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Zhang C.-L.,Changan University | Li P.,Changan University | Li T.-L.,Changan University | Zhang M.-S.,Key Laboratory for Geo hazards in Loess Area
Shuili Xuebao/Journal of Hydraulic Engineering | Year: 2014

By many artificial rainfall tests, the depth of rainfall infiltration in loess area was 4 meters at most. So, it is considered that the rainfall is hard to reach groundwater through the normal leaking way, but through vertical fissures, sinkholes or some other water paths to recharge groundwater. However, according to our investigation, these water paths only occur in unloading areas which always in the edge of loess tableland, but rare in the center. In order to make clear surface water recharge groundwater in loess area, a monitoring site was set up in Zhengning county, Gansu Province. By using the soil moisture meters which inserted into the wall of a 10-meter-deep exploratory well, the changes of volumetric moisture content of soil layers with different depth were observed continuously for one year under the condition of natural rainfall, the daily precipitation during the monitoring period was also recorded by rain gauge. The results show that the moisture content of the shallow layers within 2 meters changed in annual cycle, and the trend is consistent with the change of evaporation. When the precipitation is less than 18 mm/d, the circulation of soil moisture occurs mainly in surface layers, and has little effect on the moisture content below 20 centimeters. But when the precipitation is more than 18 mm/d, the moisture content may have a sudden increase. The greater the rainfall, the higher the growth of moisture content and the deeper the affected range will be. With the increasing depth, the growth decreases and the change of moisture content lags behind the depth. The infiltration zone of the loess is about 2 meters. In the unsaturated soil below 2 meters, water moves mainly in the form of unsaturated seepage or vapor form, the motion of vapor is very small but cannot be ignored, the water would gather on the surface as encountering paleosol layer with low permeability, weak zone will be formed after long-term accumulation, inducing loess landslides eventually.

Yu G.,Key Laboratory for Geo hazards in Loess Area | Zhang M.,Key Laboratory for Geo hazards in Loess Area | Cong K.,Gansu Institute of Geo Environment Monitoring | Pei L.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research
Quarterly Journal of Engineering Geology and Hydrogeology | Year: 2015

Critical rainfall thresholds can be the key to ensuring effective debris-flow forecasting. They are significant for the study of the trigger mechanisms of debris flows, for the forecast of the characteristics of future events, and for the development of engineering guidance for mitigation. Using a hydrological approach, we first calculated the flood peak discharge at different frequencies and then the corresponding rainfall thresholds for the initiation of different scales of debris flows in Zhouqu County, China. This was followed by the establishment of a functional relation between the intensity and duration of rainfall events that trigger debris flows of different warning levels in two initial soil conditions (dry and moist). This, in turn, yielded four warning levels and two preliminary warning levels. For the two early soil conditions (dry and moist) in the Sanyanyu Gully of Zhouqu County, the Level I (red) warning values for rainfall triggering debris flow are 56 and 51 mm h-1, respectively; the Level II (orange) warning values are 41 and 38 mm h-1, respectively; the Level III (yellow) warning values are 32 and 30 mm h-1, respectively; and the Level IV (blue) warning values are 24 and 22 mm h-1, respectively. The Level V preliminary warning values are 17 and 16 mm h-1, respectively; and the Level VI preliminary warning values are 10 and 9.5 mm h-1, respectively. The rainfall intensity and duration were found to exhibit a power function relation, I = aDß, where the values of a and ß vary with the warning levels. Rainfall events capable of triggering debris flows in the new thresholds and intensity-duration relations presented here can be used for forecasting purposes and in operational geohazard warning systems. These research results also provide a scientific basis for regional hazard mitigation and reduction in Zhouqu County. © 2015 The Author(s).

