Effects of beach slope breaks on nearshore groundwater dynamics          下载免费PDF全文

Ying Zhang  Ling Li  Dirk V. Erler  Isaac Santos  David Lockington 《水文研究》2017,31(14):2530-2540

Interactions between fresh groundwater and seawater affect significantly the nearshore pore water flow, which in turn influences the fate of nutrients and contaminants in coastal aquifers prior to discharge to the marine environment. Field investigations and numerical simulations were carried out to examine the groundwater dynamics in the intertidal zone of a carbonate sandy aquifer on the tropical island of Rarotonga, Cook Islands. The study site was featured by distinct cross‐shore slope breaks on the beach surface. Measured pore water salinities revealed different distributions under the influences of different beach profiles, inland heads, and tidal oscillations. Fresh groundwater was found to discharge around a beach slope break located in the middle area of the intertidal zone. The results indicate a strong interplay between the slope break beach morphology and tidal force in controlling the nearshore groundwater flow and solute transport. The fresh groundwater discharge location was largely determined by the beach morphology in combination with the tidal force. The nearshore groundwater flow can be very sensitive to beach slope breaks, which induce local circulation and flow instabilities. As slope breaks are a common feature of beaches around the world, these results have important, general implications for future studies of nutrients transport and transformations in nearshore aquifers and associated fluxes via submarine groundwater discharge.  相似文献   

Heat flow in the southern Chile forearc controlled by large-scale tectonic processes     

Lucia Villar-Muñoz  Jan H. Behrmann  Juan Diaz-Naveas  Dirk Klaeschen  Jens Karstens 《Geo-Marine Letters》2014,34(2-3):185-198

Between 33°S and 47°S, the southern Chile forearc is affected by the subduction of the aseismic Juan Fernandez Ridge, several major oceanic fracture zones on the subducting Nazca Plate, the active Chile Ridge spreading centre, and the underthrusting Antarctic Plate. The heat flow through the forearc was estimated using the depth of the bottom simulating reflector obtained from a comprehensive database of reflection seismic profiles. On the upper and middle continental slope along the whole forearc, heat flow is about 30–60 mW m^–2, a range of values common for the continental basement and overlying slope sediments. The actively deforming accretionary wedge on the lower slope, however, in places shows heat flow reaching about 90 mW m^–2. This indicates that advecting pore fluids from deeper in the subduction zone may transport a substantial part of the heat there. The large size of the anomalies suggests that fluid advection and outflow at the seafloor is overall diffuse, rather than being restricted to individual fault structures or mud volcanoes and mud mounds. One large area with very high heat flow is associated with a major tectonic feature. Thus, above the subducting Chile Ridge at 46°S, values of up to 280 mW m^–2 indicate that the overriding South American Plate is effectively heated by subjacent zero-age oceanic plate material.  相似文献   

Evaluation of brine discharge to rivers and streams: Methodology of rapid impact assessment     

Dirk Böhme 《Limnologica》2011,41(2):80-89

Induced by a brine discharge study for a submerse gas storage cavern project, a suitable methodology for rapid impact assessment had to be found. In this paper a simple stochastic, stationary model is described for assessment of intensity and temporal variability of chloride pollution at the regional scale of the rivershed. Chloride concentration is used as a proxy of salinity. It is assumed to be the result of deterministic process (flow-dependent) and stochastic variation (estimated for boundary conditions and tributaries by an additive error term based on PERT distribution). This approach is suited to conduct Monte Carlo simulations in order to calculate long-time means and percentiles of the prospective in-stream chloride concentration (exposure model). The biocoenoses exposed to this pollution has to be evaluated in terms of chloride tolerance. Herefore Maximum Field Distributions (MFD) of relevant species (aquatic macrophytes, macroinvertebrates, fish) were compiled and merged to Species Sensitivity Distributions (SSDs). Critical aspects of MFD data quality are discussed. Chloride model simulations representing different discharge scenarios provide exposure parameters (e.g. 90th percentile) that can be compared with SSD-derived protection levels (e.g. maximum loss of 10% of taxa) to quantify and evaluate possible adverse effects as well as potential recolonisation in case of load removal. Crosslinks to conservation issues are relevant in the selection and position of rare or protected species in the SSD. As an analysis of the German legal framework and technical guidelines revealed lack of guidance and best practices for such assessment and impact evaluation, recent experience highlights serious needs in applied research.  相似文献   

Short-term distributional consequences of climate change impacts on the power sector: who gains and who loses?     

