Effects of the North Atlantic Oscillation and wind waves on salt marsh dynamics in the Danish Wadden Sea: a quantitative model as proof of concept   总被引：2，自引：2，他引：0  

Daehyun Kim  William E. Grant  David M. Cairns  Jesper Bartholdy 《Geo-Marine Letters》2013,33(4):253-261

Long-term eustatic sea-level variation has been recognized as a primary factor affecting the hydrological and geomorphic dynamics of salt marshes. However, recent studies suggest that wind waves influenced by atmospheric oscillations also may play an important role in many coastal areas. Although this notion has been conceptually introduced for the Wadden Sea, no modeling attempts have been made yet. As a proof of concept, this study developed a simulation model using the commercially available STELLA® software, based on long-term data on water level and sedimentation collected at a back-barrier marsh on the Skallingen peninsula in Denmark. In the model, the frequency (number year^–1) of wind-driven extreme high water level (HWL) events (>130 cm Danish Ordnance Zero) was simulated in terms of the North Atlantic Oscillation (NAO) index. Then, surface accretion (cm year^–1) and submergence duration (h year^–1) were simulated for the period 1933–2007. The model showed good performances: simulated rates of surface accretion and simulated durations of submergence decreased from 1950 to 1980, the point at which the NAO shifted from its negative to its positive phase, and increased thereafter. Despite continuous increases in surface elevation, increases in simulated submergence duration were apparently due to wind-driven HWL events, which generally increased in frequency after 1980. These findings for the Danish Wadden Sea add to the growing body of evidence that the role of atmospheric oscillations—e.g., the NAO—as drivers of wind-generated water level variations merits more attention in assessing the impact of climate change on coastal marshes.  相似文献   

Temperature and abundance profiles of hot gas in galaxy groups – II. Implications for feedback and ICM enrichment     

Jesper Rasmussen †  Trevor J. Ponman 《Monthly notices of the Royal Astronomical Society》2009,399(1):239-263

We investigate the history of galactic feedback and chemical enrichment within a sample of 15 X-ray bright groups of galaxies, on the basis of the inferred Fe and Si distributions in the hot gas and the associated metal masses produced by core-collapse and Type Ia supernovae (SNe). Most of these cool-core groups show a central Fe and Si excess, which can be explained by prolonged enrichment by SN Ia and stellar winds in the central early-type galaxy alone, but with tentative evidence for additional processes contributing to core enrichment in hotter groups. Inferred metal mass-to-light ratios inside r ₅₀₀ show a positive correlation with total group mass but are generally significantly lower than in clusters, due to a combination of lower global intracluster medium (ICM) abundances and gas-to-light ratios in groups. This metal deficiency is present for products from both SN Ia and SN II, and suggests that metals were either synthesized, released from galaxies or retained within the ICM less efficiently in lower mass systems. We explore possible causes, including variations in galaxy formation and metal release efficiency, cooling out of metals, and gas and metal loss via active galactic nuclei (AGN) – or starburst-driven galactic winds from groups or their precursor filaments. Loss of enriched material from filaments coupled with post-collapse AGN feedback emerges as viable explanations, but we also find evidence for metals to have been released less efficiently from galaxies in cooler groups and for the ICM in these to appear chemically less evolved, possibly reflecting more extended star formation histories in less massive systems. Some implications for the hierarchical growth of clusters from groups are briefly discussed.  相似文献   

Quantification and regionalization of groundwater–surface water interaction along an alluvial stream     

Jesper H. Langhoff  Keld R. Rasmussen  Steen Christensen   《Journal of Hydrology》2006,320(3-4):342-358

In the present study, groundwater seepage to an alluvial stream and two tributary streams was examined at nine field sites using hydrological, geophysical, and geomorphological observations. The data indicate that seepage enters the streams in the following ways: (i) directly through the streambed; (ii) as nearly superficial flow from diffuse discharge areas on the flood plains or; (iii) as a combination of (i) and (ii). At about 40% of the sites more than 50% of seepage flows through the streambed. Moreover, it was found that the ratio C, defined as the width of the wet zone of the flood plain divided by the effective width of the stream, can be used as an indicator of the percentage of water entering the stream directly through the streambed. When C is small streambed seepage is large, while when C is large streambed seepage is small and ground water enters the stream mainly as nearly superficial or over-bank flow from the wet zone.  相似文献   

