全文获取类型
| 收费全文 | 698篇 |
| 免费 | 134篇 |
| 国内免费 | 305篇 |
专业分类
| 测绘学 | 80篇 |
| 大气科学 | 5篇 |
| 地球物理 | 308篇 |
| 地质学 | 630篇 |
| 海洋学 | 26篇 |
| 天文学 | 1篇 |
| 综合类 | 42篇 |
| 自然地理 | 45篇 |
出版年
| 2024年 | 3篇 |
| 2023年 | 4篇 |
| 2022年 | 13篇 |
| 2021年 | 34篇 |
| 2020年 | 31篇 |
| 2019年 | 32篇 |
| 2018年 | 27篇 |
| 2017年 | 27篇 |
| 2016年 | 28篇 |
| 2015年 | 51篇 |
| 2014年 | 59篇 |
| 2013年 | 58篇 |
| 2012年 | 51篇 |
| 2011年 | 61篇 |
| 2010年 | 47篇 |
| 2009年 | 74篇 |
| 2008年 | 75篇 |
| 2007年 | 78篇 |
| 2006年 | 78篇 |
| 2005年 | 53篇 |
| 2004年 | 49篇 |
| 2003年 | 53篇 |
| 2002年 | 20篇 |
| 2001年 | 20篇 |
| 2000年 | 18篇 |
| 1999年 | 18篇 |
| 1998年 | 21篇 |
| 1997年 | 13篇 |
| 1996年 | 12篇 |
| 1995年 | 5篇 |
| 1994年 | 6篇 |
| 1993年 | 5篇 |
| 1992年 | 1篇 |
| 1991年 | 3篇 |
| 1990年 | 3篇 |
| 1989年 | 4篇 |
| 1988年 | 1篇 |
| 1987年 | 1篇 |
排序方式： 共有1137条查询结果，搜索用时 265 毫秒
31.
32.
33.
34.
Molly Pohl 《The Professional geographer》2004,56(3):422-431
Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross‐sections. The relationship of average boundary shear stress (τo) to critical shear stress (τcr) served as the basis for determining channel bed material mobility under the two‐year and ten‐year flood events. The channel had the greatest channel bed mobility at the natural cross‐section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run‐of‐the‐river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River. 相似文献
35.
Channel cross‐sectional changes since construction of Livingston Dam and Lake Livingston in 1968 were studied in the lower Trinity River, Texas, to test theoretical models of channel adjustment, and to determine controls on the spatial extent of channel response. High and average flows were not significantly modified by the dam, but sediment transport is greatly reduced. The study is treated as an opportunistic experiment to examine the effects of a reduction in sediment supply when discharge regime is unchanged. Channel scour is evident for about 60 km downstream, and the general phenomena of incision, widening, coarsening of channel sediment and a decrease in channel slope are successfully predicted, in a qualitative sense, by standard models of channel response. However, there is no consistent channel response within this reach, as various qualitatively different combinations of increases, decreases or no change in width, depth, slope and roughness occur. These multiple modes of adjustment are predicted by the unstable hydraulic geometry model. Between about 60 km and the Trinity delta 175 km downstream of the dam, no morphological response to the dam is observed. Rather than a diminution of the dam's effects on fluvial processes, this is due to a fundamental change in controls of the fluvial system. The downstream end of the scour zone corresponds to the upstream extent of channel response to Holocene sea level rise. Beyond 60 km downstream, the Trinity River is characterized by extensive sediment storage and reduced conveyance capacity, so that even after dam construction sediment supply still exceeds transport capacity. The channel bed of much of this reach is near or below sea level, so that sea level rise and backwater effects from the estuary are more important controls on the fluvial system than upstream inputs. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
36.
Analysis of earth dams affected by the 2001 Bhuj Earthquake 总被引：3，自引：0，他引：3
An earthquake of magnitude of 7.6 (Mw 7.6) occurred in Bhuj, India on January 26, 2001. This event inflicted damages of varying extents to a large number of small to moderate size multi-zone earth dams in the vicinity of the epicenter. Some of the distress was due to the liquefaction of saturated alluvium in foundation. Liquefaction was relatively localized for the majority of these dams because the earthquake struck in the middle of a prolonged dry season when the reservoirs behind these dams were nearly empty and shallow alluvium soils underneath the downstream portions of the dams were partly dry. Otherwise, liquefaction of foundation soils would have been more extensive and damage to these dams more significant. Six such dams have been examined in this paper. Four of these facilities, Chang, Shivlakha, Suvi, and Tapar were within the 50 km of epicenter region. These dams underwent free-field ground motion with peak ground accelerations between 0.28g to 0.52g. Of these Chang Dam underwent severe slumping, whereas Shivlakha, Suvi, and Tapar Dams were affected severely especially over the upstream sections. Fatehgadh Dam and Kaswati Dam were affected relatively less severely. Foundation conditions underneath these dams were first examined for assessing liquefaction potential. A limited amount of subsurface information available from investigations undertaken prior to the earthquake indicates that, although the foundation soils within the top 2.0 to 2.5 m underneath these dams were susceptible to liquefaction, Bhuj Earthquake did not trigger liquefaction because of lack of saturation of these layers underneath the downstream portions of these dams. These dams were then analyzed using a simple sliding block procedure using appropriate estimates of undrained soil strength parameters. The results of this analysis for these structures were found to be in general agreement with the observed deformation patterns. 相似文献
37.
