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991.
Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m3/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection. 相似文献
992.
Land subsidence due to aquifer-system compaction accompanying groundwater extraction is a global hazard. Rising urban construction and groundwater demand necessitate increased awareness and better understanding of the geological problem. Motivated by the lack of laboratory-scale studies on this issue, an experimental investigation on the newly developed Tongchuan region, China, is presented. The study addresses the deformation behaviors of three soil samples, with the lithology of silty clay, silt, and fine sand, under the conditions of groundwater withdrawal and recharge using the GDS Consolidation Testing System. Results indicate that all three samples were characterized by elastic–plastic deformation under the conditions of withdrawing and recharging. The vertical deformation of the silty clay in the aquitard above the first confined aquifer was larger than those of the other two samples, and its deformation is a gradual and long process; thus, considerable attention should be paid to deformation in this aquitard due to the apparent creep effect and tiny rebound deformation. However, the settlement of the fine sand in the second confined aquifer cannot be ignored due to the great thickness of the aquifer. For the same soil, as the pore-water pressure declined, the unit rate of vertical deformation decreased gradually, whereas the creep effect of deformation in the later declining stage of pore-water pressure was more apparent than that in the former declining stage. These observations are highly important to the local government, which is developing measures to prevent and control subsidence. 相似文献
993.
长江水下三角洲层序地层学研究有助于全面了解长江三角洲地层特征和沉积环境演化模式。通过对长江水下三角洲下切河谷区YD0901和YD0903孔岩心的详细沉积物粒度、特征元素比值(Cl/Ti和Zr/Rb)、沉积相对比分析,恢复了冰后期以来长江水下三角洲层序地层格架。研究区冰后期以来自下而上依次出现河流相、潮汐河流相、河口湾相、浅海相和三角洲相的沉积相序。末次冰期海平面下降,古长江形成下切河谷,古河间地发育硬黏土层,构成五级Ⅰ型层序界面。之后海平面回升,分别于15 cal ka BP和8.0 cal ka BP形成最大海退和最大海侵界面,水下三角洲区域最大海侵发生时间略滞后于平原区,约为7.5 cal ka BP。据此3个层序界面将冰后期地层划分为低位体系域、海侵体系域和高位体系域。钻孔岩心记录揭示了14.8 cal ka BP海侵到达研究区;14.8~13 cal ka BP期间,受MWP-1A冰融水事件影响海平面快速上升,海岸线向陆推进速率可达71.9,km/ka;海退期间各钻孔沉积速率较低,直至2 cal ka BP开始,沉积速率明显增加。 相似文献
994.
995.
塔河油田二区奥陶系碳酸盐岩油藏储集体类型由缝洞型、高能微孔隙型、压溶孔缝型组成。通过岩心薄片观察并结合地震、测井、录井和试油等资料,查明了受次生溶蚀作用控制的岩溶缝洞型、受高能沉积相带控制的微孔隙型和受压溶作用及白云化作用控制的压溶孔缝型储层的特征和分布,并且识别出溶蚀孔洞、溶蚀缝、构造缝、粒内孔、粒间孔、铸模孔、晶间孔、晶内孔、缝合线和微裂缝10种储集空间。实际上,上述储集体主要为高能沉积相带形成的颗粒灰岩演化而成,这些颗粒灰岩经过埋藏压实和胶结等作用形成了以次生微孔隙为主的微孔隙型储集体,经过压溶和白云岩化作用形成了受成岩演化控制的压溶孔缝型储集体,在塔北奥陶系岩溶期形成了岩溶缝洞型储集体。塔河油田二区南部受沉积相展布和成岩作用影响,颗粒灰岩和云斑状灰岩发育;北部古隆起风化剥蚀作用较强,发育大型溶洞储层;东部斜坡带古河流发育,溶蚀缝洞储层多沿古河道分布;西部受深大断裂控制,溶蚀孔洞沿断裂垂向发育。 相似文献
996.
