LIVE-BED SCOUR IN LONG CONTRACTIONS   总被引：1，自引：0，他引：1  

Subhasish DEY Rajkumar V. RAIKAR 《国际泥沙研究》2006,21(2):166-170

1 INTRODUCTION Scour within a channel contraction occurs where the waterway is restricted by the foundations or road approach embankments of a bridge. Alternately, contraction scour takes place when a bridge is located at a natural contraction in the rive…  相似文献   

Timing of Cadomian and Variscan tectonothermal activity,La Hague and Alderney,North Armorican Massif: Evidence from 40Ar/39Ar mineral ages     

R. D. Dallmeyer  R. S. D'Lemos  R. A. Strachan 《Geological Journal》1994,29(1):29-44

The La Hague region of northwest France exposes Palaeo-Proterozoic Icartian gneisses which were reworked and intruded by calc-alkaline plutonic rocks during the Cadomian Orogeny (about 700–500 Ma). ⁴⁰Ar/³⁹Ar mineral cooling ages have been determined to clarify the timing of the regional metamorphism of orthogneisses and the emplacement of quartz diorite plutons in this region. Metamorphic amphiboles within Icartian gneisses display discordant ⁴⁰Ar/³⁹Ar apparent age spectra interpreted to result from limited Variscan (about 350–300 Ma) overprinting of intracrystalline argon systems which initially cooled through post-metamorphic hornblende closure temperatures during the Cadomian at about 600 Ma. Igneous hornblendes from the weakly foliated Jardeheu and Moulinet quartz diorites record isotope correlation ages of 599 ± 2 and 561 ± 2 Ma, respectively. Igneous hornblende and biotite from foliated quartz diorite on the nearby Channel Island of Alderney record isotope correlation ages of about 560 Ma. The results imply that metamorphic and plutonic events in the La Hague-Alderney region were approximately contemporaneous with those recorded on Guernsey and Sark, which are thus likely to have formed part of the same tectonic block during the Cadomian Orogeny.  相似文献   

COMPLICATED FILL-SCOUR BEHAVIORS OF THE YELLOW RIVER AND THE CHANNEL-FORMING PROCESSES   总被引：1，自引：0，他引：1  

XU JiongXin 《国际泥沙研究》1999,(2)

IINTRODUCTIONStreamflowformostriversintheworldcanberegardedasasolid-liquidtwo-phaseflow.Theexistenceofsuspendedsedimenthassomeinfluenceonthestreamflow'sphysicalandmechanicalproperties,whichinturnaffectsthestreamflow'ssedimentcarryingbehaviors(ChienandWan,1983;Chien,1989).Forriverswithrelativelylowsuspendedsedimefltconcentrations,theabove-mentionedeffectisnegligible.However,withtheincreasesinsuspendedsedimentconcentration,thiseffectbecomesincreasinglysignificant;thesediment-carryingbehavi…  相似文献   

LiDAR-based fluvial remote sensing to assess 50–100-year human-driven channel changes at a regional level: The case of the Piedmont Region,Italy     

S. Bizzi  H. Piégay  L. Demarchi  W. Van de Bund  C.J. Weissteiner  F. Gob 《地球表面变化过程与地形》2019,44(2):471-489

Remote Sensing (RS) technology has recently offered new and promising opportunities to analyze river systems. In this paper, we present a calibration of characteristic Hydraulic Scaling Law (HSL) using a regional database of river geomorphic features. We consistently linked discharge with channel geometry features for estimated Bankfull Channel Depth (eBCD), Active Channel Width (ACW), and Low Flow water Channel Width (LFCW), which are continuously available from RS data along the river course. We then used historical information and external sources of information on channel reaches that were relatively unaffected by human pressure over periods ranging from a few decades to a century (measured in comparable geographical areas) to infer relatively Unaltered HSLs (rUHSLs). Adopting rUHSL validated with available local historical evidence on channel geometry, we were able to assess historical changes in channel geometry consistently over the entire region and within the studied temporal window. The case study was conducted for the Po basin in the Piedmont Region, north-west Italy. From our analysis, it emerges that regionally 74% of the river network has riverbed incisions exceeding 1 m, while 66% of channels have halved their historical widths with a total of 617 ha of land subtracted from the active channel. LFCW is, on average, wider in Alpine rivers compared with those located in the North Apennines. Although it is currently not possible to measure the accuracy of these estimates, the evidence generated is coherent with available historical information, characteristic hydraulic scaling laws, evidence from relatively unaltered reaches and the available literature on local fluvial systems. This methodology provides robust, novel and quantitative information regarding decadal to secular channel changes that have occurred on a regional scale. This new layer of information enriches our ability to rationally address assessments of large-scale past and future channel trajectories. © 2018 John Wiley & Sons, Ltd.  相似文献   

