High Slope Angles Research Articles

Pleistocene giant slope failures offshore Arauco Peninsula, Southern Chile

Abstract: Three Pleistocene giant slope failures are observed in high-resolution bathymetric and seismic reflection data off Southern Chile, two of which extend across the full width of the continental slope from the shelf break to the trench. With mobilized volumes between 253 km 3 and 472 km 3 , these slides are among the largest submarine landslides documented at active continental margins so far. Deposits of each of the slides are imaged as chaotic sequences in seismic reflection lines buried beneath well-stratified sediments in the Chile Trench. The ages of the three slides are about 0.25, 0.41 and >0.56 Ma. The main preconditioning factor for the slope instabilities seems to be local uplift of the continental slope that results in peculiarly high slope angles of up to 30°. Uplift of the marine and continental forearc of the study area is the result of shortening across upper plate faults and therefore a long-term continuous process. Slope instability seems to be an iterative process and failure is likely to recur.

Spatial prediction of caterpillar (Ormiscodes) defoliation in Patagonian Nothofagus forests

In the temperate forests of the southern Andes, southern beech species (Nothofagus), the dominant tree species of the region, experience severe defoliation caused by caterpillars of the Ormiscodes genus (Lepidoptera: Saturniidae). Despite the recent increase in defoliation frequency in some areas, there is no quantitative information on the spatial extent and dynamics of these outbreaks. This study examines the spatial patterns of O. amphimone outbreaks in relation to landscape heterogeneity. We mapped defoliation events caused by O. amphimone in northern (ca. 40–41°S) and southern Patagonian (ca. 49°S) Nothofagus forests from Landsat imagery and analyzed their spatial associations with vegetation cover type, topography (elevation, slope angle, aspect) and mean annual precipitation using overlay analyses. We used these data and relationships to develop a logistic regression model in order to generate maps of predicted susceptibility to defoliation by O. amphimone for each study area. Forests of N. pumilio are typically more susceptible to O. amphimone outbreaks than lower elevation forests of other Nothofagus species (N. dombeyi and N. antarctica). Stands located at intermediate elevations and on gentle slopes (<15°) are also more susceptible to defoliation than higher and lower elevation stands located on high angle slopes. Stands in areas with intermediate to high precipitation, relative to the distribution of Nothofagus along the precipitation gradient, are more susceptible to O. amphimone attack than are drier areas. Our study represents the first mapping and spatial analysis of insect defoliator outbreaks in Nothofagus forests in South America.

Geologic field work on Mars: Distance and time issues during surface exploration

Based on field experience at Analogue stations MDRS and FMARS and additional theoretical computations, motorized field work traverses were planned for various surface feature types on Mars, with emphasis on their horizontal dimension, maximal slope angle and cumulative vertical surface undulation or roughness. The aim was to explore the possibilities and characteristics of field work for different terrain types. Those terrain types are reviewed in the present study that have already been analyzed based on remote sensing data, and that are described in the scientific literature. Dunes, gullies, slope streaks, cross sections of valley networks and of lava channels might be analyzed during only one extra vehicular activity (EVA, e.g., pedestrian and vehicular field work) with the objective of a first in-situ exploration along an optimized traverse, in order to provide the most valuable scientific information on their general characteristics and origin. Smaller tectonic faults, lava flows, lobate debris aprons and outcrops of polar layered deposits can be analyzed only by several EVAs together. Analysis of large landslides, calderas and interior layered deposits produce even more difficulties on Mars, and require specialized technology. In cases where several EVAs are necessary for detailed analysis along the best traverse, mobile pressurized vehicles (with pressurized cabin for astronauts without spacesuits) or other methods would be necessary. Many of the geologic structures that have been analyzed only with remotely sensed data could not be surveyed during one field campaign, and in some cases because of high slope angle and large cumulative topographic undulation, their in-situ exploration could not be accomplished with the technological capabilities available, and in-situ analysis requires more advanced technology than long distance rovers can provide now. Hence, the prior location of important sites and the usage of robotic help will be of high importance.

