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1.
Most riparian trees are phreatophytic, water table‐dependent plants which broadly differ in their tolerance to drought and permanent flooding. In semi‐arid settings, as water is limiting, inundations may be regarded as inputs rather than stresses for the survival of phreatophytes. In this study, the mortality rates and abundances of Populus alba, P. nigra, Salix alba and local Tamarix spp. were examined in 43 plots with different hydrologic conditions distributed across the floodplain of a large semi‐arid and Mediterranean river, the Ebro River (Spain). The objectives were to determine hydrologic thresholds for the maintenance of declining populations of those species, while providing novel information on their phreatophytic nature, and to examine shifts in the species composition along hydrologic gradients. All species exhibited significant relationships between mortality rates and hydrologic variables (deepest water table—WT, flood duration—FD and flood frequency—FF). S. alba was found to be the species with lowest tolerance to drier conditions (hydrologic thresholds for maintaining a mortality rate <50%: WT > ?1.22 m; FD: out of observation range; FF > 5.4 events y?1), followed by P. nigra (WT > ?2.18 m; FD > 11.1%; FF > 3.8 events y?1), Tamarix spp. (WT > ?2.96 m; FD > 3.7%; FF > 2.5 events y?1) and P. alba (WT > ?3.45 m; FD > 1.7%; FF > 2.0 events y?1). Only a significant reduction in S. alba relative abundance was observed as conditions got drier. The results provided quantitative information useful to guide management plans for the protection of Mediterranean phreatophytic tree species from further degradation and suggested that eventual natural or regulation‐induced droughts and groundwater declines would accelerate the loss of all phreatophytic species, especially S. alba. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
2.
In dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old. 相似文献
3.
Riparian or streamside woodlands include obligate riparian trees and shrubs (obligates) that are restricted to streamside zones, and facultative riparian species that are abundant in, but not restricted to the riparian areas. Due to their distinctive life history requirements, it may be predicted that the ecological specialist obligates would be more vulnerable than the facultative generalists to impacts from river damming and flow regulation. We tested this along the Snake River through Hells Canyon, USA, where two native riparian shrubs dominate: the obligate sandbar willow (Salix exigua), and the facultative, netleaf hackberry (Celtis reticulata). We assessed riparian conditions over the past century by comparing ground‐level and aerial photographs taken after 1907 and in the 1950s in advance of three dams, versus recent conditions. These comparisons revealed three changes downstream from the dams: (1) the depletion of surface sands and sandbars and (2) reductions in sandbar willow versus (3) the proliferation of hackberry in dense bands above the typical high‐water line. The willow decline probably resulted from the depletion of sand following sediment trapping by the reservoirs, combined with changes in the seasonal water flow pattern. The increase in hackberry may have resulted from a beneficial ‘irrigation effect’ of daily water releases for power generation during the summer. The opposing responses reflect the plants' differing life histories and may partially resolve impacts of river regulation on alluvial sediments versus the instream flow pattern. We consider other riparian studies that suggest that obligates such as cottonwoods (Populus angustifolia, P. deltoides and P. fremontii) are highly vulnerable to river regulation, while facultative trees and shrubs such as trembling aspen (Populus tremuloides), wolf‐willow (Elaeagnus commutata) and velvet mesquite (Prosopis velutina) are more resilient. These results suggest that conservation of riparian woodlands should emphasize the ecological specialist obligates, while facultative species may be less vulnerable to river regulation. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
4.
Stewart B. Rood;Gregory C. Hoffman;Norm Merz;Paul Anders;Rohan Benjankar;Michael Burke;Gregory Egger;Mary Louise Polzin;Scott Soults; 《河流研究与利用》2024,40(4):587-598
Like most rivers worldwide, the transboundary North American Kootenay/i River has experienced multiple impacts including watershed developments, river channelization, and floodplain clearing, draining, and diking. Construction of Libby Dam was authorized by the 1964 Columbia River Treaty (CRT) between the United States and Canada, and in 1975 began regulating downstream flows for flood risk management and hydropower generation. Following cumulative impacts, the endemic Kootenai River White Sturgeon population collapsed and was designated as endangered in 1994 (U.S. Endangered Species Act). Subsequent Biological Opinions from the U.S. Fish and Wildlife Service prescribed Libby Dam operations to provide springtime flow pulses for sturgeon spawning. These provided the unanticipated benefit of substantial seedling recruitment of native and introduced riparian cottonwoods and willows. The regulated flow regime was further adaptively managed to provide a more normative (natural) regime, to balance ecological functions with flood risk management and hydropower generation. The broadened ecological considerations would be consistent with the proposed priorities for the modernization of the international CRT. The observed responses revealed that (1) diverse aquatic and riparian organisms are dependent on common river flow characteristics; (2) a normalized flow regime provided substantial ecological benefits; and (3) due to multiple influences, hybrid ecosystems develop along regulated rivers, with a blending of natural and altered processes and communities. For other regulated rivers, we recommend that (1) high springtime flows be allowed, as feasible; (2) followed by the gradual post-peak recession; and (3) the maintenance of sufficient flows through the warm and dry interval of mid to late summer. 相似文献
5.
