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381.
对3种不同中高层砌块砌体结构(分别为10层、15层和18层砌块砌体结构)进行大量时程分析,对结构的惯性力分布进行研究,提出一种新的适用于中高层砌块砌体结构Pushover分析的侧向力分布模式-抛物线型分布模式,并用该模式分别对3种砌块砌体结构进行Pushover分析。通过将Pushover得到的结构能力曲线与时程分析方法得到的结构能力曲线进行比较,将10层砌块砌体结构的Pushover分析结果与一座1/4比例的10层砌块砌体模型结构的振动台试验结果进行比较。比较结果表明:笔者所提出的侧向力分布模式适用于中高层砌块砌体结构的Pushover分析。 相似文献
382.
383.
Dore MH 《Environment international》2005,31(8):1167-1181
The objective of this paper is to synthesize the large literature recording changing patterns of precipitation in the observed data, thus indicating that climate change is already a reality. Such a synthesis is required not only for environmental researchers but also for policy makers. The key question is the broad picture at major regional and continental levels. Some interesting conclusions for this survey are emerging. For example, the review shows increased variance of precipitation everywhere. Consistent with this finding, we observe that wet areas become wetter, and dry and arid areas become more so. In addition, the following general changing pattern is emerging: (a) increased precipitation in high latitudes (Northern Hemisphere); (b) reductions in precipitation in China, Australia and the Small Island States in the Pacific; and (c) increased variance in equatorial regions. The changes in the major ocean currents also appear to be affecting precipitation patterns. For example, increased intensity and frequency of El Ni?o and ENSO seem associated with evidence of an observed "dipole" pattern affecting Africa and Asia, although this time series is too short so far. But the changing pattern calls for renewed efforts at adaptation to climate change, as the changing precipitation pattern will also affect the regional availability of food supply. 相似文献
384.
The formation of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDDs/Fs) from carbon that was produced by the pyrolysis of paper fibers and from
wood charcoal was investigated experimentally. Fibers obtained from filter paper were pyrolyzed at 300° and 800°C to produce
low- and high-temperature carbon samples. The two types of carbon and wood charcoal were mixed with silica (SiO2) and trace copper oxide to produce three synthetic fly ash samples. Experiments to measure the formation of PCCDs/Fs from
the three ash samples were conducted using a bench-scale reactor. The two carbon samples derived from paper fibers generated
more PCDDs/Fs than was generated by the wood charcoal. The PCDDs/Fs generated by the low-temperature carbon and by the wood
charcoal were dominated by the lower-chlorinated PCDFs. Such unique homologue distribution patterns are very similar to those
generated by the open burning of household waste. The high-temperature carbon generated more highly chlorinated PCDDs/Fs.
The effect of pyrolysis temperature on the de novo formation of PCDDs/Fs from residual carbon is discussed. Paper and paper
products contained in household waste are likely to be the source of unburned carbon that contributes to high PCDD/F emissions
in the open burning of household waste. 相似文献
385.
386.
Legitimizing Fluvial Ecosystems as Users of Water: An Overview 总被引:6,自引:0,他引:6
Naiman RJ Bunn SE Nilsson C Petts GE Pinay G Thompson LC 《Environmental management》2002,30(4):455-467
We suggest that fluvial ecosystems are legitimate users of water and that there are basic ecological principles guiding the
maintenance of long-term ecological vitality. This article articulates some fundamental relationships between physical and
ecological processes, presents basic principles for maintaining the vitality of fluvial ecosystems, identifies several major
scientific challenges and opportunities for effective implementation of the basic ecological principles, and acts as an introduction
to three specific articles to follow on biodiversity, biogeochemistry, and riparian communities. All the objectives, by necessity,
link climate, land, and fresh water. The basic principles proposed are: (1) the natural flow regime shapes the evolution of
aquatic biota and ecological processes, (2) every river has a characteristic flow regime and an associated biotic community,
and (3) aquatic ecosystems are topographically unique in occupying the lowest position in the landscape, thereby integrating
catchment-scale processes. Scientific challenges for the immediate future relate to quantifying cumulative effects, linking
multidisciplinary knowledge and models, and formulating effective monitoring and assessment procedures. Additionally, forecasting
the ecological consequences of changing water regimes is a fundamental challenge for science, especially as environmental
issues related to fresh waters escalate in the next two to three decades. 相似文献
387.
Undamming Rivers: A Review of the Ecological Impacts of Dam Removal 总被引:26,自引:4,他引:22
Bednarek AT 《Environmental management》2001,27(6):803-814
Dam removal continues to garner attention as a potential river restoration tool. The increasing possibility of dam removal
through the FERC relicensing process, as well as through federal and state agency actions, makes a critical examination of
the ecological benefits and costs essential. This paper reviews the possible ecological impacts of dam removal using various
case studies.
Restoration of an unregulated flow regime has resulted in increased biotic diversity through the enhancement of preferred
spawning grounds or other habitat. By returning riverine conditions and sediment transport to formerly impounded areas, riffle/pool
sequences, gravel, and cobble have reappeared, along with increases in biotic diversity. Fish passage has been another benefit
of dam removal. However, the disappearance of the reservoir may also affect certain publicly desirable fisheries.
Short-term ecological impacts of dam removal include an increased sediment load that may cause suffocation and abrasion to
various biota and habitats. However, several recorded dam removals have suggested that the increased sediment load caused
by removal should be a short-term effect. Preremoval studies for contaminated sediment may be effective at controlling toxic
release problems.
Although monitoring and dam removal studies are limited, a continued examination of the possible ecological impacts is important
for quantifying the resistance and resilience of aquatic ecosystems. Dam removal, although controversial, is an important
alternative for river restoration. 相似文献
388.
389.
21世纪初我国粮食供需的新空间格局 总被引:41,自引:1,他引:40
利用2000-2003年县级统计数据,揭示21世纪初我国粮食供需的空间格局。我国粮食供需平衡以东西分异规律为主,其次是南北分异。地势第二级阶梯以东,主要余粮区分布最广,北方明显多于南方;胡焕庸线以西,常年缺粮区为主;两线之间,潜在缺粮区居多。东北地区、华北平原生产全国近70%的余粮。长江中下游以南地区既是我国温饱水平缺粮总数最多的地区,也是第三大余粮生产区。地势第二级阶梯上的农牧交错带,温饱水平缺粮总量列第二位,是我国粮食安全最脆弱的地区。上述地区主要位于环境变化敏感地带,粮食安全自然风险最大。因此,加强环境变化对粮食供需平衡的影响研究,对深入认识我国粮食供需平衡的稳定性具有重要的现实意义和理论价值。 相似文献
390.
利用1997~2003年ATSR卫星火点资料,采用前向气流轨迹模式及滞留时间分析方法,研究了欧亚大陆生物质燃烧排放的时空分布特征及主要生物质燃烧区域典型排放季节低层气团的输送特征,并与MOPITT一氧化碳卫星反演资料进行对比.结果表明,春季东南亚及印度半岛生物质燃烧排放的污染气团可对中国西南、华南及华东地区的大气环境造成一定影响,尤其是四川盆地、福建和台湾周边地区;中亚、东西伯利亚及蒙古高原生物质燃烧排放的污染气团主要受西风带影响而向东输送,但明显存在一条分支经中国东北向华北或更低纬度地区输送,并逐渐下沉至大气低层,在生物质燃烧排放剧烈的年份内能够对该地区的低层大气成分造成重要影响. 相似文献