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1.
We investigated a small‐scale laboratory model of a talus slope evolution. Five different size classes of basaltic rock were selected and marked with different colours. Homogenized mixtures of grains of different sizes were dropped from a fixed height onto a tilted experimental board covered with a loose granular layer. This was conducted in a series of regular sequences, and the resulting distribution on the board was studied after each sequence. At the beginning of the experiment, the grains developed a longitudinal gradation similar to natural talus slopes, where small grains settle at the top while the large ones roll down to the distal part. However, after a transient period dominated by single‐particle dynamics on the inert granular medium, the evolution proved to be more variable than expected. Due to the continuous shower of falling grains, the shear stress at the bottom of the upper granular layer increased. This resulted initially in a slow creep down slope that finally collapsed in large avalanches homogenizing the material. The slides occurred at the boundary between a weaker layer created by migration of small grains through the interstices, and marked by a vertical transition between small and large grains. We compare the experimental findings with observations from natural talus slopes, and suggest that similar experiments may be helpful in understanding the evolution of taluses. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

2.
Sections up to 3·5 m deep cut through the upper rectilinear segment of relict, vegetated talus slopes at the foot of the Trotternish escarpment reveal stacked debris-flow deposits intercalated with occasional slopewash horizons and buried organic soils. Radiocarbon dating of buried soil horizons indicates that reworking of sediment by debris flows predates 5·9–5·6 Cal ka BP , and has been intermittently active throughout the late Holocene. Particle size analyses of 18 bulk samples from these units indicates that c. 27–30 per cent of the talus deposit is composed of fine (<2 mm) sediment. Sedimentological comparison with tills excludes a glacigenic origin for the talus debris, and the angularity of constituent clasts suggests that in situ weathering has been insignificant in generating fine material. We conclude that the fine sediment within the talus is derived primarily by granular weathering of the rockwall, with syndepositional accumulation of both fine and coarse debris, implying that c. 27–30 per cent of rockwall retreat since deglaciation reflects granular weathering rather than rockfall. The abundance of fines within the talus deposits is inferred to have been of critical importance in facilitating build-up of porewater pressures during rainstorms, leading to episodic failure and flow of debris on the upper parts of the slope. A wider implication of these findings is that the mechanical properties of talus slopes cannot be regarded as those of free-draining accumulations of coarse clasts, and that models that treat talus slopes as such have limited value in explaining their form and evolution. Our findings lend support to models that envisage the upper straight slope on talus accumulations as the product of mass-transport as well as rockfall, and indicate that episodic debris flow has been the primary agent of mass-transport at this site. © 1998 John Wiley & Sons, Ltd.  相似文献   

3.
Wind tunnel experiments were conducted to investigate the effects of topography on the grain size characteristics of aeolian dust deposits. Experiments were performed on three isolated hills having various size and aspect ratios. The longitudinal profile of the median grain diameter was investigated for each hill. The longitudinal dust deposition profile was also studied for nine grain size classes of between 10 and 104 µm, as were wind and dust concentration profiles in the atmosphere upwind of, over and downwind of a hill. The wind tunnel experiments show that the grain size characteristics of aeolian dust deposits are affected by topography. Most apparent is the occurrence of a zone of reduced grain size on the leeside of hills, which extends from just upwind of the summit to a distance of several times the height of the hill. Slightly coarser than normal dust is deposited on the concave windward hill slope and in a zone downwind of the area of reduced grain size, but the increase in grain size in these zones remains very small. Although the normalized dust deposition profile for a hill does not vary substantially as a function of grain size, systematic trends are observed. The most important tendencies are: (1) a progressive extension, in the downwind direction, of a zone of decreased dust deposition on the leeside of a hill (the coarser the grains, the further downwind the zone of reduced deposition extends); (2) a progressive increase in dust deposition immediately upwind of a hill (the finer the grains, the higher the deposition value upwind of a hill becomes). Both tendencies are explained by the difference in inertia of the grains, which is controlled by grain size. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

4.
Clast size variations are reported for a relict talus slope in northwest Ireland. Alternating longitudinal zones of coarse debris below buttresses and less coarse debris below gullies give the talus pronounced lateral variations in clast size that are related to joint spacing variations in the talus source area. Longitudinal variations in clast size are also present. The talus accumulated predominantly by the process of rockfall. Other processes frequently associated with lateral size variations were not significantly effective during talus development. Talus sliding may have modified some patterns of rockfall size grading.  相似文献   

