Sheet flow hydrodynamics over a non-uniform sand bed channel   总被引：1，自引：0，他引：1  

Anurag Sharma  Bimlesh Kumar 《国际泥沙研究》2018,33(3):313-326

The current study experimentally investigates the flow characteristics and temporal variations in the sheet flow profile of a non-uniform sand bed channel. Experiments were done to explore turbulent structures in the presence of a sheet flow layer with and without seepage. The turbulent events, such as stream wise velocity, Reynolds shear stresses, and turbulence intensities were found to be increasing and vertical velocity was found decreasing with a sheet layer. The presence of a sheet layer also effects the turbulent energy production and energy dissipation. All the turbulence parameters with and without a sheet layer have also been influenced by the presence of downward seepage. The rate of sheet flow movement is increased with seepage, owing to increased turbulence with seepage. The current study used wavelet analysis on temporally lagged spatial bed elevation profiles obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuation at different spatial scales. A spatial cross correlation analysis at multiple scales, based on the wavelet coefficients, has been done on these bed elevation datasets to observe the effect of downward seepage on the dynamic behavior of sheet flow at different length scales. It is found that seepage increases average bed celerity and also increases the celerity of sheet flow of similar length scales. This increase in the celerity has been hypothesized as the increase of sheet flow movement as well as the increase in turbulent parameters with seepage, which destabilizes the bed particles resulting in a disruption in the continuous propagation pattern of the sheet flow. The increase of sheet flow celerity with seepage is confirmed from the saturation level of the wavelet power spectra of the bed elevation series. The presence of seepage also affects the non-uniformity of collective sheet material.  相似文献   

末次间冰期以来门捷列夫洋脊南部冰盖对细颗粒沉积的控制作用   总被引：1，自引：1，他引：0  

叶黎明  于晓果  章伟艳  王嵘 《海洋学报(英文版)》2020,39(9):86-95

门捷列夫洋脊南部的粘土矿物沉积具有明确的物源,为追踪该区沉积环境的演变提供了良好的条件。末次间冰期以来,ARC7-E23孔中的粘土矿物记录表现出了非常显著的变化。结合沉积物粒度的端元组份和冰阀碎屑沉积,粘土矿物的变化模式表明,东西伯利亚冰盖(ESIS)的规模可能是控制细颗粒沉积的主要因素。在氧同位素2期(MIS2)和4期(MIS4),门捷列夫洋脊南部可能被ESIS所覆盖,几乎阻挡了所有来自加拿大和拉夫贴夫海陆架的沉积物,但允许大量来自东西伯利亚海陆架的细粒沉积物输入。只有当ESIS消融后,波弗特环流和越极流的相对强度以及搬运作用才成为了控制远源沉积物输入的主要因素。MIS3期的气候条件似乎最适合远源沉积物的输入,不仅提高了表层环流的流通性,也提供了足够多的搬运介质。  相似文献   

Sea-floor scour at the mouth of Hudson Strait by deep-keeled icebergs from the Laurentide Ice Sheet     

J.M. Metz  J.A. Dowdeswell  C.M.T. Woodworth-Lynas 《Marine Geology》2008,253(3-4):149-159

Physical evidence on the dimensions of icebergs released from the mouth of Hudson Strait into the northwest Atlantic during the last Heinrich event (H-0) is presented. Side-scan sonar imagery shows scour marks up to 700 m wide and longer than 28 km. These scour marks were carved by gigantic icebergs (megabergs) with keel drafts possibly as great as 660 m capable of scouring trenches 20 to 25 m deep into the seabed. These icebergs were likely calved from the grounding margin of a thick (possibly as thick as 640 m) rapidly-flowing glacial margin during the H-0 Heinrich event (11 ka BP). Along with the relatively few megabergs released were large numbers of smaller icebergs that calved from the ice margin at the same time and were also produced from break-up of the megabergs. Scouring of the seabed by the large and smaller icebergs happened at the same time, with megabergs scouring only in the deep waters of Hatton Basin, and the smaller bergs scouring only on the eastern, shallow margin of the Basin at the continental shelf break.  相似文献   

Characteristics of turbulent boundary layers over a rough bed under saw-tooth waves and its application to sediment transport   总被引：1，自引：0，他引：1  

