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71.
In this study, a capillary barrier system was designed and tested for an arid land environment. To simulate arid land conditions
of high temperature and sub-irrigation systems, the barrier was subjected to thermal and hydraulic gradients in opposite directions;
to test the barrier system under these severe conditions, an experimental apparatus was designed and fabricated. The multilayer
capillary barrier consisted of three layers made of silica sand, a mixture of sand and bentonite in equal portions, and a
mixture of clay (25%) and aggregate (75%). Several one dimensional coupled heat and moisture tests were performed. Temperature
variations along the thickness of the barrier were recorded as a function of time, and at the end of each test, the barrier
was sliced into small sections, for the determination of volumetric water content as a function of distance from the heat
source. The experimental results were discussed in view of the barrier's intended purpose of its ability to store moisture
for long time durations.
Coupled heat and moisture flow equations were developed and solved numerically via a finite difference method. Diffusivity
parameters were calculated by using experimental results, a numerical model, and Powell's conjugate directions method of nonlinear
optimization. The model was calibrated and the results were discussed. Good agreement between calculated and experimental
results was obtained.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
72.
A coupled hydro-chemo-mechanical numerical model is developed for these coupled phenomena in many engineering fields. The
model has been applied to predicting the response of a stressed rockmass column to an injected reactive fluid (reagent) flow.
The response includes evolutions of porosity, permeability, reagent and mineral concentrations during dissolution. In the
model, the progress of dissolution is defined by the change in porosity ratio and the porosity increases with dissolution
assuming there is no precipitation. The numerical evolutions of porosity, permeability, reagent and mineral concentrations
during dissolution are validated against steady state solutions. The model results show that these evolutions are regulated
to a certain extent by the applied external loadings: an applied extensional stress enhances the progress of the dissolution
process while an applied compression stress slows the progress of the dissolution process.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
73.
中国西部干旱区生态环境演变过程 总被引:10,自引:13,他引:10
根据湖沼沉积、黄土沉积和冰积等地质记录,运用地理信息系统、遥感、生态景观学等方法.从万年、千年和百年及百年以下四个时间尺度对中国西部干旱区生态环境演变过程进行研究。结果表明,全新世以来,中国西部干旱区的气候变化经历了多次的相对暖干和相对冷湿交替变化,变化历程较为复杂。早全新世、中全新世、晚全新世气候特点因地因时而异,但总体特征以干旱化为主,280~350a来,降水量有明显的减少趋势,并表现出显著的周期性。但在近半个世纪以来,中国天山地区的气候变化特征表现为气温升高,降水量增加。总的来说,从20世纪80年代以来,新疆的气候趋于好转,植被覆盖状况改善明显,山地森林、草甸、人工绿洲的面积均呈增加趋势。研究结果还表明,在一定尺度的气候背景条件下,地表的植被覆盖对调节绿洲区域气候有着积极的作用,可增加局部地区的降水。 相似文献
74.
Abstract Spatial and temporal variations in radiative fluxes influence glacier mass‐balance in mountain areas. The primary goal of this study was to assess differences in solar radiation on three glacial cirques located in the Maladeta Mountain massif (Central Spanish Pyrenees), and analyse their implications on glacier development and morphology. A quantitative approach is adopted to obtain the values of solar radiation (direct, diffuse and global radiation), combining several field data parameters (measured at 55 control‐points) with the solar radiation modelling package Ecosim. The data obtained confirm that the morphologies of the glacial bodies developed in the three cirques have a good correlation with the spatial variation on solar radiation inputs, favouring also the conservation (Aneto and Coronas cirques) or total vanishing (Llosás cirque) of the glacial remnants analysed here. The study shows how strongly in this Alpine‐Mediterranean context solar radiation — firstly as a function of latitude and time of year, and locally as a function of topographic slope, aspect and shadowing — controls the mass‐balance and the spatial distribution of melting in small glaciers, having an effect on the development of their morphologies. 相似文献
75.
Abstract Examples of ventifacts and forms of unusual patterned ground that differ in situation, characteristics and/or origin to types reported previously are described from the Falkland Islands, South Atlantic. The ventifacts, represented by clasts projecting from an eroding tillite shore platform, have been grooved by the passage of windblown sand. The patterned ground forms comprise stone polygons on clay loam regolith, stone polygons on peat, peat‐filled desiccation cracks on peat, and clusters of on‐edge clasts (stone packings) on a gravel beach. Aeolian transport of particles is considered responsible for the infill of the polygonal cracks. The clusters of on‐edge clasts are thought to have developed as a result of wave action, in particular backwash and/or undertow. Although these features occupy small surface areas they provide useful information about local geomorphic processes and testify to the marked diversity of such features in nature. 相似文献
76.