施工进度仿真是高拱坝施工进度控制的重要手段。当前高拱坝施工进度仿真分析中,仿真施工参数基本都是根据施工经验参考近似工程选取,然而不同工程之间施工参数差距较大,可能导致仿真计算结果的失真。针对以上问题,本文提出基于贝叶斯更新理论,根据现场实际施工信息,实时更新仿真施工参数,以进行仿真计算分析。然而贝叶斯更新过程需要大量精确的统计数据,在实际工程中,由于人为及机器等多种因素,收集到的数据往往不够充分且不够精确。为此,本文将改进重抽样(Bootstrap)方法引入到贝叶斯更新中,提高了样本数据的代表性,使得样本统计参数的估计值更为准确。仿真分析结果表明,相比于传统方法,采用本文提出的施工参数实时更新方法,可以提高仿真参数精度。 相似文献
6.
高土石坝施工系统仿真模型及其集成研究 总被引：1，自引：0，他引：1
１引言土石坝施工系统是一个复杂的、具有多个随机因素的系统,用数学模型做精确的描述十分困难．系统仿真可以根据系统内部的逻辑关系和数学关系,面向系统的实际过程和系统行为构造仿真模型,从而能得到复杂随机系统的描述．本文根据系统功能将土石坝施工系统分解成坝体填筑子系统、土石料调运优化子系统和交通运输子系统．坝体填筑子系统仿真要反映整个坝体填筑的动态上升过程,土石料调运优化子系统仿真是要得到费用等的最优规划,交通运输子系统仿真要反映道路的行车密度和承受能力是否匹配．各个子系统要实现的功能各不相同,在单独进… 相似文献
7.
传统的计划评审技术(PERT)考虑的三时估计值主观性偏强,没有考虑风险因素相关性对工序活动工期的影响,影响了工期仿真与风险评价的客观性以及分析结果的可靠性。本文建立了耦合改进PERT和贝叶斯信念网络(BBNs)的心墙堆石坝施工进度风险分析模型,首先利用BBNs推理功能推算工序活动风险因素相关性,同时分析工程进度风险;其次将三角模糊数和模糊测度Me方法相结合来对PERT进行改进,引入了决策者风险偏好指标;最后将该模型应用到实际工程,得到决策者风险偏好指数-工期-完工概率分布曲线,可用于指导施工决策。工程应用结果表明:与传统的PERT方法相比,耦合改进PERT和BBNs得到的仿真结果更加可靠,可为心墙堆石坝施工进度方案设计提供理论依据。 相似文献
8.
(复合)土工膜防渗土石坝饱和-非饱和渗流特性 总被引：1，自引：1，他引：1
分别采用完全饱和渗流及饱和 非饱和渗流有限元计算理论,对(复合)土工膜防渗土石坝进行渗流场仿真分析,重点研究土工膜等效处理时不同厚度放大倍数下大坝渗流场变化规律,同时结合坝体土石料不同渗透特性,计算得到大坝渗流量和浸润线与土工膜厚度放大倍数之间的关系,并比较了按完全饱和渗流理论与饱和 非饱和渗流理论计算结果的差别。结果表明:在坝体土石料渗透特性相同的情况下,土工膜不同厚度放大倍数时浸润线仅在土工膜等效区有较大差别,在膜后坝体部位基本保持不变;大坝渗流量与土工膜厚度放大倍数及坝体填料的渗透特性相关,按饱和渗流理论计算所得的大坝渗流量大于按饱和 非饱和渗流理论计算得到的渗流量。 相似文献
9.
全球气候变化和快速城镇化打破了城市水循环平衡,造成近些年来我国城市暴雨洪涝事件频繁发生。城市暴雨洪涝(雨洪)模型是应对城市洪涝事件的重要技术保障。然而,对于城市暴雨洪涝模拟,模拟精度和计算效率往往是一对矛盾体。本文总结了目前两种应用比较广泛的建模方式,即基于部分物理机制的水文单向驱动水动力模拟方式和基于完整物理机制的全过程双向耦合模拟方式。在此基础上,为了提高防汛应急处置工作的前瞻性和准确性,本文提出基于上述两种建模方式的嵌套式模拟方法。针对昆明主城区及其局部汇水区的模拟应用研究表明,嵌套式模拟方法在一定程度上缓解了城市雨洪模型在“整体”和“局部”、“精度”和“效率”之间的矛盾,具有较好的实用价值。 相似文献
10.
Larval and juvenile Colorado pikeminnow (Ptychocheilus lucius) use shallow, low‐velocity, channel‐margin areas (backwaters) as nursery habitats. It is hypothesized that within‐day flow fluctuations caused by hydropower operations can directly affect the suitability of such habitats by altering water temperature and habitat geometry. Despite the importance of backwaters to juvenile fishes, there is a lack of established approaches for modelling how river management affects these habitats. Here, we describe a physical habitat model that predicts the effects of mainstem flow variation on backwater temperature, geometry and invertebrate availability. We specifically modelled these effects on habitat in a portion of the Green River in Utah below Flaming Gorge Dam. The overall model combines a cell‐based model of backwater geometry, a pond‐based temperature model and a model of invertebrate production. Results from a series of simulations suggest that the most important biological effects of within‐day flow fluctuations are likely to be those associated with the availability of invertebrate prey including (1) minimum wetted area, (2) the proportion of the backwater's volume exchanged with the mainstem, and, to a lesser degree, (3) backwater temperature. Taken together, such effects could have important implications for the growth and survival of juvenile fish when flow fluctuations are sufficiently large. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献