Behaviour of rubble-mound breakwater in directional and uni-directional waves     

F. Thunbo Christensen  P.C. Broberg  Stig E. Sand  P. Tryde 《Coastal Engineering》1984,8(3):265-278

The application of directional waves in design processes has clearly received much interest during recent years. Thus, in model testing with moored ships and offshore structures, significant deviations have been revealed between results obtained in traditional uni-directional wave trains and those obtained in directional wave fields. Whether the same tendency is valid in connection with the design of rubble mound breakwaters is studied in the present project. A breakwater with a front slope of 1:2 is tested at the scale 1:40. The weight and diameter of the stones in the core, filter and armour layers were carefully selected, and the sources of scatter in repeated tests were minimized. Furthermore, the incident wave energy was accurately adjusted to keep the same level in both uni-directional and directional waves. For the actual rubble-mound breakwater unambiguous results were obtained. When uni-directional waves were applied, the damage increased by 30–50% relative to the directional wave situation.  相似文献   

Velocities of a natural mid-ocean ridge basalt glass   总被引：2，自引：0，他引：2  

Yucheng Pan  Nikolas I. Christensen  Rodey Batiza  Teresa L. Coleman 《Tectonophysics》1998,290(3-4):171-180

We report, for the first time, ultrasonic velocity values for a pure (>95%), natural, submarine basalt glass of mid-ocean ridge basalt composition, from 10 to 1000 MPa at room temperature. These new data show that basalt glass, abundant in the upper oceanic crust, has the lowest velocity of any primary solid component of the oceanic crust. In addition, natural basalt glass has a steeper pressure-dependence of velocity than previously measured in more crystalline samples, indicating that cracks in natural basalt glass are weaker than in more crystalline rocks. To obtain values for the pure glass phase, we correct the natural glass data for the low-pressure closure of cracks, and the presence of minor mineralogic components and vesicles. These new data provide a baseline for evaluating the effect of abundant basalt glass and glassy mesostasis in oceanic upper crust on in situ seismic velocities. In addition, data on the elastic and seismic properties of natural glasses are useful for a better understanding of glass structure, and glass relaxation, with potential applications to submarine volcanology.  相似文献   

Prediction of Regional Ground Water Flow to Streams   总被引：2，自引：0，他引：2  

S. Christensen  K.R. Rasmussen  K. Moller 《Ground water》1998,36(2):351-360

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The THESEUS space mission: science goals,requirements and mission concept     

Amati  L.  O’Brien  P.T.  Götz  D.  Bozzo  E.  Santangelo  A.  Tanvir  N.  Frontera  F.  Mereghetti  S.  Osborne  J. P.  Blain  A.  Basa  S.  Branchesi  M.  Burderi  L.  Caballero-García  M.  Castro-Tirado  A. J.  Christensen  L.  Ciolfi  R.  De Rosa  A.  Doroshenko  V.  Ferrara  A.  Ghirlanda  G.  Hanlon  L.  Heddermann  P.  Hutchinson  I.  Labanti  C.  Le Floch  E.  Lerman  H.  Paltani  S.  Reglero  V.  Rezzolla  L.  Rosati  P.  Salvaterra  R.  Stratta  G.  Tenzer  C. 《Experimental Astronomy》2021,52(3):183-218

Experimental Astronomy - THESEUS, one of the two space mission concepts being studied by ESA as candidates for next M5 mission within its Comsic Vision programme, aims at fully exploiting Gamma-Ray...  相似文献   

Lateglacial vegetation development in Denmark – New evidence based on macrofossils and pollen from Slotseng,a small-scale site in southern Jutland     

Morten Fischer Mortensen  Hilary H. Birks  Charlie Christensen  Jørgen Holm  Nanna Noe-Nygaard  Bent Vad Odgaard  Jesper Olsen  Kaare Lund Rasmussen 《Quaternary Science Reviews》2011,30(19-20):2534-2550