Zeng L.,Changan University | Zeng L.,Key Laboratory for Geo hazards in Loess Area | Zhao G.,North China University of Water Conservancy and Electric Power | Hu W.,Key Laboratory for Geo hazards in Loess Area | Huang J.,Changan University
Geological Bulletin of China | Year: 2015

The variation of temperature and moisture inside the soil body is obvious when loess slope is freezing and melting in the loess plateau, which is located in the typical seasonal cryolithozone of Northwest China. As a result, there is a corresponding variation of water content and groundwater table inside the soil body, which tends to induce a series of geological environment problems such as landslide and collapse. In this paper, in-situ monitoring of loess temperature and water content was conducted during freezing and melting in Heifangtai loess slope of Yongjin County, Gansu Province. The authors studied variation characteristics and interrelation between temperature and water content of the freezing layer on loess slope under the freezing-thawing condition. It is found that the depth of freezing loess is around 52cm at the monitoring site, and the temperature of loess profile can be divided into three characteristic zones, namely, steady transfer zone, fluctuation zone and significant alternation of temperature. Water content was controlled by temperature during freezing-thawing of loess slope, and hence this changing process can be divided into three phases. First of all, the water content is nonlinearly increased with temperature when it is less than 2℃. Then, water content declines randomly when it is between 2℃ and 6℃. When it is greater than 6℃, water content increases slightly until stableness, and the temperature effect weight decreases. With the fully dissolving of frozen soil body, the freezing moisture melts rapidly and migrates downwards, leading to the sharp increase of water content. It is thus held that, from freezing to thawing, sharp increase of water content is the dominant factor for the slide and collapse of shallow loess. © 2015, Science Press. All right reserved.

Tang Y.M.,CAS Institute of Earth Environment | Xue Q.,Key Laboratory for Geo hazards in Loess Area | Li Z.G.,Key Laboratory for Geo hazards in Loess Area | Feng W.,Key Laboratory for Geo hazards in Loess Area
Natural Hazards | Year: 2015

Rainfall infiltration depth in an integrated and homogeneous soil mass is extremely limited. How does rainfall infiltrate into a loess slope developed with various fractures and holes? How do infiltration mechanisms impact slope stability? These are issues this paper researched and discussed. Using the in situ monitoring and field investigation method, it obtained that the infiltration depth of rainfall in integrated loess mass is generally limited to 3.0 m underground and there are often developed various fractures and holes in loess slopes, which usually form the rapid infiltration channels to make the rainwater seep quickly and vastly and to influence the loess slope stability greatly. It put forward that the rainfall infiltration inducing loess landslide can be divided into three modes: (1) superficial infiltration inducing landslide, (2) blockage infiltration inducing landslide, and (3) breakthrough infiltration inducing landslide. The first mode requires a long-time continuous rainfall; the second mode requires a sufficient antecedent accumulative rainfall, meanwhile a certain infiltration time; and the third mode requires certain rainfall intensity. © 2015, Springer Science+Business Media Dordrecht.

Li Z.,Changan University | Li Z.,Key Laboratory for Geo hazards in Loess Area | Sui L.,Changan University | Zhang M.,Key Laboratory for Geo hazards in Loess Area | Li L.,Key Laboratory for Geo hazards in Loess Area
Geological Bulletin of China | Year: 2015

Based on the spectral and shape features of loess landslide scarp, the authors segmented the high resolution remote sensing image by using regional growing and merging method, and extracted the landslide scarp area from the segmented image. The skeleton of the scarp area was obtained by way of the binary composition to characterize loess landslide. Firstly, The typical semi- circle or round-backed armchair shape characteristics and spectral characteristics became one of the important marks for loess landslide remote sensing interpretation, whereas the initial shape of the scarp was achieved by taking advantage of the regional growing and merging method with the growing parameter being 47.5 and the merging parameter being 49. Then, the holes of initial scarp of the scarp generated by segmentation process were filled. Finally, the main framework of landslide scarp was extracted using binarization skeleton algorithm, which constituted the mark of the loess landslide. This method is an improved and enhanced means for loess landslide expression in the remote sensing interpretation. © 2015, Science Press. All right reserved.

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