全文获取类型
| 收费全文 | 997篇 |
| 免费 | 200篇 |
| 国内免费 | 373篇 |
专业分类
| 测绘学 | 90篇 |
| 大气科学 | 32篇 |
| 地球物理 | 495篇 |
| 地质学 | 705篇 |
| 海洋学 | 120篇 |
| 天文学 | 1篇 |
| 综合类 | 60篇 |
| 自然地理 | 67篇 |
出版年
| 2024年 | 4篇 |
| 2023年 | 7篇 |
| 2022年 | 15篇 |
| 2021年 | 49篇 |
| 2020年 | 47篇 |
| 2019年 | 51篇 |
| 2018年 | 40篇 |
| 2017年 | 39篇 |
| 2016年 | 40篇 |
| 2015年 | 64篇 |
| 2014年 | 84篇 |
| 2013年 | 88篇 |
| 2012年 | 64篇 |
| 2011年 | 76篇 |
| 2010年 | 64篇 |
| 2009年 | 93篇 |
| 2008年 | 95篇 |
| 2007年 | 95篇 |
| 2006年 | 105篇 |
| 2005年 | 71篇 |
| 2004年 | 69篇 |
| 2003年 | 64篇 |
| 2002年 | 37篇 |
| 2001年 | 36篇 |
| 2000年 | 27篇 |
| 1999年 | 29篇 |
| 1998年 | 30篇 |
| 1997年 | 22篇 |
| 1996年 | 17篇 |
| 1995年 | 10篇 |
| 1994年 | 8篇 |
| 1993年 | 10篇 |
| 1992年 | 2篇 |
| 1991年 | 4篇 |
| 1990年 | 4篇 |
| 1989年 | 4篇 |
| 1988年 | 2篇 |
| 1987年 | 3篇 |
| 1985年 | 1篇 |
排序方式： 共有1570条查询结果，搜索用时 15 毫秒
991.
A semi‐analytical approach for predicting peak discharge of floods caused by embankment dam failures 
下载免费PDF全文
下载免费PDF全文 This study presents an analytical solution of dam‐break floods in a trapezoidal channel with detailed solution procedure. An approach predicting the peak discharge of floods caused by embankment dam failures was derived from the aforementioned analytical solution with a database of 27 historical dam failures. The prediction performance of this approach has been proved by comparing with other 14 straightforward equations for estimating the peak discharge. The proposed model with a small uncertainty of predicted peak flow rates has a high coefficient of determination and a small standard error, being ranked in the top four of the 15 methods considered in this paper. The robustness and predictive capability of the proposed model are further demonstrated in two case studies, and both were considered in the previous analyses performed by other investigators. This method provides a simple and transparent tool for engineers to predict the peak discharge and is easy to implement for trial and error calculation. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
992.
Soil loss rates due to piping erosion 总被引：1，自引：0，他引：1
E. Verachtert W. Maetens M. Van Den Eeckhaut J. Poesen J. Deckers 《地球表面变化过程与地形》2011,36(13):1715-1725
Compared with surface soil erosion by water, subsurface erosion (piping) is generally less studied and harder to quantify. However, wherever piping occurs, it is often a significant or even the main sediment source. In this study, the significance of soil loss due to piping is demonstrated through an estimation of soil volume lost from pipes and pipe collapses (n = 560) in 137 parcels under pasture on loess‐derived soils in a temperate humid climate (Belgium). Assuming a period of 5 to 10 years for pipe collapse to occur, mean soil loss rates of 2.3 and 4.6 t ha?1 yr?1 are obtained, which are at least one order of magnitude higher than surface erosion rates (0.01–0.29 t ha?1 yr?1) by sheet and rill erosion under a similar land use. The results obtained for the study area in the Flemish Ardennes correspond well to other measurements in temperate environments; they are, however, considerably smaller than soil loss rates due to subsurface erosion in semi‐arid environments. Although local slope gradient and drainage area largely control the location of collapsed pipes in the study area, these topographic parameters do not explain differences in eroded volumes by piping. Hence, incorporation of subsurface erosion in erosion models is not straightforward. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
993.
