Loess Terrain Research Articles

Causes and Measures of Soil Erosion in Loess Plateau, China

Soil erosion is one of the key research topics in the world today, and it is a serious hazard to the natural environment and economic and social development. The researcher has found that soil erosion in China is regional, widespread and high-intensity. Of all the erosion areas, the Loess plateau is the most severe and the most representative. Therefore, this paper investigates the current situation of soil erosion in China and focuses on the erosion on the Loess Plateau, for its causes and remediation measures. Loess plateau has an inferior erosion-carrying capability, so the gully slopes and hilly areas of the Loess Plateau are prone to landslides and crumbling phenomena. Other erosion reasons include the fragmented high loess terrain with a lot of gorges and ravines, heavy rainfall, and low vegetation coverage. This study also proposes remediation measures by analyzing the different natural characteristics that cause soil erosion on the Loess Plateau. It is found that soil erosion control has a regional dimension and control measures should be closely related to the regional environment and the causes of soil erosion. Practice shows that the erosion control measures proposed in this paper greatly improve erosion in the Loess Plateau region, but the current state of soil erosion on the Loess plateau remains bleak.

Evidence of metal migration over concealed gold deposit in loess terrain and its prospecting significance

There is a growing demand for sensitive penetrating geochemical techniques for prospecting transported covered terrains. However, it is difficult to determine whether geochemical anomalies are the result of deep-seated orebodies or lateral transportation. It remains unclear how metal elements migrate through the concealed layer to the surface, which has called into question these methods and limited their use. In this study, we used three approaches, including trace element patterns, isotope tracers, and microscopic observations, to provide evidence of metal migration over a concealed Au deposit in loess terrain. The results of the survey strongly suggest that the surface ore-forming elements are most likely derived from deep-seated orebodies in the study area. Metal migration to the surface resulted in the accumulation of metals in the surface media. Based on microscopic observations and discussion, we developed a model reflecting the main migration mechanism of metallic nanoparticle movement concerning ascending geogas and migration processes. Additionally, this study demonstrates that fine-fraction soil could be an effective approach for prospecting concealed deposits in loess terrains. We anticipate that this assay will enhance the confidence in using and developing penetrating geochemical techniques and enable them to play a greater role in mineral exploration.

Age and source of coastal loess in Shandong Peninsula, Bohai Sea, China: Implications for dust aggradation in respond to sea-level change

Coastal loess is a unique and intriguing eolian sediment on Earth, but understanding its formation time, provenance, composition and the geomorphic processes that resulted in its accumulation is very limited compared to its inland counterpart. In this paper, we focused on a loess section at Xiazhupan (XZP) Village, Penglai City in Shandong Peninsula, the only known coastal loess bluffs preserved in China. The coastal loess here not only has characteristics that are common to loess terrains such as being composed of predominantly silt-sized particles and the tendency to stand in vertical exposures, but it also reportedly preserves planktonic foraminifera, sizable pebbles and volcanic glass. In addition to its peculiar near-shore locality, these features make it more unusual compared to other loess deposits. Systematic optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL and pIRIR) and radiocarbon dating reveal it was deposited mostly during 60–20 ka, Marine Isotope Stage (MIS) 4-2, which correlates to the deposition of the Malan loess (L1) in the Chinese Loess Plateau. High-resolution grain size data indicate that the coastal loess is relatively coarse-grained, suggesting a source terrain that is at least partially proximal. Given its proximal source and the geochronological evidence, this coastal loess was sourced from the exposed continental shelf during the sea-level lowstand of the last glacial period, and this hypothesis is further supported by a core collected from the Bohai Sea in which silt was present and would have been subaerially exposed during MIS 4-2. In addition, this source area of exposed continental shelf is comparable to coastal loess in Europe. The coastal loess in Shandong can be regarded as a special type of desert or dryland loess, rather than glacial loess. Finally, future sea level rise will likely increase the erosion potential of these vulnerable coastal loess bluffs, making it urgent to study this special landform.