Open boundaries in short wave simulations — A new approach     

Jesper Larsen  Henry Dancy 《Coastal Engineering》1983,7(3):285-297

A new way of implementing radiation boundary conditions in finite difference schemes is reported. Instead of prescribing the incident field at the model boundary, waves are generated inside the model boundary. All outgoing waves are absorbed at open boundaries using so-called “sponge” layers.  相似文献   

Capillary Crack Imbibition: A Theoretical and Experimental Study Using a Hele-Shaw Cell     

H. Schütt  H. Spetzler 《Pure and Applied Geophysics》2001,158(4):627-646

—?We study the filling of horizontal cracks with constant aperture driven by capillary forces. The physical model of the crack consists of a narrow gap between two flat glass plates (Hele-Shaw cell). The liquid enters the gap through a hole in the bottom plate. The flow is driven purely by the force acting on the contact lines between solid, liquid, and gas. We developed a theoretical model for this type of flow on the basis of Darcy's law; it allows for the consideration of different surface conditions.¶We run the experiment for two surface conditions: Surfaces boiled in hydrogen peroxide to remove initial contamination, and surfaces contaminated with 2-propanol after boiling in hydrogen peroxide. The flow rate depends on the gap aperture and on the interaction of the liquid with the air and the solid surfaces: The smaller the aperture, the lower the flow rate due to viscous resistance of the liquid. The flow rate is also reduced when the glass surfaces are contaminated with 2-propanol. The contact line force per unit length is approximately 60% higher on clean glass surfaces than it is on glass surfaces with the 2-propanol contamination. These experimental results are in agreement with our theoretical model and are confirmed by independent measurements of the liquid-solid interaction in capillary rise experiments under static conditions with the same Hele-Shaw cell.¶Another aspect of this study is the distribution of the liquid for the different surface conditions. The overall shape is a circular disk, as assumed in the theoretical model. However, a pronounced contact line roughness develops in case of the surfaces contaminated with 2-propanol, and air bubbles are trapped behind the contact line. A further analysis of the flow regime using the capillary number and the ratio of the viscosities of the involved fluids (water and air) reveals that the experiments take place in the transition zone between stable displacement and capillary fingering, i.e., neither viscous nor capillary fingers develop under the conditions of the experiment. The contact line roughness and the trapped air bubbles in the contaminated cell reflect local inhomogeneities of the surface wettability.  相似文献   

Coastal eutrophication and the Danish national aquatic monitoring and assessment program     

Daniel J. Conley  Stiig Markager  Jesper Andersen  Thomas Ellermann  Lars M. Svendsen 《Estuaries and Coasts》2002,25(4):848-861

Nutrient over-enrichment and cultural eutrophication are significant problems in the Danish marine environment. Symptoms of eutrophication include periods of hypoxia and anoxia in bottom waters, death of benthic-dwelling organisms during anoxia, long-term reductions in the depth distribution of macrophyte communities, changes in the species composition of macrophyte communities, and increases in reports of harmful algal blooms. In 1987 the Action Plan on the Aquatic Environment was adopted to combat nutrient pollution of the aquatic environment with the overall goal of reducing nitrogen loads by 50% and point source phosphorus loads by 80%. The Danish Aquatic Nation-wide Monitoring Program was begun in 1988 in order to describe the status of point sources (industry, sewage treatment plants, stormwater outfalls, scattered dwellings, and fish farms), ground water, springs, agricultural watersheds, streams, lakes, atmospheric deposition, and the marine environment. Another important aspect of the program was to document the effects on the aquatic environment of the measures and investments taken for nutrient reduction as outlined in the Action Plan. The monitoring program should determine if reductions in nutrients are achieved by the measures taken and should help decision makers choose appropriate additional measures to fulfill the objectives. Coordination with international programs and commissions is an important component of the monitoring program to meet internationally agreed upon reductions in nutrient inputs. The future and direction of the Danish National Aquatic Monitoring and Assessment Program will be to a large extent shaped by both the Water Framework Directive and Habitat Directive adopted by the European Union.  相似文献   

