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711.
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Sedimentation and morphological changes at Yuantuojiao Point,estuary of the North Branch,Changjiang River 总被引:3,自引:1,他引:2
The North Branch, separated by the Chongming Island, was once the main channel in the estuary of the Changjiang River. Reclamation and a decrease in runoff to the North Branch had led to the narrowing and shallowing of the channel. The Yuantuojiao Point is located at the intersecting point connecting the North Branch of the Changjiang River and the Jiangsu coastline. Erosion cliffs are developed between the typical silty-muddy tidal flat and the salt marsh occupied by Spartina alterniflorea, and this has changed rapidly over the past few years. The sediment grain size analysis results of the surficial and two core samples indicate that the Yuantuojiao Point tidal flat experienced continuous accretional processes. Based upon 137 Cs analysis results of the YT and YY Cores sampled from the tidal flat at the Yuantuojiao Point, the average sedimentation rate of the YT Core was 2.30 cm/a from 1963 to 2007, and 2.38 cm/a from 1954 to 2007 for the YY Core. The sedimentation rates of both core locations have declined since the 1960s corresponding to the seaward reclamation at the Yuantuojiao Point. The average sedimentation rates at the Yuantuojiao Point were similar to that of the silty-muddy tidal flat at the northern Jiangsu coast, but lower than that of the south of the Changjiang River Estuary. According to field morphological investigations from 2006 to 2008 on the salt marsh at the Yuantuojiao Point, cliffs retreated markedly by storm surges and disappeared gradually because of the rapid sedimentation on the silty-muddy tidal flat. The maximum annual retreat reached 10 m. The recent sedimentation and morphological changes of the Yuantuojiao Point tidal flat not only displayed the retreat of the salt marsh and the disappearance of cliffs, but also was accompanied by rapid sedimentation of the silty-muddy tidal flat and the salt marsh, indicating the responses to the tidal currents, storm surges, Spartina alterniflorea trapping sediments and large-scale reclamation. The sediment grain size and their trends, southward coastal flow, and sandspits of the longshore bars suggest that the main sediment source at the Yuantuoijao Point, estuary of the North Branch was possibly from the Changjiang River before 1958, since then, it has been from the south of the submarine radial sand ridges of the southern Huanghai Sea (Yellow Sea). 相似文献
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715.
借助137Cs估算滇池沉积量 总被引:4,自引:0,他引:4
The water quality of Dianchi Lake declines quickly and the eutrophication is getting serious. To identify the internal pollution load of Dianchi Lake it is necessary to evaluate its sediment accumulation. Sedimentation rates of Dianchi Lake are determined by 137Cs dating. However, 137Cs vertical distribution in sediment cores of Dianchi Lake has special character-istics because Dianchi Lake is located on the southeast of the Qinghai-Tibet Plateau, the Kunming quasi-stationary front is over the borders of Yunnan and Guizhou where the specific precipitation is distributed. Besides 1954, 1963 and 1986 137Cs marks can be determined in sediment cores, a 137Cs mark of 1976 representing the major period of 137Cs released from China unclear test can be determined and used for an auxiliary dating mark. Meanwhile Di-anchi Lake is divided into seven sections based on the water depth, basin topography, hy-drological features and supplies of silt and the lakebed area of each section is calculated. The mean annual sedimentation rates for seven sections are 0.0810, 0.1352, 0.1457, 0.1333, 0.0904, 0.1267 and 0.1023 g/cm2 a in 1963-2003, respectively. The gross sediment accu-mulation of the lake is 26.18×104 t/a in recent 17 years and 39.86×104 t/a in recent 50 years. 相似文献
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本文首先阐明了基于旋转坐标系的频率域正演算法只能适用于相同横纵向空间采样间隔的局限性,并发展了一种新的基于平均导数方法(average-derivative method,简称ADM)的25点有限差分格式来实现声波方程频率域高精度正演.这种基于平均导数方法的算法将声波频率域方程中空间导数项的差分近似表示为正交方向上5个网格点的加权平均形式,能适用于不同的横纵向空间采样间隔,因此能作为四阶声波频率域正演的一种统一格式,具有很好的适用性.通过优化方法求取空间导数项和加速度项的加权优化系数,从而使数值频散达到极小化,每个波长所需要的网格点数在1%的误差范围内仅为2.78个网格点数.本文通过引入完全匹配层(perfectly matched layer,简称PML)吸收边界条件,有效地消除了人工边界反射.数值模拟结果验证了本文25点ADM算法的有效性和准确性. 相似文献
719.
改进的Backus平均模型已被证明能较好地模拟Bakken页岩,对于同样的对象,本文采用各向异性DEM模型进行分析,分别模拟了干酪根作为岩石背景基质与内部包含物时岩石的等效弹性张量随干酪根含量的变化规律,发现将干酪根看作岩石基质时模拟结果更接近目标页岩的实验室测量数据,与目标岩样的电镜扫描结果一致;继而将粘土矿物的纵横比以及干酪根与粘土矿物的弹性模量看作自由参数,通过最小二乘法求取了各向异性DEM模型的最佳输入参数,由此得到的预测结果优于改进的Backus平均模型,说明采用各向异性DEM模型来分析有机质的含量和分布对岩石弹性性质的影响是有效可行的.最后,拟合生成了目标页岩的实际刚度系数与含氢指数的关系曲线,得出岩石刚度与干酪根成熟度之间具有正相关性. 相似文献
720.