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Incision of alluvial channels in response to a continuous base level fall: Field characterization, modeling, and validation along the Dead Sea 总被引:5,自引:1,他引:4
The dramatic lake level drop of the Dead Sea during the twentieth century ( 30 m) provides a field-scale experiment in transport-limited incision of gravel-bed channels in response to quasi-continuous base level fall at approximately constant rate. We apply a one-dimensional numerical incision model based on a linear diffusion equation to seven ephemeral channels draining into the Dead Sea. The model inputs include the measured twentieth century lake level curve, annual shoreline location (i.e., annual channel lengthening following the lake level drop), reconstructed longitudinal profiles of each of the channels based on mapped and surveyed terraces, and the current profiles of the active channels. The model parameters included the diffusion coefficient and the upstream-derived sediment flux. Both were first calibrated using a set of longitudinal profiles of known ages and then validated using additional sets of longitudinal profiles.The maximum at-station total incision observed at each of the studied channels was significantly less then the total lake level drop and varied in response to both drainage area and lake bathymetry. The model applied predicted degradation rates and the pattern of degradation with high accuracy. This suggests that sediment flux in the modeled channels is indeed linearly dependent on slope. Further support for this linear dependency is provided by a linear correlation between the diffusion coefficient and the mean annual rain volume over each basin (a proxy for discharge). The model presented could be a valuable tool for planning in rapid base level fall environments where incision may risk infrastructure. 相似文献
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A model of a heterogeneous neighborhood 总被引:1,自引:0,他引:1
Tel Aviv is a highly ethnic city in which ethnicity is deeply embedded in people’s perceptions of their social milieus. Shapira,
as one of the most heterogeneous neighborhoods in Tel Aviv, supplies a unique demonstration of the inadequacy of the Chicago
model, which assumes the emergence of homogeneous neighborhoods. The study shows that ethnicity is exercised as a major force
in determining social life in Shapira. Interactive segregation indices reveal that residents of Shapira tend to prefer intra-ethnic
social networks although two-thirds of them maintain inter-ethnic networks as well. Despite this, residential and activity
spaces are highly heterogeneous. Residential spaces are heterogeneous on all scales from residents’ immediate surroundings
to the block and the neighborhood as a whole. In most buildings one may find neighbors from two or three different ethnic
groups with only non-Jewish residents excluded from publicly owned buildings. Most residents perform large part of their everyday
life outside the neighborhoods in ethnically heterogeneous surroundings, but nonetheless in choosing their more meaningful
partners for social networks they choose them from their ethnic groups. The fact that residents tend to perform a large part
of their activities out of their neighborhood, and to live in heterogeneous surroundings in social categories that are perceived
by them salient to their daily life, does not undermine the relevance of the neighborhood as a socially constituted entity
in the urban field. People organize and act in order to improve life conditions in the neighborhood, they feel sense of attachment
to the neighborhood and they develop some local social networks in the neighborhood. 相似文献
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Gdaliahu Gvirtzman Moshe Wieder Ofer Marder Hamudi Khalaily Rivka Rabinovich Hagai Ron 《Geoarchaeology》1999,14(2):101-126
An Early Paleolithic site was recently discovered within a sequence of paleosols in the Revadim Quarry, central coastal plain of Israel. The section is composed of three superimposed soils in a continuous sequence, but separated by two unconformity surfaces. The uppermost paleosol is a modern Dark Brown Grumusol (Vertisol), the middle is a Quartzic Gray Brown Soil (Haploxeralf), and the lower is a Red Hamra (Rhodoxeralf). Normal magnetic polarity was detected in the two lower soils, indicating that they are younger than the Brunhes–Matuyama boundary (<780 ka). A human occupation bed, enriched in secondary carbonate nodules, forms the lower part of the Quartzic Gray Brown Soil and overlies the Red Hamra. The living floor is located on top of the unconformity surface, separating the Red Hamra from the overlying Quartzic Gray Brown Soil. Middle to Late Acheulian handaxes, choppers, cores, and flake tools, including tools made by the Levallois technique, and man-laid flint pebbles were excavated in the human occupation bed. In addition, two elephant tusks, an elephant pelvis, an elephant tooth (Palaeoloxodon antiquus), tusk splinters, and bones of equid, suid, cervid, bovid, felid, and rodents were also collected. Based on well-documented nearby boreholes and on regional correlation, it appears that the underlying dune sands, the parent materials from which the Red Hamra developed, were deposited probably during a phase of high-stand sea level of Isotope Stage 9. The Red Hamra developed simultaneously with the human occupation of the site, probably during a phase of low-stand sea-level of Isotope Stage 8, before some 300–245 ka. The overlying dune sands, the parent materials from which the Quartzic Gray Brown Soil developed, were deposited probably during a phase of high-stand sea level of Isotope Stage 7. The climate prevailing in the area during Stage 8, as well as during the human habitation, was moist, with a dense vegetation cover of grassland and probably scattered trees. A small lake of trapped fresh water at a junction of two small tributaries of the Soreq River drainage system near the area occupied was available to hominids and animals. © 1999 John Wiley & Sons, Inc. 相似文献