Dirk Rübbelke  Stefan Vögele 《Climatic change》2013,116(2):191-206

Climate change tends to negatively affect the power sector, inter alia, by causing cooling problems in power plants and impairing the water supply required for hydropower generation. In the future, when global warming is expected to increase, autonomous adaptation to climate change via international electricity markets inducing reallocations of power generation may not be sufficient to prevent supply disruptions anymore. Furthermore, the consequent changes of supply patterns and electricity prices might cause an undesirable redistribution of wealth both between individual power suppliers and between suppliers and consumers. This study ascertains changes in European power supply patterns and electricity prices caused by on-going global warming as well as the associated redistribution of wealth for different climate change scenarios. The focus of the analysis is on short-term effects. Our results confirm that autonomous adaptation in the power sector should be complemented by planned public adaptation in order to preserve energy security and to prevent undesired distributional effects.  相似文献   

The effect of impact obliquity on shock heating in planetesimal collisions     

Thomas M. Davison  Fred J. Ciesla  Gareth S. Collins  Dirk Elbeshausen 《Meteoritics & planetary science》2014,49(12):2252-2265

Collisions between planetesimals in the early solar system were a common and fundamental process. Most collisions occurred at an oblique incidence angle, yet the influence of impact angle on heating in collisions is not fully understood. We have conducted a series of shock physics simulations to quantify oblique heating processes, and find that both impact angle and target curvature are important in quantifying the amount of heating in a collision. We find an expression to estimate the heating in an oblique collision compared to that in a vertical incidence collision. We have used this expression to quantify heating in the Rhealsilvia‐forming impact on Vesta, and find that there is slightly more heating in a 45° impact than in a vertical impact. Finally, we apply these results to Monte Carlo simulations of collisional processes in the early solar system, and determine the overall effect of impact obliquity from the range of impacts that occurred on a meteorite parent body. For those bodies that survived 100 Myr without disruption, it is not necessary to account for the natural variation in impact angle, as the amount of heating was well approximated by a fixed impact angle of 45°. However, for disruptive impacts, this natural variation in impact angle should be accounted for, as around a quarter of bodies were globally heated by at least 100 K in a variable‐angle model, an order of magnitude higher than under an assumption of a fixed angle of 45°.  相似文献   

On the origin of the unusual orbit of Comet 2P/Encke   总被引：1，自引：0，他引：1  

Harold F. Levison  Dirk Terrell  Luke Dones 《Icarus》2006,182(1):161-168

The orbit of Comet 2P/Encke is difficult to understand because it is decoupled from Jupiter—its aphelion distance is only 4.1 AU. We present a series of orbital integrations designed to determine whether the orbit of Comet 2P/Encke can simply be the result of gravitational interactions between Jupiter-family comets and the terrestrial planets. To accomplish this, we integrated the orbits of a large number of objects from the trans-neptunian region, through the realm of the giant planets, and into the inner Solar System. We find that at any one time, our model predicts that there should be roughly 12 objects in Encke-like orbits. However, it takes roughly 200 times longer to evolve onto an orbit like this than the typical cometary physical lifetime. Thus, we suggest that (i) 2P/Encke became dormant soon after it was kicked inward by Jupiter, (ii) it spent a significant amount of time inactive while rattling around the inner Solar System, and (iii) it only became active again as the ν₆ secular resonance drove down its perihelion distance.  相似文献   

Precipitation of pyroxenes and Mg₂SiO₄ from majoritic garnet: simulation of peridotite exhumation from great depth     

Larissa F. Dobrzhinetskaya  Harry W. Green  II  Alex P. Renfro  Krassimir N. Bozhilov  Dirk Spengler  Herman L. M. van Roermund 《地学学报》2004,16(6):325-330

