372
Climatic Geomorphology
(i.e., gelifraction), rock avalanches (i.e., overturns), gravity slides (i.e., landslides, mud
flows) and debris flows.
The rocky materials coming from the upper parts of the drainage basin can reach the
proximal area of the fans. Rock avalanches take place by the abrupt fall of a large bedrock
mass that can disintegrate into boulders and clasts of diverse size, which move downslope
as a high-velocity granular flow. In the rock avalanche of the Monte Huascar~in (Cordillera
Blanca, Peruvian Andes), that took place in May 1970, a mean speed of 280 km/h was
reached. These granular flows may feed the fans (Nicoletti and Sorriso-Valvo, 1991) and
can reach run-out distances of several kilometres. Their resultant deposits, denoted as
"megabreccias" by some authors, display a characteristic coarsening-upwards bedding
with blocks up to 20 m at the surface, and matrix-rich lower layers. In plan view they
present a lobate shape, as in the case of the rock avalanches of the Sierras Pampeanas
in NW Argentina, studied by Fauque and Strecker (1988). They were 3 to 4 km long,
0.5 to 1.5 km wide, and displayed a front rim of 8 to 10 m high. Inside these lobes older
similar deposits were recognized.
Gravity slides (landslides) correspond to rapid downslope movements of the colluvial
slopes and/or the rocky substrate along a preferential shear surface. These include
rotational and translational slides. In these cases the mobilized material is a coherent and
homogeneous block, opposite to the granular flows involved in rock avalanches. These
processes are important because they directly or indirectly feed the fans. Also, many times
the generated landslide scars work as the nucleation zones for gully or stream channel
generation (Sorriso-Valvo, 1988). As in the case of rock avalanches, seismic shaking can
trigger most of these slope movements, whereas the debris flows are commonly generated
by the progressive addition of water to the colluvial slopes that eventually fail by plastic
flow.
Debris flows are one of the most characteristic deposits of arid and semi-arid zones in
both modem and ancient alluvial records. They constitute flows of sediments formed by
a mixture of coarse clasts embedded in a fine-grained matrix with a variable content of
water and air (Figure 15.28) (Johnson, 1970; Innes, 1983; Costa, 1984, 1988a,b; Johnson
and Rodine, 1984; Takahashi, 1991; Coussot and Meunier, 1996). Their source areas are
normally constituted by the colluvial slopes or high weathered zones of the drainage
basins. In the mountainous deserts, a complementary source area is constituted by
alluvial materials deposited during previous stages of fan activity. The water required to
generate the debris flows usually comes from high intensity storms, but sometimes also
from the rapid thaw or rainfall events on the snow covers located in the upper zones of
the drainage basins. These facts produce rapid overland flows that may generate debris
flows where they incorporate the heterometric fragments and silt/clay-size particles
mantling the hillslopes. Where the flow velocity increases and the internal shear
resistance reduces, they work as a viscous fluid (Johnson, 1970). The viscosity of debris
flows may vary considerably, and when they present a high viscosity and volume, the
channel-floor erosion can give rise to sharp levees along the channel banks. On the other
hand, with a low viscosity, their downslope movement over grassland slopes produces a
very limited erosion and smaller levees (Selby, 1993). All these processes are governed
by a flow of wet sediments moving downslope within the stream channels. The
morphology of debris flows is variable, but generally displays a set of common features.
Their most frequent morphologies are constituted by hummocky surfaces with lobate
Climatic Geomorphology
(i.e., gelifraction), rock avalanches (i.e., overturns), gravity slides (i.e., landslides, mud
flows) and debris flows.
The rocky materials coming from the upper parts of the drainage basin can reach the
proximal area of the fans. Rock avalanches take place by the abrupt fall of a large bedrock
mass that can disintegrate into boulders and clasts of diverse size, which move downslope
as a high-velocity granular flow. In the rock avalanche of the Monte Huascar~in (Cordillera
Blanca, Peruvian Andes), that took place in May 1970, a mean speed of 280 km/h was
reached. These granular flows may feed the fans (Nicoletti and Sorriso-Valvo, 1991) and
can reach run-out distances of several kilometres. Their resultant deposits, denoted as
"megabreccias" by some authors, display a characteristic coarsening-upwards bedding
with blocks up to 20 m at the surface, and matrix-rich lower layers. In plan view they
present a lobate shape, as in the case of the rock avalanches of the Sierras Pampeanas
in NW Argentina, studied by Fauque and Strecker (1988). They were 3 to 4 km long,
0.5 to 1.5 km wide, and displayed a front rim of 8 to 10 m high. Inside these lobes older
similar deposits were recognized.
Gravity slides (landslides) correspond to rapid downslope movements of the colluvial
slopes and/or the rocky substrate along a preferential shear surface. These include
rotational and translational slides. In these cases the mobilized material is a coherent and
homogeneous block, opposite to the granular flows involved in rock avalanches. These
processes are important because they directly or indirectly feed the fans. Also, many times
the generated landslide scars work as the nucleation zones for gully or stream channel
generation (Sorriso-Valvo, 1988). As in the case of rock avalanches, seismic shaking can
trigger most of these slope movements, whereas the debris flows are commonly generated
by the progressive addition of water to the colluvial slopes that eventually fail by plastic
flow.
Debris flows are one of the most characteristic deposits of arid and semi-arid zones in
both modem and ancient alluvial records. They constitute flows of sediments formed by
a mixture of coarse clasts embedded in a fine-grained matrix with a variable content of
water and air (Figure 15.28) (Johnson, 1970; Innes, 1983; Costa, 1984, 1988a,b; Johnson
and Rodine, 1984; Takahashi, 1991; Coussot and Meunier, 1996). Their source areas are
normally constituted by the colluvial slopes or high weathered zones of the drainage
basins. In the mountainous deserts, a complementary source area is constituted by
alluvial materials deposited during previous stages of fan activity. The water required to
generate the debris flows usually comes from high intensity storms, but sometimes also
from the rapid thaw or rainfall events on the snow covers located in the upper zones of
the drainage basins. These facts produce rapid overland flows that may generate debris
flows where they incorporate the heterometric fragments and silt/clay-size particles
mantling the hillslopes. Where the flow velocity increases and the internal shear
resistance reduces, they work as a viscous fluid (Johnson, 1970). The viscosity of debris
flows may vary considerably, and when they present a high viscosity and volume, the
channel-floor erosion can give rise to sharp levees along the channel banks. On the other
hand, with a low viscosity, their downslope movement over grassland slopes produces a
very limited erosion and smaller levees (Selby, 1993). All these processes are governed
by a flow of wet sediments moving downslope within the stream channels. The
morphology of debris flows is variable, but generally displays a set of common features.
Their most frequent morphologies are constituted by hummocky surfaces with lobate