Earth Surface Processes and Landforms最新文献

Most ways of predicting flow resistance in shallow rivers with a partial or complete cover of coarse sediment use a bed-sediment grain diameter as a roughness length scale. However, beds with the same grain size distribution differ in roughness and flow resistance depending on how the larger grains are arranged, the nature of any bedforms and the possible complications of bedrock or rough banks. This has led to interest in predicting flow resistance using metrics of the topographic roughness of the bed. Some researchers have used the standard deviation of bed elevation as a roughness length scale. An alternative for channels containing boulders is to regard the bed as an array of large roughness elements. Fluvial research to date using these two approaches is limited and inconclusive. We review potentially relevant findings from the much more extensive literature in boundary-layer meteorology and various branches of engineering and note links between the distribution-statistics and element-array approaches. The skewness of the elevation distribution is widely seen as important but it is unclear how best to use it for flow prediction. Other open questions include the scale dependence of topographic metrics, and what type of flow resistance equation to use them in. Calibration and testing of new prediction methods require flow data from reaches with known roughness statistics. This need should be met partly by measurements at field sites or in flume models of them, but also by flume experiments and numerical simulations using synthetic roughness.

Glacial debris flows, which are induced by ice and rock avalanches, pose an increasing threat to human life and critical infrastructure under climate change. Therefore, the movement characteristics of debris flows induced by ice and rock avalanches have attracted increasing attention, but the understanding of the effects of ice and rock avalanches on the mechanism of debris flow mobility remains incomplete. To study the mechanisms of the enhanced mobility of debris flows and simulate the impacts of ice and rock avalanches, we employed a servo-hydraulic-controlled ring–shear apparatus to examine the undrained shear behaviour of moraine material under monotonic and impact loading conditions. It is found that impact loading caused a significant reduction in the shear strength of moraine, with the mobilized friction angle decreasing by 16.1% (from 20.6 to 17.3°) and the residual angle of internal friction decreasing by 52.9% (from 7.0 to 3.3°). After moraine failure (at the peak shear strength), the moraine material exhibited rapid shear strength reduction behaviour under impact loading conditions owing to the lower strength weakening coefficient and smaller slip-weakening distance. Compared with the monotonic loading ring–shear test results, the liquefaction potential was greater, and the shear energy needed to reach the same pore water pressure ratio and ultimate pore water pressure was much lower under impact loading. This suggests that under impact loading, moraine is more susceptible to liquefaction and exhibits mobility characteristics. Our results provide new insights and increase the understanding of the mechanism underlying the enhanced mobility of glacial debris flows induced by ice and rock avalanches.

The erodibility of bedrock and rock masses is an important parameter for understanding landform development, landscape evolution modelling and engineering applications. Yet, complex geotechnical properties and the difficulty of directly quantifying erodibility limit the theoretical understanding and prediction of erosion processes. Several proxy methods have been suggested to assess bedrock erodibility by fluvial impact erosion. Yet, none of these proxy methods have been rigorously benchmarked with direct laboratory or field measurements. Here, we assess the usefulness of proxy methods described in the literature in the quantitative prediction of fluvial impact erosion. We compare four proxy methods – Mohs' hardness, the Schmidt hammer rebound value, Annandale's erodibility index and the Selby score – to erodibility laboratory data measured using erosion mills. We assess these methods using three statistical parameters: Kendall's tau and Spearman's rho rank correlation coefficients, and the adjusted R² from an exponential fit. We distinguish between three applications, which require increasing correlation strength. These are (i) trend detection (sorting groups of data by their relative erodibility), (ii) quantitative ranking (relative erodibility of groups of data can be quantitatively assessed), and quantitative prediction (erodibility for individual sites can be quantitatively assessed). Mohs' hardness, Schmidt hammer measurements and Annandale's method are suitable for trend detection, while Selby's method is not. None of the methods is suitable for quantitative prediction. As such, none of the methods is a suitable proxy for estimating erodibility in fluvial bedrock erosion at a particular location. For quantitative ranking, we suggest to use either Mohs' hardness or Schmidt hammer measurements, because of (i) the correlation with mill-measured erodibility, (ii) their ease and quickness of application in the field and (iii) the minimum of required training. When applying these methods, investigators should obtain data both from the same and from different lithological units at many sites. Then, the results can then be used for bulk assessment, but not for individual sites.