Suntoyo  Hitoshi Tanaka  Ahmad Sana   《Coastal Engineering》2008,55(12):1102-1112

A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, φ and the acceleration coefficient, a_c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k–ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by Watanabe and Sato [Watanabe, A. and Sato, S., 2004. A sheet-flow transport rate formula for asymmetric, forward-leaning waves and currents. Proc. of 29th ICCE, ASCE, pp. 1703–1714.]. Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found.  相似文献   

Formalising an inversion methodology for reconstructing ice‐sheet retreat patterns from meltwater channels: application to the British Ice Sheet     

Sarah L. Greenwood  Chris D. Clark  Anna L. C. Hughes 《第四纪科学杂志》2007,22(6):637-645

  相似文献   

Preliminary 10Be chronology for the last deglaciation of the western margin of the Greenland Ice Sheet     

Vincent Rinterknecht  Yuri Gorokhovich  Joerg Schaefer  Marc Caffee 《第四纪科学杂志》2009,24(3):270-278

The now acknowledged thinning of the Greenland Ice Sheet raises concerns about its potential contribution to future sea level rise. In order to appreciate the full extent of its contribution to sea level rise, reconstruction of the ice sheet's most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment. We measured ¹⁰Be concentrations in 12 samples collected along longitudinal and altitudinal transects from Sisimiut to within 10 km of the Isunguata Sermia Glacier ice margin on the western coast of Greenland. Along the longitudinal transect, we collected three perched boulders and two bedrocks. In addition, we sampled seven perched boulders along a vertical transect in a valley within 10 km of the Isunguata Sermia Glacier ice margin. Our pilot dataset constrains the height of the ice sheet during the Last Glacial Maximum (LGM) between 500 m and 840 m (including the 120 m relative sea level depression at the time of the LGM, 21 ka BP). From the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12.3 ± 1.5 ¹⁰Be ka (n = 2) and 8.3 ± 1.2 ¹⁰Be ka (n = 3) to be ～6 cm a^?1 over this time period. Direct dating of the retreat of the western margin of the Greenland Ice Sheet has the potential to better constrain the retreat rate of the ice margin, the thickness of the former ice sheet as well as its response to climate change. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Coastal lake records add constraints to the age and magnitude of the Younger Dryas ice‐front oscillation along the Skagerrak coastline in southern Norway     

Anders Romundset  Thomas R. Lakeman  Fredrik Hgaas 《第四纪科学杂志》2019,34(2):112-124

  相似文献   

Deglacial meltwater drainage and glaciodynamics: inferences from Laurentide eskers, Canada     

Dr. Tracy A. Brennand   《Geomorphology》2000,32(3-4)

This paper evaluates current knowledge of Laurentide eskers in Canada in the light of developments in glacier hydrology and glacial sedimentology. Questions regarding the morpho-sedimentary relations of eskers, the synchroneity and operation of R-channel systems, the role of supraglacial meltwater input and proglacial water bodies, the controls on esker pattern, and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed. A morphologic classification of eskers is proposed. Five types of eskers are identified and investigated. Type I eskers likely formed in extensive, synchronous, dendritic R-channel networks under regionally stagnant ice that terminated in standing water. Type II eskers likely formed in short, subaqueously terminating R-channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation. Type III eskers plausibly formed in short R-channels that drained either to interior lakes in, or tunnel channels under, regionally stagnant ice. Type IV eskers may have formed as time-transgressive segments in short, subaerially terminating R-channels (or reentrants) that developed close to the ice margin as the ice front underwent stagnation-zone retreat or downwasted and backwasted regionally (stagnant ice); however, formation in synchronous R-channels cannot be discounted on the basis of reported observations. Type V eskers may have formed in H-channels that terminated subaerially. The spatial distribution of these esker types is discussed. The factors that determined Laurentide R-channel pattern and operation were likely a complex combination of (i) supraglacial meltwater discharge, (ii) the number and location of sink holes, (iii) the ice surface slope, thickness and velocity, and (iv) the permeability, topography and rigidity of the bed. These factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle.  相似文献   

The influence of ice marginal setting on early Holocene retreat rates in central West Greenland          下载免费PDF全文

SAMUEL E. KELLEY  JASON P. BRINER  SUSAN R. H. ZIMMERMAN 《第四纪科学杂志》2015,30(3):271-280

  相似文献   

A Lateglacial 10Be production rate from glacial lake shorelines in Scotland          下载免费PDF全文

DAVID SMALL  DEREK FABEL 《第四纪科学杂志》2015,30(6):509-513

  相似文献