Geomorphic natural hazard on loess terrain: expansion on the gully sidewall

Geomorphic natural hazard on loess terrain: expansion on the gully sidewall

Shear strength and mesoscopic character of undisturbed loess with sodium sulfate after dry-wet cycling

The deterioration of shear strength of loess under dry-wet cycling is one of the serious threats to natural and artificial slopes in loess terrains in China, especially when soluble salts are involved. Three types of salt weathering spalling were noted from field survey, i.e., strip-, flake-, and concave-shaped spalling. These are closely correlated with the migration of both water and salt as well as the deterioration of shear strength of the soil. A novel saline infiltration method was proposed to prepare undisturbed saline loess specimens by slowly infiltrating sodium sulfate solution into undisturbed loess through a perforated film and medium dense sponge. Conventional triaxial test results prove that the stress-strain curves are all strain hardening. Both the cohesion and internal friction angle decrease after dry-wet cycling, but the magnitude of reduction gradually decreases until a stable value is reached, manifesting as an exponential decay. The two indexes approximately attenuate linearly at higher salt contents. Moreover, the cohesion-based damage ratio was selected to quantitatively estimate the damage caused by dry-wet cycles and salt weathering, respectively. The damage ratio due to dry-wet cycling is higher than that by salt weathering. As dry-wet cycling proceeds, the ratio of dry-wet to salt weathering–induced damage initially increases followed by a decrease, which is approximately parabolic. The dry-wet-based damage ratio reduces at higher salt contents while the proportion by salt weathering grows. By quantifying the apparent surface cracks, the crack ratio grows after more cycles or at higher salt contents. A growing fractal dimension can also be observed. Based on the computed tomography (CT), a damage variable was determined for mesoscopic cracks. Easily, it can be noticed that the changes in crack ratio are synchronized with the attenuation of cohesion of loess as well as the mean CT value (ME).

Loess geohazards research in China: Advances and challenges for mega engineering projects

Loess is a meta-stable, cemented assemblage of mainly silt and clay-sized particles of low plasticity. When dry it behaves like a brittle material, but when wetted up the fabric rapidly collapses. Unique geomorphological features include extensive surface erosion, soil piping (loess ‘karst’), catastrophic landslides, and widespread collapse (hydro-consolidation). The Chinese Loess Plateau is a more or less continuous drape of thick loess covering some 440,000 km2. It is one of China's regions that is most prone to geohazards. This paper reviews advances in the research related to loess geohazards, drawing particular attention to the need to apply research findings to recent, very large (mega-)construction projects in loess terrain such as the Mountain Excavation and City Construction in Yan'an levelling 78 km2 for urban expansion, the Lanzhou New District creating 246 km2 of new level ground for construction, and large engineered interventions in the landscape for gully control and land reclamation such as those in Shaanxi and Gansu generating agricultural land covering an area of some 8000 km2. These projects are in response to increasing pressures to facilitate expansion of urban centres, their interconnecting infrastructures and their agricultural support systems. It is argued that, where proper application of scientific knowledge for engineering control (e.g. densification, drainage) of these new landscapes is absent, these projects could generate a substantial, and costly geohazard legacy for future generations.

Loess slide susceptibility assessment using frequency ratio model and artificial neural network

Landslide susceptibility assessment is essential for disaster management. The aim of this study is to select a reliable and accurate model for loess slide susceptibility assessment. We use a frequency ratio model and artificial neural network to develop loess slide susceptibility maps. We analysed the relationships between loess slide frequency and conditioning factors including elevation, slope gradient, aspect, profile curvature, thickness of loess, rainfall, topographic wetness index, valley depth, distance to rivers and land use. We developed a landslide inventory consisting of 223 loess slides by the interpretation of remote sensing images from earlier published or unpublished reports and from intensive field surveys. From these 223 loess slides, 178 (80%) were selected for training the models and the remaining 45 (20%) slides were used for validating the developed models. The validation was carried out by using receiver operating characteristic (ROC) curves. From the analysis, it is seen that both the frequency ratio model and artificial neural network performed equally well, although the frequency ratio method is much easier to apply. The loess slide susceptibility maps can be used for land use planning and risk mitigation in loess terrain.

Mechanism of slope failure in loess terrains during spring thawing

Slope failure in loess terrains of Northern China during spring thawing period is closely related to the freeze-thaw cycling that surface soils inevitably experienced. Field surveys were carried out on natural and artificial slopes in thirteen surveying sites located in the Northern Shaanxi, the center of Loess Plateau, covering five characteristic topographic features including tablelands, ridges, hills, gullies and valleys. Based on the scale that is involved in freeze-thaw cycling, the induced failures can be classified into three main modes, i.e., erosion, peeling and thaw collapse, depending on both high porosity and loose cementation of loess that is easily affected. Model tests on loess slopes with gradients of 53.1°, 45.0° and 33.7° were carried out to reveal the heat transfer, water migration and deformation during slope failure. The surface morphology of slopes was photographed, with flake shaped erosion and cracks noted. For three slope models, time histories for the thermal regime exhibit three obvious cycles of freeze and thaw and the maximum frost depth develops downwards as freeze-thaw cycling proceeds. Soil water in the unfrozen domain beneath was migrated towards the slope surface, as can be noticed from the considerable change in the unfrozen water content, almost synchronous with the variation of temperature. The displacement in both vertical and horizontal directions varies over time and three obvious cycles can be traced. The residual displacement for each cycle tends to grow and the slopes with higher gradients are more sensitive to potentially sliding during freeze-thaw cycling.

