A field test of tubino's (1991) model of alternate bar formation     

Mark R. Welford 《地球表面变化过程与地形》1994,19(4):287-297

This study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses a physically based non-linear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream. The study site is an artificially straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, bed sediment size and flow stage over a 2 year study period. Both linear and non-linear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. But these theories do not predict bar development for unsteady flow conditions, which typically occur in natural alluvial channels. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood; (2) a stage of modest bar decay near the peak of the flood; and (3) a stage of non-linear bar growth during the prolonged falling stage of the flood. Bars developed during the falling limb of a hydrograph, and exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. As flow stage decreased, short, low, fine-grained bars were superimposed on long, high and coarser-grained bars that developed under preceding high flow stages. These results suggest that the process of bar formation in artificially straightened natural streams with heterogeneous bed material may occur under different flow conditions and in a different manner than predicted by theoretical models. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes.  相似文献   

N个薄层介质组合引起的视各向异性     

Zhang Zhongjie He Qiaodeng 《吉林大学学报(地球科学版)》1990,(2)

作者详细推导了N个薄层介质组合时所引起的视各向异性介质的弹性参数,扩展了J.E.white先生在《Underground Sound》一书中关于双层介质组合的视各向异性介质的弹性参数应用范围,并计算了2个薄层各向同性的弹性介质组合时视各向异性介质中地震波速度随波动传播方向而变化的关系曲线,说明了薄层组合确实可以引起介质的视各向异性。  相似文献   

Surface wave propagation in a transversely isotropic elastic layer overlying a liquid saturated porous solid half-space and lying under the uniform layer of liquid     

M. D. Sharma  Rajneesh Kumar  M. L. Gogna 《Pure and Applied Geophysics》1990,133(3):523-539

Dispersion of Rayleigh-type surface wave is studied in a homogeneous transversely isotropic elastic layer overlying a nondissipative liquid-saturated porous solid half-space and lying under a uniform layer of homogeneous liquid. The frequency equation in the form of ninth-order determinant is obtained.Special cases have been deduced by reducing the depth of the layers to zero and by changing the transverse isotropic layer to an isotropic layer. Dispersion curves for the phase velocity have been plotted for a particular model.  相似文献   

大跨预应力混凝土空间框架结构的地震振动台试验研究     

熊学玉  耿耀明  李春祥  黄鼎业 《地震工程与工程振动》2005,25(1):102-107

利用地震振动台对大跨预应力混凝土井式梁空间框架结构的抗震性能进行了实验研究。两个模型的实验结果表明：(1)大跨预应力井式梁空间框架结构可以提供大的楼层平面内刚度；(2)对称大跨预应力井式梁空间框架结构，可以不考虑耦合地震反应；(3)大跨预应力井式梁空间框架结构的竖向地震反应中心大而周边小；(4)在相同加速度但不同的地震波作用下，结构的竖向地震反应不同。  相似文献   

关于吉林地区挖孔桩设计问题的探讨     

李向群  袁铁铮 《吉林大学学报(地球科学版)》2001,31(4):372-375

探讨了在吉林地区高层建筑中有关桩基规范执行及设计施工过程中常见的问题。通过对不同直径的桩端进入持力层深度的对比 ,明确 2d与 4d的对比要求 ,即持力层厚度的保证比进入持力层深度更重要。针对通长配筋“一通到底”的设计给施工带来的不便及浪费提出了新的建议。通过实践给出“α”角的建议值 ,既能减小扩大桩头斜面上受到卸荷后侧土压力 ,又可减少施工时的坍方。最后对降水及护壁提出了结合实际地质情况的 4种方法。  相似文献   

Gradient irregularity in the herbert gorge of Northeastern Australia     

Ellen E. Wohl 《地球表面变化过程与地形》1992,17(1):69-84

The seventy-kilometre-long Herbert Gorge of northeastern Australia preserves a record of past floods in slackwater deposits and palaeostage indicators. Step-backwater modelling of water-surface profiles indicates that discharges ranging from 11000 to 17000 m³s^?1 have occurred six times in the gorge during the last 900 years. These flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology. In particular, the hydraulics of extreme flows can be related to boulder transport, and to the location of large boulder bars. Large boulder bars occur throughout the Herbert Gorge, being best developed at loci of stream power minima along the inside of bends, at tributary junctions, and at obstructions in the channel caused by bedrock highs. Only the flows exceeding approximately 8000 m³ s^?1 are competent to transport the boulders which constitute the bars. In the straight channel reaches, the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls. The bars provide an efficient means of energy dissipation, and they are interpreted as a result of the inherent high turbulence of flow in a steep channel. The regular spacing of the bars, and their correspondence with the hydraulics of large flows, suggest that the bars and associated bedrock highs may represent a self-regulating mechanism akin to the pool-riffle sequence of alluvial channels. It may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges.  相似文献   

