Earth Surface Processes and Landforms最新文献

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.

An unprecedented precipitation event in the hyper-arid Atacama Desert of Northern Chile occurred in March 2015. Geomorphic alterations to the river channel and the coastal zone, coupled with the exceptional magnitude of the rainfall, caused catastrophic damage and loss of life. On the coast of the El Salado watershed, legacy mine tailings infilled the watershed-ocean connection, while the river channel was altered both by tailings and urbanization. The consequences of this event resulted from the coupling of anthropogenic geomorphic changes with an unusual climate event. Lack of field data, complex geomorphology and sediment loads influenced by human activity make analysing floods in these regions especially challenging. The objective of this work is to improve our understanding of the factors that control flood hazards by using numerical simulations to reconstruct the 2015 flood in El Salado. We carry out unsteady two-dimensional simulations fully coupled with the sediment concentration to identify the influence of tailing deposits, considering high-resolution data of the pre- and post-2015 flood topography. The results highlight the importance of specific event-based studies, using models that can help designing better strategies for climate change adaptation and risk mitigation, while providing information for risk reduction and channel restoration.

Despite artificial rainfall simulation proves invaluable for the study of soil erosion processes and model construction, it still fails to fully replicate the characteristics of natural rainfall. Currently, most artificial rainfall experiments have carried out a large number of continuous high-intensity rainfall due to the focus on the characteristics of short duration and high intensity of natural rainstorm but have ignored the erosion effects caused by intermittent rainstorm with low intensity and long duration. In this study, two sets of artificial rainfall simulation experiments of intermittent low-intensity rainstorm (RR1) and continuous high-intensity rainstorm (RR2) were conducted to evaluate the effects of rainfall characteristics on erosion morphology, runoff generation and soil loss. The evolution morphology monitored by a digital close-range photogrammetry technology demonstrated the difference between the two rainstorm regimes. The soil surface was damaged more seriously under rainfall of RR2, and the rill morphological indicators of RR1 were all less than that of RR2. As rainfall proceeded, morphological indicators except for rill width-depth ratio gradually increased. As a result, the runoff rate and sediment yield between two regimes were distinct. The segmented and total soil loss, average runoff rate and sediment concentration of RR1 were all less than that of RR2, with the total soil loss of the two rainstorm regimes being 275 and 683 kg, respectively. Water infiltration, rainfall intensity, duration and frequency may be the main factors leading to the difference in soil loss and erosion morphology between two rainstorm regimes. The inconsistency of these factors can easily cause the deviation of understanding of soil erosion mechanism. Therefore, the comparison of erosion effects under different rainstorm regimes has important implications for the improvement of natural rainstorm simulation and the comprehensive understanding of erosion mechanism.

Experimental floods have been increasingly used as a promising practice to rehabilitate river ecosystems downstream of dams; however, the morphological and habitat dynamics they determine under different sediment supply conditions still poses relevant research and management questions. This study investigates the morphological and fish mesohabitat dynamics following an experimental flood, in two river reaches subject to different sediment supply regimes. We chose the lower Spöl River (Switzerland) as a relevant case study, subject to an experimental flood program for several years. Downstream of the dam, a tributary supplies large amounts of sediment to the Spöl dividing the study area into two homogeneous reaches with different sediment availability but similar flow conditions during the experimental flood. We analyzed and quantified the changes in morphology and fish habitat suitability for the Brown Trout (Salmo trutta) at the mesoscale in these two reaches caused by the 2021 experimental flood, which lasted 11 h and had a peak magnitude corresponding to a 1-year return interval in the pre-dam flow regime. We found almost no correlation between changes in the channel morphology and in habitat suitability for this event. In the upstream reach, located immediately downstream of the dam, we observed a narrower channel with a regular longitudinal sequence featuring nearly immobile coarse rapids, interspersed with more dynamic, finer riffles. Here, reach-scale morphodynamics and the shifts of the mesohabitat mosaic and the suitable habitats were below 10%. Conversely, the downstream reach, characterised by a wider channel and much higher sediment supply of well-sorted, finer bed material, was dominated by alternate bar instability and migration at the reach scale, which caused a 45% shift in its pre-flood habitat mosaic. Nevertheless, in the same reach, the overall suitability of habitats remained relatively unchanged. We attributed these different dynamics to two main factors: (i) more prolonged bedload mobility conditions and (ii) the occurrence of bar migration in the downstream reach compared to the upstream one. This study (i) underscores the critical importance of considering sediment supply from downstream tributaries when designing and monitoring the effects of experimental floods, (ii) supports the use of morphodynamic models in the related planning and monitoring phases and (iii) shows the relevance of integrating morphodynamics and eco-hydraulic analysis to support the implementation of such flow restoration programs.