Study on the Stability of Underground Constructions with Different Locations in Alpine and Gorge Region

The reasonable selection of a location for underground structures has a great influence on the stability of surrounding rock masses, especially when the construction is built in the alpine and gorge regions. In general, the higher and steeper the mountains are, the more significant the effect is. In this paper, numerical analysis was carried out to study the stress distribution characters in the mountain with different slope angles of 30 °, 45 ° and 60 °. The results show that, the initial vertical component of stress field can not be directly determined by the buried depth when the slope angle is greater than 30 °. Meanwhile, numerical results indicate that is unfavorable for the structural stability when the underground caverns are constructed in the stress concentration areas of mountains with high slope angle. Moreover, some conclusions and recommendations were proposed for the design of underground constructions.

Non-Circular Rock-Fall Motion Behavior Modeling by the Eccentric Circle Model

The geometry of a block is a major factor affecting its rock-fall motion. A two-dimensional disc or three-dimensional sphere is often used to numerically simulate rock-fall motion, though circular or spherical blocks poorly represent the real geometry of rock falls. An eccentric circle model is specifically proposed to simulate the behavior of non-circular blocks in rock-fall motion using the distinct element method. In the eccentric circle model, the moment originates from the eccentricity. Three types of block shape, circle, ellipse, and eccentric circle, were used to simulate the fall of a single block through four different slope shapes to impact on a defense wall. The results of the simulation revealed that when the eccentricity falls below 0.5, the rock-fall motion is close to that of the ellipse model. As eccentricity grows, the rock-fall motion is closer to that of a flat piece, more stable, and tending to slide. When the block impacts the defensive wall using the circle model, a higher force and energy head in the wall is obtained. This case tends to be conservative, with the exception of the high slope angle case. The bouncing height using the circle model falls just between those of the ellipse and the eccentric circle models and tends to be unconservative. In conclusion, the rock-fall motion simulation using the proposed eccentric circle model appears to approach the non-circular block trajectory and energy loss, making it useful for rock-fall risk prevention and mitigation.

Landslides in the Rumphi District of Northern Malawi: characteristics and mechanisms of generation

Landslides are a common phenomenon in all the regions of Malawi. A number of historical landslides have been documented and are summarized here. This paper examines the occurrence of landslides in the Rumphi District of Northern Malawi. The study is based on a detailed study of 98 landslides that occurred in 2003 at Ntchenachena and Chiweta areas in the Rumphi District. This paper examines factors that contributed to and caused these landslides. The paper suggests that landslides were triggered by 206 mm of rain received in 2 days. The high percentage of medium to fine sand and abrupt rise in pore pressure accelerated the process of liquefaction at Ntchenachena. Cleft water pressure at the point between regolith and soil mass caused a number of slides in the Chiweta areas. High slope angles, deep weathering of the basement and high annual total rainfall contributed to the slope instability. Human activities through cultivation on steep slope, slope remodeling, and deforestation significantly altered the conditions of the slopes, thereby increasing the degree of landslide hazard present in these areas.

Global distribution of bedrock exposures on Mars using THEMIS high‐resolution thermal inertia

We investigate high thermal inertia surfaces using the Mars Odyssey Thermal Emission Imaging System (THEMIS) nighttime temperature images (100 m/pixel spatial sampling). For this study, we interpret any pixel in a THEMIS image with a thermal inertia over 1200 J m−2 K−1 s−1/2 as “bedrock” which represents either in situ rock exposures or rock‐dominated surfaces. Three distinct morphologies, ranked from most to least common, are associated with these high thermal inertia surfaces: (1) valley and crater walls associated with mass wasting and high surface slope angles; (2) floors of craters with diameters &gt;25 km and containing melt or volcanics associated with larger, high‐energy impacts; and (3) intercrater surfaces with compositions significantly more mafic than the surrounding regolith. In general, bedrock instances on Mars occur as small exposures (less than several square kilometers) situated in lower‐albedo (&lt;0.18), moderate to high thermal inertia (&gt;350 J m−2 K−1 s−1/2), and relatively dust‐free (dust cover index &lt;0.95) regions; however, there are instances that do not follow these generalizations. Most instances are concentrated in the southern highlands, with very few located at high latitudes (poleward of 45°N and 58°S), suggesting enhanced mechanical breakdown probably associated with permafrost. Overall, Mars has very little exposed bedrock with only 960 instances identified from 75°S to 75°N with likely &lt;3500 km2 exposed, representing ≪1% of the total surface area. These data indicate that Mars has likely undergone large‐scale surface processing and reworking, both chemically and mechanically, either destroying or masking a majority of the bedrock exposures on the planet.

