Tree establishment on bars in low‐order gravel‐bed mountain streams     

Isabelle Charron  Edward A. Johnson  Y. E. Martin 《地球表面变化过程与地形》2011,36(11):1522-1533

In many large alluvial rivers, trees often recruit and survive along laterally accreted sediments on bars. This produces a gradient of tree ages and composition with distance from the active channel. However, in low‐order, gravel‐bed mountain streams, such as the stream investigated in this study, it is suggested that vertical accretion results in sediment deposition patterns on bars that are often highly patchy. Consequently, tree species and ages are also heterogeneously distributed, rather than having distinct linear or arcuate banding patterns with distance from the channel. In addition, overall age patterns of trees on these bars follow the distribution of floods, with numerous young trees and few older trees. Recruitment is fairly continuous on these bars and is not correlated with high water years, suggesting that even flows close to bankfull levels are capable of transporting fine sediment to the bars on which trees establish. This pattern of sediment deposition/erosion and the resulting tree recruitment and survival seem to be a result of valley confinement and the lack of lateral accretion in these smaller, mountainous channels. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

设有圈梁与构造柱的砌体结构预应力筋抗震加固试验研究     

陈红媛  房贞政 《地震工程与工程振动》2011,31(4):120-126

通过对设有圈梁与构造柱的砌体结构采用体外预应力筋加固和未加固模型进行双向拟动力试验,对墙体裂缝开展、结构耗能、结构损伤等指标进行对比分析,探讨体外预应力筋加固对砌体结构抗震能力的影响.试验结果表明:体外预应力筋产生的约束力对限制裂缝开展、防止圈梁与预制楼板的脱空效果明显砌体结构在体外预应力筋加固后的总滞回耗能大于未加固...  相似文献   

Morphology and evolution of bars in a wandering gravel-bed river; lower Fraser river, British Columbia, Canada   总被引：2，自引：0，他引：2  

STEPHEN P. RICE  MICHAEL CHURCH§  COLIN L. WOOLDRIDGE†  EDWARD J. HICKIN‡ 《Sedimentology》2009,56(3):709-736

A hierarchical typology for the channels and bars within aggradational wandering gravel-bed rivers is developed from an examination of a 50 km reach of lower Fraser River, British Columbia, Canada. Unit bars, built by stacking of gravelly bedload sheets, are the key dynamic element of the sediment transfer system, linking sediment transport during individual freshets to the creation, development and remoulding of compound bar platforms that have either a lateral or medial style. Primary and secondary unit bars are identified, respectively, as those that deliver sediment to compound bars from the principal channel and those that redistribute sediment across the compound bar via seasonal anabranches and smaller channels. The record of bar accretion evident in ground-penetrating radar sequences is consistent with the long-term development of bar complexes derived from historical aerial photographs. For two compound bars, inter-annual changes associated with individual sediment transport episodes are measured using detailed topographic surveys and longer-term changes are quantified using sediment budgets derived for individual bars from periodic channel surveys. Annual sediment turnover on the bars is comparable with the bed material transfer rate along the channel, indicating that relatively little bed material bypasses the bars. Bar construction and change are accomplished mainly by lateral accretion as the river has limited capacity to raise bed load onto higher surfaces. Styles of accretion and erosion and, therefore, the major bar form morphologies on Fraser River are familiar and consistent with those in gravelly braided channels but the wandering style does exhibit some distinctive features. For example, 65-year histories reveal the potential for long sequences of uninterrupted accretion in relatively stable wandering rivers that are unlikely in braided rivers.  相似文献   

Recognition and significance of sharp‐based mouth‐bar deposits in the Eocene Green River Formation,Uinta Basin,Utah     

EDWIN ROBERT SCHOMACKER  AUDUN VESTHEIM KJEMPERUD  JOHAN PETTER NYSTUEN  JENS SIGURD JAHREN 《Sedimentology》2010,57(4):1069-1087

Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth‐bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive‐transgressive sequence. Individual sandstone bodies are texturally sharp‐based towards mudstone substratum. In proximal parts, the mouth‐bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth‐bar clinothems. The clinothems pass gradually into greenish‐grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex‐upward sandstone bars. The mouth‐bar deposits are connected to terminal distributary channel deposits. Together, these mouth‐bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth‐bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.  相似文献   

带交叉钢筋T形截面短柱抗震性能试验研究   总被引：3，自引：0，他引：3  

黄选明  曹万林  崔建升  郑汉兵  马景春 《地震工程与工程振动》2003,23(1):90-94

在提出带暗柱异形柱基础上,进一步提出钢筋混凝土带交叉钢筋异形桩这一设计方法。通过3根T形截面短柱抗震性能的试验研究,分析比较了普通T形短柱、带暗柱T形短柱和带交叉钢筋T形短柱的屈服荷载、极限荷载、屈服位移、弹塑性位移、刚度、延性、滞回特性、耗能能力等性能。试验表明加配交叉钢筋可明显提高T形截面短柱的抗震性能。  相似文献   

Modélisation du festonnage des barres sableuses d'avant-côte : application à la côte aquitaine,France     

Bruno Castelle  Philippe Bonneton  Rémi Butel 《Comptes Rendus Geoscience》2006,338(11):795-801

