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排序方式: 共有3925条查询结果,搜索用时 15 毫秒
891.
Alexandre Cunha Costa Saskia Foerster José Carlos de Araújo Axel Bronstert 《水文研究》2013,27(7):1046-1060
Scarcity of hydrological data, especially streamflow discharge and groundwater level series, restricts the understanding of channel transmission losses (TL) in drylands. Furthermore, the lack of information on spatial river dynamics encompasses high uncertainty on TL analysis in large rivers. The objective of this study was to combine the information from streamflow and groundwater level series with multi‐temporal satellite data to derive a hydrological concept of TL for a reach of the Middle Jaguaribe River (MJR) in semi‐arid north‐eastern Brazil. Based on this analysis, we proposed strategies for its modelling and simulation. TL take place in an alluvium, where river and groundwater can be considered to be hydraulically connected. Most losses certainly infiltrated only through streambed and levees and not through the flood plains, as could be shown by satellite image analysis. TL events whose input river flows were smaller than a threshold did not reach the outlet of the MJR. TL events whose input flows were higher than this threshold reached the outlet losing on average 30% of their input. During the dry seasons (DS) and at the beginning of rainy seasons (DS/BRS), no river flow is expected for pre‐events, and events have vertical infiltration into the alluvium. At the middle and the end of the rainy seasons (MRS/ERS), river flow sustained by base flow occurs before/after events, and lateral infiltration into the alluvium plays a major role. Thus, the MJR shifts from being a losing river at DS/BRS to become a losing/gaining (mostly losing) river at MRS/ERS. A model of this system has to include the coupling of river and groundwater flow processes linked by a leakage approach. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
892.
Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak‐T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high‐grade Higher Himalayan Crystalline Sequences (HHCS) over the low‐grade Lesser Himalaya Sequences (LHS) in far‐eastern Nepal. Maximum recorded pressure conditions occur just above the MCT (~11 kbar), and decrease southward to ~6 kbar in the garnet zone and northward to ~7 kbar in the kyanite ± staurolite zone. The inferred nearly isothermal loading path for the LHS in the staurolite zone may have resulted from the underthrusting of the LHS beneath the HHCS. In contrast, the increasing temperature path during both loading and decompression (i.e. clockwise path) from the lowermost HHCS in the staurolite to kyanite ± staurolite transitional zone indicates that the rocks were fairly rapidly buried and exhumed. Exhumation of the lowermost HHCS from deeper crustal depths than the flanking regions, recording a high field pressure gradient (~1.2–1.6 kbar km?1) near the MCT, is perhaps caused by ductile extrusion along the MCT, not the emplacement along a single thrust, resulting in the P–T path discontinuities. These observations are consistent with the overall scheme of the model of channel flow, in which the outward flowing ‘HHCS’ and inward flowing ‘LHS’ are juxtaposed against each other and are rapidly extruded together along the ‘MCT’. A rapid exhumation by channel flow in this area is also suggested by a nearly isothermal decompression path inferred from cordierite corona surrounding garnet in gneiss of the upper HHCS. However, peak metamorphic temperatures show a progressive increase of temperature structurally upward (~570–740 °C) near the MCT and roughly isothermal conditions (~710–810 °C) in the upper structural levels of the HHCS. The observed field temperature gradient is much lower than those predicted in channel flow models. However, the discrepancy could be resolved by taking into account heat advection by melt and/or fluid migration, as these can produce low or nearly no field temperature gradient in the exhumed midcrust, as observed in nature. 相似文献
893.
Zhixue Zhao Xiaoguang Wang Yonghong Hao Tongke Wang Abderrahim Jardani Herve Jourde Tian‐Chyi Jim Yeh Ming Zhang 《水文研究》2019,33(19):2551-2560
The study on the hydraulic properties of coastal aquifers has significant implications both in hydrological sciences and environmental engineering. Although many analytical solutions are available, most of them are based on the same basic assumption that assumes aquifers extend landward semi‐infinitely, which does not necessarily reflect the reality. In this study, the general solutions for a leaky confined coastal aquifer have been developed that consider both finitely landward constant‐head and no‐flow boundaries. The newly developed solutions were then used to examine theoretically the joint effects of leakage and aquifer length on hydraulic head fluctuations within the leaky confined aquifer, and the validity of using the simplified solution, which assumes the aquifer is semi‐infinite. The results illustrated that the use of the simplified solution may cause significant errors, depending on joint effects of leakage and aquifer length. A dimensionless characteristic parameter was then proposed as an index for judging the applicability of the simplified solution. In addition, practical application of the general solution for the constant‐head inland boundary was used to characterize the hydraulic properties of a leaky confined aquifer using the data collected from a field site at the Seine River estuary, France, and the versatility of the general solution was further justified. 相似文献
894.
Daniel A. Hepp Dierk Hebbeln Stefan Kreiter Hanno Keil Christian Bathmann Jürgen Ehlers Tobias Mörz 《第四纪科学杂志》2012,27(8):844-853
A combination of a dense reflection seismic grid and up to 50‐m‐long records from sediment cores and cone penetration tests was used to study the geometry and infill lithology of an E–W‐trending buried tunnel valley in the south‐eastern North Sea. In relation to previously known primarily N–S‐trending tunnel valleys in this area, the geometry and infill of this 38‐km‐long and up to 3‐km‐wide valley is comparable, but its E–W orientation is exceptional. The vertical cross‐section geometry may result from subglacial sediment erosion of advancing ice streams and secondary incision by large episodic meltwater discharges with high flow rates. The infill is composed of meltwater sands and reworked till remnants on the valley flanks that are overlain by late Elsterian rhythmic, laminated, lacustrine fine‐grained sediments towards the centre of the valley. A depression in the valley centre is filled with sediments most likely from the Holsteinian transgression and a subsequent post‐Holsteinian lacustrine quiet‐water setting. The exceptional axis orientation of this tunnel valley points to a regional N–S‐oriented ice front during the late Elsterian. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
895.
