occur mostly on the middle and upper hillslope segments. This landslide type is triggered during the wet season by rainfalls of long duration and moderate intensity or at the end of the wet season during heavy storms [25]. Failure may result from the increase in pore-water pressure or from the elimination of soil-suction and the reduction of the apparent cohesion [88, 46]. Debris flows, on the other hand, are triggered in the late rain season in hillslope hollows, on the lower slope segments and on steep hillslopes when the regolith is saturated with water. The incidence of debris flows is associated with highintensity rainfall occurring in the late rain season and appears to be strongly associated with a destruction of the vegetation cover [19].

### *2.2.4. Urbanization, environmental change and landslide hazards*

A back calculation of the slope failures indicates that most valley side slopes are stable in a dry state, but tend to become instable at pore pressure ratios of 0.1 to 0.5 (Fig. 3). The close coincidences between the slope angle of the maximum segments, the threshold slope angle for failure and the threshold regolith depth indicates that the long-term formative process on the hillslopes is landsliding. This implies that as long as river incision enables the mainte‐ nance of steep slope angles, all hillslopes are likely to be affected by reoccurring landslides in the same places as long as weathering processes supply enough material to cross the threshold regolith thickness for slope failure with respect to the slope angle and the geotech‐ nical properties again. However, the study also indicates that the form-process relationship is associated with events that are characterized by a low frequency and high magnitude re‐ occurring at temporal scales of several decades to centuries rather than being the result of continuously acting formative processes. It is easy to suppose that landscapes originating from such a process-response system where hillslope evolution resulted in the development of slopes close to the threshold of slope failure tend to respond violently to environmental

Apart from human interferences, the high relative relief, steep hillslopes and the thick weathering layers, the most decisive factor contributing to landsliding is high rainfall. In the Serra do Mar landslide events are likely to occur independently of antecedent rainfalls and regardless of the vegetation cover and human interferences where rainfall exceeds 250 mm/24h [37]. Furthermore, the occurrence of landslides is promoted on most hillslopes

Since 1928, the Serra do Mar has been affected by about 25 to 30 extreme landslide disasters due to intense rainfall events, which have caused thousands of deaths and extensive dam‐ age to the infrastructure and various structures, though many smaller landslide events re‐ sulting in various degrees of damage tend to occur every year [23, 32, 19]. In the period from 1988 to 2000, the number of landslide fatalities in Santa Catarina, São Paulo, Rio de Janeiro, Minas Gerais, Bahia and Pernambuco averaged between 13 to 50 and locally, in coastal areas, between 51 to 364 [5]. About 85 percent of the landslide disasters occurred during the summer season, and most of the larger events that are documented in the scientific literature concentrate on the period between December and March [46, 53, 19, 68]. However, an extra‐ ordinary rainfall event was recorded in the winter of 2004. The event was caused by a cold frontal passage which became stationary in the coastal area of south-eastern Brazil [68]. Once the initially cold post-frontal anticyclone had acquired barotropic equivalent charac‐ teristics, a persistent southerly and south-easterly flow of winds became established which was impeded along the rise of the Serra do Mar causing advection and high rainfall. The event caused serious flooding and landslides along the coastal region of São Paulo [68].

Although any generalization of the functional relationships between the incidence, type and rate of movements may be overridden by local site-specific factors, the results of studies on landsliding in south-eastern Brazil suggest that most landslides occur in the late rainy sea‐ son when the accumulation of moisture in the regolith has attained a temporal maximum [1,

changes and human interferences.

12 Environmental Change and Sustainability

which are steeper than 40% [32].

*2.2.3. Extreme rainfall events and landsliding*

The rapidly growing population in the cities in south-eastern Brazil, the unplanned growth of urban areas and the inability to house the growing number of people have resulted in hu‐ man occupation of geologically and topographically hazardous terrains, which are often characterized by an inappropriate infrastructure and precarious residences [5, 53]. The com‐ bined sum of these changes has also increased the risk of landsliding even in urban areas with a much lower natural susceptibility to landsliding.

The areal extent of the alterations in urban areas has often resulted in a reinforcement of the intensity of the hillslope processes as the affected subsystems tend to work synergistically. Urbanization is associated with a sealing of the surface, a lowering of the infiltration rate, a reduction of the water storage, and an increase in surface runoff. Soil erosion resulting from vegetation-clearing measures causes the development of gullies. Large gullies tend to affect the flow pattern of rivers by decreasing the baseflow whilst the stormflow is increased [21]. This leads to more intense floods and more events where hillslopes are undercut by rivers. Deforestation of hillslopes, on the other hand, tends to increase the likelihood of debris flows as a function of the decrease in root strength [19]. Road cuts or excavations destabilize hillslopes as the material supporting the regolith or rocks on the slopes is removed. Human settlement along streams and in valleys with houses perched on steep valley side slopes next to rivers increases the risk of disasters as hillslopes are undercut by rivers. The destruc‐ tion caused during a landslide event also varies with the type, size and rate of movements. Disastrous effects are often associated with large debris flows which are induced in the late rainy season by heavy rainfalls once the material on the hillslopes has become saturated with water. During the 2011 landslide disaster in the vicinity of Rio de Janeiro, cascades of mudflows and debris flows destroyed houses and buildings. As the slipped debris moved downslope, water contribution from the surrounding areas resulted in an increased fluidiza‐ tion of the debris, which moved rapidly into the valleys and caused an increase in sediment load and in the flooding. Flooding and landsliding resulted from unusually persistent rain and an interspersed extreme storm rainfall event which had a devastating impact along the south-eastern Brazilian coast.