High-resolution mapping of glacial landforms in the North Alpine Foreland, Austria

In this study results from traditional field mapping were merged with precise elevation information from airborne LiDAR (Light detection and ranging) surveys. Morphological and sedimentological data provide new results from the Austrian (eastern) part of the Salzach piedmont glacier during times of and shortly after the Last Glacial Maximum (LGM). The variations in meltwater discharge had a major impact on the development of glacial landforms. In areas with high meltwater supply erosional or debris reworking processes play a major role, represented by drainage channels, drumlins and kettled, low relief hummocky moraine with low slope angles. Low discharge areas are associated with distinct depositional forms such as high relief end moraines (up to 30 m) and hummocky moraine (averaging 20 m) with high slope angles. Isolated conical kames may reach heights up to 45 m. Fluvial activity is supposed to rise towards the end of the glacial cycle causing high melting rates and comprehensive debris reworking. The formation of terminal lakes and associated widespread, inorganic lake clays are the last deposits within the study area before the Salzach Glacier completely receded to its main valley. The survey of glacial landforms through the combination of field mapping and high-resolution DEM derived from airborne LiDAR missions gives precise information on transport and deposition during the last glacial cycle of the eastern Salzach Glacier piedmont lobe.

Topographic registers of paleo-valleys on the southeastern Brazilian continental shelf

The evolution of paleo-incised-valleys in the São Paulo State region of the southeastern Brazilian continental shelf is presented in this study in relation to the post Last Glacial Maximum (LGM) sea-level rises based on the submarine topography modeled by a detailed Digital Elevation Model and evidences noted in high resolution seismic profiles. The hypothesis that has guided this study is that the set of paleo-valley characteristics (i.e. the fluvial parameters of modern coastal drainage systems, the topographical shape and dimensions of the valleys and of the subsurface channels) may indicate aspects of the relation between the influence of the fluvial and the eustatic variation regime in geomorphological-stratigraphic registers. Models described in the literature sustain the view that faster marine transgressions tend to increase erosion in estuaries, which may explain the lack of registers of paleo-drainage both in topography and the sub-surface in areas with wider shelves. On the other hand, on narrower shelves, with a higher slope angle, the transgression process can preserve, or even enhance, the incised valley registers during shoreface retreat. In the area studied, we observed that the dimensions and form of the continental shelf varies from the northern to the southern part of the area, affecting aspects of the geomorphological registers of the submerged incised valleys.

Mapeamento do comprimento de rampa em microbacias com Sistema de Informação Geográfica

São apresentados procedimentos para a geração de imagens de comprimento de rampa em microbacias por Sistemas de Informação Geográfica (SIG). Os métodos de geoprocessamento foram avaliados com análises de regressão entre os resultados digitais e um conjunto de dados cartográficos medidos manualmente numa grade regular de 200 m de espaçamento, sobre toda a área da microbacia do córrego São Joaquim (Pirassununga, Estado de São Paulo). A função de análise de custos anisotrópicos do SIG foi adaptada para modelar o comprimento de rampa, explorando-se a semelhança dos conceitos e a analogia entre seus elementos e feições topográficas. Os passos de extração desses elementos do Modelo Digital de Elevação (DEM), divisores, direção de rampa, força de rampa e distância unitária, assim como todos os procedimentos desenvolvidos, foram programados em um arquivo de linguagem específica do SIG usado. A ocorrência de erros nas determinações de comprimento de rampa prevaleceu em áreas de relevo acidentado com declividades altas e rampas curtas, o que indica a necessidade de se estabelecerem procedimentos mais sofisticados de ajuste para os resultados de SIG

Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia

Abstract This paper examines post-fire erosion response in a sub-alpine environment in south-eastern Australia for a period of 2.2 years. Few studies have examined fire impacts on sediment transfer in this environment. Erosion pins were used in grids located at upper, mid and lower slope positions on adjacent burnt and unburnt hillslopes to assess fire effects on the extent of surface level change. The results indicated that there was a significant difference between the surface level change regimes on the burnt and unburnt hillslopes. Estimated erosion rates for the burnt slope over the study period ranged from 2.7 to 94.3 t ha − 1 , which could be considered low given the high slope angles, high precipitation and moderate fire severity. Slope position was critical in modifying post-fire erosion response, as it controlled slope angle and the rate of surface cover regrowth. Analysis of lower slope sites, for which more detailed data was available, indicated a second delayed erosion peak after the initial elevated post-fire response during the following spring snowmelt period. Surface recovery on the lower burnt site was slow, with vegetation cover still comparatively low 2.3 years after the fire. Evidence of post-fire sediment supply limitation was found on this site, with a declining rate of increase in the magnitude of total surface level change, despite limited regrowth and an increasing number of precipitation events > 20 mm for measurement intervals since the fire. Modification of the hillslope surface by fire leads to changing hillslope erosion process dominance in this environment. The post-fire hillslope undergoes erosion by direct rain-drop impact and overland flow, whereas the unburnt slope rarely experiences overland flow due to the thick ground cover. As a result surface level change on the unburnt slope was largely influenced by wetting–drying effects rather than sediment transfer by surface flow. Downslope biotransfer appears to be the dominant sediment movement process in the unburnt sub-alpine forest environment.

An Application of GIS Techniques to Assess the Risk of Disturbance of Archaeological Sites by Mass Movement and Marine Flooding in Auyuittuq National Park Reserve, Nunavut

Coastal regions within Auyuittuq National Park Reserve (ANPR) are sensitive to mass movement processes and threatened by flooding in response to sea level rise. These processes pose a risk to culturally significant archaeological sites within ANPR. Sites at risk of disturbance need to be identified and protected to conserve valuable archaeological resources. Since the costs of identifying and monitoring sites at risk in remote areas are substantial, modern technologies such as Geographic Information Systems (GIS) can be used to create a more rapid and cost-effective means to monitor coastal environments and manage coastal resources. This study examines the application of GIS technology to assess the risk of disturbance of 44 coastal archaeological sites by mass movement and marine flooding within ANPR. Data on surficial materials and slope angles are combined in an overlay analysis to assess terrain sensitivity to mass movement. The output from this analysis is a coarse regional assessment of mass movement potential as it relates to the strength of materials on slopes. The overall risk of disturbance for archaeological sites within ANPR is assessed by combining the risk of mass movement and the risk of marine flooding. Twenty-eight sites within ANPR are identified as being at considerable risk to disturbance: these sites are located largely on glaciomarine sediments at moderate or high slope angles and are at substantial risk to flooding (less than two metres above sea level).

Land use changes detection and spatial distribution using digital and satellite data, case study: Farim drainage basin, Northern of Iran

Land use change may influence many natural phenomena and ecological processes, including runoff, soil erosion, sedimentation and soil conditions. Decreasing of forest area in the North of Iran is one of the critical problems in recent years. The aims of this study are to detect land use changes between 1967 to 2002 using satellite images of Land Sat 7 ETM+ (2002), aerial photos and digital topographic maps (1967 and 1994) and to investigate the effect of some physical and socio economical factors on land use dynamic. The forest maps of 1967 and 1994 were collected from 1:25000 digital maps in Micro Station and then Arc/View 3.2 software. The interpretation of the maps of other land uses were derived using aerial photos. ETM+ satellite data were used to generate land use map dated 2002. The images quality assessment and georeferencing were performed on images. Different suitable spectral transformations such as rationing, PCA, Tasseled Cap transformation and data fusion were performed on the images in ENVI and IDRISI software. Image classification was done using supervised classification maximum likelihood and minimum distance classifier utilizing original and synthetic bands resulted from diverse spectral transformation and the forest area was separated from non forest area. Unsupervised classification was used to separate other types of land use. Change detection has shown that the forest area decreased between 1967 and 2002 by 2.99% from 7322.22 to 6947.23 ha. Also, the area with irrigated land farms have been increased to 202.01 ha (1.61%) and the dry land farming area decreased to 9.2%. Overlaying the map of land use change with roads and residential maps showed that by increasing the distance from roads and residential areas and villages, deforestation rate and conversion of forest to arable lands were reduced, but conversion of arable lands to released lands increased. Also, the most quantity of deforestation was observed in lower slope angle, but the dry land farming converted to release lands was observed in higher slope angle.

