首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   58篇
  免费   5篇
测绘学   1篇
大气科学   1篇
地球物理   34篇
地质学   16篇
海洋学   2篇
天文学   5篇
综合类   2篇
自然地理   2篇
  2019年   2篇
  2017年   3篇
  2014年   4篇
  2013年   1篇
  2012年   2篇
  2011年   5篇
  2010年   2篇
  2009年   3篇
  2008年   2篇
  2007年   4篇
  2006年   5篇
  2005年   2篇
  2004年   2篇
  2003年   3篇
  2002年   1篇
  2000年   3篇
  1999年   1篇
  1998年   1篇
  1997年   2篇
  1995年   1篇
  1994年   2篇
  1993年   2篇
  1992年   1篇
  1991年   1篇
  1989年   3篇
  1988年   1篇
  1987年   1篇
  1986年   1篇
  1983年   1篇
  1982年   1篇
排序方式: 共有63条查询结果,搜索用时 31 毫秒
11.
12.
13.
Despite the occurrence of riffle–pool sequences in many rivers there are few data concerning riffle–pool unit morphology. Of many criteria proposed to identify riffle–pool units, only three methods can be regarded as objective and robust. These are the ‘zero‐crossing’, the ‘spectral analysis’ and the ‘control‐point’ methods. In this paper statistics are developed using the first two of these methods to describe the streamwise morphology of 275 riffles and 285 pools which form a continuous 32·1 km reach of the bed of the River Severn in Shropshire, England. Yalin's theoretical relationship between the average riffle:pool unit length ( λ p) and channel width ( W ), λ p = 3 W , applies to the River Severn. Reach‐average riffle height ( H ) is a constant proportion of bankfull depth ( h ); typically H ≅ 0·16 h . Riffle height is a positive function of riffle length. Pool depth is a positive function of pool length. However, both riffle length and pool length increase more rapidly than the bed‐level amplitude, such that long riffles or pools are relatively ‘flat’. As channel gradient reduces, bedforms flatten and become more asymmetric as riffle stoss sides and the proximal slope of pools lengthen at the expense of riffle lee sides and pool distal slopes. The statistical relationships between riffle steepness (H/L) and water depth are similar to those for equilibrium subaqueous dunes. The Severn data are consistent with Yalin's theoretical analysis relating riffle bedform length (L r) to water depth, i.e. L r = α2π h, wherein α ≅ 1 for steep near‐equilibrium bedforms but α ≅ 2 to 3 as the relative depth decreases and riffles become long, low features. Theoretical consideration and turbulence data indicate that the frequency of coherent turbulent‐flow structures associated with the riffle–pool mixing length in the Severn should be of the order of 50 to 100 s. The morphological similarity of the steepest River Severn riffles with dunes raises intriguing questions with respect to self‐similar, convergent organization of periodic alluvial bedforms and to bedform dynamic classification particularly. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   
14.
The general nature of bulk flow within bedrock single‐channel reaches has been considered by several studies recently. However, the flow structure of a bedrock‐constrained, large river with a multiple channel network has not been investigated previously. The multiple channel network of the Siphandone wetlands in Laos, a section of the Mekong River, was modelled using a steady one‐dimensional hydraulic model. The river network is characterized by a spatially‐varying channel‐form leading to significant changes in the bulk flow properties between and along the channels. The challenge to model the bulk flow in such a remote region was the lack of ideal boundary conditions. The flow models considered both low flow, high inbank and overbank flows and were calibrated using SPOT satellite sensor imagery and limited field data concerning water levels. The application of the model highlighted flow characteristics of a large multi‐channel network and also further indicated the field data that would be required to properly characterize the flow field empirically. Important results included the observation that adjacent channels within the network had different water surface slopes for the same moments in time; thus calibration data for modelling similar systems needs to account for these significant local differences. Further, the in‐channel hydraulic roughness coefficient strongly varied from one cross‐section to the next (Manning's ‘n’ range: 0·01 to 0·10). These differences were amplified during low flow but persisted in muted form during high discharges. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   
15.