Strength behaviors and meso-structural characters of loess after freeze-thaw

Freeze-thaw induced variations of the shear strength of loess is critical for evaluating the stability of natural and artificial slopes in loess terrains. The frequently encountered loess in Northern China was taken as the study object and two types of specimens were produced for direct shear testing including the undisturbed and the remolded. Strength behaviors of loess that has suffered preset cycles of freeze and thaw were investigated based on the particular correlations with freeze-thaw cycles, water content and dry density. Cohesion of the loess varied with the cycles of freeze and thaw until a residual value was reached while little change can be noted from the angle of internal friction. The apparent surface that was the first to be affected by freeze-thaw was compared with a quantified index of surface cracks determined by image processing. The meso-structural characters for loess after freeze-thaw cycling were explored such as the equivalent diameter, the particle orientation, the degree of circularity and the ratio of porosity area, with a damage variable that depends on the ratio of porosity area proposed. The coefficient of safety for a natural loess slope was calculated by considering the coupled effect of freeze-thaw cycles, frozen depth and the initial water content. This may provide guidance in predicting the stability of slopes in loess terrains when the freeze-thaw is included.

Loess terrain segmentation from digital elevation models based on the region growth method

Regional automatic segmentation – automatic terrain segmentation according to terrain features – is significant for modern geographical analysis. We propose a new approach of terrain region segmentation based on the region growth method. This method features actual runoff nodes as seed points. The corresponding growth threshold is defined based on statistical analysis of quantitative indexes of topographic features. Terrain segmentation of some regions is completed using the growth threshold. The corresponding edge boundaries of different terrain regions are extracted by image processing. Thus, the automatic segmentation of the terrain region is realized by the edge boundary. The application of the method to a typical Chinese loess landform area and automatic segmentation of three types of terrain regions – gully interfluve land, gully slope land, and gully groove land – are achieved by analyzing characteristics of the curvature structure of surface profiles. Segmentation results, compared with results of visual interpretation from a high-precision digital orthophoto map, show an average accuracy of segmentation of 93.51%. Topographic factor features of segmentation results are statistically analyzed. This study presents an effective and practical approach for segmenting terrain regions. This approach may be incorporated into the theory and method system of digital terrain analysis.

Analiza wyznaczania konsystencji lessów lubelskich na podstawie wyników sondowań statycznych CPT

W pracy przedstawiono porównanie wyników badań laboratoryjnych konsystencji gruntów lessowych Lublina z wynikami badań otrzymanych w wyniku sondowań statycznych CPT. Wyznaczone wartości stopni plastyczności porównano z wartościami oszacowanymi z sondowań na podstawie interpretacji z różnych źródeł bibliograficznych. Dodatkowo wyprowadzono własną formułę interpretacyjną. Stwierdzono, że dla podłoża lessowego jako kryterium wydzielenia geotechnicznego odpowiednie jest przyjęcie parametru wiodącego w postaci wartości qc, a nie IL, które najczęściej jest oznaczane dla gruntów spoistych. Przez wzgląd na możliwość uzyskania bardziej miarodajnych parametrów determinujących nośność podłoża istotne jest rozpoznanie zależności pomiędzy określonymi parametrami.

Slope spectrum critical area and its spatial variation in the Loess Plateau of China

Slope spectrum has been proved to be a significant methodology in revealing geomorphological features in the study of Chinese loess terrain. The determination of critical areas in deriving slope spectra is an indispensable task. Along with the increase in the size of the study area, the derived spectra are becoming more and more alike, such that their differences can be ignored in favor of a standard. Subsequently, the test size is defined as the Slope Spectrum Critical Area (SSCA). SSCA is not only the foundation of the slope spectrum calculation but also, to some extent, a reflection of geomorphological development of loess relief. High resolution DEMs are important in extracting the slope spectrum. A set of 48 DEMs with different landform areas of the Loess Plateau in northern Shaanxi province was selected for the experiment. The spatial distribution of SSCA is investigated with a geo-statistical analysis method, resulting in values ranging from 6.18 km2 to 35.1 km2. Primary experimental results show that the spatial distribution of SSCA is correlated with the spatial distribution of the soil erosion intensity, to a certain extent reflecting the terrain complexity. The critical area of the slope spectrum presents a spatial variation trend of weak-strong-weak from north to south. Four terrain parameters, gully density, slope skewness, terrain driving force (T d ) and slope of slope (SOS), were chosen as indicators. There exists a good exponential function relationship between SSCA and gully density, terrain driving force (T d ) and SOS and a logarithmic function relationship between SSCA and slope skewness. Slope skewness increases, and gully density, terrain driving force and SOS decrease with increasing SSCA. SSCA can be utilized as a discriminating factor to identify loess landforms, in that spatial distributions of SSCA and the evolution of loess landforms are correlative. Following the evolution of a loess landform from tableland to gully-hilly region, this also proves that SSCA can represent the development degree of local landforms. The critical stable regions of the Loess Plateau represent the degree of development of loess landforms. Its chief significance is that the perception of stable areas can be used to determine the minimal geographical unit.