Modelling patterned ground distribution in finnish lapland: an integration of topographical, ground and remote sensing information

New data technologies and modelling methods have gained more attention in the field of periglacial geomorphology during the last decade. In this paper we present a new modelling approach that integrates topographical, ground and remote sensing information in predictive geomorphological mapping using generalized additive modelling (GAM). First, we explored the roles of different environmental variable groups in determining the occurrence of non‐sorted and sorted patterned ground in a fell region of 100 km2 at the resolution of 1 ha in northern Finland. Second, we compared the predictive accuracy of ground‐topography‐ and remote‐sensing‐based models.The results indicate that non‐sorted patterned ground is more common at lower altitudes where the ground moisture and vegetation abundance is relatively high, whereas sorted patterned ground is dominant at higher altitudes with relatively high slope angle and sparse vegetation cover. All modelling results were from good to excellent in model evaluation data using the area under the curve (AUC) values, derived from receiver operating characteristic (ROC) plots. Generally, models built with remotely sensed data were better than ground‐topography‐based models and combination of all environmental variables improved the predictive ability of the models. This paper confirms the potential utility of remote sensing information for modelling patterned ground distribution in subarctic landscapes.

Evidence for late Holocene activity along the seismogenic fault of the 1999 Izmit earthquake, NW Turkey

Abstract During the strong 1999 Izmit (Kocaeli) earthquake, 100 km long east-west-striking (N80°–100°), right-lateral, fault traces were formed. In the epicentral area the seismic ruptures did not follow any known or mapped fault traces, but morphology and tectonostratigraphic evidence from trenches reveal pre-existing earthquake-related features, e.g. elongated valleys, shutter ridges, high-angle slopes, scarplets and stream offsets. In the Gölcuk Peninsula a characteristic NW-SE-trending extensional fault segment emerged at the surface with a 1.5–2 m maximum vertical displacement and a 0.30 m right-lateral component. The resulting coseismic fault scarp was mapped in detail, and two trenches were excavated at the Dëniz Evler site. The 1999 displacement at this site was 1.50 m, whereas the penultimate event displaced the same sediments by 0.70 m, and a previous event by 0.20 m. Displacement is not characteristic, as fault-associated soft (recent) deltaic deposits, and the fault itself, are typically not coseismic, but rather a secondary accommodation structure in geometric consistency with the right-lateral main displacement zone. The data were compared with similar results from the Aşãgı Yuvacik, Kular Yaylacik and Acisu sites between Izmit and Sapanca Lake. The same fault segment seems to have been activated and produced surface ruptures including during the earthquakes of AD 1509, AD 989 and AD 554, plus two prehistoric events. The palaeoseismological results provide clear evidence for repeated reactivation of the same fault or fault segments during historical seismic events.

The statistical relationship between unconfined compressive strengths and the frequency distributions of slope gradients — A case study in northern Hungary