5.
Cinder Cone, an undissected, 200 m high Holocene cone in Lassen Volcanic National Park, California, is mantled by basaltic blocks and bombs, including abundant large spherical accretionary bombs. Types of pyroclasts, ranging from light brown sideromelane droplets to blocky, crystalline tachylite fragments, appear to reflect the vent history; when the vent was clear, an abundance of lava was erupted at higher temperature and lower viscosity, producing predominantly rapidly chilled sideromelane droplets. When the vent was blocked by pooling of lava or by slumping of talus from crater walls, intermittent Strombolian eruptions ejected more viscous, crystalline to tachylitic fragments and comminuted talus. Such activity has been observed at Mt. Etna, Italy and Heimaey, Iceland. Avalanching of debris into the crater and down outer slopes, one of the main processes in cinder cone formation, isalso responsible for thevarieties of pyroclast types formed during Strombolian eruptions.  相似文献   

6.
The movement of unconsolidated materials near the Earth's surface is often driven by disturbances that occur at a range of spatial and temporal scales. The nature of these disturbances ranges from highly variable, such as tree turnover, to periodic and predictable, such as frost heave or creep. To explore the effect of probabilistic disturbances on surface processes, we formulated a granular creep model with analogy to rate process theory (RPT) used for chemical reactions. According to the theory, individual particles must be energized to a height greater than adjacent particles in order for grain dilation and transport to occur. The height of neighbouring particles (which is akin to activation energy in chemical reactions) varies with slope angle such that energy barriers get smaller in the downslope direction as slopes steepen. When slopes approach the friction‐limited angle of repose, the height of energy barriers approaches zero and grains ?ow in the absence of disturbance. An exponential function is used to describe the probability distribution of particle excitation height although alternative distributions are possible. We tested model predictions of granular dynamics in an experimental sandpile. In the sandpile, acoustic energy serves as the disturbance agent such that grains dilate and shear in response. Particle velocities are controlled by the frequency of energy pulses that result in grain displacement. Using tracer particles, we observed a convex‐upward velocity pro?le near the surface of the sandpile, consistent with predictions of our RPT‐based velocity model. In addition, we depth‐integrated the velocity model to predict how ?ux rates vary with inclination of the sandpile and observed non‐linear ?ux–gradient curves consistent with model predictions. By varying the acoustic energy level in the experimental sandpile, we documented changes in the rate of grain movement; similar changes in modelled velocities were achieved by varying the exponent of the particle excitation probability distribution. The general agreement between observed and modelled granular behaviour in our simple laboratory sandpile supports the utility of RPT‐based methods for modelling transport processes (e.g. soil creep, frost heave, and till deformation), thus enabling us to account for the probabilistic nature of disturbances that liberate sediment in natural landscapes. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

7.
Soil surface roughness is a dynamic property which determines, to a large extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investigate the effect of rock fragment content, rock fragment size and initial moisture content of the fine earth on the evolution of interrill surface roughness during simulated rainfall. Surface elevations of simulated plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5 mm, I = 70 mm h−1) with a laser microreliefmeter. The results were used to investigate whether systematic variations in interrill surface roughness along stony hillslopes in southeastern Spain could be attributed to rock fragment cover and rock fragment size. Soil surface elevations were measured along the contour lines (50 cm long transects) with a contact microreliefmeter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlation length (L) derived from exponential fits of the autocorrelation functions. The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is not valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7–2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfall. For soils containing large rock fragments (7.7 cm), RR increased with rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly with rock fragment content. The correlation length for soils containing small rock fragments decreases with rock fragment content and is significantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protected against raindrop impact by a water film in the depressions between the rock fragments. On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly with slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is even more pronounced. The simultaneous increase of rock fragment cover and rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy misaschists and those covered with cubic andesites. The variations in correlation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughness of interrill areas covered by rock fragments using slope angle and lithology.  相似文献   

8.
Various parameters of the internal structure of a debris-avalanche deposit from ancestral Mount Shasta (size and percentage of block facies in each exposure, number and width of jigsaw cracks, and number of rounded clasts in matrix facies) were measured in order to study flow and emplacement mechanisms. Three types of coherent blocks were identified: blocks of massive or brecciated lava flows or domes, blocks of layered volcaniclastic deposits, and blocks of accidental material, typically from sedimentary units underlying Shasta Valley. The mean maximum dimension of the three largest blocks of layered volcaniclastic material is 220 m, and that of the lava blocks, 110 m. This difference may reflect plastic deformation of blocks of layered volcaniclastic material; blocks of massive or brecciated volcanic rock deformated brittly and may have split into several smaller blocks. The blocks in the deposit are one order of magnitude larger, and the height of collapse 1100 m higher, than the Pungarehu debris-avalanche deposit at Mount Egmont, New Zealand, although the degree of fracturing is about the same.This suggests either that the Shasta source material was less broken, or that the intensity of any accompanying explosion was smaller at ancestral Mount Shasta. The Shasta debris-avalanche deposit covered the floor of a closed basin; the flanks of the basin may have retarded the opening of jigsaw cracks and the formation of stretched and deformed blocks such as those of the Pungarehu debris-avalanche deposit.  相似文献   