地震废墟环境下Wi-Fi无线信号传播模型参数测量     

武有文  洪利  姚振静  韩智明  李亚南 《震灾防御技术》2017,12(1):210-219

本文主要针对地震废墟下手机Wi-Fi无线定位,分析实际废墟环境下Wi-Fi 2.4G无线信号的传播特性。通过对废墟结构的分析,使用矢量网络分析仪进行信道测量,对大尺度和小尺度衰减两种特性进行信道分析。采取理论预测和实际测量分析相结合的方法,对2008年5·12汶川地震的极重灾区遗址进行了废墟实地测量。对大尺度衰减进行路径损耗和钢筋混凝土吸收损耗分析,对小尺度衰减进行了平均超量时延和均方根时延分析,利用加窗、计算滑动均值,得到废墟下2.4G无线信号的传播规律,为下一步定位工作提供理论基础。  相似文献   

Review on Subaqueous Sediment Gravity Flow and Submarine Fan     

Hanpu Fu  Qun Liu  Xiumian Hu 《地球科学进展》2020,35(2):124-136

Subaqueous sediment gravity flow is the volumetrically most important process transporting sediment across our planet, which forms its largest sediment accumulations (submarine fan). Based on the previous studies, we tried to clear up the concept, classification and identification of subaqueous sediment gravity flow, and introduced the progress of modern direct observation and submarine fan model. Turbidity current and debris flow are two of the most important parts of the gravity flow, the former deposits layer by layer with normal gradation while the latter is en masse settling with chaotic disorder. The turbidity current transformed into the debris flow during the transportation is called hybrid flow. The hyperpycnal flow is the turbidity current formed by flood discharges into the ocean/lake. Modern direct observations show that the turbidity current can contain dense basal layers and last for a week. The structure of turbidity current can be different from those surge-like turbidity current observed in laboratory. Submarine fans are mainly composed of channel, levee, lobe, background deposits and mass transport deposits, which should be studied by architecture analysis and hierarchical classification. The channel deposits extend narrowly with abundant erosion structures; levee deposits are composed of thin layer mud-silty turbidites, wedge thinning laterally; the lobe deposits extend well laterally with narrow range of grain size. The hierarchy of channel deposits is channel unit, channel complex and channel complex system. The hierarchy of lobe deposits is bed, lobe element, lobe and lobe complex.  相似文献   

Marine ecosystem management: Fish abundance and size under exploitation     

Joe Horwood 《Marine Policy》2010

This study is targeted on policy makers and environmental scientists to illustrate the typical historical scale of depletion of our fish stocks, and what current and emerging legislation might mean for fisheries management and the metrics of fish stocks. The population demography of the Bristol Channel sole is described since 1820. Their decline in abundance, and change in length compositions, are modelled. By 2000, the mature stock had been depleted to less than 5% of its original size, and larger sole were rarely caught. The implications of maximum sustainable yield targets, and of good environmental status, are examined.  相似文献   

Fine-grained Pleistocene deepwater turbidite channel system on the slope of Qiongdongnan Basin, northern South China Sea   总被引：2，自引：0，他引：2  

Yuan Shengqiang  Wu Shiguo  Lüdmann Thomas  Yao Genshun  Lv Fuliang  Cao Feng  Wang Hairong  Li Li 《Marine and Petroleum Geology》2009,26(8):1441-1451

