Effects of wave–current interaction on the current profile     

Maitane Olabarrieta  Raul Medina  Sonia Castanedo 《Coastal Engineering》2010

Wave–current laboratory experiments have shown that the logarithmic current profile observed in pure current flows is modified due to the presence of waves. When waves propagate opposite the current, an increase in the current intensity is achieved near the mean water level, while a reduction is obtained for following waves and currents. With the aim of analyzing these nonlinear effects along the whole water column, an Eulerian wave–current model is presented. In contrast to previously presented wave–current models, the present is able to include the variation of the free surface elevation due to the wave motion and the effect of a non hydrostatic pressure field. Therefore it does not restrict its application to waves in shallow waters. Moreover, the model is able to simulate all the possible angles between waves and currents.  相似文献   

Analysis of Manning coefficient for small‐depth flows on vegetated beds     

R. García Díaz 《水文研究》2005,19(16):3221-3233

The limitations of Manning's formula are analysed when it is in extreme conditions, and more specifically with small‐depth flows on natural‐vegetation beds. A thorough analysis is made of research carried out on macro‐rough beds, placing particular emphasis on vegetated beds. Research carried out to date on the roughness of vegetated beds and macro‐roughness is commented on, including that carried out at the Laboratory of Hydraulics and Hydrology of the Forestry Engineering Faculty (Polytechnic University of Madrid). The work was done in two phases, the first in a laboratory channel with artificial vegetation and the second in natural beds. The results of the experimental research allow the development of a new approximate method of determining Manning coefficient according to the Froude number. This method may be applied in extreme conditions, both in small depths and steep slopes. It was proved that the Manning coefficient depends not only on roughness height, but also on depth and slope values; thus, it is advisable to choose the appropriate method for its calculation. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

岩体结构面粗糙度系数JRC的定向统计研究   总被引：5，自引：2，他引：5  

杜时贵 《工程地质学报》1994,(3)

本文回顾了岩体结构面粗糙度系数ＪＲＣ的研究成果，分析了各种ＪＲＣ研究方法的应用范围。在野外实际结构面形态的详细调查和深入研究的基础上，发展了Ｂａｒｔｏｎ直边法，并提出按岩性定向统计研究结构面粗糙度系数ＪＲＣ的科学思想。  相似文献   

Simulation of flow over pool-riffle topography: A consideration of the velocity reversal hypothesis     

P. A. Carling  N. Wood 《地球表面变化过程与地形》1994,19(4):319-332

Computer flow simulations using the HEC-2 step-backwater routine are used to demonstrate the effect of systematically varying river channel width, riffle spacing and channel roughness on the shear velocity, section-mean velocity and energy slope in fixed-bed pool-riffle sequences. Initial scaling is obtained by utilizing published information on hydraulic parameters within reaches of the River Severn. Subsequently this restriction is relaxed and the effect of varying parameter combinations within realistic limits is explored. The purpose of this exercise is to isolate those scenarios which may preclude or promote the occurrence of a competence ‘reversal’, such that pools scour at high flow whilst deposition occurs on riffles. It is concluded that rivers in which pools are hydraulically rougher than riffles are likely to demonstrate a competence reversal. For prescribed conditions, the critical discharge at which a reversal occurs is a negative function of riffle spacing and riffle width relative to pool width. Downstream variation in hydraulic roughness also has implications for the phase relationship of shear velocity maxima and minima in relation to the extremes in pool-riffle topography.  相似文献   

跃变型扩散的实验研究          下载免费PDF全文

张霭琛  刘树华  叶文虎 《大气科学》1993,17(1):39-43

本文为一次沿海岸边翻越二维山丘的扩散实验.结果表明:在一定条件下,假若两层空气的湍流强度和尺度的比值不大,当烟羽从上层侵入到下层较强混合气层时,不一定产生燻烟型扩散,仅表现为扩散参数在一定水平距离上的跃变.本文同时介绍了两组风洞实验的结果,其目的在于说明扩散参数跃变存在的事实.  相似文献   

