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961.
962.
Yongqin David Chen Tao Yang Chong-Yu Xu Qiang Zhang Xi Chen Zhen-Chun Hao 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(1):9-18
This paper presents a visually enhanced evaluation of the spatio-temporal patterns of the dam-induced hydrologic alteration
in the middle and upper East River, south China over 1952–2002, using the range of variability approach (RVA) and visualization
package XmdvTool. The impacts of climate variability on hydrological processes have been removed for wet and dry periods,
respectively, so that we focus on the impacts of human activities (i.e., dam construction). The results indicate that: (1)
along the East River, dams have greatly altered the natural flow regime, range condition and spatial variability; (2) six
most remarkable indicators of hydrologic alteration induced by dam-construction are rise rate (1.16), 3-day maximum (0.91),
low pulse duration (0.88), January (0.80), July (0.80) and February (0.79) mean flow of the East River during 1952–2002; and
(3) spatiotemporal hydrologic alterations are different among three stations along Easter River. Under the influence of dam
construction in the upstream, the degree of hydrologic changes from Lingxia, Heyuan to Longchuan station increases. This study
reveals that visualization techniques for high-dimensional hydrological datasets together with RVA are beneficial for detecting
spatio-temporal hydrologic changes. 相似文献
963.
The problem of flow separation around islands is investigated using a dynamically adaptive finite element model to allow for
resolution of the shear layers that form in the advent of separation. The changes in secondary circulation and vertical motion
that occur in both attached and separated flows are documented, as is the degree of closure of the wake eddies. In the numerical
experiments presented, the strongest motion always takes place at the sides of the idealised island, where flow curvature
and shear act together to induce ascent. In contrast, it is the slower motion within the wake eddies that allow streamlines
to extend from the bottom to the surface. We find no evidence for closure of the wake eddies. Rather, all of our separated
experiments show that streamlines that pass through the eddies originate outside of the shear layers and frictional boundary
layers on the upstream side of the idealised island. The numerical experiments demonstrate the potential for dynamically adaptive,
unstructured meshes to resolve the separated shear layers that occur downstream of the idealised island, as well as the narrow
boundary layers that form on the island itself. 相似文献
964.
Elizabeth Cottrell Michael J. Walter David Walker 《Earth and Planetary Science Letters》2010,289(3-4):631-634
965.
Philippe Sorrel Bernadette Tessier François Demory Agnès Baltzer Firas Bouaouina Jean-Noël Proust David Menier Camille Traini 《Continental Shelf Research》2010,30(10-11):1250-1266
The late Holocene is of particular interest to our understanding of the evolution of coastal sedimentary systems because this period encompasses warmer and cooler periods, and rising sea level in northern Europe. Based on an approach combining AMS 14C, sedimentological and rock magnetic analyses on sediment cores complemented with seismic data collected in the macrotidal Bay of Vilaine (south Brittany), we document the depositional history of the inner bay coeval to the mid- to late-Holocene transgression in south Brittany. Correlation between sedimentary archives revealed the main sedimentary infilling phases during the last 6000 years. Four units (U1–U4) are recognized in the coastal sediment wedge of the system, corresponding to the stepwise marine invasion of the bay. We show that (1) marine inundation, due to the steep morphology of the bedrock, is diachronous between distal and proximal records. A time lag of ~1000 years is inferred over a distance of less than 5 km; (2) in the outer areas, the sedimentation has been condensed since 3000 years; (3) proximal estuarine archives offer the best record of sedimentary processes covering the last 2000 years, including the Medieval Warm Period (MWP).Correlations in proximal records in the Bay of Vilaine assess the connection between coastal sedimentary dynamics, climatic conditions and anthropogenic activities during the MWP. We match the preservation of clay deposits to increased river-borne suspended matter transported to the estuary probably as a result of accelerated land-use development (higher soil erosion) in the catchment area between ca. 880 and 1050 AD. Because the preservation of estuarine sedimentary successions is favoured when coastal wave sediment reworking is minimal, it is proposed that the prevailing climatic regime in south Brittany during the MWP likely resembled to that of the preferred negative phase of the North Atlantic Oscillation (NAO). Our data are fairly consistent with other late Holocene records from northern Europe including the Atlantic seaboard. However, they outline the difficulty in interpreting climatic and anthropogenic signatures in coastal sedimentary records where high-resolution chronologies required to unravel their respective influences are still missing. 相似文献
966.