Glacial kettles are surficial depressions that form in formerly glaciated terrain when buried stagnant ice melts within pro-glacial sediments, often deposited by meltwater streams. Kettles, like other glacial landforms, provide insight into the impact of climate on landscape evolution, such as the extent and timing of glaciations. The geometry of kettle features is variable, but existing theory does not explain the range of observed morphologies. Our study aims to establish a quantitative relationship between the depth of ice burial and the resulting morphology of terrain collapse in kettle depressions. To do so, we simulated kettle formation in the laboratory by burying ice spheres of four sizes in well-sorted coarse sand at four different depths. As the spheres melt at room temperature, a glacial kettle analog forms at the surface. We scanned the resulting kettle topography with a portable LiDAR scanner to produce 3D digital elevation models of each depression, from which we measured each depression's depth and width and, in one instance, the time series of kettle formation. Using this data, we quantified the relationship between the sphere diameter, burial depth and resulting dimensions of the kettle by developing a set of equations, which we then applied to full-scale features. Our results indicate that ice burial deeper than one sphere diameter corresponds to a decrease in depression depth and an increase in depression width. This application offers insight into the interdependence of ice burial depth and kettle geometry.

This study evaluated six open-access digital elevation models (DEMs) for the Del Azul Creek Basin in the Argentine Chaco-Pampean Plain: Shuttle Radar Topography Mission, Advanced Land Observing Satellite Phased Array L-Band Synthetic Aperture Radar, TerraSAR-X Add-On for Digital Elevation Measurements (TanDEM-X), NASADEM Global DEM, Forest and Building height biases were removed from Copernicus GLO 30 DEM V1-0 (FABDEM), and TanDEM-X 30 m Edited DEM (EDEM). Statistical metrics were calculated for (i) residuals between DEMs and the Ice, Cloud and Land Elevation Satellite-2 (ICESat-2); (ii) the minimum distance between DEM-derived drainage networks and those from the Buenos Aires Provincial Water Authority; and (iii) DEM-derived slopes in shallow water bodies compared with the Joint Research Centre's Global Surface Water Mapping product. Analyses were performed for four elevation and seven slope bands. TanDEM-X had the smallest errors compared to ICESat-2 (median 0.19 m, NMAD 0.38 m), followed by FABDEM (median 0.31 m, NMAD 0.23 m). EDEM performed best in drainage networks (median 99.45 m, NMAD 117.16 m), followed by FABDEM. In general, the vertical error increased with elevation and the accuracy of the drainage network estimates improved. The vertical accuracy decreased with steeper slopes, with FABDEM performing the best across all slope ranges. FABDEM exhibited the best performance in determining seasonally dispersed shallow water bodies, demonstrating its overall usefulness for hydrological applications in large-scale plains characterized by aeolian geoforms of lowland accumulation and erosion. Assessing freely available products provides valuable resources for researchers and professionals and can guide decision making for managing hydrological resources, including flood risk and infrastructure development.

Controls on the physical and chemical architecture of the subsurface critical zone are somewhat controversial, with multiple hypotheses proposed to account for variations in the depth of weathering between sites, and with landscape position at a site. In the Piedmont region of the Mid-Atlantic US weathering of crystalline bedrock has been observed to extend tens of meters below the surface and groundwater in a'bow-tie’ shape – i.e. weathering extends to lower elevations below ridges than below channels. The chemical and physical structure of a hillslope transect in the Maryland Piedmont was explored with a 45 m borehole in the ridge, as well as shallow bedrock boreholes at the toe of the slope and valley. Chemical weathering fronts were characterized using elemental abundances and mineralogical analysis. The ridge borehole did not extend deeper than the chemically and physically weathered rock. Surface and borehole geophysics and density measurements were used to characterize the weathered rock and saprolite. Na and Ca results suggest that plagioclase feldspar weathering is similar between samples collected from 45 m under the ridge and 2.2 m under the valley bottom. A narrow Fe oxidation garnet weathering front co-insides with the transition from weathered bedrock to saprolite, suggesting that this reaction may generate initial saprolite porosity. Muscovite weathering co-occurs with complete depletion of plagioclase a few meters above the Fe oxidation front. These nested weathering fronts in the saprolite appear to follow a subdued version of the surface topography. The location and shape of the nested saprolite weathering fronts may be controlled by the feedback between the transport of reactants and solutes and reaction-generated porosity, consistent with the conceptual “valve” hypothesis. Differing dominant control mechanisms on deep bedrock weathering and saprolite initiating reactions may explain the thickness and structure of the critical zone at our site.

Observing the subsurface architecture of the deep Critical Zone (CZ), which lies beyond the uppermost layer of accessible soil, is a complex but crucial task. Near-surface geophysics offers an alternative to accessing the deep CZ at scales relevant to fluid, nutrient and gas transport. As geophysical instruments are sensitive to different subsurface physical properties, their combination can enhance insight into CZ architecture. However, the agreement between and complementarity of multiple geophysical techniques has not been widely assessed for CZ-related questions. This study employed geophysics to image a highly weathered lateritic hill rich in iron oxides developed from Archean granite within the Avon River Critical Zone Observatory, Western Australia. Data gathered from an electrical resistivity tomography (ERT) and horizontal-to-vertical-spectral-ratio (HVSR) passive seismic transect were used to visualise CZ architecture through specific resistivity values and ambient noise contrasts. Both techniques revealed a notable degree of lateral variability consistent with the formation of the ~3–4 m thick duricrust-capped hilltop, the creation of gullies in the sodic material of the pallid zone exposed along the slope and the deposition of ~11 m thick colluvial sediment at the foot slope. Calculated bedrock depth was consistent between the HVSR and ERT instruments along the hilltop plateau but varied from ~23 m to 31 m on the slope and 32 m to 39 m at the foot slope, respectively. Overall, the vertical variation depicted by the ERT, including the differentiation of two layers within the lateritic weathering profile - the pallid zone and saprolite – made up for the inaccuracy of the HVSR technique in depicting layers of similar composition. Moreover, the HVSR method clearly depicted bedrock depth, overcoming the partial masking of the bedrock by saline groundwater in the ERT model. The complementarity of these two methods allowed the development of a detailed conceptual model of subsurface CZ architecture within a saline lateritic weathering profile.