Landslide hazards mapping using uncertain Naïve Bayesian classification method

Landslide hazard mapping is a fundamental tool for disaster management activities in Loess terrains. Aiming at major issues with these landslide hazard assessment methods based on Naive Bayesian classification technique, which is difficult in quantifying those uncertain triggering factors, the main purpose of this work is to evaluate the predictive power of landslide spatial models based on uncertain Naive Bayesian classification method in Baota district of Yan’an city in Shaanxi province, China. Firstly, thematic maps representing various factors that are related to landslide activity were generated. Secondly, by using field data and GIS techniques, a landslide hazard map was performed. To improve the accuracy of the resulting landslide hazard map, the strategies were designed, which quantified the uncertain triggering factor to design landslide spatial models based on uncertain Naive Bayesian classification method named NBU algorithm. The accuracies of the area under relative operating characteristics curves (AUC) in NBU and Naive Bayesian algorithm are 87.29% and 82.47% respectively. Thus, NBU algorithm can be used efficiently for landslide hazard analysis and might be widely used for the prediction of various spatial events based on uncertain classification technique.

Initiation and Evolution of Storm over Loess Plateau for Weak Synoptic Forcing Situations

Using Thunderstorm Identification Tracking Analysis and Nowcasting System( TITAN) that developed by NCAR,the storm initiation and evolution over the loess plateau were analyzed. The results showthat:( 1) Forced by weak synoptic system in summer,the most common initiation mechanism of convective cloud over the loess plateau was terrain forcing,the remaining triggering mechanism followed by dry line,convergence line,warm area and clod pool.( 2) Appropriate humidity is a key condition for triggering convective. If there is the humidity gradient values,even though dynamic force and heating power are weak,much convective cloud may form. However,when the uplifting power is weak,the convective cloud can' t develop further.( 3) The effect of topography,warm area and boundary layer convergence line is to speed up the occurring of the convective cloud and to strengthen it. However,all these factors affect it differently in different time of a day,topographic lifting having strong influence in the morning,the force of warm areas increasing around 12: 00( local time),boundary layer convergence line functioning mainly in the afternoon. By analyzing many similar cases,a conceptual model of storm initiation and evolution is also presented.

Pilot study of partial extraction geochemistry for base metal exploration in a thick loess-covered region

The Jiaolongzhang polymetallic deposit in Eastern Gansu is located under loess terrains with thicknesses ranging from <50m to >200m. The mineralization zones are completely covered by loess and by a red soil layer. A soil survey along Line 48 by using the total digestion technique at a depth of 30cm showed no responses for deposit-related elements Pb, Zn, Cu, and Ag. Different top soil leachants and leaching procedures were tested, and several elements were analyzed by using ICP–MS to decipher the leaching geochemistry in a loess terrain. Deionized water, NH4Cl, and HOAc did not detect any signals for the buried deposits, whereas 2% HCl leaching effectively detected buried deposits. Strong multi-point anomalies were found in many elements above the concealed ore bodies. The highest Cu, Pb, and Zn concentrations reached 800, 2000, and 8000ng/g in the anomalous samples, respectively, and reached 200, 70, and 400ng/g in the background samples, respectively. The elemental distribution patterns corresponded with the results of the metals-in-soil-gas survey. Most pH values of soil in background and anomalous samples were higher and lower than 8.45, respectively; thus, the pH values corresponded to a pattern of reduced chimneys.