Nearshore crescentic bars play a key role in nearshore morphodynamics. These bars are observed all along the Aquitanian Coast, with a mean wavelength of about 700 m. A non-linear stability analysis is undertaken to simulate the development of crescentic patterns. Results show that self-organization mechanism can lead alone to the development of these alongshore rhythmic features. Simulated wavelengths are in agreement with observations on the Aquitanian Coast. To cite this article: B. Castelle et al., C. R. Geoscience 338 (2006).  相似文献   

Regulation of sand transport in the Colorado River by changes in the surface grain size of eddy sandbars over multi-year timescales     

DAVID J. TOPPING  DAVID M. RUBIN†  JOHN C. SCHMIDT‡ 《Sedimentology》2005,52(5):1133-1153

In settings where the transport of sand is partially or fully supply limited, changes in the upstream supply of sand are coupled to changes in the grain size of sand on the bed. In this manner, the transport of sand under the supply-limited case is ‘grain-size regulated’. Since the closure of Glen Canyon Dam in 1963, the downstream reach of the Colorado River in Marble and Grand Canyons has exhibited evidence of sand-supply limitation. Sand transport in the river is now approximately equally regulated by changes in the discharge of water and changes in the grain sizes of sand on the channel bed and eddy sandbars. Previous work has shown that changes in the grain size of sand on the bed of the channel (driven by changes in the upstream supply of sand owing to both tributary floods and high dam releases) are important in regulating sand transport over timescales of days to months. In this study, suspended-sand data are analysed in conjunction with bed grain-size data to determine whether changes in the grain size of sand on the bed of the channel or changes in the grain size of sand on the surface of eddy sandbars have been more important in regulating sand transport in the post-dam Colorado River over longer, multi-year timescales. The results of this study show that this combined theory- and field-based approach can be used to deduce which environments in a complicated setting are the most important environments for regulating sediment transport. In the case of the regulated Colorado River in Marble and Upper Grand Canyons, suspended-sand transport has been regulated mostly by changes in the surface grain size of eddy sandbars.  相似文献   

三峡永久船闸衬砌墙分水平缝对结构及结构锚杆受力影响研究     

苏超  姜弘道  钱向东 《工程地质学报》2001,9(3):267-271

采用考虑三维接触非线性的结构温度徐变仿真计算方法, 针对三峡永久船闸 (全衬式和混合式)闸室墙整体和分缝两种结构形式进行了计算分析, 研究表明:闸室墙分水平缝可以有效地降低全衬式和混合式结构混凝土铅直向拉应力, 但使最大锚杆拉应力值和平均锚杆拉应力值增加, 分布的离散性增大, 在横缝和纵缝交点附近出现锚杆拉应力集中现象。  相似文献   

Formation of stationary alternate bars in a steep channel with mixed-size sediment: A flume experiment     

Thomas E. Lisle  Hiroshi Ikeda  Fujiko Iseya 《地球表面变化过程与地形》1991,16(5):463-469

Alternate bars were formed by sediment transport in a flume with Froude-modelled flow and relative roughness characteristic of gravel-boulder channels with steep slopes. The flume (0.3 m wide × 7.5 m long) was filled with a sand-gravel mixture, which was also fed into the top of the flume at a constant rate under constant discharge. Channel slope was set at 0.03. Initially, coarse particles accumulated on incipient bar heads near one side of the flume and diverted flow and bedload transport across the flume toward a pool scoured against the opposite flume wall downstream. Sorting in the pool directed coarse particles onto the next bar head downstream. Alternate sequences of pools and coarse bar heads were thereby linked down the entire flume by interactions of sediment sorting, flow, and channel morphology. During episodes of bar construction, unsorted bedload invaded interior bar surfaces and was deposited. Persistent deposition of coarse particles on bar heads prevented downstream migration of bars by inhibiting bar-head erosion and bedload transport over bars. Likely factors leading to bar-head stabilization in modelled gravel-bed channels are coarse mixed-size sediment, steep channel gradients, and shallow depths.  相似文献   

香港海湾沙坝发育过程中的风沙作用     

王为 《中国沙漠》1996,16(2):120-126

海湾沙坝是香港海岸一种主要的沉积地貌，以大屿山岛贝澳湾的沙坝最为典型。关于贝澳湾古（内）沙坝的成因曾有过较多的争论，但各种成因假说都缺乏有力的证据支持。本文分析了湾内沉积物特征和沉积过程与海平面变化的关系，揭示了风沙在海湾沙坝发育过程中所起的作用。古（内）沙坝的沉积成分随着内沙坝向上加积贝壳含量急剧减少，其上部沉积成分已明显不同于外沙坝和现代海滩，而与其前后的干泻湖上部相似，显示了沉积过程中沉积物来源的变化。内沙坝的沉积物粒径向上变细，分选向上变好，而粒径最小处不在沙坝的最上部，则是在贝壳含量开始向上减少的高度，表明沉积动力随着沉积来源的变化而改变。电镜扫描也显示内沙坝上部的部分颗粒不是来自海滩，具有磨圆差、缺乏水下撞击痕迹以及上翻解理与贝壳状断口共存的特征。内外沙坝下部海滩岩的高度、位置与年代还表明内外沙坝间的沉积高度与海平面变化无关。研究表明：香港海湾沙坝发育过程向上加积的动力由原来的海动力变为风动力；随岸线向海推进内沙坝的沉积物来源由来自海滩物质变为同时来自海滩物质及干泻湖上层物质，只有风才能堆积这些物质；内外沙坝间的沉积高度差只是风积高度不同的结果，与海平面变化无关；海湾内风沙作用并不显著。  相似文献