Our experimental simulations of the exhumation path of mantle peridotites show that high‐temperature (1400 °C) decompression of lherzolite from 14 to 13 and 12 GPa results in exsolution of interstitial blebs of diopside and Mg₂SiO₄ (wadsleyite) lamellae from majoritic garnet. At lower pressures (from 8 to 5 GPa, at T = 1400 °C) only enstatite exsolves as blebs at garnet boundaries. Continuous high‐temperature decompression from 14 to 7 GPa produces zoned majoritic garnet containing blebs of exsolved pyroxenes inside garnet rims. No intracrystalline precipitation of pyroxene was observed in garnet, although such lamellae are found in some natural garnet peridotites. The explanation appears to be the three orders of magnitude difference in grain size between experimental and natural specimens. Our data suggest that Mg₂SiO₄ and diopside exsolutions reflect the deepest point of the exhumation path of garnet peridotites, whereas enstatite precipitation may be restricted to garnets with less majoritic component at shallower depths.  相似文献   

Ancient charcoal as a natural archive for paleofire regime and vegetation change in the Mayumbe,Democratic Republic of the Congo     

Wannes Hubau  Jan Van den Bulcke  Peter Kitin  Florias Mees  Geert Baert  Dirk Verschuren  Laurent Nsenga  Joris Van Acker  Hans Beeckman 《Quaternary Research》2013

Charcoal was sampled in four soil profiles at the Mayumbe forest boundary (DRC). Five fire events were recorded and 44 charcoal types were identified. One stratified profile yielded charcoal assemblages around 530 cal yr BP and > 43.5 cal ka BP in age. The oldest assemblage precedes the period of recorded anthropogenic burning, illustrating occasional long-term absence of fire but also natural wildfire occurrences within tropical rainforest. No other charcoal assemblages older than 2500 cal yr BP were recorded, perhaps due to bioturbation and colluvial reworking. The recorded paleofires were possibly associated with short-lived climate anomalies. Progressively dry climatic conditions since ca. 4000 cal yr BP onward did not promote paleofire occurrence until increasing seasonality affected vegetation at the end of the third millennium BP, as illustrated by a fire occurring in mature rainforest that persisted until around 2050 cal yr BP. During a drought episode coinciding with the ‘Medieval Climate Anomaly’, mature rainforest was locally replaced by woodland savanna. Charcoal remains from pioneer forest indicate that fire hampered forest regeneration after climatic drought episodes. The presence of pottery shards and oil-palm endocarps associated with two relatively recent paleofires suggests that the effects of climate variability were amplified by human activities.  相似文献   

Ichnodiversity and ichnoabundance: Revealing depositional trends in a confined turbidite system   总被引：1，自引：0，他引：1  

Dirk Knaust  Michał Warchoł  Ian A. Kane 《Sedimentology》2014,61(7):2218-2267

Ichnological studies are still in their infancy when it comes to the interpretation of deep‐marine deposits. The Eocene–Oligocene turbidite system of the Grès d'Annot Formation in south‐east France is well‐studied sedimentologically, but its trace‐fossil content is poorly known. Here, an integrated ichnological–sedimentological study is presented from the Annot sub‐basin for the first time, which demonstrates its value for interpreting proximal to distal and axial to marginal trends in confined turbidite systems. A comprehensive trace‐fossil data set was collected from seven outcrops situated in the southern part of the basin. These data are presented following a morphology‐based classification scheme to allow easy recognition and characterization of ichnotaxa. Ichnodiversity and the abundance of ichnotaxa are regarded as important parameters in such interpretations. Instead of simply counting ichnotaxa per outcrop or stratigraphic unit, an equation has been developed in which the ‘ichnoabundance’ (new term) of each counted ichnotaxon is calculated. An exponential growth factor is applied to the increase of the frequency of trace fossils, and is assumed in this equation to better reflect the population dynamics of benthic organisms. A comparison of the solution for pre‐turbidite and post‐turbidite trace‐fossil suites seems to be more suitable for revealing regional and stratigraphic trends compared with conventional approaches. Despite varying size and conditions of the studied outcrops, the results achieved from the Grès d'Annot Formation can help in the reconstruction of sedimentary processes acting in this confined turbidite basin.  相似文献   

Impacts of the use of the geological subsurface for energy storage: an investigation concept     

Sebastian Bauer  Christof Beyer  Frank Dethlefsen  Peter Dietrich  Rainer Duttmann  Markus Ebert  Volker Feeser  Uwe Görke  Ralf Köber  Olaf Kolditz  Wolfgang Rabbel  Tom Schanz  Dirk Schäfer  Hilke Würdemann  Andreas Dahmke 《Environmental Earth Sciences》2013,70(8):3935-3943

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