预应力混凝土双压花锚具性能的低周重复荷载试验研究     

赵均  张志强  张高峰 《世界地震工程》2006,22(2):144-148

通过对预应力混凝土一种新型双压花锚具足尺寸试件的试验,研究了这种锚具在低周重复荷载下的性能及其特点、锚具受周期荷载后再次承受静力荷载作用时的性能以及周期荷载的不同应力循环幅度对锚具性能的影响。试验结果表明:双压花锚具达到了国家标准规定的周期荷载性能要求;随着周期荷载循环次数的增加,锚具每个循环的荷载与变形关系呈近似线弹性关系;锚具在历经50次重复加卸荷后,其静载性能发生了很大变化,而最大承载力和极限变形能力没有降低。  相似文献   

M L Determination for Local and Regional Events Using a Sparse Network in Southwestern Germany     

Stefan Stange 《Journal of Seismology》2006,10(2):247-257

A method for the determination of consistent local magnitude M _L values (Richter scale, or M _WA) for earthquakes with epicentral distances ranging from 10 km through 1000 km is demonstrated. The raw data consists of nearly 1300 amplitude readings from a network of six digital seismographs in Baden–Württemberg (Southwestern Germany) during 26 months starting in 1995, later extended by another 1000 amplitude readings until 1999. Relying on most of the basics introduced by C.F. Richter a three-parameter attenuation curve (distance correction, magnitude-distance relation) for Baden–Württemberg and adjacent areas is presented. Station corrections are evaluated and the attenuation curve is calibrated with respect to other agencies for distances greater than 650 km. Reasonable parametrisations are discussed and meaningful error bars are attributed. Finally, a seventh station is incorporated by means of its station correction alone, without needing to update the attenuation curve.  相似文献   

Some foundation stability problems of the railway bridge over the Mala Rijeka     

Ž. Radosavljević  O. Marković 《Rock Mechanics and Rock Engineering》1977,9(1):55-64

SummarySome Foundation Stability Problems of the Railway Bridge over the Mala Rijeka The authors presented an outlay of problems occurring in estimating the stability of rocky slopes loaded by large vertical forces. General attitudes, kept by the authors during the stability analysis for an actual case — foundation of piers No. 3 and 4 of the railway bridge across the Mala Rijeka, are also presented. The results of calculation as well as the advantages of the approach applied are outlined in the paper. Finally, general conclusions are drawn regarding the method to be kept in solving the stability of slopes loaded by large artificial loading.With 7 Figures  相似文献   

Assessment of intermediate fine sediment storage in a braided river reach (southern French Prealps)     

O. Navratil  C. Legout  D. Gateuille  M. Esteves  F. Liebault 《水文研究》2010,24(10):1318-1332

This paper presents a field investigation on river channel storage of fine sediments in an unglaciated braided river, the Bès River, located in a mountainous region in the southern French Prealps. Braided rivers transport a very large quantity of bedload and suspended sediment load because they are generally located in the vicinity of highly erosive hillslopes. Consequently, these rivers play an important role because they supply and control the sediment load of the entire downstream fluvial network. Field measurements and aerial photograph analyses were considered together to evaluate the variability of fine sediment quantity stored in a 2·5‐km‐long river reach. This study found very large quantities of fine sediment stored in this reach: 1100 t per unit depth (1 dm). Given that this reach accounts for 17% of the braided channel surface area of the river basin, the quantities of fine sediment stored in the river network were found to be approximately 80% of the mean annual suspended sediment yields (SSYs) (66 200 t year^?1), comparable to the SSYs at the flood event scale: from 1000 t to 12 000 t depending on the flood event magnitude. These results could explain the clockwise hysteretic relationships between suspended sediment concentrations and discharges for 80% of floods. This pattern is associated with the rapid availability of the fine sediments stored in the river channel. This study shows the need to focus on not only the mechanisms of fine sediment production from hillslope erosion but also the spatiotemporal dynamics of fine sediment transfer in braided rivers. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献