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1.
基于多波束测深和高分辨率多道反射地震数据,以及遥感资料研究南海北部东沙海域深水沙丘,总结底形形态分布特征,分析其水动力成因.按照空间分布形态将沙丘分为平行沙丘、波状沙丘和网状沙丘,本文首次发现水下网状沙丘,重点讨论网状沙丘的特征分布和形成机制.网状沙丘发育在东沙环礁东北部水深约352~420 m之间,研究发现其延伸方向共有三个走向：NS、NE-SW和EW,前两个方向为西向传播的内波所致,而最后一个则很有可能是由洋盆西向传播的内波遇到东沙环礁后形成衍射,向北传播的内波引起了向南的近海底底流,冲刷海底底层,剥蚀出足够的沉积物颗粒,从而形成EW走向的水下沙丘.值得一提的是,在多边形网状沙丘的上方,反射地震剖面揭示水层中发育与下方沙丘相当波长的高频震荡内波,并且在遥感资料上也发现相关的衍射和内波干涉区域,而内波干涉很有可能导致高频震荡,从而形成网状沙丘.本研究丰富了水下沙丘形态类型,探讨了新型水下沙丘的形成机制,在陆坡底沙运动及工程应用中具有重要的价值. 相似文献
2.
The velocities with which grains were observed to emerge from a sand bed after an intersaltation collision at u* = 40 cm s?1 are reported for four bed attitudes, from horizontal bed to adverse bed slope 15°. The principal effect of bed angle is to alter the magnitude and direction of the ricochet velocity. However, emergent velocities of dislodged grains are consistent with reptation path lengths comparable to the length of the upwind face of ripples in the corresponding wind. Calculations of the loss of forward momentum at collision, using the data for the range of bed attitudes studied suggest that creep is most vigorous on the sloping upwind face of the ripple and diminishes at the crest. As a result, the crest would be expected to accumulate the coarse material which moves predominantly by creep. The saltations originating in ricochet from the sloping back of the ripple are more vigorous and more concentrated in plan than are those originating at the crest. However, the saltation path length is at least an order of magnitude greater than the ripple wavelength and the probability distribution of path lengths is quite dispersed. Consequently it is very unlikely that these spatial patterns of ricochet are preserved sufficiently distinctly in the saltation cloud and subsequent collision distribution to be the agent of ripple development. This study therefore supports a view of moving grain interaction with the bed in which saltation provides the power to mobilize grains but ripple growth is associated with reptation and particularly with a pattern of impact which develops with the bed relief. Creep is more active on upwind facing slopes than at the crest, which therefore is a zone of net creep grain deposition. 相似文献
3.
Shear wave splitting is a well-known method for indication of orientation, radius, and length of fractures in subsurface layers. In this paper, a three component near offset VSP data acquired from a fractured sandstone reservoir in southern part of Iran was used to analyse shear wave splitting and frequency-dependent anisotropy assessment. Polarization angle obtained by performing rotation on radial and transverse components of VSP data was used to determine the direction of polarization of fast shear wave which corresponds to direction of fractures. It was shown that correct implementation of shear wave splitting analysis can be used for determination of fracture direction. During frequencydependent anisotropy analysis, it was found that the time delays in shearwaves decrease as the frequency increases. It was clearly demonstrated throughout this study that anisotropy may have an inverse relationship with frequency. The analysis presented in this paper complements the studied conducted by other researchers in this field of research. 相似文献
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Spatial backshore processes were investigated through field observations of topography and median sand grain size at a sandy beach facing the Pacific Ocean in Japan. A comparison of the backshore profile and cross‐shore distribution of the median sand grain size in 1999 and 2004 revealed an unusual sedimentary process in which sand was coarsened in a depositional area in the 5‐year period, although sediment is generally coarsened in erosional areas. In support of these observations, monthly spatial field analyses carried out in 2004 demonstrated a remarkable backshore coarsening process triggered by sedimentation in the seaward part of the backshore during a storm event. In order to elucidate mechanisms involved in the backshore coarsening process, thresholds of movable sand grain size under wave and wind actions (a uniform parameter for both these cases) in the onshore and offshore directions were estimated using wave, tide, and wind data. The cross‐shore distributions of the estimated thresholds provided reasonable values and demonstrated a coarsening mechanism involving the intermediate zone around the shoreline under alternating wave and wind actions as a result of which coarse sand was transported toward the seaward part of the backshore by large waves during storms and then toward the landward part by strong onshore winds. The 5‐year backshore coarsening is most certainly explained by repetition of short‐term coarsening mechanisms caused by wave‐induced sand transport occurring from the nearshore to the intermediate zone. Copyright © 2010 John Wiley & Sons, Ltd 相似文献
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R. G. D. Davidson‐Arnott B. O. Bauer I. J. Walker P. A. Hesp J. Ollerhead C. Chapman 《地球表面变化过程与地形》2012,37(11):1227-1241
Wind flow and sand transport intensity were measured on the seaward slope of a vegetated foredune during a 16 h storm using an array of sonic anemometers and Wenglor laser particle counters. The foredune had a compound seaward slope with a wave‐cut scarp about 0.5 m high separating the upper vegetated portion from the lower dune ramp, which was bare of vegetation. Wind direction veered from obliquely offshore at the start of the event to obliquely onshore during the storm peak and finally to directly onshore during the final 2 h as wind speed dropped to below threshold. Sand transport was initially inhibited by a brief period of rain at the start of the event but as the surface dried and wind speed increased sand transport was initiated over the entire seaward slope. Transport intensity was quite variable both temporally and spatially on the upper slope as a result of fluctuating wind speed and direction, but overall magnitudes were similar over the whole length. Ten‐minute average transport intensity correlates strongly with mean wind speed measured at the dune crest, and there is also strong correlation between instantaneous wind speed and transport intensity measured at the same locations when the data are smoothed with a 10 s running mean. Transport on the beach for onshore winds is decoupled from that on the seaward slope above the small scarp when the wind angle is highly oblique, but for wind angles <45° from shore perpendicular some sand is transported onto the lower slope. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
7.