We investigate the growth of the northern Tibetan Plateau and associated climate change by applying oxygen and carbon isotopic compositions in Cenozoic strata in the southwestern Qaidam basin. The X-ray diffraction and isotopic studies reveal that the carbonate minerals are mainly authigenic and they do not preserve any evidence for detrital carbonate and diagenesis. The isotope data show large fluctuations in the δ18O and δ13C values in the middle–late Eocene, indicating relatively warm and seasonal dry climate. The positive correlation of the δ18O and δ13C values in the Oligocene and the positive shift of the δ13C values from the Eocene to Oligocene suggest that the climate changed to arid in the Oligocene. However, the δ18O values show negative shift, which is closely related to the global cooling event. During the Miocene, the δ13C values vary between –2‰ and –4‰, whereas the δ18O values show continuous negative shift. The mean δ18O values decrease from –8.5‰ in the early Miocene to –10.0‰ in the late Miocene. The stable isotope-based paleoaltimetry results suggest that the elevation of the southwestern Qaidam basin was approximately 1500 m in the middle–late Eocene and Oligocene. Subsequently, during Miocene the crustal uplift process started and the elevation reached approximately 2000 m in the early Miocene and 2500 m in the late Miocene, which suggests large-scale growth of the northern Tibet Plateau during the Miocene. 相似文献
997.
通过对滇西NW—SE向牟定—香格里拉大地电磁测深剖面的反演并结合地质构造、小震精定位资料,综合分析了剖面经过地区的深部构造.在滇西北地区深部发育一规模巨大的近水平产出地壳高导层,此高导层从香格里拉一直延伸到永胜盆地以东,沿剖面水平延伸超过200km.地壳高导层又可分为两段,两段高导层在剖面上形成两个香蕉型相连的复合形态,高导层之上的高阻体则形成两个碗型相连的复合结构,碗型内部有向东倾斜的相对低阻带存在.滇西北地区整体构造格架可以解译为一套两个主推覆面构成的叠瓦式推覆系统,两个主推覆面高低起伏形成总长度超过250km的底部滑脱带.第一个主推覆面的长度超过150km,并在丽江东部上翘到接近地表,小金河断裂是其延伸到地表的主要破裂带.第二个主推覆面长度超过120km,过程海断裂后上翘并在永胜以东地区出露地表.根据反向低阻带影像,可以推断推覆体反冲构造发育,在玉龙山至丽江以东和永胜盆地西缘至平川盆地东缘形成两处冲起构造.推覆面在剖面上呈现出平缓-陡峭-再平缓-再陡峭的断坪和断坡相间的阶梯状结构. 相似文献
998.
999.
通过对欧盟独立交易登记系统(CITL)的抓取、识别、清洗与融合构建了全样本的交易大数据集合,完整地重现了欧盟碳排放权交易体系(EU ETS)在试验阶段的市场微观结构。进而从市场内部微观视角出发,对EU ETS的市场微观体系、排放企业的微观交易行为,以及市场供需微观演化等予以分析,结果表明在市场建立初期:排放企业的交易大多是以配额履约为目的;交易行为呈现活跃度低、季节性和同向性的特点;市场上少量的高排放企业凭借其在配额总量、资金和信息上的优势在交易中占据了主导地位。此外,微观行为数据还反映了金融部门在碳配额交易中的作用:配额在前期流向金融部门使得供过于求的状况有所缓解,但当配额流出金融部门时,市场供需失调的情况被迅速放大。在碳市场建立初期,理解EU ETS市场参与者微观行为特点以及潜在的风险,对中国即将启动的全国统一碳排放权交易体系的机制设计有重要的启示意义。 相似文献
1000.
本文利用主成分分析、震间位错反演和小波技术分析了鲜水河断裂西北段的跨断层形变资料(1986—2013).结果表明:以左旋走滑为主的断层长期运动为跨断层资料的主要信息,且符合负指数函数的运动规律,随着断层深度的增加,滑动量逐渐减弱.从炉霍段、道孚段到乾宁段,断层闭锁程度逐渐增强.2001年昆仑山M8.1地震发生后,鲜水河断裂西北段地壳浅层(地表以下30km)的左旋走滑明显减弱,到2013年的累积减弱量为3~13mm.5·12汶川M8.0地震发生前,断层滑动出现周期4~5年的增强信号.4·20芦山M7.0地震发生前,断层滑动首先出现低频信号(4~5年周期)增强,随着地震发生临近,强信号频率逐渐升高,直到出现周期为1年的强信号. 相似文献