TY - JOUR A1 - Zhang, Zhuodong A1 - Wieland, Ralf A1 - Reiche, Matthias A1 - Funk, Roger A1 - Hoffmann, Carsten A1 - Li, Yong A1 - Sommer, Michael T1 - Identifying sensitive areas to wind erosion in the xilingele grassland by computational fluid dynamics modelling JF - Ecological informatics : an international journal on ecoinformatics and computational ecolog N2 - In order to identify the areas in the Xilingele grassland which are sensitive to wind erosion, a computational fluid dynamics model (CFD-WEM) was used to simulate the wind fields over a region of 37 km(2) which contains different topography and land use types. Previous studies revealed the important influences of topography and land use on wind erosion in the Xilingele grassland. Topography influences wind fields at large scale, and land use influences wind fields near the ground. Two steps were designed to implement the CFD wind simulation, and they were respectively to simulate the influence of topography and surface roughness on the wind. Digital elevation model (DEM) and surface roughness length were the key inputs for the CFD simulation. The wind simulation by CFD-WEM was validated by a wind data set which was measured simultaneously at six positions in the field. Three scenarios with different wind velocities were designed based on observed dust storm events, and wind fields were simulated according to these scenarios to predict the sensitive areas to wind erosion. General assumptions that cropland is the most sensitive area to wind erosion and heavily and moderately grazed grasslands are both sensitive etc. can be refined by the modelling of CFD-WEM. Aided by the results of this study, the land use planning and protection measures against wind erosion can be more efficient. Based on the case study in the Xilingele grassland, a method of regional wind erosion assessment aided by CFD wind simulation is summarized. The essence of this method is a combination of CFD wind simulation and determination of threshold wind velocity for wind erosion. Because of the physically-based simulation and the flexibility of the method, it can be generalised to other regions. KW - Sensitive areas KW - Wind erosion KW - Computational fluid dynamics KW - Grassland KW - Surface roughness Y1 - 2012 U6 - https://doi.org/10.1016/j.ecoinf.2011.12.002 SN - 1574-9541 VL - 8 IS - 5 SP - 37 EP - 47 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Purinton, Benjamin A1 - Bookhagen, Bodo T1 - Multiband (X, C, L) radar amplitude analysis for a mixed sand- and gravel-bed river in the eastern Central Andes JF - Remote sensing of environment : an interdisciplinary journal N2 - Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These backscatter signals are often exploited to assess the roughness of plowed agricultural fields and water surfaces, and less so to complex, heterogeneous geological surfaces. The bedload of mixed sand- and gravel-bed rivers can be considered a mixture of smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river in the eastern Central Andes with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we present theory and hypotheses regarding radar response to an alluvial channel bed. We test our hypotheses by comparing backscatter responses over vegetation-free endmember surfaces from inside and outside of the active channel-bed area. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the local variability of sand-dominated patches is analyzed using Fourier frequency analysis, by fitting stretched-exponential and power-law regression models to the 2-D power spectrum of backscatter amplitude. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR amplitude signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength L-band radar, whereas C- and X-band is sensitive only to sand variability. Because the spatial extent of smooth sand patches in our study area is typically< 50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity in alluvial rivers with aerially exposed bedload using SAR amplitude. KW - SAR amplitude KW - Radar backscatter KW - Surface roughness KW - Fluvial KW - geomorphology KW - TerraSAR-X/TanDEM-X KW - Sentinel-1 KW - ALOS-2 PALSAR-2 Y1 - 2020 U6 - https://doi.org/10.1016/j.rse.2020.111799 SN - 0034-4257 SN - 1879-0704 VL - 246 PB - Elsevier CY - New York ER - TY - JOUR A1 - Korzeniowska, Karolina A1 - Pfeifer, Norbert A1 - Landtwing, Stephan T1 - Mapping gullies, dunes, lava fields, and landslides via surface roughness JF - Geomorphology : an international journal on pure and applied geomorphology N2 - Gully erosion is a widespread and significant process involved in soil and land degradation. Mapping gullies helps to quantify past, and anticipate future, soil losses. Digital terrain models offer promising data for automatically detecting and mapping gullies especially in vegetated areas, although methods vary widely measures of local terrain roughness are the most varied and debated among these methods. Rarely do studies test the performance of roughness metrics for mapping gullies, limiting their applicability to small training areas. To this end, we systematically explored how local terrain roughness derived from high-resolution Light Detection And Ranging (LiDAR) data can aid in the unsupervised detection of gullies over a large area. We also tested expanding this method for other landforms diagnostic of similarly abrupt land-surface changes, including lava fields, dunes, and landslides, as well as investigating the influence of different roughness thresholds, resolutions of kernels, and input data resolution, and comparing our method with previously published roughness algorithms. Our results show that total curvature is a suitable metric for recognising analysed gullies and lava fields from LiDAR data, with comparable success to that of more sophisticated roughness metrics. Tested dunes or landslides remain difficult to distinguish from the surrounding landscape, partly because they are not easily defined in terms of their topographic signature. KW - Gullies KW - Surface roughness KW - Curvature KW - Digital terrain model (DTM) KW - LiDAR KW - Geomorphometry Y1 - 2017 U6 - https://doi.org/10.1016/j.geomorph.2017.10.011 SN - 0169-555X SN - 1872-695X VL - 301 SP - 53 EP - 67 PB - Elsevier Science CY - Amsterdam ER - TY - JOUR A1 - Cheng, Chaojie A1 - Milsch, Harald T1 - Hydromechanical investigations on the self-propping potential of fractures in tight sandstones JF - Rock mechanics and rock engineering N2 - The hydromechanical properties of single self-propping fractures under stress are of fundamental interest for fractured-rock hydrology and a large number of geotechnical applications. This experimental study investigates fracture closure and hydraulic aperture changes of displaced tensile fractures, aligned tensile fractures, and saw-cut fractures for two types of sandstone (i.e., Flechtinger and Fontainebleau) with contrasting mechanical properties, cycling confining pressure between 5 and 30 MPa. Emphasis is placed on how surface roughness, fracture wall offset, and the mechanical properties of the contact asperities affect the self-propping potential of these fractures under normal stress. A relative fracture wall displacement can significantly increase fracture aperture and hydraulic conductivity, but the degree of increase strongly depends on the fracture surface roughness. For smooth fractures, surface roughness remains scale-independent as long as the fracture area is larger than a roll-off wavelength and thus any further displacement does not affect fracture aperture. For rough tensile fractures, these are self-affine over a larger scale so that an incremental fracture wall offset likely leads to an increase in fracture aperture. X-ray microtomography of the fractures indicates that the contact area ratio of the tensile fractures after the confining pressure cycle inversely correlates with the fracture wall offset yielding values in the range of about 3-25%, depending, first, on the respective surface roughness and, second, on the strength of the asperities in contact. Moreover, the contact asperities mainly occur isolated and tend to be preferentially oriented in the direction perpendicular to the fracture wall displacement which, in turn, may induce flow anisotropy. This, overall, implies that relatively harder sedimentary rocks have a higher self-propping potential for sustainable fluid flow through fractures in comparison to relatively soft rocks when specific conditions regarding surface roughness and fracture wall offset are met. KW - Self-propping fracture KW - Mechanical aperture KW - Hydraulic aperture KW - Normal KW - stress KW - Fracture wall offset KW - Surface roughness Y1 - 2021 U6 - https://doi.org/10.1007/s00603-021-02500-4 SN - 0723-2632 SN - 1434-453X VL - 54 IS - 10 SP - 5407 EP - 5432 PB - Springer CY - Wien ER -