Response of Coastal Groundwater Table to Offshore Storms   总被引：1，自引：0，他引：1  

李凌  N.Cartwright  P.Nielsen  D.Lockington 《中国海洋工程》2004,18(3):423-431

Large groundwater table fluctuations were observed in a coastal aquifer during an offshore storm. The storm induced significant changes of the mean shoreline elevation, characterized by a pulse-like oscillation. This pulse propagated in the aquifer, resulting in the water table fluctuations. A general analytical solution is derived to quantify this new mechanism of water table fluctuation. The solution is applied to field observations and is found to be able to predict reasonably well the observed storm-induced water table fluctuations. Based on the analytical solution, the damping characteristics and phase shift of the oscillation as it propagates inland are examined.  相似文献   

Impacts of – and farmers’ adaptation to – land allocation policies in the north central uplands of Vietnam     

Rikke Folving Ginzburg  Thomas Theis Nielsen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2018,118(1):36-55

A series of policies on land tenure have been implemented in Vietnam following the doi moi restructuring. This study assesses the impacts of these policies, the extent to which they have had intended effects and coping strategies adopted by farmers. Data were collected in five upland villages in Nghe An Province, North Central Vietnam. The intended effects of the land allocation policies were poverty alleviation, intensification of agriculture, agricultural modernization and forest protection. The policies implemented to achieve these goals include changes in land allocation, purportedly to increase land tenure security. However, the policies have been implemented in a very uneven manner and the effects differ widely due to differences in the local contexts in which they have been implemented. In general, farmers perceive the impacts of policies as adverse and have attempted to cope with their impact in a variety of ways. The paper argues that land allocation policies have: (1) decreased the amount of land available, (2) not improved land tenure security and (3) had a limited impact on farming practices. The differences between the five villages are great, demonstrating the very different results produced by national policies depending on the specific implementation modality and the local context.  相似文献   

Foreword     

Alan Green  Lars Nielsen  Christopher Juhlin 《Journal of Applied Geophysics》2010,70(4):267

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A new shoreline displacement model for the last 7 ka from eastern James Bay, Canada     

Ionel Florin Pendea  André Costopoulos  Colin Nielsen  Gail Lois Chmura 《Quaternary Research》2010,73(3):474-484

The shoreline displacement history of the eastern James Bay lowlands in the last 7 ka has been investigated by means of AMS radiocarbon dating of sediments cored from wetlands. We present twelve radiocarbon dates on macrofossils from six sites spread along a gradient of increasing land age and elevation. Palynomorph analysis (pollen, spores, and dinoflagellate cysts) was used to define the isolation stratigraphy. During the last 7 ka the shoreline elevation has regressed at a decreasing rate. The rate of shoreline emergence was initially rapid (6. 5 m/ 100 yr) between 6850 and  6400 cal yr BP then slowed down to 1.4– 2 m/ 100 yr during the late Holocene. Examination of previous relative sea level data based upon mollusc shells reveals high levels of uncertainty that mask potential temporal variability.  相似文献   

Compound response of marine and terrestrial ecosystems to varying climate: pre-anthropogenic perspective from bivalve shell growth increments and tree-rings     

Helama S  Schöne BR  Kirchhefer AJ  Nielsen JK  Rodland DL  Janssen R 《Marine environmental research》2007,63(3):185-199

Annual growth increments were examined from shells of the ocean quahog (Arctica islandica L.) from northwest Norway and from tree-ring samples of the Scots pine (Pinus sylvestris L.) from nearby coastal areas. The reconstructed annual growth increments were used to compare growth variability in marine and terrestrial ecosystems. Spatiotemporal comparison of the growth records showed statistically significant correlation during the 19th century A.D., indicative of ecosystem-independent response to pre-anthropogenic climate variations. Geographical correlation between marine and terrestrial records was only observed at the local scale. Years with particularly low winter or high summer North Atlantic Oscillation (NAO) indices showed the best synchronization of marine and terrestrial growth. Despite strong correlation during historical time, our palaeoecological evidence suggests that marine and terrestrial ecosystems may show dissimilar growth reaction to recently observed positive winter-NAO phases.  相似文献   

Geoacoustic characterization of a range-dependent ocean environment using towed array data     

Fallat  M.R. Nielsen  P.L. Dosso  S.E. Siderius  M. 《Oceanic Engineering, IEEE Journal of》2005,30(1):198-206

This paper examines geoacoustic inversion over a range-dependent multiple-layer seabed using a towed acoustic source and towed horizontal array. The approach is based on combining the results of a series of short-range, range-independent inversions to form a range-dependent representation of the environment. The data were collected in the Strait of Sicily during the MAPEX 2000 experiment. Issues such as the resolvability of multilayer structure and the sensitivity of various geoacoustic parameters are investigated by inversion of simulated data and by comparison of the MAPEX 2000 inversion results to a high-resolution seismic profile and to sediment core measurements. It appears that two, and in some cases possibly three, sediment layers can be resolved.  相似文献   