A sediment mass balance constructed for a 16‐km reach of the Snake River downstream from Jackson Lake Dam (JLD) indicates that river regulation has reduced the magnitude of sediment mass balance deficit that would naturally exist in the absence of the dam. The sediment budget was constructed from calibrated bed load transport relations, which were used to model sediment flux into and through the study reach. Calibration of the transport relations was based on bed load transport data collected over a wide range of flows on the Snake River and its two major tributaries within the study area in 2006 and 2007. Comparison of actual flows with unregulated flows for the period since 1957 shows that operations of JLD have reduced annual peak flows and increased late summer flows. Painted tracer stones placed at five locations during the 2005 spring flood demonstrate that despite the reduction in flood magnitudes, common floods are capable of mobilizing the bed material. The sediment mass balance demonstrates that more sediment exits the study reach than is being supplied by tributaries. However, the volume of sediment exported using estimated unregulated hydrology indicates that the magnitude of the deficit would be greater in the absence of JLD. Calculations suggest that the Snake River was not in equilibrium before construction of JLD, but was naturally in sediment deficit. The conclusion that impoundment lessened a natural sediment deficit condition rather than causing sediment surplus could not have been predicted in the absence of sediment transport data, and highlights the value of transport data and calculation of sediment mass balance in informing dam operations. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
994.
995.
A two-dimensional (2D) numerical model has been developed to solve shallow water equations for simulation of dam-break flows. The spatial derivatives are discretized using a well-balanced explicit central upwind conservative scheme. The scheme is Riemann solver free and guarantees the positivity of the flow depth over complex topography if the Courant number is kept less than 0.25. The time integration is performed by Euler’s scheme. The model is verified against analytical results for water surface elevation and discharge for three benchmark test cases. A good agreement between analytical solutions and computed results is observed. The property of well-balancing in still water over an uneven bottom is also confirmed. The model is then validated by simulating a laboratory experiment in which a dam break flow propagates over a triangular obstacle. The model performance was found to be satisfactory. A dam break laboratory experimental test case on a frictionless horizontal bottom is also simulated for 2D validation of the model, and good agreement between simulation and the experimental data is observed. The suitability of the proposed model for real life applications is demonstrated by simulating the Malpasset dam-break event, which occurred in 1959 in France. The computed arrival time of the flood wave front and the maximum flow depths at various observation points matched well with the measurements on a 1/400 scale physical model. The overall performance indicates that this model can be applied for simulation of dam-break waves in real life cases. 相似文献
996.
在高应力状态下坝料的流变较为明显。为研究坝料流变对混凝土面板坝应力变形的影响,采用长科院九参数幂级数流变模型及其试验参数,对某高混凝土面板堆石坝进行应力、变形分析。结果表明,坝料流变使坝体变形明显增加,坝体应力有所减小。考虑坝料的流变特性后的面板法向位移(挠度)明显增加,面板坝轴向和顺坡向应力极值增加。对于分期浇筑面板和分期蓄水的高混凝土面板堆石坝,选用合适的流变本构模型正确地模拟堆石体的流变特性,可以为大坝填筑进度及面板分期浇筑时间的确定提供参考,并有助于正确地预测大坝的应力变形。 相似文献
997.
998.
Andrew L.R. Jackson 《Global Environmental Change》2011,21(4):1195-1208
The European Union's (EU) network of nature conservation areas – Natura 2000 – covers almost 18% of EU territory, and is subject to strict legal protection, which is enforced by the European Commission, a supranational authority. Given the Natura 2000 network's size, conflicts between Natura 2000 and renewable energy projects are inevitable, particularly as countries push to meet their 2020 energy and emissions reduction targets by pursuing more – and larger – renewable energy projects. Focusing on two cases in the renewable energy sector – a hydroelectric dam in Portugal's Sabor valley, and a large tidal barrage in the UK's Severn estuary – this article shows that the EU's strict biodiversity protection regime could necessitate the rejection of many large renewable energy projects. That is, it may not be possible as a matter of EU law for national authorities to grant permission for such projects. The potential for such difficulties will be shown to be highly visible to policymakers, and could, this article argues, trigger negative impacts in terms of the rule of law, and negative feedbacks on nature conservation policies in the EU and, by way of precedent, globally. The legal issues presented here should not, this article argues, be regarded as insurmountable problems, nor as a trigger for reforms aimed at weakening biodiversity protections. Rather, these issues are better regarded as an opportunity for an open, informed, global debate regarding the relationship between biodiversity and climate change policies, and the hierarchy, if any, between them. 相似文献
999.
1000.