A simple method of estimating rock mass porosity and permeability     

Cheng Haw Lee  Ian. W. Farmer 《Geotechnical and Geological Engineering》1990,8(1):57-65

Conclusion A simple method of estimating fracture porosity and permeability based on empirical relations between fracture aperture andJRC andJCS can be developed. This shows very close correlation with existing data, using simple discontinuity models. There is quite a good correlation between fracture porosity and permeability and depth for larger initial apertures.  相似文献   

Fractal properties of simulated bed profiles in coarse-grained channels   总被引：2，自引：0，他引：2  

André Robert 《Mathematical Geology》1991,23(3):367-382

Bed roughness characteristics in coarse-grained channels are fairly complex. A hierarchy of roughness elements can be observed, ranging from variable particle sizes and shapes and small-scale sedimentary structures, to large-scale bedforms such as riffle-pool sequences. The effects of these scales of roughness on the flow geometry still remain to be thoroughly investigated. The semivariogram has been suggested in the past as a means of quantifying bed roughness effects on streamflow, as well as for distinguishing between scales of roughness. However, field measurements are rather time-consuming. The low number of bed profiles measured in the field precludes the identification of generally applicable relationships between the statistical properties derived from the semivariograms (such as the Hausdorff dimensions and the scale of autocorrelation corresponding to each fractal band) and the bed configuration itself (geometrical and sedimentological properties). Simulation results of gravel-bed profiles are, therefore, presented in order to complement the original investigation of Robert (1988a). The simulation experiments, based on grain characteristics of sizes and shapes and on morphological properties of small-scale bedforms, yield very significant information on boundary roughness at the microscale and give insight into the interpretation of empirical semivariograms (derived from field measurements). Bed-material sorting, variable grain shapes, and height and spacing of cluster bedforms control the fractal dimensions obtained from the semivariograms, as well as the location of the break of slope and the range of the process.  相似文献   

Modeling forced pool–riffle hydraulics in a boulder-bed stream, southern California     

Lee R. Harrison  Edward A. Keller 《Geomorphology》2007,83(3-4):232-248

The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force.  相似文献   

基于卫星遥感数据改进区域尺度的显热通量模型     

宋小宁  赵英时  冯晓明 《地理与地理信息科学》2007,23(1):36-38

以西北半干旱地区的内蒙古农牧交错地带锡林郭勒盟为研究区，以不同退化程度的草地为研究对象，通过叶面积指数计算各像元的动力传输粗糙度长度，以改进显热通量算法；并结合植被／土壤组分温度分别计算基于亚像元的植被／土壤的显热通量。由敏感性分析和模型验证表明，改进的显热通量算法能够提高区域尺度的能量平衡计算精度。  相似文献   

CHARACTERISTICS OF VEGETATION COVER,ROUGHNESS AND ALBEDO DISTRIBUTION OVER CHINA   总被引：1，自引：0，他引：1  

张正秋  周秀骥  李维亮  徐兴奎 《Acta Meteorologica Sinica》2001,(1)

To build land surface dataset for climate model,with application of remote sensing techniqueas well as the Geographic Information System(GIS),the data of surface type,roughness andalbedo over China in 1997 were retrieved,resolutions being 10 km×10 km.Based on these data,an analysis is conducted on the geographic distributions and seasonal variations of surfacevegetation cover and roughness as well as albedo over China.Results show that surface vegetationcover is mainly located to the south of Yangtze River,in Southwest and Northeast China andsparse vegetation cover is in the Northwest.The variation of land surface cover affects thevariations of land surface roughness and albedo.High albedo occurred in the north of XinjiangAutonomous Region,the north of Northeast China and the Qinghai-Xizang Plateau in winter,incorrespondence with the location of snow cover.For most part of China,surface roughness decreases and albedo increases in winter,while theroughness increases and the albedo decreases in summer,which could mainly result from landsurface cover(snow cover and vegetation cover)and soil moisture changes.This shows that thegeographic distribution and seasonal variation of the albedo are almost opposite to those of theroughness,in agreement with theoretical results.Temporally,the amplitude of surface roughnesschange is quite small in comparison with the roughness itself.  相似文献