Rapid climatic changes cause permafrost to thaw, initiating thermokarst landforms such as lakes and ponds. These waterbodies cover large extents of the northern circumpolar permafrost region and are significant sources of greenhouse gases. For the assessment of current and potential future waterbody development, continuous monitoring and analyses of the driving factors are required. In Dávvavuopmi, a permafrost peatland located in the sporadic permafrost zone of northern Sweden, high-resolution imagery of the first two decades of the 21st century is available. This study combined field, GIS and statistical methods to explain spatiotemporal pond dynamics by investigating pond morphology and regional climate characteristics. Erosion affected 42% of the shorelines, and the erosion intensity was significantly correlated with the height and slope of bluffs facing the waterbodies. Along some sections, active erosion was causing shoreline retreat, but the dominant trend in this landscape was pond drainage and terrestrialisation/fen vegetation ingrowth. Between 2003 and 2021 the thermokarst pond area and number decreased by 6%/decade and 27%/decade, respectively. Inter- and intra-annual climatic parameters could not be directly linked to thermokarst pond dynamics. Instead, the climate conditions (MAAT/snow depth) control permafrost degradation, causing enhanced hydrological connectivity in the landscape, which drives the pond drainage trend.

The quantification of pebble shape has been of interest to geomorphologists for decades. Several authors developed parameters to describe pebble shapes from their images. The extraction of this information from images involves two steps: the segmentation of pebble contours and the application of a computational geometry algorithm to estimate shape parameters. When images are taken in the field, unavoidable shadows might hinder the possibility of using automatic segmentation methods. This paper introduces a new method for automatic segmentation of pebbles that improves segmentation accuracy in the presence of shadows. The method is based on the Canny edge detection algorithm which uses a double thresholding process to provide a classification of the strength of the detected edges. The proposed method applies this algorithm with an ensemble of thresholding values, estimating, for each pixel, the probability of being an edge. The resulting pebble contours were analysed using two computational geometry algorithms to obtain shape parameters. The algorithm was calibrated on a sample of five pebbles and then validated on a sample of 1696 pebbles. Its accuracy has been estimated by comparing the resulting shape parameters with those obtained using reference software, which was used as ground truth (GT). The proposed segmentation method was capable of accurately segmenting around 91% of the sample with a relative error for roundness of −1.7% and −0.4%; for elongation of −0.2% and −0.3% and for circularity of 0.2% and 0.1%, when shape parameters were computed using the algorithms of Zheng or Roussillon, respectively. The method could therefore be used to segment images of pebbles collected in the field with low contrast and shadowing, providing comparable accuracy with ‘manual’ segmentation, while removing operator bias.

We here describe the results of stratigraphic and sedimentological examinations of debris flow deposits at Breidokk, Gol, southern Norway. The deposits are situated at the valley floor, below a steep slope with three large and several smaller debris flow channels incised into the thick till cover. The study area is populated and with abundant infrastructure such as roads, public and private buildings and other types of infrastructure, including underground water pipes and cables. Six, 10–15 m long and 1–3 m deep trenches were dug out with an excavator and examined. The sediments in the trenches consist of moraine-, glaciofluvial/fluvial- and debris flow deposits. The latter consist of matrix supported, unsorted, massive beds from 1 cm to more than 1 m in thickness, with clasts up to 80 cm in diameter. A total of 16 post glacial debris flow beds are identified in five of the six trenches, representing a minimum of eight individual debris flow events. This is probably an underestimation of the debris flow activity through postglacial times as the location of the trenches was in large determined by infrastructure and were not optimally placed for mapping all debris flow deposits in the area. Also, correlation between trenches proved difficult. A total of 37 radiocarbon ages of buried soil and other organic material situated above and below debris flow deposits, together with the sedimentological and stratigraphical interpretation, show that debris flow activity has prevailed throughout the Holocene, also within the last 1000 years. A possible increase in activity within the last 3–4000 years BP has been noted. This is important knowledge to aid in the interpretation of the Quaternary history of the area but also to determine the hazard zones.