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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.
Christof Müller 《Marine Geophysical Researches》2005,26(2-4):197-206
Between 1997 and 1999 several marine seismic surveys were carried out in Kiel Bay aimed towards the development of a three-dimensional
acquisition and interpretation technique for small scale subsurface structures using high-frequency sources and multichannel
streamers. The data set was recently revisited by the author and reprocessed to obtain a multichannel stacked seismic data
cube. Nominal hydrophone positions are deduced by determining offsets from first arrival times and estimating the hydrophone
positions under consideration of the ships track. Processing towards a ‘seismic cube’ mainly comprised CMP sorting, constant
velocity NMO correction and stacking. The resulting VHR 3-D seismic ‘brute stack cube’ reveals rich structural details. The
fluvial Pleistocene channel system already documented in an earlier publication was tracked further to the north. It is situated
below a flat cover of gas-bearing Holocene sediments, which locally constitute the seafloor. This till-horizon is superimposed
on a second till layer showing strong topographic variations. Seismic signal phase and shielding effects indicate the possible
presence of gas in these formations. This case history demonstrates that the VHR 3-D seismic method is a feasible tool for
low budget investigation and research. 相似文献
73.
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
74.
Charles E. Tilburg Jennie E. Seay T. Dale Bishop Harlan L. Miller III Christof Meile 《Estuarine, Coastal and Shelf Science》2010
Since transport of planktonic larvae is essential to the maintenance and expansion of many marine species, we examined the spatial and temporal distribution of green porcelain crab Petrolisthes armatus (Gibbes, 1850) larvae and the possible underlying physical and behavioral mechanisms using a combination of field observations and numerical modeling. The field study consisted of observations of larval abundance and distribution as well as hydrographic surveys of the Satilla River estuary on the east coast of the USA in August 2006. Larvae were found throughout the water column within the tributaries but primarily at depth in the main river. A numerical model was used to examine the effect of flow and possible larval behavior responsible for the observed distribution and the consequences for larval retention in the estuary. Model results that included downward larval movement are consistent with the field observations, supporting the hypothesis that P. armatus larvae vertically migrate within the water column, which aids in their retention within the estuary. 相似文献