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1.
Critical to restoring the nature conservation value of many river corridors is an understanding of how alluvial landscapes will respond to cessation of river management and land use practices that have previously degraded the environment. This paper analyses changes in valley floor landforms and vegetation patch dynamics, in relation to fluvial disturbance, over a period of almost 100 years following flood embankment abandonment on a wandering gravel‐bed river, namely the River Tummel, Scotland. Such rivers were once typical of many draining upland areas of northern maritime Europe. Prior to abandonment the valley floor landscape was agriculturally dominated and the river for the most part was single thread confined between flood embankments. The pattern of landform change and vegetation patch development over time following a decision in 1903 not to maintain embankments was tracked by geomorphic and land cover mapping utilizing successive sets of aerial photography for the period 1946 to 1994. A historical context for these changes was also feasible because the channel planform in 1900 and earlier channel planform changes dating back to 1753 were known due to the availability of old maps and earlier geomorphic studies. The land cover mapping was validated by comparison of results produced from the interpretation work on the 1994 aerial photographs with the field‐based UK National Vegetation Classification protocol. The findings of the study illustrate that bordering the River Tummel fluvial landforms and vegetation patch mosaics, presumably resembling those that occurred before valley floor land use intensification, evolved in less than 50 years after flood embankment abandonment with a resultant increase in habitat diversity. The change relates primarily to flood‐induced channel planform change and moderate levels of fluvial disturbance. The general significance of this change to plant species diversity on the valley floor of the River Tummel and elsewhere is discussed as is possible implications of the upstream impoundment and scenarios for climatically induced changes in flood frequency and magnitude. The overall outcome is the strong possibility that simple changes in river management and land use practices could result in re‐establishment of the nature conservation value of similar river corridors in Europe over the medium term without active restoration efforts. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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将群桩筏板基础离散成桩和弹性板,桩筏基础与地基的共同作用转化为桩、弹性板与地基之间的力与位移协调分析。对不同的地基模型求解相应的地基柔度系数,同时将桩作为弹性杆件求解其柔度系数,并将筏板视为地基上的四边自由矩形板,求出其在地基反力、桩顶反力和外荷载以及简支边广义位移共同作用下的位移,最后通过力和位移协调建立桩、筏、地基之间的共同作用方程并求解。算例对比表明半解析半数值方法具有较好的精度,能满足实际工程计算需要。 相似文献
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数值模拟方法已成为研究桩承式路堤中土拱最重要的手段,其关键在于路堤填土要采用合理的本构模型。建立桩承式路堤平面土拱分析的弹塑性有限元模型,考虑摩尔-库伦模型(MC)、硬化土模型(HS)和小应变硬化土模型(HSS)3种不同的路堤土本构模型,用有限元方法模拟不同路堤土本构模型下桩承式路堤中的土拱形态和土拱效应。计算结果表明:3种不同路堤土本构模型下平面土拱的形态都是半个椭圆。路堤土采用HS和HSS模型,获得的土拱形态、效应和桩帽-土差异沉降相同。较之HS和HSS模型,路堤土采用MC模型时计算得到的桩帽-土差异沉降较小,桩帽荷载分担比略大。当路堤高度较小时,采用MC模型获得的土拱远小于HS和HSS模型下的计算结果。土拱效应的数值模拟中路堤土可采用简单的MC模型,但土拱形态的数值模拟中路堤土宜采用HS模型。 相似文献
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分别以天然地基筏板和4组变刚度桩筏基础模型对比试验为基础,分析基桩平面布置、几何尺寸与基础沉降的关系,从而为桩筏基础变刚度调平概念设计提供试验依据。试验研究表明:与上部结构共同作用的桩筏基础,总体上其沉降分布表现为中心区域沉降较大而边角位置沉降较小;筏下均匀布桩较之天然地基,基础沉降量的最大值大约减少70%;基础中心区域桩长增加1倍或者桩数增加1/3,沉降量的最大值约减少50%;上部结构与核心筒下局部加大基桩几何尺寸和适当增加桩数能够有效地减少沉降量的最大值以及沉降差,从而达到调平的效果;筏下布桩有助于减少基础周边的地表沉降,可使其影响局限于1/4基础宽度范围之内。图11表3参12 相似文献
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