早期碳酸盐胶结作用对砂岩孔隙演化的影响——以塔里木盆地满加尔凹陷志留系砂岩为例

塔里木盆地满加尔凹陷志留系砂岩为无障壁的潮坪、滨岸及辫状河三角洲沉积的岩屑砂岩和岩屑质石英砂岩,对满北和满东、满南地区志留系砂岩成岩作用序次尤其是碳酸盐胶结的时间、方式和特征、物性的对比研究认为:砂岩中碳酸盐胶结的时间早晚对砂岩孔隙演化具有重要影响,满北地区志留系砂岩发育有大量早期泥晶碳酸盐胶结,显微镜下研究表明这种胶结作用发生在岩石受到充分压实之前,呈基底式胶结,岩石颗粒呈点接触或漂浮状分布于早期碳酸盐胶结物之中,这种胶结作用抑制了岩石后期的压实作用。尽管目前其埋藏达5 000～6 500 m,但其经历的压实作用并不强烈,粒间发育大量早期的碳酸盐胶结物,后来这些胶结物被溶蚀,形成了大量次生孔隙。而在满东和满南地区的志留系砂岩,碳酸盐胶结发生在岩石经过充分压实之后,晚期的碳酸盐胶结物仅分布于岩石经充分压实后的粒间孔隙中,含量较前者低,后期可供溶蚀的碳酸盐比前者少,因而溶蚀形成的次生孔隙有限。因此,尽管满东满南地区志留系埋藏比满北浅（3 700～5 000 m）,碳酸盐胶结物含量也低,也发育晚期碳酸盐胶结物的溶蚀,但岩石的压实作用比满北强,物性比满北低差。这主要与碳酸盐胶结的早晚有关,早期碳酸盐胶结对孔隙演化具有明显控制作用,有利于岩石孔隙的保存,为后来的溶蚀形成次生孔隙提供了条件。     

重要性不同的现有结构抗震加固设计地震动参数研究

提出了重要性不同的现有结构抗震加固设计基准期取值原则,给出了现有结构抗震加固设防水准的定义。基于地震危险性特征分区,利用地震动参数的危险性曲线公式对重要性不同的现有结构抗震加固设计地震动参数进行了分析。同时,为了便于工程设计人员使用,给出了现有结构设计地震动参数取值的简化方法。最后,通过算例解释了理论方法和简化计算方法中抗震加固设计地震动参数的取值方法和步骤。     

The post-industrialising city: political perspectives and cultural biases

Many Western cities face marked changes that are associated with post-industrialisation of the urban environment. In this article, four political perspectives on the threats and opportunities associated with post-industrialisation are discussed: liberal productivism; communitarianism; institutional perfectionism and participative democracy. In cultural terms these four approaches appear to be remarkably biased. As such, they are unlikely to match the complicated, confusing and unpredictable processes that are taking place in Western cities. The complexities of post-industrialisation demand cultural variety in the public domain. This, in turn, requires connectivity and openness in the institutions that structure urban politics and urban policy making. This revised version was published online in August 2006 with corrections to the Cover Date.     

SPM response to tide and river flow in the hyper-turbid Ems River

In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.     

土壤水盐运动模式研究及其在干旱区农田的应用

采用预估校正法求解土壤水分运动的h型Richards方程,此方法可以控制一定的迭代步数,但同样可达到二阶精度。同时采用具有二阶高精度的差分法求解土壤盐分运移的对流扩散方程,此格式可避免采用一般差分格式时出现的数值弥散问题。并将上述数学模式对干旱区绿洲棉田,在蒸散条件下土壤水分运动和土壤盐分向上运动积累过程进行数值模拟,模拟结果与测量值吻合较好,可为预报土壤盐碱化提供科学依据。     

Groundwater impact on environmental flow needs of streams in sandy catchments in the Netherlands

Abstract

During recent years, water managers and water users in the Netherlands experienced water shortages in numerous streams. Besides low rainfall amounts and high temperatures, anthropogenic alterations to the groundwater system are also responsible for the reduced baseflow in streams. These alterations may reduce resilience and increase a risk to streams as more droughts are expected in the Netherlands due to climate change. We propose a methodology to assess the impact of groundwater-related alterations and climate change on baseflow and environmental flow needs (EFN). Application of this methodology for two sandy catchments showed that, under average meteorological conditions, baseflow in the main streams still meets the EFN requirements. During dry years, baseflow is probably insufficient in the upper parts of the catchments. Anthropogenic alterations show a significant impact: drainage caused 25–40% baseflow reduction, groundwater abstractions caused 5–28% and climate change will potentially cause an additional reduction of 33–70% by 2050.