Recently, as oil exploitation has become focused on deepwater slope areas, more multi-channel high resolution 2D and 3D seismic data were acquired in the deepwater part of the Qiongdongnan Basin, northern South China Sea. Based on 3D seismic data and coherence time slice, RMS and 3D visualization, a series of deepwater channels were recognized on the slope that probably developed in the late Quaternary period. These channels trend SW–NE to W–E and show bifurcations, levees, meander loops and avulsions. High Amplitude Reflections (HARs), typical for channel–levee complexes, are of only minor importance and were observed in one of the channel systems. Most of the detected channels are characterized by low-amplitude reflections, and so are different from the typical coarse-grained turbidite channels that had been discovered worldwide. The absence of well data in the study area made it difficult to determine the age and lithology of these channels. Using a neighboring drill hole and published data about such depositional systems worldwide, the lithology of these channels is likely to be dominated by mudstones with interbedded thin sandstones. These channels are formed by turbidity currents originated from the little scale mountain river of mid-Vietnam in SW direction and were probably accompanied by a relative sea level drop in the last glacial age. These channels discovered on the northern South China Sea slope are likely to be fine-grained, mud-dominant and low N:G deposits in a deepwater paleogeographic setting.  相似文献   

Fish composition and assemblage structure in three Eastern English Channel macrotidal estuaries: A comparison with other French estuaries     

Jonathan Selleslagh  Rachid Amara  Pascal Laffargue  Sandric Lesourd  Mario Lepage  Michel Girardin 《Estuarine, Coastal and Shelf Science》2009

This study has analysed for the first time fish composition and assemblage structures of three small macrotidal estuaries of the Eastern English Channel (EEC) and has explored the influences of 19 biotic and abiotic variables on the fish assemblages. Fish from Canche, Authie and Somme estuaries were collected during spring (June 2006 and May 2007) and autumn (September 2006) along the estuarine gradients using a 1.5 m beam trawl. Using identical sampling protocols, the study also analysed and compared for the first time taxonomic and functional aspects of the fish assemblages in 15 estuaries located along the Atlantic and English Channel coasts. SIMPER analysis showed high similarities in fish assemblages in the three EEC estuaries and during either spring or autumn periods. However, intra-estuary similarities were relatively low, indicating that fish assemblage structures (species richnesses or abundances) were more variable within the estuary (salinity gradient) than between estuaries and/or seasons (spring vs autumn). Although numerous environmental variables were included in the study, only 47% of the variability observed in the fish distribution was explained. Fish spatial variations in the EEC estuaries are mostly driven by abiotic variables as opposed to biological interactions. As indicated by CCA, salinity and muddy sediments were the two most important factors structuring the fish assemblages. The macrobenthos being very abundant in the EEC estuaries (580–1121 ind. m⁻²), the availability of potential prey is probably not a limiting factor in the utilization of estuaries by fish. Contrary to the majority of French estuaries dominated by estuarine species (ES), the fish assemblages of the EEC estuaries are clearly dominated by marine migrant (MM) species (65% on average) with high abundance of juveniles (mostly young-of-the-year). Cluster and SIMPROF's analyses distinguished the functional structure of the 15 estuarine fish assemblages into different clusters. The three EEC estuaries form a similar group with the largest French estuaries (Seine, Loire and Gironde). The latters were characterized by higher MM/ES ratio compared to the other French estuaries. Freshwater flow and tidal range were the main factors associated with this group of estuaries. The results of the present study do not indicate that anthropogenic impacts (chemical contamination) on estuaries affect their ecological functioning as described by the fish ecological guilds. It is suggested that the guild approach may not be useful to provide valuable information on the ecological status of estuaries.  相似文献   

舟山海域特定水道潮流能估算   总被引：2，自引：0，他引：2  

王智峰  周良明  张弓贲  王爱芳 《中国海洋大学学报(自然科学版)》2010,40(8)

对舟山海域高亭水道5个站位、灌门水道8个站位的连续26 h实测潮流资料进行分析。结果表明:高亭水道和灌门水道均为不规则半日潮,各站总的垂向平均流速在1 m/s以上。介绍了国际上常用的估算潮流能的方法:Farm方法和Flux方法,并用这2种方法分别对高亭水道和灌门水道潮流能进行估算。近似得到高亭水道可开发潮流能功率在4.67~5.31 MW之间,灌门水道可开发潮流能功率在7.92~9.37 MW之间。综合2水道可开发潮流能,换算成1 a的发电量,约占舟山市2005年用电总量的6%~7%。  相似文献