Absorption of strain waves in porous media at seismic frequencies     

T. L. Chelidze  H. A. Spetzler  G. A. Sobolev 《Pure and Applied Geophysics》1996,147(1):25-55

An understanding of strain wave propagation in fluid containing porous rocks is important in reservoir geophysics and in the monitoring in underground water in the vicinity of nuclear and toxic waste sites, earthquake prediction, etc. Both experimental and theoretical research are far from providing a complete explanation of dissipation mechanisms, especially the observation of an unexpectedly strong dependence of attenuationQ ^–1 on the chemistry of the solid and liquid phase involved. Traditional theories of proelasticity do not take these effects into account. In this paper the bulk of existing experimental data and theoretical models is reviewed briefly in order to elecidate the effect of environmental factors on the attenuation of seismic waves. Low fluid concentrations are emphasized. Thermodynamical analysis shows that changes in surface energy caused by weak mechanical disturbances can explain observed values of attenuation in real rocks. Experimental dissipation isotherms are interpreted in terms of monolayered surface adsorption of liquid films as described by Langmuir's equation.In order to describe surface dissipation in consolidated rocks, a surface tension term is added to the pore pressure term in the O'Connell-Budiansky proelastic equation for effective moduli of porous and fractured rocks. Theoretical calculations by this modified model, using reasonable values for elastic parameters, surface energy, crack density and their geometry, lead to results which qualitatively agree with experimental data obtained at low fluid contents.  相似文献   

Complexities of rock fracture and rock friction from deformation of Westerly granite     

Ganglin Chen  Hartmut Spetzler 《Pure and Applied Geophysics》1993,140(1):95-121

A series of rock friction experiments has been carried out to study the complexities in rock fracture and rock friction. Intact Westerly granite samples were loaded to shear failure in a laboratory polyaxial loading apparatus. The resultant fractured samples were reloaded to cause frictional sliding. Both polyaxial loading (₁ > ₂ > ₃ > 0) and equal confining condition (₁ > ₂ = ₃ > 0) were used. The deformation processes were monitored by macroscopic axial stress-strain, optical holography, and ultrasonic velocity measurements.Intense localized deformation along the fracture occurred very early in the loading of fractured samples. Contacts on the fracture surfaces continuously broke during loading. No acoustic velocity anomaly was observed for the fractured sample, in contrast to a 25% drop in the velocity before the failure of the corresponding intact sample. The current study and previous research suggest that the deformation localization is an important process in governing the instability of rock friction. Instability analysis of rock friction needs to include not only the deformation processes along the sliding surfaces, but also those adjacent to the fractures such as the localized deformation along the fractures observed in the current study. The instability analysis of rock friction with rate- and state-dependent friction laws does not specifically include the deformation localization adjacent to the faults and thus ignores an important class of instability as described byRudnicki (1977).A dependence of frictional strengths on the stress components normal to the sliding and in the plane of the fracture surface was observed. This dependence can be understood by considering the loading of the irregular fracture surface under polyaxial loading conditions. This observation requires the friction laws in the macroscopic scale to be modified for those cases where the three principal stresses (₁, ₂, and ₃) are significantly different.  相似文献   

Sharing responsibilities in Danish fisheries management—experiences and future directions     

Jesper Raakjær Nielsen  Anne-Sofie Christensen 《Marine Policy》2006

During the last decade, devolution of responsibilities of fisheries management has increasingly become an issue in the political debate. The CEC and national governments have expressed an interest in sharing responsibilities for fisheries management with stakeholders. This article investigates the dynamics and constraints of the devolution process in general and in relation to Danish demersal fisheries by answering the following questions: What are the important factors to consider with respect to sharing responsibilities in fisheries management; how can this delegation and/or devolution of responsibilities be institutionalised? And what kind of decisions should be delegated to stakeholders and to whom?  相似文献   

作物相对水分利用效率与理论节水潜力          下载免费PDF全文

代小平  周雯晶  韩宇平  窦明  冯吉  Svensson Jesper 《水科学进展》2019,30(5):682-690

作物水分利用效率是评价农业用水效率的重要指标,由于气候差异,采用作物水分利用效率评价不同区域的用水效率可能存在一定误差,尚缺乏深入研究。基于潜在水分利用效率指标和气象数据,计算了2014年中国小麦、玉米和水稻在充分灌溉条件下的潜在水分利用效率,分析了区域气候差异对作物水分利用效率的影响,提出了相对水分利用效率和理论节水潜力的概念和计算方法,并进行了实例研究。结果表明:各类作物在不同站点的潜在水分利用效率的标准差为0.49~1.01 kg/m3,多数作物潜在水分利用效率的空间差异大于实际水分利用效率的空间差异;主要作物的平均相对水分利用效率为50.7%,其理论节水潜力为884.8~4 064.5 m3/hm2;作物水分利用效率和广义节水潜力指标可能高估或低估作物的用水效率和节水潜力。研究认为在比较区域间作物的水分利用效率时不能忽略气候差异的影响;由于考虑了区域气候差异,相对水分利用效率和理论节水潜力指标更合理。  相似文献