A Middle Weichselian interstadial lake deposit on Sejerø, Denmark: macrofossil studies and dating     

Ole Bennike  Michael Houmark‐Nielsen  Peter Wiberg‐Larsen 《第四纪科学杂志》2007,22(6):647-651

Studies of lake sediments on Sejerø in southern Kattegat indicate a treeless arctic environment with the dwarf shrub Salix polaris, herbs and wetland mosses. The mean July temperature was around 8 to 10°C. An AMS radiocarbon age determination of a Salix twig yielded an age of around 36 000 ¹⁴C yr BP, which is in accordance with previous conventional dating of bulk sediment samples. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Determination of Precise and Accurate 51V/50V Isotope Ratios by MC‐ICP‐MS,Part 1: Chemical Separation of Vanadium and Mass Spectrometric Protocols     

Sune G. Nielsen  Julie Prytulak  Alex N. Halliday 《Geostandards and Geoanalytical Research》2011,35(3):293-306

We present the first technique to obtain precise and accurate vanadium (V) stable isotope compositions by chemical isolation and multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS). Separation of V from matrix elements was achieved via five separate ion exchange columns. The procedure quantitatively removed Ti and Cr, which contain direct isobaric interferences on the minor isotope ⁵⁰V. Isotope compositions were determined using a conventional standard solution‐sample bracketing technique. The V isotope composition for an in‐house secondary standard solution from BDH Chemicals was δ⁵¹V = ?1.19 ± 0.12‰ (2s, n = 600), measured as the per mil deviation relative to the composition of a widely available Specpure Alfa Aesar (AA) vanadium solution. This represents an improvement in measurement precision on previous techniques of almost two orders of magnitude. The effects of adding Cr, Ti and S to standard solutions were explored to determine the robustness of protocols. Only very low levels of these elements could be tolerated to obtain precise and accurate isotope compositions and was achieved with the chemical purification procedure. Standard solutions from AA and BDH processed as unknowns through the entire chemical separation and measurement protocols returned 100% yields and the same isotopic compositions as those of unprocessed standard solutions.  相似文献   

Downward transfer of momentum by wind-driven waves     

Peter Nielsen  David P. CallaghanTom E. Baldock 《Coastal Engineering》2011

A model for the downward transfer of wind momentum is derived for growing waves. It is shown that waves, which grow due to an uneven pressure distribution on the water surface or a wave-coherent surface shear stress have horizontal velocities out of phase with the surface elevation. Further, if the waves grow in the x-direction, while the motion is perhaps time-periodic at any fixed point, the Reynolds stresses associated with the organized motion are positive. This is in agreement with several field and laboratory measurements which were previously unexplained, and the new theory successfully links measured wave growth rates and measured sub-surface Reynolds stresses. Wave coherent air pressure (and/or surface shear stress) is shown to change the speed of wave propagation as well as inducing growth or decay. From air pressure variations that are in phase with the surface elevation, the influence on the waves is simply a phase speed increase. For pressure variations out of phase with surface elevation, both growth (or decay) and phase speed changes occur. The theory is initially developed for long waves, after which the velocity potential and dispersion relation for linear waves in arbitrary depth are given. The model enables a sounder model for the transfer to storm surges or currents of momentum from breaking waves in that it does not rely entirely on ad-hoc turbulent diffusion. Future models of atmosphere-ocean exchanges should also acknowledge that momentum is transferred partly by the organized wave motion, while other species, like heat and gasses, may rely totally on turbulent diffusion. The fact that growing wind waves do in fact not generally obey the dispersion relation for free waves may need to be considered in future wind wave development models.  相似文献   

Advances in monitoring vegetation and land use dynamics in the Sahel     

Cheikh Mbow  Rasmus Fensholt  Thomas Theis Nielsen  Kjeld Rasmussen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2014,114(1):84-91

Vegetation dynamics of the West African Sahel has attracted great scientific interest over the last 40?years because of the dramatic inter-decadal variability observed in the resource base of the region directly impacting on the livelihoods of the West African population. From farmers to pastoralists, agro-pastoralists and forest-users, all depend on the availability of vegetation resources and are affected by fluctuations in the available vegetation resource. Vegetation dynamics are controlled by both natural and human factors, including climate change and variability, increased concentration of CO₂ in the atmosphere, grazing pressure, bush fires and agricultural expansion or contraction. The use of satellite data in combination with field data played a major role in the monitoring of vegetation dynamics and land use in the Sahel, since the mega drought of the 1970s and the 1980s. This paper briefly reviews the advance of satellite-based monitoring of vegetation dynamics over these 40?years. We discuss the promises of current and likely future data sources and analysis tools, as well as the need to strengthen in situ data collection to support and validate satellite-based vegetation and land use monitoring and modelling.  相似文献