9.
We analyzed a combination of airborne and terrestrial LiDAR, high‐resolution photography, seismic, and acoustic data in order to gain insights into the initiation, dynamics, and talus deposition of a complex rock fall. A large (46 700 m3) rock fall originated from near Ahwiyah Point in eastern Yosemite Valley and fell a total of 730 m to the valley floor on 28 March 2009. Analyses of remote sensing, seismic, and acoustic data were integrated to reconstruct the rock fall, which consisted of (1) the triggering of a 25 400 m3 rock block in an area of intersecting and sometimes highly weathered joint planes, (2) the sliding and subsequent ballistic trajectory of the block from a steeply dipping ledge, (3) dislodging of additional rock from the cliff surface from beneath the rock fall source area, (4) a mid‐cliff ledge impact that detached a volume of rock nearly equivalent in volume to the initial block, (5) sliding of the deteriorating rock mass down the remainder of the cliff, and (6) final impact at the base of the cliff that remobilized the existing talus downward and outward and produced an airblast that knocked down hundreds of trees. The depositional geomorphology indicates that the porosity of the fresh talus is significantly lower than that expected for typical blocky talus slopes, likely because the rock debris from this event was pulverized into smaller, more poorly sorted fragments and densified via dynamic compaction when compared to less energetic, fragmental‐type rock falls. These results suggest that accumulation of individual rock‐fall boulders tends to steepen talus slopes, whereas large, energetic rock falls tend to flatten them. Detachment and impact signals were recorded by seismic and acoustic instruments and highlight the potential use of this type of instrumentation for generalized rock fall monitoring, while LiDAR and photography data were able to quantify the cliff geometry, rock fall volume, source and impact locations, and geomorphological changes to the cliff and talus. Published in 2012. This article is a US Government work and is in the public domain in the USA.  相似文献   

10.
Both from a systemic and natural hazard perspective, it is essential to understand the causes and frequency of rockfalls in mountain terrain and to predict the block sizes deposited at specific locations. Commonly, rockfalls are studied either retrospectively, using talus slopes, or directly by rockwall surveys. Nevertheless, our understanding of rockfall activity, particularly at the lower magnitude spectrum, is still incomplete. Moreover, the explanatory framework is rarely addressed explicitly. In this study, we investigate two rockwall–talus systems in the Swiss Alps to estimate the rockfall frequency–magnitude pattern and their key controls. We present a holistic approach that integrates deductive geotechnical and thermal investigations of the source rockwalls with abductive talus‐based explanations of rockfall volume and frequency. The rockwalls' three‐dimensional (3D) joint pattern indicates that 75% of the blocks may be released as debris fall (< 14 m3) and boulder falls (14–61 m3), which is mirrored in the corresponding talus material. Using two‐year records of near‐surface rockwall temperatures as input for a 1D heat conduction model underlines the destabilizing role of seasonal ice segregation. Deepest frost cracking of 300 cm may occur on the north‐northeast (NNE)‐exposed, snow‐rich rockwall, with peaks at the outermost surface. The synthesis of all data suggests that infrequent, large planar slides (approximately every 250 years) overlain by smaller, more frequent wedge and toppling failures (approximately every 17–50 years) as well as high‐frequency flake‐like clasts (3–6 events/year) characterize the rockfall frequency–magnitude pattern at Hungerli Peak. Here, we argue that small‐size rockfalls need more scientific attention, particularly in discontinuous permafrost zones. Our study emphasizes that future frequency–magnitude research should ideally incorporate site‐specific structural and thermal properties, rather than just focusing on climatic or meteorological triggers. We discuss how holistic rockwall–talus approaches, as proposed here, could help to increase our process understanding of rockfalls in mountain environments. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