This paper focuses on the question of whether there is a deterministic connection between the slope gradient and unconfined compression strength (UCS) as a lithological factor and on describing the nature of this connection. Moreover, the authors determined the sensitive statistical parameters in the statistical surface analyses. The surface analyses were carried out in an area extending over 1500 km 2 in northern Hungary and containing parts of the uplifted Palaeozoic and Mesozoic basement and the semi-consolidated material of the Palaeogene and Neogene molasse sediments. The 67 geological formations of the area were grouped into 10 petrophysical categories characterised by unconfined compressive strength as a petrophysical parameter. The geological database was the digitalized geological map of North Hungarian Region (1 : 100 000). The digital topographic database was based on 10 m contour lines of 1 : 50 000 maps, the digital elevation model was generated by kriging interpolation. Three topographic models were created with resolutions of 25 × 25, 50 × 50 and 100 × 100 m per pixels. Evident correlation can be shown between the UCS and the relative frequency of the slope gradient. The adequate regression procedure is the power regression for low slope category values while logarithmic regression is applicable at high slope angles. Based on the characteristic of the relationship and the value of r 2, slope category intervals can be identified the relative frequency of which is proven to be determined by the UCS. These intervals are found to be between 4–10%, 10–16%, 16–22%, 22–44% and over 44%. Using the determination equations of slope gradient between 4–10% and over 44%, the UCS of the bedrock can be calculated approximately as the average value of the two calculated results. So the quotient of the frequency of these two category intervals can be regarded as an important morphometric index for a given bedrock. From the aspect of petrophysical characteristics, the rock with UCS between 6 and 86 MPa were proven to be deterministic for the slope development i.e. in the determination of the slope category frequencies. Applying the relative frequency of slope gradients a relative erosion resistance of the petrophysical categories can be calculated. Considering the determination coefficients, among the statistical parameters of the distribution of slope category values the standard deviation, mode, mean and median proved to be determined by the UCS of the bedrock.

Stratigraphy, age, and correlation of voluminous debris‐avalanche events from an ancestral Egmont Volcano: Implications for coastal plain construction and regional hazard assessment

Two previously unrecognised debris‐avalanche deposits have been identified on the eastern flanks of Egmont Volcano beneath a thick mantle of tephric and andic soil material that has mostly subdued their topographic expression. The Ngaere Formation is a c. 23 14C ka large volume (>5.85 km3) debris‐avalanche deposit that is widely distributed over 320–500 km2 of the north‐east, south‐east, and south portions of the Egmont ring plain. The second deposit, Okawa Formation, is a c. 105 ka large volume (>3.62 km3) debris‐avalanche deposit that has been mapped over a minimum area of 255 km2 in northern and north‐eastern Taranaki. Both debris‐avalanche formations contain axial facies with hummocks composed mainly of block‐supported brecciated andesitic debris. A less conspicuous marginal facies, texturally resembling a mudflow, is more extensive. A third debris‐avalanche deposit (Motunui Formation) is extensively preserved along the north Taranaki coast where it is truncated by a c. 127 ka wave cut surface (NT2) and closely overlies a c. 210 ka wave cut surface (NT3). The source of this debris‐avalanche deposit is unknown. Side‐scan sonar and shallow seismic profiling have been useful in accurately delineating the distribution of combined Okawa and Motunui debris‐avalanche deposits in the offshore environment but cannot distinguish between the two deposits or enable onshore spatial and volumetric estimates for each unit to be revised. However, the widespread occurrence of debris‐avalanche rock material offshore does emphasise the importance of this lag material altering the orientation of the coast influencing both wave climate and rates of coastal erosion. Similarly, the extensive onshore occurrence of debris‐avalanche rock material appears to be a significant factor in widening of the north Taranaki coastal plain and preservation of the NT2 and NT3 uplifted marine terrace surfaces. Initiation of collapse by magmatically‐induced seismicity is apparently common at many stratovolcanoes. Emplacement of Ngaere Formation was immediately preceded by a magmatic fall unit and is directly overlain by a closely spaced sequence of 13 fall units. In contrast, there is no evidence to indicate that an eruptive event triggered or immediately followed the Okawa debris‐avalanche event, but seismically induced gravitational sliding cannot be discounted. Egmont Volcano has repetitively collapsed over its c. 127 ka history and has generated at least five voluminous landscape‐forming debris‐avalanche deposits. Probabilistically‐based return times are calculated at c. 1967 14C yr for volumes ≥0.15 km3 and c. 21 000 14C yr for volumes ≥7.5 km3. Despite lower return times in comparison to tephra emission, Egmont Volcano is an inherently unstable cone because it comprises interbedded lavas and unconsolidated volcaniclastic deposits with a high slope angle overlying a faulted basement of Tertiary sediments. Should eruptive activity recommence and coincide with significant upper cone dilation, then the likelihood of a gravitational cone collapse is expected to increase although critical thresholds remain to be modelled. Fortunately, the Taranaki Regional Volcanic Contingency Plan is based on pre‐emptive evacuation which is intended to minimise loss of life in advance of an eruptive and/or cone collapse event occurring.