This paper presents the first unambiguous terrestrial palaeoecological record for the late glacial “Bølling warming” in Denmark. Pollen and macrofossil stratigraphies from pre-Bølling to 10,800 cal yr BP are presented from a small kettle hole in Southwest Denmark, during which the lake basin developed from an immature stage after the deglaciation to complete infilling in the early Holocene. Results show that the recently deglaciated landscape bore a discontinuous vegetation of pioneer plants. After the Bølling warming, an open Dryas octopetala-Betula nana community developed with Helianthemum oelandicum. Subarctic species were dominant and local successions were probably delayed by relatively unstable and infertile soils. There is no indication of a climate cooling during the period corresponding to the Older Dryas, but the occurrence of several drought tolerant and steppe species indicates that the period was relatively dry. In the Allerød period the Dryas-B. nana vegetation was initially replaced by an open Salix and grass dominated vegetation and some 400 years later, the first tree birches were documented presumably occupying moist and sheltered soils while drier land remained open. In the Younger Dryas period trees disappeared and the vegetation became open again and dominated by subarctic species. Following climate warming at the Younger Dryas–Holocene transition a shrub community of Empetrum and Juniperus developed. After approximately 200 years it was replaced by birch forest. Overall, the late-glacial vegetation cover had a more open and patchy character than inferred from previous pollen studies as assessment of the vegetation succession based on macrofossil evidence is essential. The inferred general vegetation development corresponds well with results of other studies in the region. Canonical ordinations (RDA) indicate that vegetation changes at the landscape scale during the Lateglacial period were driven by changes in climate, soils and competition for light.  相似文献   

Latent Variable Modeling for Integrating Output from Multiple Climate Models     

William F. Christensen  Stephan R. Sain 《Mathematical Geosciences》2012,44(4):395-410

Numerical models of atmosphere–ocean circulation are widely used to understand past climate and to project future climate change. Although the same laws of physics, chemistry, and fluid dynamics govern any general circulation model, each model’s formulations and parameterizations are different, yielding different projections. Notwithstanding, models within an ensemble will have varying degrees of similarity for different outputs of interest. Multi-model ensembles have been used to increase forecast skill by using simple or weighted averages where weights have been obtained by considering factors such as estimated model bias and consensus with other models (Giorgi and Mearns, J. Clim. 15:1141–1158, 2002, Geophys. Res. Lett. 30:1629–1632, 2003; Tebaldi et al., Geophys. Res. Lett. 31:L24213, 2004, J. Clim. 18:1524–1540, 2005). This paper considers an alternative view of multi-model ensembles. For use with the North American Regional Climate Change Assessment Program (NARCCAP), multivariate statistical models are employed to characterize modes of similarity within the members of an ensemble. Specifically, we propose a spatially-correlated latent variable model which facilitates the exploration of when, where, and how regional climate models are similar, and what factors best predict observed locations of model convergence.  相似文献   

A concept for estimating depth of the redox interface for catchment-scale nitrate modelling in a till area in Denmark     

A. L. Hansen  B. S. B. Christensen  V. Ernstsen  X. He  J. C. Refsgaard 《Hydrogeology Journal》2014,22(7):1639-1655

A concept was developed for distributing the depth of the redox interface at catchment scale in tills for use in nitrate modelling. The hypothesis was that the redox interface has been developing since the beginning of the Holocene due to oxidation of inherent reduced compounds in the sediment by oxygen in recharging water. Key to the concept is estimation of the spatial pattern of the redox interface, inferred from variability in groundwater recharge and the amount of reduced compounds in the sediments. The concept was tested for the 101-km² Norsminde fjord catchment in Denmark using a catchment-scale nitrate model, where two parameters controlling the estimated redox interface were calibrated based on the observed amount of nitrate transported to the fjord. Estimated depths of the redox interface across the catchment enabled reproduction of the cumulative distribution of measured depths across the catchment, but they did not fit well with the observations at individual wells. It is therefore concluded that the concept may be useful for simulation of nitrate transport and reduction in the saturated zone at catchment scale, but it is not able to correctly predict nitrate reduction at point scale.  相似文献   

Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)     

Søren Erbs Poulsen  Keld Rømer Rasmussen  Niels Bøie Christensen  Steen Christensen 《Hydrogeology Journal》2010,18(1):161-171

A monitoring system, including five groups of piezometers and five vertical multielectrode profiling probes (VMEP), has been installed in an aquifer beneath a coastal dune in Denmark. In order to assess the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multielectrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a one-dimensional, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01–0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and salt water overlying freshwater close to the sea. In June, the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn, aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.  相似文献   

Trophic modelling of the Peruvian upwelling ecosystem: Towards reconciliation of multiple datasets     

Sylvie Gunette  Villy Christensen  Daniel Pauly 《Progress in Oceanography》2008,79(2-4):326

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Preface to the Special Issue on Airborne Geophysics     

Niels B. Christensen 《Journal of Applied Geophysics》2009,67(3):193

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