11.
Forecasts of water level during river floods require accurate predictions of the evolution of river dune dimensions, because the hydraulic roughness of the main channel is largely determined by the bed morphology. River dune dimensions are controlled by processes like merging and splitting of dunes. Particularly the process of dune splitting is still poorly understood and – as a result – not yet included in operational dune evolution models. In the current paper, the process of dune splitting is investigated by carrying out laboratory experiments and by means of a sensitivity analysis using a numerical dune evolution model. In the numerical model, we introduced superimposed TRIAS ripples (i.e. triangular asymmetric stoss side‐ripples) on the stoss sides of underlying dunes as soon as these stoss sides exceed a certain critical length. Simulations with the model including dune splitting showed that predictions of equilibrium dune characteristics were significantly improved compared to the model without dune splitting. As dune splitting is implemented in a parameterized way, the computational cost remains low which means that dune evolution can be calculated on the timescale of a flood wave. Subsequently, we used this model to study the mechanism of dune splitting. Literature showed that the initiation of a strong flow separation zone behind a superimposed bedform is one of the main mechanisms behind dune splitting. The flume experiments indicated that besides its height also the lee side slope of the superimposed bedform is an important factor to determine the strength of the flow separation zone and therefore is an important aspect in dune splitting. The sensitivity analysis of the dune evolution model showed that a minimum stoss side length was required to develop a strong flow separation zone. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

12.
Detailed observation of the microstructural features of 11 fault gouge and 3 fault breccia samples collected from Tianjingshan-Xiangshan fault zone has revealed that fault gouge can be classified into 3 types: flow banded granular gouge, foliated gouge and massive gouge. The determination of the shape preferred orientation (SPO) of survivor grains in fault gouges indicates that the foliated gouge displays a profound SPO inclined to the shear zone boundary, similar to theP-foliation; flow banded granular gouge displays a SPO parallel to the shear zone boundary, while massive fault gouge and fault breccia display a random SPO. All these fault gouges fall in different fields of shear rate ternary diagram.  相似文献   

13.
Pyroclastic flow deposits of the 1991 eruption of Volcán de Colima,Mexico   总被引：1，自引：1，他引：0  
The April 16, 1991, eruption of Volcán de Colima represents a classical example of partial dome collapse with the generation of progressively longer-runout, Merapi-type pyroclastic flows that traveled up to 4 km along the El Cordoban gullies (East, Central and West). The flows filled the gullies with block-and-ash flow deposits up to 10 m thick, of which, after 7 years of erosion, only remnants remained in the El Cordoban West and East gullies. The El Cordoban Central gully, however, provided a well-preserved and incised longitudinal section of the 1991 deposits. The deposits were emplaced as proximal and distal facies, separated by a change in slope angle from >30° to <20°. The proximal facies consists of massive, clast-supported flow units (up to 1 m thick) with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading. The distal facies consists of a massive, matrix-supported deposit up to 8 m thick, which contains dispersed andesite blocks in a fine ash matrix. In the distal facies, a train of blocks marks flow-unit upper boundaries and, although sorting is poor, some grading is present. Thin, finely stratified, or dune-bedded layers of fine ash material are locally present above or below units of both facies. Sedimentologic parameters show that the size or fraction of large pyroclasts (larger than –1 ) decreases from proximal to distal facies, as the percentage of matrix (0 to 4 ) increases, especially immediately beyond the break in slope. We propose that the propagation of the Colima pyroclastic flows is critically dependent on local slope angle, the presence of erodible slope debris, and the decrease in grain size with distance from the vent. The progressive fining is probably caused by some combination of erosion, clast breakup and deposition of larger pyroclasts, and is itself influenced by the slope angle. In the proximal region, the flows moved as granular avalanches, in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud. The maximum runout distance of the avalanches was controlled by the angle of repose of the material, and the volume and grain size of source and eroded material. Because the slope angle is close to the repose angle for this debris, granular avalanches were not able to propagate far beyond the change in slope. If, however, an avalanche had enough mass in finer grain size fractions, at least part of the flow continued beyond the break in slope and across the volcano apron, propagating in a turbulent state and depositing surge layers, or in an otherwise settling-modified state and depositing block-and-ash flow layers.Editorial responsibility: T Druitt  相似文献   

