Fuzzy factorial analysis of snow avalanches     

Claude Jaccard 《Natural Hazards》1990,3(4):329-340

The mutual influence of 21 factors pertaining to terrain, weather, forest and snowpack have been discussed by 10 experts. The semantic (nil, weak, medium, high) evaluations are translated as membership degrees of fuzzy sets, and averaged between the experts by taking their fuzzy expectation value, yielding a 21 × 21 fuzzy matrix for direct interactions. Fuzzy successive multiplications and additions of the matrix give the indirect interactions. The activity and the passivity of the factors with respect to avalanches is represented by directed weighted graphs, and the average semantic values by a 4 × 4 matrix: the activity (passivity) is weak (strong) for tree damage; medium (nil) for altitude; medium (weak) for ground shape; medium (medium) for vegetation, soil, wind, microclimate, tree type, vertical distribution and mechanics of trees, snowpack distribution and snow gliding; strong (nil) for ground inclination and exposition, weather; strong (medium) for heat, precipitation, horizontal distribution of trees, snowpack constitution and stability.  相似文献   

System parameters of soil foundations for time domain dynamic analysis     

Wen-Yu Jean  Tsung-Wu Lin  Joseph Penzien 《地震工程与结构动力学》1990,19(4):541-553

Soil-structure interaction analysis is usually carried out in the frequency domain, because the compliance functions of the half-space are known only in the frequency domain. Since non-linear analysis cannot be carried out in the frequency domain, a system with frequency independent parameters is used to represent the half-space soil medium so that a nonlinear analysis in the time domain becomes possible. The objective of this paper is to propose a system with lumped parameters, which are independent of frequency, to represent the half-space soil medium. The proposed frequency independent system consists of a number of real discrete structure elements; thus the existing dynamic analysis programs may be adoptable with little modification. In this paper, the parameters are found by minimizing the sum of the squares of deviations between the steady-state responses of the theoretical half-space model and those of the lumped parameter system over a specified frequency range. Once the parameters have been found, the lumped parameter system can be used in practical applications for time domain dynamic analysis of either linear or non-linear structures. In comparison with the dynamic response of the theoretical half-space model, the lumped parameter system yields satisfactory results.  相似文献   

Depositional mechanics of turbulent nuées ardentes (surges) from their grain sizes     

Fran?ois P Brissette  Jean Lajoie 《Bulletin of Volcanology》1991,53(1):60-66

The ability of turbulent nuées ardentes (surges) to transport coarse pyroclasts has been questioned on the basis that settling velocities of coarse fragments in the deposits are much too high for them to have been supported by turbulence in a dilute gas suspension. A computer model is used to evaluate the settling velocity of pyroclasts in suspensions of varying concentration and temperature. Since suspension of grains in low-concentration surges occurs if the shear velocity exceeds the settling velocity, the shear velocities related to the 16th and 84th percentiles, and the mean of the grain-size distribution are compared in surge deposits of the Vulsini, with the shear velocity necessary to move the coarsest grain on the bed surface (the Shields criterion). The results show that the settling velocities do not vary significantly in gaseous suspensions having volume concentrations lower than 15%, and that an increase in concentration to 25% is not sufficient to decrease the settling velocity of the coarser fraction, if it represents flow shear velocity. It is shown that the settling velocity of the mean grain size (M _z ) best depicts the shear velocity of a dilute turbulent suspension. Applying the results to the May 1902 paroxysmal nuées ardentes of Mount Pelée shows that the estimated mean velocities are well within the observed velocities, and sufficient to support all the clasts in dilute, turbulent suspensions.  相似文献   

Carboxylic monoacids in the air of mayombe forest (Congo): Role of the forest as a source or sink     

Jean Servant  Georges Kouadio  Bernard Cros  Robert Delmas 《Journal of Atmospheric Chemistry》1991,12(4):367-380