Colin V. Murray-Wallace Robert P. Bourman John R. Prescott Frances Williams David M. Price Antonio P. Belperio 《Quaternary Geochronology》2010,5(1):28-49
A geochronological framework for the sequential development of coastal barrier aeolianite complexes in the mouth region of the River Murray, Australia's largest river system is presented based on amino acid racemization and thermoluminescence dating. The sedimentary successions represent a foreshortened and condensed sequence of coastal barriers compared with those of the Coorong Coastal Plain in southern South Australia where the barrier complexes are more widely separated in response to tectonic uplift. The barriers have formed during interglacial sea-level highstands and are correlatives of genetically equivalent landforms of the Coorong Coastal Plain. Thermoluminescence dating and the extent of amino acid racemization in aeolianite ‘whole-rock’ sediment samples, reveal a general increase in age of the barriers landwards from the modern coastline. In detail, however, the individual barriers represent composite structures having formed in more than one interglaciation, due to the reoccupation of Pleistocene shoreline positions during sea-level highstands of similar amplitude, in a zone of gradual basin subsidence. The most seaward Pleistocene aeolianite at Surfer Beach is of interstadial age (Marine Isotope Stage 5c, 105 ± 5 ka; MIS 5c), and correlates with the Robe Range of the Coorong Coastal Plain. The last interglacial shoreline (130 ± 15 ka; MIS 5e) is particularly well-defined in the River Murray mouth region. It is represented by a complex association of coastal parabolic dunes superimposed on a transverse dune system, which runs parallel with the former coastline, and also includes associated estuarine, lagoonal and open ocean beach facies. Landward of the last interglacial succession are distinct barriers relating to the penultimate interglaciation (215 ± 35 ka; MIS 7), as well as earlier interglaciations (350 ± 65 ka; MIS 9 or 11 and 470 ± 70 ka; MIS 11 or 13). The coastal barriers have been successively breached by the ancestral River Murray at times of lower sea level during glacial cycles. Former mouths of the River Murray during interglacial sea-level highstands are likely to have existed near Tauwitchere Island during MIS 7, and between Goolwa and Hindmarsh Island and near the southern-most part of Lake Albert during the last interglacial (MIS 5e). The River Murray mouth region represents a failed delta as the limited sediment brought to this area since late middle Pleistocene time has been either rapidly incorporated within aeolian deposits during sea-level highstands, or transported to the edge of the Lacepede Shelf during glacial maxima. The Holocene and modern River Murray has not established a marine delta, but deposits its load in the settling basins of the terminal lakes. Only a small digitate delta has formed where the river enters Lake Alexandrina. 相似文献
967.
David F. Boutt 《Ground water》2010,48(4):580-592
Short-term changes in the hydraulic head of surface water bodies are known to influence the shallow response of hydraulically connected groundwaters. Associated with these fluctuations is the physical increase in stream water creating a mechanical load on the ground surface. This load is supported by the geologic materials (sediment or rock) and the pore fluid contained within the pores. Changes in this surface load have a direct effect on the total stress of the aquifer causing either a change in effective stress or fluid pressure. This response, predicted by the framework of linear poroelasticity, is a well-understood phenomenon in geologic materials. Currently, field measurements of the hydraulic response (i.e., fluid pressure) of aquifer materials are undergoing poroelastic loading due to dam releases in the Deerfield River Watershed in Massachusetts. An increase in stream stage from upstream dam releases causes an instantaneous pore fluid pressure increase at multiple depths and locations in the aquifer. This increase lasts anywhere from 15 to 40 minutes depending on the magnitude of the rise in the stream stage. Pore-pressure changes are well correlated to stream stage fluctuations for all of the recorded events. Poroelastic models created using basin stratigraphy and hydraulic properties of the aquifer response match the field observations well. Model results suggest that the overall stratigraphy is important in controlling the magnitude and duration of the poroelastic response. An improved understanding of responses such as these can be used to constrain uncertainties in model calibration and simulations of the contaminant migration in low permeability fine-grained (compressive) materials. 相似文献
968.
