Application of FLATModel, a 2D finite volume code, to debris flows in the northeastern part of the Iberian Peninsula

FLATModel is a two-dimensional shallow-water approximation code with corrections and modifications that create a simulation tool adapted to debris-flows behaviour. FLATModel uses the finite volume method with the numerical implementation of the Godunov scheme and includes correction terms regarding the effect of flow over high slopes and curvature. Additionally, the stop-and-go phenomenon, the basal entrainment and a correction regarding the front inclination of the final deposit are incorporated into FLATModel. In addition, different flow resistance laws were integrated in the numerical code including Bingham, Herschel–Bulkley and Voellmy fluid model. Firstly, our numerical model was validated using analytical solutions of a dam-break scenario and published data on a laboratory experiment. Secondly, three real events, which occurred in the northeastern part of the Iberian Peninsula, were back-calculated. Although field observations of the three events are not very detailed, the back-analyses revealed interesting patterns on the flow dynamics, and the numerical results generally showed good agreement with field data. Comparing the different flow resistance laws, the Voellmy fluid model presents the best behaviour regarding both the flow behaviour and the deposit characteristics. Preliminary simulation runs incorporating the effect of basal entrainment offered satisfactory results, although the final volume is rather sensitive on the selected friction angle of channel-bed material. The outcomes regarding the correction of the calculated front inclination of the final deposit showed that this implementation strongly improves the simulation results and better represents steep fronts of final deposits.     

鄂尔多斯高原地下水流系统的多层结构循环模式—来自深孔中PACKER系统分层水头测定的证据

地下水流系统和循环模式分析是研究地下水形成机理的基础,对正确认识和评价地下水资源具有重要意义。鄂尔多斯高原在含水系统和众多地下水排泄区的控制下,形成了多个不同规模、不同循环深度、相互独立的地下水流系统。PACKER系统分层试验测定的不同深度水头的数据证明,鄂尔多斯高原地下水流系统存在托斯多层水流模式,区域性水流系统一般包含浅循环、中间循环和深循环3个循环系统。浅循环系统的发育深度在200m以内,深循环系统的发育深度大于400m。     

Inversely-Mapped Analytical Solutions for Flow Patterns around and within Inclined Elliptic Inclusions in Fluid-Saturated Rocks

In this paper, an inverse mapping is used to transform the previously-derived analytical solutions from a local elliptical coordinate system into a conventional Cartesian coordinate system. This enables a complete set of exact analytical solutions to be derived rigorously for the pore-fluid velocity, stream function, and excess pore-fluid pressure around and within buried inclined elliptic inclusions in pore-fluid-saturated porous rocks. To maximize the application range of the derived analytical solutions, the focal distance of an ellipse is used to represent the size of the ellipse, while the length ratio of the long axis to the short one is used to represent the geometrical shape of the ellipse. Since the present analytical solutions are expressed in a conventional Cartesian coordinate system, it is convenient to investigate, both qualitatively and quantitatively, the distribution patterns of the pore-fluid flow and excess pressure around and within many different families of buried inclined elliptic inclusions. The major advantage in using the present analytical solution is that they can be conveniently computed in a global Cartesian coordinate system, which is widely used in many scientific and engineering computations. As an application example, the present analytical solutions have been used to investigate how the dip angle of an inclined elliptic inclusion affects the distribution patterns of the pore-fluid flow and excess pore-fluid pressure when the permeability ratio of the elliptic inclusion is of finite but nonzero values.     

Micromechanical approach to unsaturated water flow in structured geomaterials by two-scale computations

This article presents a micromechanical approach to the problem of unsaturated water flow in heterogeneous porous media in transient conditions. The numerical formulation is based on the two-scale model obtained previously by periodic homogenization. It allows for a coupled solution of the non-linear flow equations at macroscopic and microscopic scales and takes into account the macroscopic anisotropy of the medium and the local non-equilibrium of the capillary pressure. The model was applied to simulate two-dimensional water infiltration at constant flux into an initially dry medium containing inclusions of square and rectangular shapes. The numerical results showed the influence of the inclusion–matrix conductivity ratio and the local geometry on the macroscopic behavior. The influence of the conductivity ratio manifested itself by the acceleration or retardation of the onset of the macroscopic water flux at the outlet, while the local geometry (anisotropy) significantly affected the macroscopic spatial distribution of the water flux. Such type of approach can be extended to simulate coupled phenomena (for example hydro-mechanical problems) with evolving local geometry.     

中强地震前后能量分布的自仿射特性

根据简化的地震波传播理论,“漫化”地震能量在空间的分配,减弱能量空间分布概率的极端情形及消除统计域为“空”的情形,由此构造类似于二维ｆＢｍ的地震能量空间分布概率曲面。与起伏不平的地形相似,由于基在水平方向及垂直方向的变化不是等比例的,因而这一分形曲面可能是统计自仿射的。     