In the tropical rain forests of the Congo during the dry season, from June to September 1987, carboxylic acid partial pressures (P _gas) in the air above the canopy, at ground level, and at the boundary layer, were estimated from water samples such as fog and rainwater. The concentrations of these acids were also measured in the sap of tree leaves. Tree leaves act as a sink or as a source if the acid P _gas is greater of lower than the acid concentrations in molecular form in sap. For each of these soluble gases, there is a value of P _gas where the exchange is nul. This is called the compensation point. Values of the compensation point for some tree leaves were evaluated according to Henry's law. Henry's law coefficients at ppm levels were redetermined for formic (HCOOH), acetic (CH₃COOH), propionic (CH₃CH₂COOH), and isobutyric (CH₃CH(CH₃)COOH) acids.By comparison of P _gas and compensation points, it is concluded that the forest was a potential source for these acids. The soil-or the litter-acts as a significant source of a carboxylic acid of C₃ or C₄ atoms in the aliphatic chain. This carboxylic acid, not yet fully characterized, could play an important role in the rain acidity in forested zones of the equatorial areas.The direct emission of these carboxylic acids by vegetation was the main source in the boundary layer above the forest. The average emissions were 3.1×10⁹, 7.8×10⁹, and 8.4×10⁹ molecules cm^-2 s^-1 for HCOOH, CH₃COOH, and CH₃CH₂COOH, respectively. The savanna is an exogenous source of HCOOH and CH₃CH₂COOH during moderately rainy days for 30% of the time. The ozonolysis of isoprene seems to be a small source of HCOOH.  相似文献   

The African rain forest vegetation and palaeoenvironments during late quaternary   总被引：4，自引：0，他引：4  

Jean Maley 《Climatic change》1991,19(1-2):79-98

This review paper presents first the main pollen results on the vegetation history of the rain forest during the late Quaternary.- The Lake Bosumtwi record (Ghana) shows the disappearance of rain forest from the base of the core (ca. 28 000 yr BP) to ca. 9000 yr BP. During this time interval the vegetation was of montane type with sparse clumps of trees. There is synchronism between montane vegetation disappearance and rain forest reappearance. This phenomenon occurred abruptly around 9000 yr BP.- The Lake Barombi Mbo record (West Cameroon) shows clearly that from ca. 24 000 yr BP until the present time, rain forest persisted with limited variations, and thus, this area represents a refuge area.From these data and other, one concludes that Afromontane vegetation extended to lowland during cool and humid phases.Other palaeoenvironmental data were obtained by diverse geological analyses of the lacustrine sediments. For Bosumtwi, the relatively precise reconstruction of lake-level fluctuations permitted several palaeoclimatic interpretations for the main Holocene phases.For Barombi Mbo, the evolution of total organic carbon (TOC) and total nitrogen (TON) seems to be related mainly to temperature evolution. By comparison with present-day mountain environments, TOC and TON increase in cool environments, but decrease when warmth and humidity increase, as during Holocene time, because the recycling processes speed up in the topsoil. For the same period the alteration of the soils in the catchment produced a strong increase of kaolinite. All these change intervened ca. 9500 yr BP, which is a key date in tropical Africa.In conclusion, climatic correlations between equatorial and dry north tropical Africa illustrate how changes in the forest block must have important effects on adjacent climatic zones.  相似文献   

Volcanic hazards in the surroundings of Pico de Orizaba,eastern Mexico     

Ármann Höskuldsson  Jean Marie Cantagrel 《Natural Hazards》1994,10(3):197-219

Volcanic hazards from Pico de Orizaba volcano are presented here tor the first time. Some 1.3 million people live within the hazard zone, which in the most severe case would encompass the Mexican Gulf coast, east of the volcano. Three major cities located in the eastern part of the hazard zone account for 800 000 of this population and about 200 000 people live within a 20 km radius of the volcano. Probability calculations are presented as an attempt to quantify the hazards in the surroundings of the volcano. Such quantification can be of use in planning for future land use within the hazard zones.A zone of about 10 km radius centred on the top crater is a high hazard zone for gravity-driven flows and fallout ejecta. For large volume eruptions, the radius could be extended to 120 km to the east and 60 km to the west. The asymmetrical distribution is related to the topography of the volcano. Hazards from Pyroclastic-fall deposits are principally to the west of the volcano, since easterly winds are dominant in the area lava-flow hazards are greatest within a 10 km radius from the summit crater. Pyroclastic flow hazards are high up to 20 km from the volcano summit.In the case of reactivation of the volcano, melting of a glacier covering the summit of Pico de Orizaba having a volume equivalent to some 45 × 10⁹ litres of water, would produce lahars which would descend the flanks of the volcano.  相似文献   