Sustaining the human ecological benefits of surface water requires carefully planned strategies for reducing the cumulative
risks posed by diverse human activities. Municipal governments in Aksaray City play a key role in developing solutions to
surface water management and protection problems. The responsibility to provide drinking water and sewage works, regulate
the use of private land, and protect public health provides the mandate and authority to take action. A large part of Aksaray
City uses Mamasın dam water as its primary source for drinking water. Several point sources of contamination may result from
direct wastewater discharges from Melendiz and Karasu rivers, which recharge the Mamasın dam watershed. Relevant studies were
carried out for monitoring the eutrophication process, which usually occurs in the static water mass of the Mamasın dam lake.
This process may be caused by the continual increase in nutrients and decrease of O2 levels, causing anaerobic conditions. Stimulated algae growth in these water bodies consequently reduces water quality. Hydrochemical
parameters were evaluated to estimate the types of pollution sources, the level of pollution, and its environmental impacts
on the Mamasın dam drinking water reservoir. 相似文献
38.
广西南丹地区尾矿分布于大厂、车河、芒场等矿区。尾砂来自以锡石为主的多金属矿床，尾砂中除Sn，Pb，Zn，Ag，Sb的质量分数达到工业指标外，Au亦达到综合利用指标。通过试验证明，对尾矿中的细微粒锡石可采用风力分级选矿、磁选和化学选矿等方法回收；尾砂中的非金属资源可作为填充材料、水泥原料利用，使尾砂型人工矿床充分的资源化。 相似文献
39.
An investigation and remediation of instability along upstream cut slopes for an earthfill dam in differentially weathered rock in southern Turkey is described. The major instability problem was a 45-m high and 200-m long previously cut slope next to the main axis of the dam, above the diversion tunnels and water outlet structures. The slope was first designed and excavated in 1986 based on the temporary berm approach. Rising water level in the reservoir would change the shear strength parameters and the pore-water pressure in the slope; thus, probable deep failure would damage the entrance of the diversion tunnels and water outlet structures, as well as the earthfill embankment of the dam. In June 1996, the slope face was re-excavated and protected against wave erosion by placement of a layer of rock riprap over a layer of bedding and a filter material. A strong earthquake (MS = 6.2) occurred during a period of rapid drawdown in 27 June 1998. The slope remained stable, although numerous tension cracks developed in Quaternary terrace deposits near the reservoir area. 相似文献
40.
Subsurface dams are rather effective and used for the prevention of saltwater intrusion in coastal regions around the world. We carried out the laboratory experiments to investigate the elevation of saltwater wedge after the construction of subsurface dams. The elevation of saltwater wedge refers to the upward movement of the downstream saltwater wedge because the subsurface dams obstruct the regional groundwater flow and reduce the freshwater discharge. Consequently, the saltwater wedge cannot further extend in the longitudinal direction but rises in the vertical profile resulting in significant downstream aquifer salinization. In order to quantitatively address this issue, field-scale numerical simulations were conducted to explore the influence of various dam heights, distances, and hydraulic gradients on the elevation of saltwater wedge. Our investigation shows that the upward movement of the saltwater wedge and its areal extension in the vertical domain of the downstream aquifer become more severe with a higher dam and performed a great dependence on the freshwater discharge. Furthermore, the increase of the hydraulic gradient and the dam distance from the sea boundary leads to a more pronounced wedge elevation. This phenomenon comes from the variation of the freshwater discharge due to the modification of dam height, location, and hydraulic gradient. Large freshwater discharge can generate greater repulsive force to restrain the elevation of saltwater wedge. These conclusions provide theoretical references for the behaviour of the freshwater–seawater interface after the construction of subsurface dams and help optimize the design strategy to better utilize the coastal groundwater resources. 相似文献