14.
We present a two‐dimensional model of the development of scree slopes using the discrete‐element method. We concentrate on the dynamics of the accumulating debris at the cliff foot rather than on the failure modes of the cliff‐face or shape of the underlying rock surface. The evolution of this unconsolidated material is intermittent and systematically changing over time, with an early high disturbance regime, dominated by a characteristic event size (where 65% of particles in the debris are in motion to some extent), replaced at later times by many shallow slides interspersed with infrequent large events that involve motion through almost the full scree depth. These large slides lead to a stratigraphy in which the layers of material are stretched almost horizontal near the base of the slope. The scree surface thus shows a gradient in age with most recent rock‐fall close to the cliff and the oldest rock‐fall debris outcropping at the foot. The final surface slope tends to show little curvature, and the final mean slope is well correlated with the angle of internal friction of the particles, although the change is very small over a wide range of friction angles [final slope (in degrees relative to horizontal) ~ 0.043 × internal friction angle + 17.49, with a correlation coefficient of 0.89, p‐value 0.0001]. Some weak size‐segregation of the debris is found, but this seems to have little to do with individual particles bounding down the slope. The shape of the rock core agrees largely with the analytic forms given by Fisher–Lehmann and Bakker–Le Heux expressions, but the original simple Fisher quadratic can give the best fit. Overall the evolution shows a remarkable insensitivity to the model parameters, suggesting that the controls on dry scree‐slope evolution are primarily geometric in character. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

15.
Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge   总被引：1，自引：0，他引：1  
The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The ‘breaking up’ of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault’s history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.  相似文献   

16.
Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.  相似文献   

17.
Detailed slope surveys were made in seven karst depressions in central Belize. Four major slope types were identified—inclined bedrock slopes, staircases, broken cliffs, and talus slopes. Depression bases are dominated by inclined bedrock and talus slopes, midslopes largely by broken cliffs, and near hill summits by staircases. Erosional weight loss tablets deployed in 1975–1981 and 1980–1985 show significant variations between the different slope types, with a maximum near depression bases. Throughfall measurements in 1980 and 1985 show that midslope and near-summit sites receive 10–20 per cent less water than do depression bases. Surface runoff, monitored using coloured dyes, is of limited distance and duration but still contributes to surface corrosion and morphology. Vadose seepage, monitored by tracing water from the surface to a small, shallow cave, is rapid, and subsurface corrosion exceeds that at the surface. Overall, near-surface form and process are consistent with hydrological models of depression development (Williams, 1983, 1985) and surface processes contribute significantly to overall depression form. Moreover, surface activity complements that in the subcutaneous zone by directing recharge into the epikarstic aquifer.  相似文献   

18.
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19.
A comparison was made between the distal ends of twenty-two avalanche and fifteen non-avalanche slopes in the San Juan Mountains of Colorado, U.S.A. All slopes occurred in the subalpine zone. Six characteristics were used for analysis: type of slope, surface material, longitudinal profile, perched debris or debris tails, avalanche impact on opposite valley wall, and transverse profile. Both fan and roadbank avalanche slope types were found along with the non-avalanche slopes. Almost all slopes were turf covered rather than talus since the work was done below treeline. Twenty avalanche slopes had a distinctive concave longitudinal profile. Little debris of any kind was found since the slopes were in an area of insignificant amounts of detritus. Many of the larger and two of the smaller avalanche slopes showed evidence of impact upon the opposite slope. Eighteen of the avalanche slopes had convex transverse profiles.  相似文献   

20.
Physical modelling experiments have been carried out in a cold room to test on a small scale, the effects of water supply during the thaw of an experimental slope with permafrost. Permafrost was maintained at depth and a thin active layer was frozen and thawed from the surface. Data from the experiments relate to two different conditions, first with moderate rainfall, and second with heavy rainfall during the thaw period. When moderate rainfall is applied during thaw phases, the experimental slope is slightly degraded. At the scale of the experiment, erosion processes involve frost jacking of the coarse blocks, frost creep and gelifluction that induce slow and gradual down slope displacements of the active layer, but also small landslides leading to large but slow mass movements with short displacements. Changes in experimental slope morphology are marked by the initiation of a small‐scale drainage network and the development of a little crest line which shows a progressive upslope migration. With such boundary conditions, there is not enough water supply to evacuate downslope the whole of the eroded material and a topographic smoothing is observed. When heavy rainfall is applied during thaw periods, rapid mass wasting (small mud‐flows and debris flows) become prominent. Slope failures are largely controlled by the water saturation of the active layer and by the occurrence of steeper slopes. At the scale of the experiment, rates of erosion and maximum incision increase by about 100% leading to significant slope degradation with marked and specific scars comparable to gullying. These morphological changes are dependant on both the size and the frequency of catastrophic events. These experiments provide detailed data that could improve the knowledge of the physical parameters that control the initiation, at a small‐scale, of erosion processes on periglacial slopes with a thin active layer and/or with thin cover of mobilizable slope deposits. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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