The dynamics and thermodynamics of large ash flows

 Ash flow deposits, containing up to 1000 km³ of material, have been produced by some of the largest volcanic eruptions known. Ash flows propagate several tens of kilometres from their source vents, produce extensive blankets of ash and are able to surmount topographic barriers hundreds of metres high. We present and test a new model of the motion of such flows as they propagate over a near horizontal surface from a collapsing fountain above a volcanic vent. The model predicts that for a given eruption rate, either a slow (10–100 m/s) and deep (1000–3000 m) subcritical flow or a fast (100–200 m/s) and shallow (500–1000 m) supercritical flow may develop. Subcritical ash flows propagate with a nearly constant volume flux, whereas supercritical flows entrain air and become progressively more voluminous. The run-out distance of such ash flows is controlled largely by the mass of air mixed into the collapsing fountain, the degree of fragmentation and the associated rate of loss of material into an underlying concentrated depositional system, and the mass eruption rate. However, in supercritical flows, the continued entrainment of air exerts a further important control on the flow evolution. Model predictions show that the run-out distance decreases with the mass of air entrained into the flow. Also, the mass of ash which may ascend from the flow into a buoyant coignimbrite cloud increases as more air is entrained into the flow. As a result, supercritical ash flows typically have shorter runout distances and more ash is elutriated into the associated coignimbrite eruption columns. We also show that one-dimensional, channellized ash flows typically propagate further than their radially spreading counterparts. As a Plinian eruption proceeds, the erupted mass flux often increases, leading to column collapse and the formation of pumiceous ash flows. Near the critical conditions for eruption column collapse, the flows are shed from high fountains which entrain large quantities of air per unit mass. Our model suggests that this will lead to relatively short ash flows with much of the erupted material being elutriated into the coignimbrite column. However, if the mass flux subseqently increases, then less air per unit mass is entrained into the collapsing fountain, and progressively larger flows, which propagate further from the vent, will develop. Our model is consistent with observations of a number of pyroclastic flow deposits, including the 1912 eruption of Katmai and the 1991 eruption of Pinatubo. The model suggests that many extensive flow sheets were emplaced from eruptions with mass fluxes of 10⁹–10¹⁰ kg/s over periods of 10³–10⁵ s, and that some indicators of flow "mobility" may need to be reinterpreted. Furthermore, in accordance with observations, the model predicts that the coignimbrite eruption columns produced from such ash flows rose between 20 and 40 km. Received: 25 August 1995 / Accepted: 3 April 1996     

SOME NEW UNDERSTANDINGS OF LAND-SURFACE PROCESSES IN ARID AREA FROM THE HEIFE*

The survey for the HEIFE(Atmosphere-Land Surface Processes Experiment at Heihe River Basin,Western China) is given in the paper.The following basic subjects for land-surface processes in arid areas are studied:(1) the general characteristics of the energy budget on ground surface in arid areas;(2) the parameterization of the land surface processes;(3) the interaction between oasis and its desert circumstances,a special phenomenon in arid areas.The analysis shows that the sensible heat flux in the surface energy budget is in the majority,and the latent heat flux may be neglected.The influence of atmospheric stratification stability on the turbulent transfer of energy and substance must be considered in parameterization of land surface processes in arid areas.The "cold island effect" phenomenon in oasis and the "humidity inversion" phenomenon in desert near oasis are the result of the interaction between them.The results would improve the understanding of land surface processes in arid areas.     

Accelerating ground warming in the Canadian Prairie provinces: Is it a result of global warming?

The results of precision temperature logs made to depths of several hundred meters in some 80 wells in Western Canada, most of which are located in the Prairie Provinces, show evidence of warming at the ground surface in the 0.5 K to 3.5 K range (average=2.2±0.7 K, for 80 unevenly distributed sites). Modeling shows that this warming mostly pertains to this century and it has been most substantal in the last four decades if the ramp function of the linear increase of surface temperature is assumed. Using the step function model's increase of surface temperature (land clearing, forest fires, etc.) the calculated onset of warming would pertain mostly to the last two decades. Contour maps of ground temperatures currently and previously and a contour map of the ground warming magnitude dilineate a large regional character of the ground temperature change at the southern marigin of permafrost for the large area of the Prairie Provinces. In many cases however, the magnitude of ground warming is much larger than the magnitude of air warming. This is especially evident for the northern areas of Alberta in the boreal forest ecoprovince. The magnitude of ground warming is equal to the magnitude of surface air warming in southern Alberta in the grassland and aspen parkland ecoprovinces. The analysis of the temperature depth response to the surface warming from well data shows the integrated effect of surface air warming together with the increases in ground temperature due to natural terrain effects and other anthropogenical changes to the surface of the earth.     

Energy balance of simple elastodynamic sources

Complete relations are derived for energy and energy flux of elastic waves generated by an isotropic and double-couple source in a perfectly elastic, homogeneous, isotropic, and unbounded medium. In the energy balance of elastodynamic sources near-field waves play an essential role, transforming static energy into wave energy, andvice versa. For explosive and dislocation sources, the sources surface radiates a positive wave energy that is partially distributed to the medium transforming into static energy. For implosive and antidislocation sources, the source surface generates elastic waves, but it does not necessarily imply that it also radiates a positive wave energy. The energy transported by waves can originate in gradual transformation of the static-to-wave energy during propagation of waves through a stressed medium.On leave from Geophysical Institute, Czech Academy of Sciences, Boní II/1401, 41 31, Praha 4 Czech Republic     

Textural segmentation of digital rock images into bedding units using texture energy and cluster labels

The texture of digital rock images, as recorded, for instance, with borehole imaging devices, is shown to reflect different bedding types. Textural segmentation of borehole images, therefore, subdivides the recorded sequence into bedding units. We show that a textural segmentation algorithm based on the concept of texture energy achieves good results when compared with synthetic as well as real data in which petroleum geologists have performed zonations on cores. Texture energy involves filtering of the original image with a set of texture sensitive masks. The filtering is done as a finite convolution over the size of the masks. On the resulting images the variance is computed over a relatively large sliding window, which, in its practical implementation, covers the full width of the image. The resulting nine one-dimensional curves are then clustered hierarchically into a user-determined number of image texture or lithological bedding classes. Principal component analysis previous to clustering can be used to reduce redundancy in the data. A recurring and relatively ill-defined problem in this field are macro-textures, i.e., the cyclic interbedding of two or more bedding types. We show that sliding Fourier transforms and variable mask scale can successfully address the zonation of macro-textures. In general, the method gives best results with mask sizes equivalent to 2–4 centimeters, reflecting the length scale at which the investigated geological bedding seems to have its highest variation.