Role of hydrological regime and floodplain sediments in channel instability of the Bhagirathi River,Ganga-Brahmaputra Delta,India     

Sanat Kumar Guchhait  Aznarul Islam  Balai Chandra Das  Nishith Kumar Maji 《自然地理学》2016,37(6):476-510

Abstract

This research deals with the surface dynamics and key factors – hydrological regime, sediment load, and erodibility of floodplain facies – of frequent channel shifting, intensive meandering, and lateral instability of the Bhagirathi River in the western part of the Ganga-Brahmaputra Delta (GBD). At present, the floodplain of the Bhagirathi is categorized as a medium energy (specific stream power of 10–300 W m^?2), non-cohesive floodplain, which exhibits a mixed-load and a meandering channel, an entrenchment ratio >2.2, width–depth ratio >12, sinuosity >1.4, and channel slope <0.02. In the study area, since 1975, four meander cutoffs have been shaped at an average rate of one in every 9–10 years. In the active meander belt and sand-silt dominated floodplains of GBD, frequent shifting of the channel and meander migration escalate severe bank erosion (e.g. 2.5 × 10⁶ m³ of land lost between 1999 and 2004) throughout the year. Remote sensing based spatio-temporal analysis and stratigraphic analysis reveal that the impact of the Farakka barrage, completed in 1975, is not the sole factor of downstream channel oscillation; rather, hydrogeomorphic instability induced by the Ajay–Mayurakshi fluvial system and the erodibility of floodplain sediments control the channel dynamics of the study area.  相似文献   

Pattern,style and timing of British–Irish Ice Sheet advance and retreat over the last 45 000 years: evidence from NW Scotland and the adjacent continental shelf     

Tom Bradwell  Derek Fabel  Chris D. Clark  Richard C. Chiverrell  David Small  Rachel K. Smedley  Margot H. Saher  Steven G. Moreton  Dayton Dove  S. Louise Callard  Geoff A. T. Duller  Alicia Medialdea  Mark D. Bateman  Matthew J. Burke  Neil McDonald  Sean Gilgannon  Sally Morgan  David H. Roberts  Colm ó Cofaigh 《第四纪科学杂志》2021,36(5):871-933

Predicting the future response of ice sheets to climate warming and rising global sea level is important but difficult. This is especially so when fast-flowing glaciers or ice streams, buffered by ice shelves, are grounded on beds below sea level. What happens when these ice shelves are removed? And how do the ice stream and the surrounding ice sheet respond to the abruptly altered boundary conditions? To address these questions and others we present new geological, geomorphological, geophysical and geochronological data from the ice-stream-dominated NW sector of the last British–Irish Ice Sheet (BIIS). The study area covers around 45 000 km² of NW Scotland and the surrounding continental shelf. Alongside seabed geomorphological mapping and Quaternary sediment analysis, we use a suite of over 100 new absolute ages (including cosmogenic-nuclide exposure ages, optically stimulated luminescence ages and radiocarbon dates) collected from onshore and offshore, to build a sector-wide ice-sheet reconstruction combining all available evidence with Bayesian chronosequence modelling. Using this information we present a detailed assessment of ice-sheet advance/retreat history, and the glaciological connections between different areas of the NW BIIS sector, at different times during the last glacial cycle. The results show a highly dynamic, partly marine, partly terrestrial, ice-sheet sector undergoing large size variations in response to sub-millennial-scale climatic (Dansgaard–Oeschger) cycles over the last 45 000 years. Superimposed on these trends we identify internally driven instabilities, operating at higher frequency, conditioned by local topographic factors, tidewater dynamics and glaciological feedbacks during deglaciation. Specifically, our new evidence indicates extensive marine-terminating ice-sheet glaciation of the NW BIIS sector during Greenland Stadials 12 to 9 – prior to the main ‘Late Weichselian’ ice-sheet glaciation. After a period of restricted glaciation, in Greenland Interstadials 8 to 6, we find good evidence for rapid renewed ice-sheet build-up in NW Scotland, with the Minch ice-stream terminus reaching the continental shelf edge in Greenland Stadial 5, perhaps only briefly. Deglaciation of the NW sector took place in numerous stages. Several grounding-zone wedges and moraines on the mid- and inner continental shelf attest to significant stabilizations of the ice-sheet grounding line, or ice margin, during overall retreat in Greenland Stadials 3 and 2, and to the development of ice shelves. NW Lewis was the first substantial present-day land area to deglaciate, in the first half of Greenland Stadial 3 at a time of globally reduced sea-level c. 26 kabp , followed by Cape Wrath at c. 24 kabp. The topographic confinement of the Minch straits probably promoted ice-shelf development in early Greenland Stadial 2, providing the ice stream with additional support and buffering it somewhat from external drivers. However, c. 20–19 kabp , as the grounding-line migrated into shoreward deepening water, coinciding with a marked change in marine geology and bed strength, the ice stream became unstable. We find that, once underway, grounding-line retreat proceeded in an uninterrupted fashion with the rapid loss of fronting ice shelves – first in the west, then the east troughs – before eventual glacier stabilization at fjord mouths in NW Scotland by ~17 kabp. Around the same time, ~19–17 kabp , ice-sheet lobes readvanced into the East Minch – possibly a glaciological response to the marine-instability-triggered loss of adjacent ice stream (and/or ice shelf) support in the Minch trough. An independent ice cap on Lewis also experienced margin oscillations during mid-Greenland Stadial 2, with an ice-accumulation centre in West Lewis existing into the latter part of Heinrich Stadial 1. Final ice-sheet deglaciation of NW mainland Scotland was punctuated by at least one other coherent readvance at c. 15.5 kabp , before significant ice-mass losses thereafter. At the glacial termination, c. 14.5 kabp , glaciers fed outwash sediment to now-abandoned coastal deltas in NW mainland Scotland around the time of global Meltwater Pulse 1A. Overall, this work on the BIIS NW sector reconstructs a highly dynamic ice-sheet oscillating in extent and volume for much of the last 45 000 years. Periods of expansive ice-sheet glaciation dominated by ice-streaming were interspersed with periods of much more restricted ice-cap or tidewater/fjordic glaciation. Finally, this work indicates that the role of ice streams in ice-sheet evolution is complex but mechanistically important throughout the lifetime of an ice sheet – with ice streams contributing to the regulation of ice-sheet health but also to the acceleration of ice-sheet demise via marine ice-sheet instabilities.  相似文献   