David W. Townsend Nathan D. RebuckMaura A. Thomas Lee Karp-BossRachel M. Gettings 《Continental Shelf Research》2010
Recent oceanographic observations and a retrospective analysis of nutrients and hydrography over the past five decades have revealed that the principal source of nutrients to the Gulf of Maine, the deep, nutrient-rich continental slope waters that enter at depth through the Northeast Channel, may have become less important to the Gulf's nutrient load. Since the 1970s, the deeper waters in the interior Gulf of Maine (>100 m) have become fresher and cooler, with lower nitrate (NO3) but higher silicate (Si(OH)4) concentrations. Prior to this decade, nitrate concentrations in the Gulf normally exceeded silicate by 4–5 μM, but now silicate and nitrate are nearly equal. These changes only partially correspond with that expected from deep slope water fluxes correlated with the North Atlantic Oscillation, and are opposite to patterns in freshwater discharges from the major rivers in the region. We suggest that accelerated melting in the Arctic and concomitant freshening of the Labrador Sea in recent decades have likely increased the equatorward baroclinic transport of the inner limb of the Labrador Current that flows over the broad continental shelf from the Grand Banks of Newfoundland to the Gulf of Maine. That current system now brings a greater fraction of colder and fresher deep shelf waters into the Gulf than warmer and saltier offshore slope waters which were previously thought to dominate the flux of nutrients. Those deep shelf waters reflect nitrate losses from sediment denitrification and silicate accumulations from rivers and in situ regeneration, which together are altering the nutrient regime and potentially the structure of the planktonic ecosystem. 相似文献
969.
David Ching-Fang Shih Yih-Min Wu Gwo-Fong Lin Jyr-Ching Hu Yue-Gau Chen Chien-Hsin Chang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2008,22(3):401-410
A transition and subduction zone adjacent to the Ryukyu Arc, Ryukyu Trench, and Okinawa Trough, extends between southern Japan
and northeastern Taiwan. It is generated during the northwestward subduction of the Philippine Sea Plate, which lies the Eurasian
Plate along the Ryukyu Trench. The movement of the Philippine Sea Plate is hindered at the northeastern corner of Taiwan,
which causes complicated structure of the Philippine Sea Plate at the western end of the Ryukyu subduction zone. Development
of the active subduction and transition boundary near the western Ryukyu Arc is evaluated statistically by using displacements
derived from GPS site data. The statistical model shows that the absolute displacement derived from GPS measurements of nearly
8 years indicates a maximum spatial variation of 0.625 m. Three trends are observed for such long-term progress, and use of
linear regression also reveals quite good consistency between the data and statistic models. Such rate is also elevated following
the trend development. Southeastern and nearly horizontal movement is suggested to the main development of for the site movements,
it is likely related to the tensional activity adjacent to this boundary. 相似文献
970.
This article examines the required spatial discretization perpendicular to the fracture-matrix interface (FMI) for numerical simulation of solute transport in discretely fractured porous media. The discrete-fracture, finite-element model HydroGeoSphere ( Therrien et al. 2005 ) and a discrete-fracture implementation of MT3DMS ( Zheng 1990 ) were used to model solute transport in a single fracture, and the results were compared to the analytical solution of Tang et al. (1981) . To match analytical results on the relatively short timescales simulated in this study, very fine grid spacing perpendicular to the FMI of the scale of the fracture aperture is necessary if advection and/or dispersion in the fracture is high compared to diffusion in the matrix. The requirement of such extremely fine spatial discretization has not been previously reported in the literature. In cases of high matrix diffusion, matching the analytical results is achieved with larger grid spacing at the FMI. Cases where matrix diffusion is lower can employ a larger grid multiplier moving away from the FMI. The very fine spatial discretization identified in this study for cases of low matrix diffusion may limit the applicability of numerical discrete-fracture models in such cases. 相似文献