Editor D. Koutsoyiannis; Guest editor M. Acreman     

Species pool versus site limitations of macrophytes in urban waters

Biodiversity in urban areas is affected by a multitude of stressors. In addition to physico-chemical stress factors, the native regional species pool can be greatly reduced in highly urbanized landscapes due to area loss and fragmentation. In this study, we investigated how macrophyte composition and diversity in urban water systems are limited by the regional species pool and local environmental conditions. Canonical correspondence analysis of the macrophyte species composition revealed that urban and semi-natural water systems differed and differences could be related to local abiotic variables such as pH and iron concentrations. Macrophytes in the semi-natural area were typical for slightly acid and oligotrophic conditions. In urban water systems, exotic species characteristic of eutrophic conditions were present. In the semi-natural areas, the number of macrophyte species exceeded the number of species expected from species–area relationships of artificial water bodies in rural areas. In urban areas, the number of macrophyte species was similar to artificial water systems in rural areas. Macrophyte species present in the study areas also were generally found within 20–30 km distance to the study area. Macrophyte species composition in urban water systems and semi-natural water systems appeared to be influenced by the regional species pool within approximately 30 km of the locations. Nevertheless, site limitation ultimately determined the local macrophyte species composition and diversity in urban water systems and in semi-natural water systems.     

The role of overland flow and subsurface flow on the spatial distribution of soil moisture in the topsoil

Many investigations show relationships between topographical factors and the spatial distribution of soil moisture in catchments. However, few quantitative analyses have been carried out to elucidate the role of different hydrological processes in the spatial distribution of topsoil moisture in catchments. A spatially distributed rainfall—runoff model was used to investigate contributions of subsurface matric flow, macropore flow and surface runoff to the spatial distribution of soil moisture in a cultivated catchment. The model results show that lateral subsurface flow in the soil matrix or in macropores has a minor effect on the spatial distribution of soil moisture. Only when a perched groundwater table is maintained long enough, which is only possible if the subsurface is completely impermeable, may a spatial distribution in moisture content occur along the slope. Surface runoff, producing accumulations of soil moisture in flat flow paths of agricultural origin (field boundaries), was demonstrated to cause significant spatial variations in soil moisture within a short period after rainfall (<2 days). When significant amounts of surface runoff are produced, wetter moisture conditions will be generated at locations with larger upstream contributing areas. Copyright © 2001 John Wiley & Sons, Ltd.     

A Velocity-Based Approach to Visco-Elastic Flow of Rock

In structural geology, viscous creep is generally recognized as the major deformation mechanism in the folding of rock layers through geological time scales of hundreds of thousands of years. Moreover, since deformation of rock salt by creep takes already place on relatively small time scales—weeks to months, say—creep is a relevant phenomenon when studying salt mining, notably the convergence of mine cavities and the land subsidence caused by it. While creep is the dominant process on relatively long time scales, elasticity plays a dominant role in processes that take place on relatively short time scales. The elastic response to a stress is a displacement; the shape of the rock is deformed instantaneously with respect to its initial shape. However, the viscous response of a rock to a stress is a relatively low velocity in the order of millimeters per months or years, say. In this paper we consider the two deformation phenomena creep and elasticity. In general, elasticity is a compressible phenomenon, while creep is incompressible. Here we approximate creep by the introduction of a negligibly small amount of compressibility, which makes creep velocity calculations similar to conventional elastic displacement calculations. Using this procedure, a standard finite element package for elasticity can be applied to viscous problems, also in combination with elasticity. The method has been demonstrated to upscaling of creep viscosities.