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851.
852.
From the mid 1980s through the late 1990s, the channel of the lower Yellow River experienced serious shrinkage, which has decreased the flood conveyance of the channel and the sediment carrying capacity of the flow, raised the water levels of floods, and, thus, severely threatened the safety of flood control along the river. The completion of Xiaolangdi Dam in 1999 could help mitigate the channel shrinkage problem, but the situation has not changed yet. This paper analyses the characteristics, mechanisms, and conditions resulting in channel shrinkage, points out channel instabilities, and puts forward approaches of channel rehabilitation. 相似文献
853.
Y S O’Reilly 《中国科学D辑(英文版)》2006,49(9):926-937
The partial melting and metasomatism are two most important geological processes of changing chemical compositions of mantle. Extracting partial melt from primitive mantle leads residual mantle to deplete in- compatible elements, whereas the metasomatism … 相似文献
854.
Wang Sheng-zu 《中国地震研究》2006,20(3):326-338
INTRODUCTIONAccording to the“netlike plastic-flow (NPF)”continental dynamics model ( Wang, 1993a ,1993b; Wang,et al .,2001c) ,the continental lithosphere generally includes 4 tectonic deformationlayers frombottomtotop:the NPFlayer (i .e .thelowerlithosphere ,includingthelithospheric mantleandlower crust) ,the transitional weak layer distributed discontinuously (i .e .the so-called low-velocity/high-conductivitylayer) ,the shear fracturinglayer (i .e .the seismogenic layer inthe upperc… 相似文献
855.
856.
南海晚新生代地层结构特征、沉积厚度及沉积模式 总被引:1,自引:0,他引:1
本文综合地震、钻井及区域地质资料,通过分析地震相参数等,研究南海晚新生代(E3-Q)地层结构特征和沉积厚度变化,建立了晚新生代沉积模式,所取得的研究成果可为南海油气的进一步勘探开发提供有效的基础资料。 相似文献
857.
858.
Sand transport in the Colorado River in Marble and Grand canyons was naturally limited by the upstream supply of sand. Prior to the 1963 closure of Glen Canyon Dam, the river exhibited the following four effects of sand supply limitation: (1) hysteresis in sediment concentration, (2) hysteresis in sediment grain size coupled to the hysteresis in sediment concentration, (3) production of inversely graded flood deposits, and (4) development or modification of a lag between the time of a flood peak and the time of either maximum or minimum (depending on reach geometry) bed elevation. Construction and operation of the dam has enhanced the degree to which the first two of these four effects are evident, and has not affected the degree to which the last two effects of sand supply limitation are evident in the Colorado River in Marble and Grand canyons. The first three of the effects involve coupled changes in suspended-sand concentration and grain size that are controlled by changes in the upstream supply of sand. During tributary floods, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. Also associated with these changes in sand supply are changes in the fraction of the bed that is covered by sand. Thus, suspended-sand concentration in the Colorado River is likely regulated by both changes in the bed-sand grain size and changes in the bed-sand area. A physically based flow and suspended-sediment transport model is developed, tested, and applied to data from the Colorado River to evaluate the relative importance of changes in the bed-sand grain size and changes in the bed-sand area in regulating suspended-sand concentration. Although the model was developed using approximations for steady, uniform flow, and other simplifications that are not met in the Colorado River, the results nevertheless support the idea that changes in bed-sand grain size are much more important than changes in bed-sand area in regulating the concentration of suspended sand. 相似文献
859.
Christian Winter 《Sedimentary Geology》2007,202(3):562-571
The application of numerical models for the simulation of coastal hydro-and sediment dynamics requires model verification, calibration and validation with field data. Yet, no commonly accepted rules for the evaluation of sediment transport models exist. This paper discusses the significance of statistical parameters and their limitations considering common time lags in tidal environments. It is shown that the occasionally used discrepancy ratio lacks quantitative and qualitative information on model performance, as the time context information on time series characteristics is lost. As an initial measure of association, the simple linear correlation coefficient r2 is proposed. To account for time lag errors in suspended transport models, a separate cross-correlation analysis for the flood and ebb tidal phase is proposed. For a comparison with other model applications, a concluding rating of model performance can be expressed by a dimensionless error definition which takes into account the quality of field data. 相似文献
860.
YUAN Xuyin DENG Xu SHEN Zhiyong GAO Yun 《中国地球化学学报》2007,26(4):384-393
The species of Cu, Pb, Zn, Cd and Cr in sediments of the Taihu Lake, China, have been analyzed using the sequential chemical extraction method. Variations in the chemical fractions of these metals and their geographic distributions have also been studied. For all five metals, the residual fraction is highest but the exchangeable fraction is lowest among all the fractions. Compared to other metals, Cd has the highest percentage in the exchangeable fraction, and Cr is associated mainly with the residual fraction. Cu in the organic fraction and Pb in the Fe-Mn fraction are the important species, whereas the lowest percentages are found for Cd in the organic fraction, Cu in the Fe-Mn oxide fraction and Pb in the carbonate fraction. With respect to spatial differences, the total contents in the non-residual fractions of the metals in bay sediments are found to be higher than those in other sediments. The fractions of Cd, Cu and Cr showed significant variations in different regions. The fractions of Pb and Zn, however, did not show significant variations in spatial distribution, suggesting different amounts and different paths of anthropogenic input for the metals. Comparisons of the metal speciation indicated that Cd might be the most bioavailable metal, followed by Pb. 相似文献