Estimating overland flow erosion capacity using unit stream power   总被引:2,自引:0,他引:2  
Soil erosion caused by water flow is a complex problem. Both empirical and physically based approaches were used for the estimation of surface erosion rates. Their applications are mainly limited to experimental areas or laboratory studies. The maximum sediment concentration overland flow can carry is not considered in most of the existing surface erosion models. The lack of erosion capacity limitation may cause over estimations of sediment concentration. A correlation analysis is used in this study to determine significant factors that impact surface erosion capacity. The result shows that the unit stream power is the most dominant factor for overland flow erosion which is consistent with experimental data. A bounded regression formula is used to reflect the limits that sediment concentration cannot be less than zero nor greater than a maximum value. The coefficients used in the model are calibrated using published laboratory data. The computed results agree with laboratory data very well. A one dimensional overland flow diffusive wave model is used in conjunction with the developed soil erosion equation to simulate field experimental results. This study concludes that the non-linear regression method using unit stream power as the dominant factor performs well for estimating overland flow erosion capacity.  相似文献   
16.
The channel boundary conditions along the Lower Yellow River (LYR) have been altered significantly since the 1950s with the continual reinforcement and construction of both main and secondary dykes and river training works. To evaluate how the confined complex channel–floodplain system of the LYR responds to floods, this study presents a detailed investigation of the relationship between the tempo‐spatial distribution of sedimentation/erosion and overbank floods occurred in the LYR. For large overbank floods, we found that when the sediment transport coefficient (ratio of sediment concentration of flow to flow discharge) is less than 0.034, the bankfull channel is subject to significant erosion, whereas the main and secondary floodplains both accumulate sediment. The amount of sediment deposited on the main and secondary floodplains is closely related to the ratio of peak discharge to bankfull discharge, volume of water flowing over the floodplains, and sediment concentration of overbank flow, whereas the degree of erosion in the bankfull channel is related to the amount of sediment deposited on the main and secondary floodplains, water volume, and sediment load in flood season. The significant increase in erosion in the bankfull channel is due to the construction of the main and secondary dykes and river training works, which are largely in a wide and narrow alternated pattern along the LYR such that the water flowing over wider floodplains returns to the channel downstream after it drops sediment. For small overbank floods, the bankfull channel is subject to erosion when the sediment transport coefficient is less than 0.028, whereas the amount of sediment deposited on the secondary floodplain is associated closely with the sediment concentration of flow. Over the entire length of the LYR, the situation of erosion in the bankfull channel and sediment deposition on the main and secondary floodplains occurred mainly in the upper reach of the LYR, in which a channel wandering in planform has been well developed.  相似文献   
17.
Local sea level can deviate from mean global sea level because of both dynamic sea level (DSL) effects, resulting from oceanic and atmospheric circulation and temperature and salinity distributions, and changes in the static equilibrium (SE) sea level configuration, produced by the gravitational, elastic, and rotational effects of mass redistribution. Both effects will contribute to future sea level change. To compare their magnitude, we simulated the effects of Greenland Ice Sheet (GIS) melt by conducting idealized North Atlantic “water-hosing” experiments in a climate model unidirectionally coupled to a SE sea level model. At current rates of GIS melt, we find that geographic SE patterns should be challenging but possible to detect above dynamic variability. At higher melt rates, we find that DSL trends are strongest in the western North Atlantic, while SE effects will dominate in most of the ocean when melt exceeds ~20 cm equivalent sea level.  相似文献   
18.
19.
In one‐dimensional mathematical models of ?uvial ?ow, sediment transport and morphological evolution, the governing equations based on mass and momentum conservation laws constitute a hyperbolic system. Succinctly, the hyperbolic nature excludes dispersion or diffusion operators, which is well known in the context of differential equations. There is no doubt that the so‐called ‘dispersion’ argument for bed material wave evolution is questionable, as we have explicitly asserted. Surprisingly, in a recent communication, the authors of the ‘dispersion’ argument suggest that dispersion is not precluded in hyperbolic systems. We provide herein further perspectives to help explain that the dispersion argument is neither appropriate nor necessary for interpreting bed material wave evolution. Also the continuity equations involved are addressed to prompt wider understanding of their signi?cance. In particular, the continuity equation of the water–sediment mixture proposed by the authors of the ‘dispersion’ argument is proved to be incorrect, and inevitably their reasoning based on it is problematic. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
20.
The literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections. However, there are few, if any, comprehensive reviews of the literature. Surprisingly, given the economic importance of placers, the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory. In this wide-ranging review, the history of the development of principles of placer formation is explored as far as possible in a non-technical fashion. A consideration is given to the hydrodynamics of physical grain sorting above lower-stage and upper-stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed. Finally, examples of the depositional environment of diamond, tin and gold placers are considered.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号