A cellular automata model for simulating the evolution of positive–negative terrains in a small loess watershed

Cellular automata (CA) have been used increasingly to simulate complex geographical phenomena. This paper proposes a CA model for simulating the evolution of dynamic positive and negative (P–N) terrains in a small loess watershed. The CA model involves a large number of attributes, including the state of P–N terrains, distance to the shoulder-line, neighbourhood condition and topographic factors. Topographic factors include the slope gradient, aspect, slope length, slope variation, aspect variation, plan curvature, profile curvature, relief amplitude and flow accumulation. The CA model was applied to simulate the evolution of P–N terrains in an indoor, small loess watershed under artificial rainfall. The transition rules for CA were constructed automatically using a decision-tree algorithm. The derived transition rules are explicit for decision-makers and helpful for generating more reliable terrains. The simulation produces encouraging results in terms of numeric accuracy and spatial distribution, in agreement with natural P–N terrains. The iterative processes show that loess negative terrains continuously erode positive terrains. The development of a loess sinkhole near the centre gully head was reproduced as well, which shows the function of loess sinkholes in the formation of loess channel systems.

Landform‐oriented flow‐routing algorithm for the dual‐structure loess terrain based on digital elevation models

AbstractThe loess landform in the Loess Plateau of China is with typical dual structure, namely, the upper smooth positive terrain and the lower cliffy negative terrain (P–N terrain for short). Obvious differences in their morphological feature, geomorphological mechanism, and hydrological process could be found in the both areas. Based on the differences, a flow‐routing algorithm that separately addresses the dual‐structure terrain would be necessary to encompass this spatial variation in their hydrological behaviour. This paper proposes a mixed flow‐routing algorithm to address aforementioned problems. First, the loess landform surface is divided into P–N terrains based on digital elevation models. Then, specific catchment area is calculated with the new algorithm to simulate the water flows in both positive and negative terrain areas. The mixed algorithm consists of the multiple flow‐routing algorithm (multiple‐flow direction) for positive areas and the D8 algorithm for negative areas, respectively. The approach is validated in two typical geomorphologic areas with low hills and dense gullies in the northern Shaanxi Loess Plateau. Four indices are used to examine the results, which show that the new algorithm is more suitable for loess terrain in simulating the spatial distribution of water accumulation, as well as in modeling the flow characteristics of the true surface by considering the morphological structures of the terrain. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Q3 Loess-Like Silt Bearing Capacity of Foundation Partition Statistical Analysis in the Shanxi Yuncheng

Currently it is still a lack of reliable evidences on the evaluation of the bearing capacity of foundation in highway construction of Shanxi Province. Mainly reason is that the method widely used to determine in bearing capacity of loess foundation in building structures is not completely suitable to length and wide range of highway subgrade project. Because Shanxi loess region terrain is very complex, the capacity of Q3 loess-like silt in Yuncheng of Shanxi is collected and analyzed, based on statistics of a large number of bearing capacity from site loading tect. A linear regression relationship is established between loess foundation bearing capacities and water content, liquid limit, void ratio. Another linear regression relationship is established between saturation and liquid limit void ratio . Meanwhile, it compares these two sets of linear regression relationship with the measured bearing capacity and the carrying capacity by specification. Applied result show that the relative errors of the bearing capacities computed by the formulas are less than 10%, compared with other standard methods to determine the loess bearing capacity. It is significantly improvement for meeting the requirements of the highway construction.

Research on the slope spectrum of the Loess Plateau

A new concept dealing with digital analysis of loess terrain, slope spectrum, is presented and discussed in this paper, by introducing its characteristic, representation and extracting method from DEMs. Using 48 geomorphological units in different parts of the loess as test areas and 5 m-resolution DEMs as original test data, the quantitative depiction and spatial distribution of slope spectrum in China’s Loess Plateau have been studied on the basis of a series of carefully-designed experiments. In addition, initial experiment indicates a strong relationship between the slope spectrum and the loess landform types, displaying a potential importance of the slope spectrum in geomorphological studies. Based on the slope spectrums derived from the 25 m-resolution DEM data in whole loess terrain in northern part of Shaanxi, 13 slope spectrum indices were extracted and integrated into a comprehensive layer with image integration method. Based on that, a series of unsupervised classifications was applied in order to make a landform classification in northern Shaanxi Loess Plateau. Experimental results show that the slope spectrum analysis is an effective method in revealing the macro landform features. A continuous change of slope spectrum from south to north in northern Shaanxi Loess Plateau shows an obvious spatial distribution of different loess landforms. This also proves the great significance of the slope spectrum method in describing the terrain roughness and landform evolution as well as a further understanding on landform genesis and spatial distribution rule of different landforms in the Loess Plateau.

Landslide in the Thick Loess Terrain of North-West China

Landslide in the Thick Loess Terrain of North-West China