This study investigates the potential of field-based induced polarization (IP) methods to provide in-situ estimates of soil cation exchange capacity (CEC). CEC influences the fate of nutrients and pollutants in the subsurface. However, estimates of CEC require sampling and laboratory analysis, which can be costly, especially at large scales. Induced polarization (IP) methods offer an alternative approach for CEC estimation. The sensitivity of IP measurements to the surface properties of geological materials ought to make them more appropriate than DC resistivity and electromagnetic induction methods, that are sensitive to bulk electrical properties. Such abilities of IP are well demonstrated in the laboratory; however, applications are lacking at field scales. In this work, the ability of field-based IP to characterize the CEC of floodplain soils is assessed by implementing a methodology that allows for direct comparison between IP and soil parameters. In one field, soil polarization and CEC exhibited the expected positive correlation; but multi-frequency measurements showed no clear advantage over single-frequency measurements. In another field, coarser soils (with low CEC) exhibited a high polarization. These coarser soils were characterized by anomalous magnetic susceptibility values, and hence the polarization was attributed to the presence of magnetic minerals. Although better than order-of-magnitude estimates of CEC were possible in soils without substantial magnetic minerals, better characterization of porosity, saturation, cementation and saturation exponents, and pore fluid conductivity would improve predictions. However, the measurement of these parameters would require similar efforts as direct CEC measurements. This study contributes to bridging the gap between laboratory-derived relationships and their applicability in field applications. Overall, this work provides valuable insight for future studies seeking to understand polarization mechanisms in soils at the field scale.

Accurate estimations of the sediment transport capacity (T_c) are essential for soil erosion modelling. However, the applicability of existing T_c equations to colluvial soils with high steep slopes and high gravel content is limited. In this study, the variation of T_c with shear stress (τ), unit stream power (P) and stream power (ω) is investigated for different slopes and flow discharge. We also evaluate the applicability of the Yalin, Govers and GUEST equations (based on τ, P and ω, respectively) for estimating T_c on steep–slope colluvial deposits. Experiments were conducted using colluvial soil in a non-erodible rill flume. The results reveal that T_c follows a power function with τ and ω and a linear function with P. The regression results of the three hydrodynamic parameters and T_c agree with the T_c equation forms of the corresponding equations. The Yalin equation, developed based on gently sloping erodible bed conditions, simulates overall low T_c values for steep sloping non-erodible bed conditions (P.O._0.5–2 = 42.8%). The accuracy is significantly improved by correcting the sediment transport coefficient K_t (P.O._0.5–2 = 100%). The accuracy of the Govers-simulated T_c values under steep slope conditions, based on gentle slope conditions, decreases with increasing slope gradient (P.O._0.5–2 = 37.14%), which is attributed to the large amount of coarse-grained sediment present in this study. Thus, we retained the original form of the equation and further improved its accuracy by adjusting the coefficients (P.O._0.5–2 = 94.29%). As T_c increases, the GUEST equation can accurately simulate T_c. The accuracy is improved by calibrating the F-value (P.O._0.5–2 = 100%). Using dimensional analysis, the equations built based on hydraulic conditions (excess current power, shear stress, flow velocity, etc.) and median particle size can accurately simulate T_c values (P.O._0.5–2 = 100%). These findings provide a basis for the development of erosion models for avalanche deposits on steep slopes.