微生物-CMC无固相钻井液固壁作用与机理初探          下载免费PDF全文

李之军  赵淦  向浩天  杜泽华  陈俊秀  刘阔 《探矿工程》2021,48(S1):249-253

松散破碎性地层孔壁失稳一直是困扰钻探工程界的难题之一,增强该类地层的胶结性,提高其力学性能是有效解决孔壁失稳的技术关键。本文将微生物诱导碳酸钙沉积（MICP）技术与CMC无固相钻井液相结合,构建微生物-CMC无固相钻井液体系。通过岩心浸泡实验、X射线衍射实验（XRD）以及扫描电镜分析两种微观分析手段对微生物-CMC无固相钻井液的固壁作用与机理进行了初探。结果表明：微生物-CMC无固相钻井液对松散破碎性地层具有较明显的加固作用,且作用时间越长,初始菌种浓度越高,钙源浓度越大,固壁效果越好。在固壁过程中,微生物随钻井液渗透进入试样内部,在松散颗粒之间诱导生成碳酸钙晶体,填充孔隙空间,将松散颗粒胶结成整体,并具有一定的力学强度,从而达到加固孔壁的目的。本研究结果为解决松散破碎性地层孔壁失稳提供了新的钻井液技术方案。  相似文献   

Properties and stability of a meso-scale line-form disturbance   总被引：1，自引：0，他引：1  

沈新勇  丁一汇  赵南 《大气科学进展》2006,23(2):282-290

By using the 3D dynamic equations for small- and meso-scale disturbances, an investigation is performed on the heterotropic instability (including symmetric instability and traversal-type instability) of a zonal line-like disturbance moving at any angle with respect to basic flow, arriving at the following results: (1) with linear shear available, the heterotropic instability of the disturbance will occur only when flow shearing happens in the direction of the line-like disturbance movement or in the direction perpendicular to the disturbance movement, with the heterotropic instability showing the instability of the internal inertial gravity wave; (2) in the presence of second-order non-linear shear, the disturbance of the heterotropic instability includes internal inertial gravity and vortex Rossby waves. For the zonal line-form disturbance under study, the vortex Rossby wave has its source in the second-order shear of meridional basic wind speed in the flow and propagates unidirectionally with respect to the meridional basic flow. As a mesoscale heterotropic instable disturbance, the vortex Rossby wave has its origin from the second shear of the flow in the direction perpendicular to the line-form disturbance and is independent of the condition in the direction parallel to the flow; (3) for general zonal line-like disturbances, if the second-order shear happens in the meridional wind speed, i.e., the second shear of the flow in the direction perpendicular to the line-form disturbance, then the heterotropic instability of the disturbance is likely to be the instability of a mixed Rossby–internal inertial gravity wave; (4) the symmetric instability is actually the instability of the internal inertial gravity wave. The second-order shear in the flow represents an instable factor for a symmetric-type disturbance; (5) the instability of a traversal-type disturbance is the instability of the internal inertial gravity wave when the basic flow is constant or only linearly sheared. With a second or nonlinear vertical shear of the basic flow taken into account, the instability of a traversal-type disturbance may be the instability of a mixed vortex Rossby – gravity wave.  相似文献   

高地应力围岩分区碎裂化的时间效应分析和相关参数研究   总被引：2，自引：0，他引：2  

李英杰  潘一山  章梦涛 《地质力学学报》2006,12(2):252-260

本文阐述了高地应力围岩分区碎裂化现象的形成需要经过一定的时间,但这段时间不会很长;采用蠕变理论对岩石分区碎裂化的时间效应进行了分析;推导了能够描述加速蠕变阶段的流变模型-改进的西原模型的本构方程和蠕变方程,在此基础上求出了岩石分区碎裂化发生时破裂带半径(破碎带距巷道中心的距离)的公式。   相似文献   