Postglacial topographic evolution of glaciated valleys: a stochastic landscape evolution model 总被引:1,自引:0,他引:1
The retreat of valley glaciers has a dramatic effect on the stability of glaciated valleys and exerts a prolonged influence on the subsequent fluvial sediment transport regime. We have studied the evolution of an idealized glaciated valley during the period following retreat of ice using a numerical model. The model incorporates a stochastic process to represent deep‐seated landsliding, non‐linear diffusion to represent shallow landsliding and an approximation of the Bagnold relation to represent fluvial sediment transport. It was calibrated using field data from several recent surveys within British Columbia, Canada. We present ensemble model results and compare them with results from a deterministic linear‐diffusion model to show that explicit representation of large landslides is necessary to reproduce the morphology and channel network structure of a typical postglacial valley. Our model predicts a rapid rate of fluvial sediment transport following deglaciation with a subsequent gradual decline, similar to that inferred for Holocene time. We also describe how changes in the model parameters affect the estimated magnitude and duration of the paraglacial sediment pulse. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
77.
Introduction The Xiuyan earthquake(MS=5.4)on Nov.29,1999,occurred between Hushan town and Pi-anling town at the southeast end of the aftershocks of Haicheng earthquake(MS=7.3).These two earthquakes′sequences are highly similar in the way that the foreshocks are very frequent,focal mechanisms of the main shocks are about the same and fault strikes are approximately in the same direction.It is important to study these earthquakes for further research on seismogenesis to fore-cast earthquake… 相似文献
78.
G. Molchan 《Pure and Applied Geophysics》2005,162(6-7):1135-1150
This paper presents an analysis of the distribution of the time τ between two consecutive events in a stationary point process.
The study is motivated by the discovery of unified scaling laws for τ for the case of seismic events. We demonstrate that
these laws cannot exist simultaneously in a seismogenic area. Under very natural assumptions we show that if, after rescaling
to ensure Eτ =1, the interevent time has a universal distribution F, then F must be exponential. In other words, Corral’s unified scaling law cannot exist in the whole range of time. In the framework
of a general cluster model we discuss the parameterization of an empirical unified law and the physical meaning of the parameters
involved.
An erratum to this article is available at . 相似文献
79.
80.
Relief-rejuvenation and topographic length scales in a fluvial drainage basin, Napf area, Central Switzerland 总被引:2,自引:2,他引:2
This paper explores how, and to what extent, a phase of relief-rejuvenation modifies the mode of surface erosion in an approximately 63 km2 drainage basin located at the northern border of the Swiss Alps (Luzern area). In the study area, the retreat of the Alpine glaciers at the end of the Last Glacial Maximum (LGM) caused base level to lower by approximately 80 m. The fluvial system adapted to the lowered base level by headward erosion. This is indicated by knickzones in the longitudinal stream profiles and by the continuous upstream narrowing of the width of the valley floor towards these knickzones. In the headwaters above these knickzones, processes are still to a significant extent controlled by the higher base level of the LGM. There, frequent exposure of bedrock in channels and especially on hillslopes implies that sediment flux is to a large extent limited by weathering rates. In the knickzones, however, exposure of bedrock in channels implies that sediment flux is supply-limited, and that erosion rates are controlled by stream power.The morphometric analysis reveals the existence of length scales in the topography that result from distinct geomorphic processes. Along the tributaries where the upstream sizes of the drainage basins exceed 100,000–200,000 m2, the mode of sediment transport and erosion changes from predominantly hillslope processes (i.e., landsliding, creep of regolith, rock avalanches and to some extent debris flows) to processes in channels (fluvial processes and debris flows). This length scale reflects the minimum size of the contributing area for channelized processes to take over in the geomorphic development (i.e., threshold size of drainage basin). This threshold size depends on the ratio between production rates of sediment on hillslopes, and export rates of sediment by processes in channels. Consequently, in the headwaters, erosion rates and sediment flux, and hence landscape evolution rates, are to a large extent limited by weathering processes. In contrast, in the lower portion of the drainage basin that adjusts to the lowered base-level, rates of channelized erosion and relief formation are controlled mainly by stream power. Hence, this paper shows that base-level lowering, headward erosion and establishment of knickzones separate drainage basins in two segments with different controls on rates of surface erosion, sediment flux and relief formation. 相似文献