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71.
This paper examines a variety of recirculation flow patterns that develop in the groyne fields on rivers. A comprehensive data set was obtained from flume experiments at Delft University of Technology and field measurements performed on the Elbe River in Germany. The analysis focuses on patterns of velocity, scour and deposition, and corresponding change of riverbed morphology. The results show that velocity patterns in the groyne fields depend mainly on the aspect ratio between groyne length and length of groyne field. When the aspect ratio is greater than 0·5, a one‐gyre pattern of recirculation develops, while at groyne fields with aspect ratios less than 0·5 a two‐gyre recirculation pattern emerges. The spatial distribution of fine‐sediment deposition between the groynes coincides with the locations of gyres. Moreover, the thickness of the fine‐sediment layer decreases toward the gyre margins where recirculation velocities are greatest. Although the total concentration of suspended sediment in the river does not change considerably as the flow moves through the groyne field, the biological and gravimetrical composition of the suspended material changes substantially within the groyne field. These changes are due to preferential deposition of coarse mineral particles and the replacement of those minerals with finer organic material. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
72.
Inter-Annual Variability and Decadal Trends in Alpine Spring Phenology: A Multivariate Analysis Approach 总被引:1,自引:0,他引:1
Plant phenological observations are of increasing value as indicators of climate change and variability. We developed a robust
multispecies estimate for Swiss Alpine spring phenology for the period 1965–2002 by applying empirical orthogonal function
(EOF) analysis on a combination of 15 spring phases. The impact of climate parameters such as temperature and precipitation
on the phenological development was investigated using a multivariate statistical model. This multispecies estimate proved
to be a good approach to assess the pattern of spring appearance during the last 40 years. It revealed an earlier onset of
spring in recent years, mainly since 1988 when a clear shift in spring appearance occurred. The mean overall trend of 1.5
days per decade was clearly driven by winter and spring temperatures whereas precipitation showed no significant influence.
The dominant EOF patterns suggested a general climate forcing for the observed inter-annual variability independent of single
plant phases. A more regional phenology signal was found in the second EOF mode, indicating slightly weaker phenological trends
in southern Switzerland as well as in higher altitudes. Both, temperature and precipitation contributed to this pattern significantly.
Analysis of single phases confirmed the pattern of the multispecies estimate. All species showed trends towards earlier appearance
ranging from −1 to −2.8 days per decade and the appearance dates had a very high covariance with temperature. 相似文献
73.
Astrid Lyså Eiliv A. Larsen Johanna Anjar Naki Akçar Morgan Ganerød Asbjørn Hiksdal Roelant Van Der Lelij Christof Vockenhuber 《Boreas: An International Journal of Quaternary Research》2021,50(1):6-28
The volcanic island of Jan Mayen, remotely located in the Norwegian-Greenland Sea, was covered by a contiguous ice cap during the Late Weichselian. Until now, it has been disputed whether parts of the island south of the presently glaciated Mount Beerenberg area were ever glaciated. Based on extensive field mapping we demonstrate that an ice cap covered all land areas and likely also extended onto the shallow shelf areas southeast and east of the island. Chronological interpretations are based on K-Ar and 40Ar/39Ar dating of volcanic rocks, cosmogenic nuclide (36Cl) surface exposure dating of bedrock and glacial erratics, and radiocarbon dating. We argue that ice growth started after 34 ka and that an initial deglaciation started some 21.5–19.5 ka in the southern and middle parts of the island. In the northern parts, closer to the present glaciers, the deglaciation might have started later, as evidenced by the establishment of vegetation 17–16 cal. ka BP. During full glaciation, the ice cap was likely thickest over the southern part of the island. This may explain a seemingly delayed deglaciation compared with the northern parts despite earlier initial deglaciation. In a broader context, the new knowledge of the Late Weichselian of the island contributes to the understanding of glaciations surrounding the North Atlantic and its climate history. 相似文献
74.