Two controlled flow events were generated by releasing water from a reservoir into the Olewiger Bach, located near Trier, Germany. This controlled release of near bank‐full flows allowed an investigation of the fine sediment (<63 µm) mobilized from channel storage. Both a winter (November) and a summer (June) release event were generated, each having very different antecedent flow conditions. The characteristics of the release hydrographs and the associated sediment transport indicated a reverse hysteresis with more mass, but smaller grain sizes, moving on the falling limb. Fine sediment stored to a depth of 10 cm in the gravels decreased following the release events, indicating the dynamic nature and importance of channel‐stored sediments as source materials during high flow events. Sediment traps, filled with clean natural gravel, were buried in riffles before the release of the reservoir water and the total mass of fine sediment collected by the traps was measured following the events. Twice the mass of fine sediment was retained by the gravel traps compared with the natural gravels, which may be due to their altered porosity. Although the amount of fine sediment collected by the traps was not significantly related to measures of gravel structure, it was found to be significantly correlated to measures of local flow velocity and Froude number. A portion of the traps were fitted with lids to restrict surface exchange of water and sediment. These collected the highest amounts of event‐mobilized sediments, indicating that inter‐gravel lateral flows, not just surface infiltration of sediments, are important in replenishing and redistributing the channel‐stored fines. These findings regarding the magnitude and direction of fine sediment movement in gravel beds are significant in both a geomorphic and a biological context. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
896.
提出了一种基于非曼哈顿结构的单元上(Over-The-Cell,OTC)布线算法。算法第一次在单元上布线中采用非曼哈顿布线结构,并根据算法特点提出了线网和边相关系数的概念。实践证明,得益于单元区线网选择时对线网和边相关系数的应用,算法和传统标准单元详细布线算法相比,可以进一步降低通道高度和减少通孔数量。 相似文献
897.
This study focused on the effects of upstream reservoir thermal dynamics and downstream tidal influences on temperatures in a 25-km reach of Alouette River (coastal British Columbia, Canada) below a control dam and upstream of its confluence with Pitt River. Temperature was monitored during summer 2013 using 25 sensors. Water was released from the reservoir through a low level outlet at approximately 2.7 m3 s−1, except during late spring when a higher flow was released over the dam spillway. Temperature variations in the lowest section of Alouette River, and in the lower portion of a tributary, were distinct from those further upstream due to backwatering effects associated with a semi-diurnal tide, which can cause flow reversals in Pitt River. An internal seiche was identified in the reservoir during mid-summer that resulted in oscillating releases of warmer and cooler water with an amplitude of up to 6°C and a period of approximately 12 hr. Wavelet analysis and band-pass filtering indicated that the 12-hr signal declined in strength with downstream distance, but remained detectable about 15 km below the dam. In contrast, the 24-hr diel signal increased in strength with distance below the dam. Travel times computed via cross-correlation of the 12-hr signals with that at the low level outlet were within ±10% of those estimated from measured mean velocities. Lagrangian tracking of water parcels using the derived travel times indicated that the cooling effect of periodic releases of cold water during the seiching period persisted to the lower extent of the non-tidal reach. The tidally influenced locations experienced higher temperatures than those recorded in the non-tidal portion of Alouette River, although the relative roles of heating in the upstream tidal reach versus upstream advection of water associated with tide-driven flow reversals in Pitt River require further study. 相似文献
898.
As with most Italian rivers, the Reno River has a long history of human modification, related also to morphological changes of the lower Po River since Roman times, but in the last decades, significant land use changes in the headwaters, dam construction, torrent control works and extensive bed material mining have caused important channel morphology and sediment budget changes. In this paper, two main types of channel adjustment, riverbed incision and channel narrowing, are analysed. Riverbed degradation is discussed by comparing four different longitudinal profiles surveyed in 1928, 1951, 1970 and 1998 in the 120 km long reach upstream of the outlet. The analysis of channel narrowing is carried out by comparing a number of cross‐sections surveyed in different years across the same downstream reach. Field sediment transport measurements of seven major floods that occurred between 2003 and 2006 are compared with the bedload transport rates predicted by the most renowned equations. The current low bedload yield is discussed in terms of sediment supply limited conditions due to land use changes, erosion‐control works and extensive and out of control bed material mining that have affected the Reno during the last decades. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
899.
底流在陆缘深水环境下广泛存在,可对深水沉积过程及砂体分布产生重要影响。前人对重力流与底流的交互作用机制及沉积产物开展了大量研究,但目前有关底流改造型的海底扇储层构型模式仍然研究不够深入。东非鲁伍马盆地是当前重力流—底流交互作用研究的热点地区,文中以其代表性的下始新统海底扇水道体系为例,综合岩心、测井及三维地震资料开展储层构型精细表征,建立重力流—底流交互作用下的海底扇水道体系构型模式。研究表明,目标水道体系内部发育水道、溢岸及朵叶3种构型要素,其中水道可分为水道复合体、单一水道及其内部不同级次的构型单元。底流对细粒物质的搬运可形成非对称的溢岸沉积,导致水道复合体之间呈逆底流侧向迁移叠置样式,其间泥岩隔层容易保存; 单一水道之间呈顺水道纵向迁移或逆底流侧向迁移样式,其中纵向迁移部位水道切叠连通,而侧向迁移部位容易保存泥质侧向隔挡体。受重力流沉积演化的影响,单一水道内部充填由砂泥交互型逐渐演化为富砂型,且在水道弯曲段的轴部砂体最为发育。 相似文献
900.