Book review     

Jean O. Dickey  H. Schmid 《Journal of Geodesy》1989,63(3):VI-X

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Convective instabilities in a variable viscosity fluid cooled from above     

Claude Jaupart  Barry Parsons 《Physics of the Earth and Planetary Interiors》1985,39(1):14-32

Whether in the mantle or in magma chambers, convective flows are characterized by large variations of viscosity. We study the influence of the viscosity structure on the development of convective instabilities in a viscous fluid which is cooled from above. The upper and lower boundaries of the fluid are stress-free. A viscosity dependence with depth of the form ν₀ + ν₁ exp(?γ.z) is assumed. After the temperature of the top boundary is lowered, velocity and temperature perturbations are followed numerically until convective breakdown occurs. Viscosity contrasts of up to 10⁷ and Rayleigh numbers of up to 10⁸ are studied.For intermediate viscosity contrasts (around 10³), convective breakdown is characterized by the almost simultaneous appearance of two modes of instability. One involves the whole fluid layer, has a large horizontal wavelength (several times the layer depth) and exhibits plate-like behaviour. The other mode has a much smaller wavelength and develops below a rigid lid. The “whole layer” mode dominates for small viscosity contrasts but is suppressed by viscous dissipation at large viscosity contrasts.For the “rigid lid” mode, we emphasize that it is the form of the viscosity variation which determines the instability. For steep viscosity profiles, convective flow does not penetrate deeply in the viscous region and only weak convection develops. We propose a simple method to define the rigid lid thickness. We are thus able to compute the true depth extent and the effective driving temperature difference of convective flow. Because viscosity contrasts in the convecting region do not exceed 100, simple scaling arguments are sufficient to describe the instability. The critical wavelength is proportional to the thickness of the thermal boundary layer below the rigid lid. Convection occurs when a Rayleigh number defined locally exceeds a critical value of 160–200. Finally, we show that a local Rayleigh number can be computed at any depth in the fluid and that convection develops below depth z_r (the rigid lid thickness) such that this number is maximum.The simple similarity laws are applied to the upper mantle beneath oceans and yield estimates of 5 × 10¹⁵?5 × 10¹⁶ m² s^?1 for viscosity in the thermal boundary layer below the plate.  相似文献   

Petrology of the mafic rocks of the Xigaze ophiolite,Tibet   总被引：2，自引：0，他引：2  

Jacques Girardeau  Jean Claude C Mercier  Wang Xibin 《Contributions to Mineralogy and Petrology》1985,90(4):309-321

The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.  相似文献   

Geological setting of hydrothermal activity at 12°50′N on the East Pacific Rise: A submersible study     

R.D. Ballard  Roger Hekinian  Jean Francheteau 《Earth and Planetary Science Letters》1984,69(1):176-186

A detailed submersible investigation of a 20-km segment of the East Pacific Rise near 12°50′N between the Orozco and Clipperton fracture zones has resulted in the localization of 24 active hydrothermal vent fields and over 80 sites of sulfide accumulations. The active vents range from low-temperature vents characterized by exotic benthic communities to high-temperature “black smokers” and the deposition of polymetallic sulfides. The study is based upon a combination of fine scale topography obtained using the SEABEAM sonar system on N/O “Jean Charcot”, camera lowerings along the axis using the RAIE vehicle, and 32 dives by the submersible “Cyana” operating from N/O “Le Suroit”. The observations made between the Orozco and Clipperton fracture zones show topographic highs situated along the strike of the accreting plate segment separated by a small ridge offset at 11°49′N. This offset divides this portion of the ridge into two separate spreading segments each of which has a primary topographic high along strike. Secondary highs are associated with each segment of the ridge separated by either small offsets (or relay zones) or in some cases, zones where spreading centers overlap. Dives made on the tops of both primary highs (12°50′N and 11°30′N) confirm the presence inferred from previous surface work of high-temperature vent fields while one reconnaissance dive (14°20′N) near the Orozco fracture zone/ridge axis intersection reveals the absence of any hydrothermal activity in the present or recent past. The vast majority of vent fields investigated were found at the topographic high near 12°50′N, are associated with the most recent period of volcanism, and are confined to lava ponds situated within the axial graben.  相似文献