山西2006年雨季天气气候特征分析     

姚彩霞  杨英  任璞  郭俊龙  郝建萍 《气象科技》2008,36(1):74-77

2005年6月17～24日,华南地区发生了连续多日的暴雨天气过程,其显著特征是存在着南北两条雨带,北支雨带(福建中北部)由准静止的梅雨锋造成,南支雨带(广东中东部)发生在锋前暖区之中,这种连续多日共存的双雨带现象引起了气象学家的广泛关注.为了探究锋面和锋前暖区暴雨的成因,加深这两类不同性质暴雨的认识,利用NCEP每6 h一次的1°×1°经纬度再分析资料以及华南地区加密观测的逐小时地面降水等资料,以此次连续多日维持的双雨带降水过程为例,详细分析了锋面附近与锋前暖湿区内暴雨系统的主要物理差异.结果发现:梅雨锋暴雨和锋前暖区暴雨不仅在中尺度雨团活动、系统动力结构、大气不稳定机制和大气加热结构等存在明显的差异,而且在水汽输送、中尺度环境以及与暴雨有关的垂直环流之间也存在着不同点,这些差异可能是造成锋前暖区暴雨难以模拟和预报的主要原因.  相似文献   

海南雷暴气候特征及大气环流背景分析   总被引：12，自引：2，他引：10  

郭冬艳  辛吉武  吴胜安  姜涛  杨昌贤  陈红 《气象科技》2008,36(4):404-409

利用1966～2005年海南地区18个市、县逐日雷暴观测资料及NECP再分析资料和EOF分析方法,分析了海南地区雷暴天气的气候特征及其气候变化的可能影响因素.结果表明:海南雷暴的年际变化呈较明显的下降趋势;空间分布丰要旱现北部内陆地区多、南部沿海地区少的特点;全省雷暴集中发牛在4～10月,盛发期在5～9月;雷暴异常年5～9月平均大气环流与同期500 hPa大气环流特征的关系表现在雷暴频繁年低纬地区的位势高度距平场出现大范围的负距平.当年夏季西太平洋副热带高压较弱,位置偏东,雷暴偏少年则副高较强,脊线偏西.不稳定凶子K指数和TT指数对海南的雷暴有一定预报能力.  相似文献   

华南锋面与暖区暴雨个例对比分析   总被引：14，自引：1，他引：13  

赵玉春  李泽椿  肖子牛 《气象科技》2008,36(1):47-54

分析了1961～1999年亚洲热带近海海温与云南降水的关系.通过研究云南夏季降水对近海海温异常的响应,发现云南初夏的降水与亚洲热带近海海温有明显的负相关关系,这种降水减少海温正距平的情况在北印度洋西部和阿拉伯海表现比孟加拉湾和南海更显著,但是在盛夏相关关系不显著.同时发现云南西南部的夏季降水与前期盂加拉湾海温有显著的正相关,而与其它海域的海温相关不显著.滇东南的夏季降水只与前期南海的海温有显著的正相关.  相似文献   

地电阻率供电回路故障检测板的研制与使用     

郅红魁  赵彦旭  付靖  谢佳兴  尚淑珍 《地震地磁观测与研究》2021,42(1):69-75

地电阻率仪器在工作中容易出现无电流、电流不稳等故障现象,不易排查维修.通过对仪器工作原理和供电回路过程进行研究,发现供电回路中间仪器的内部或外部各接头接触不良是造成上述现象的主要因素.根据存在的问题,洛阳地震台研制出供电回路故障检测板.该检测板可利用其继电器上LED灯的工作状态和动作时序,快速查找、排除相关供电回路故障...  相似文献   

Coupled analysis for response and instability of sloping seabed under wave action     

《Applied Ocean Research》2019

Wave-induced instability of seabed may cause damage to coastal and offshore structures. This issue has been investigated mostly for mildly sloping ($<5^{°}$) seabed considering uncoupled or one-way coupled response of wave and seabed interaction. However, some of the marine structures are founded on seabed with steeper slopes. In this study, the wave-induced response and instability of sloping seabed are evaluated using a coupled finite element model. The interaction between fluid and porous seabed accounting for the effect of fluid motion on the seabed response, and conversely the effect of seabed response on the fluid motion (but not on the surface wave profile) is considered. The results indicate that the system response (fluid pressure, stresses, etc.) and the extent of instantaneously liquefied zone within the sloping seabed with significant steepness are lesser than those for horizontal seabed. Moreover, for typical sediment and wave characteristics, for the flat seabed, the response obtained from fully coupled analysis is not significantly different from those obtained by uncoupled analysis. For the sloping bed, such difference is slightly greater as compared to that for the flat bed.  相似文献