Evaluation of current statistical approaches for predictive geomorphological mapping

Predictive models are increasingly used in geomorphology, but systematic evaluations of novel statistical techniques are still limited. The aim of this study was to compare the accuracy of generalized linear models (GLM), generalized additive models (GAM), classification tree analysis (CTA), neural networks (ANN) and multiple adaptive regression splines (MARS) in predictive geomorphological modelling. Five different distribution models both for non-sorted and sorted patterned ground were constructed on the basis of four terrain parameters and four soil variables. To evaluate the models, the original data set of 9997 squares of 1 ha in size was randomly divided into model training (70%, n=6998) and model evaluation sets (30%, n=2999). In general, active sorted patterned ground is clearly defined in upper fell areas with high slope angle and till soils. Active non-sorted patterned ground is more common in valleys with higher soil moisture and fine-scale concave topography. The predictive performance of each model was evaluated using the area under the receiver operating characteristic curve (AUC) and the Kappa value. The relatively high discrimination capacity of all models, AUC=0.85–0.88 and Kappa=0.49–0.56, implies that the model's predictions provide an acceptable index of sorted and non-sorted patterned ground occurrence. The best performance for model calibration data for both data sets was achieved by the CTA. However, when the predictive mapping ability was explored through the evaluation data set, the model accuracies of CTA decreased clearly compared to the other modelling techniques. For model evaluation data MARS performed marginally best. Our results show that the digital elevation model and soil data can be used to predict relatively robustly the activity of patterned ground in fine scale in a subarctic landscape. This indicates that predictive geomorphological modelling has the advantage of providing relevant and useful information on earth surface processes over extensive areas, such data being unavailable through more conventional survey methods.

Surface of young Jupiter family comet 81P/Wild 2: view from the Stardust Spacecraft.

Images taken by the Stardust mission during its flyby of 81P/Wild 2 show the comet to be a 5-kilometer oblate body covered with remarkable topographic features, including unusual circular features that appear to be impact craters. The presence of high-angle slopes shows that the surface is cohesive and self-supporting. The comet does not appear to be a rubble pile, and its rounded shape is not directly consistent with the comet being a fragment of a larger body. The surface is active and yet it retains ancient terrain. Wild 2 appears to be in the early stages of its degradation phase as a small volatile-rich body in the inner solar system.

Effect of topography and water repellent layer on the non-uniform development of planted trees in a sandy arid area

A Tamarix aphilla forest was planted on arid sand dunes in the northern Negev, where average annual rainfall is 150 mm. Field observations, 40 years after planting, revealed a non-uniform spatial survival and development of trees. Trees planted on steep dunes are well developed with large canopies and a high survival rate, whereas trees planted on a relatively flat sandy area are small, thin and scattered. A thick litter cover, underlain by a sandy water repellent layer, developed below the large trees. Both layers are almost completely absent in the area of the small trees. The study focused on the role of the water repellent layer on the water regime over steep and low angle dunes. Water repellency was determined by the Water Drop Penetration Time method. The monitoring of rainfall–runoff relationships was based on runoff plots established on a steep and a low dune. Sand samples for water content analysis were taken with a soil auger, down to a depth of 2.5 m. Red dyed waters were used for the identification of the flow pattern of infiltrated water. Data obtained show a striking difference between steep and low dunes in all aspects studied, namely the degree of water repellency, frequency and magnitude of runoff generation, infiltration depth and water content. The values of all variables monitored were found higher on steep than on low-angle dunes. Results obtained show that two opposite feedback mechanisms occurred after tree plantation. A positive impact developed over the steep dunes. The high slope angle enhanced subsurface lateral flow towards the base of the dunes, resulting in water concentration and rapid tree growth. The large trees shed a substantial amount of leaves, whose decay developed a water-repellent layer. Runoff generation over this layer further enhanced deep-water infiltration through the phenomenon of “finger flow”. At the same time, a negative impact occurred over the low angle dunes. Due to lack of the water repellent layer runoff did not develop, preventing the positive effect of water concentration and deep-water infiltration required for a sustainable development of planted trees.