《Advances in water resources》1998,21(3):193-202
In this paper we develop a new hypothesis which relates the formation of sand waves in open flow to the effect of stationary (lee) waves on flow with smooth nonhomogeneous downstream depth or behind an obstacle. The lee wave moves upstream with the phase velocity equal to the absolute value of the flow velocity, hence the wave crest does not move. Results of our experiments show that sand waves on the flow bed appear only below the lee waves and characteristics of sand waves are determined by the wave properties. We investigated the nonlinear stationary waves for subcritical and supercritical Froude numbers. These results allow us to predict sand waves characteristics for a particular flow. 相似文献
8.
Elastic and electromagnetic waves are commonly used to investigate various soil characteristics. The goal of this study is to estimate the elastic moduli and the void ratio based on both the compressional and shear wave velocities, and the electrical resistivity measured by field velocity resistivity probe (FVRP). The compressional and shear waves are measured by piezoelectric disk elements and bender elements installed at the end of the FVRP frame tip. The electrical resistivity is determined by the electrical resistivity probe installed at the tip of the FVRP frame. The FVRP tests are carried out in a clay–sand mixture prepared in a calibration chamber and in silty sand to silty clay soils in the field. The elastic waves and electrical resistivity are measured at every 1 cm. The field tests are carried out at a depth of 6–20 m, at 10 cm intervals, at the Southern coastal area of the Korean peninsula. The measured data are converted into the constraint and shear moduli based on the elastic waves. Void ratios are evaluated based on the elastic wave velocities and the electrical resistivity, and these void ratios match the volumetric void ratio well. This study suggests that the FVRP may effectively determine the elastic moduli and void ratio. 相似文献
9.
剪切波速(VS)与标贯击数(N)之间存在相关关系,受地区土壤条件影响很大。对临沂地区场地实测得到的砂土的剪切波速和标贯击数之间的关系进行了统计分析,得到了砾砂、粗砂、中砂和细砂相应的关系曲线。结果发现受沉积环境的影响,砂土层粒径与埋深呈正相关,砂土粒径越大其密实段样本点数量越多。为消除不同密实程度段之间的相互影响,以密实度为划分标准进一步进行分区段统计分析,得到了不同密实程度的四类砂土的相关关系方程,通过实际钻孔数据对比了分段与不分段的统计分析结果,分段模拟能更好地反映两者之间的相关关系。将砂土根据密实度进行划分再给出剪切波速和标贯击数的回归关系可以提高分析结果的准确性,同时可以考虑土体特性的影响,更为科学。此研究为临沂地区提供了一种简便预估剪切波速的方法,对相关地区的工程建设和科学研究也具有参考价值。 相似文献
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当喷泉效应较弱而双峰结构发展不充分的时候,可能在赤道异常区仅能够观测到一个电子密度的峰值,称之为单峰现象.本文利用CHAllenging Minisatellite Payload卫星在2001-2010年的电子密度数据给出了单峰的发生规律,单峰在地方时早上08：00-10：00和下午16：00-19：00发生率高,发生位置在经度上呈现多波数分布,尤其在10：00-18：00明显：在分季时多呈现四波,而在冬至季时以三波为主.单峰发生多的经度,正好对应着双峰的结构特征较弱之处.究其原因,是非迁移潮的DE2和DE3分量调制了背景风场和大气发电机电场,在电场和喷泉效应减弱的经度,双峰结构难以形成时,就会表现为单峰结构.本文扩展了对单峰现象的地方时、季节和经度分布等规律的了解,明确了非迁移潮在其中施加的影响,由此,单峰同双峰现象一样可以用于研究非迁移潮对热层-电离层的作用. 相似文献
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Concepts derived from previous studies of offshore winds on natural dunes are evaluated on a dune maintained for shore protection during three offshore wind events. The potential for offshore winds to form a lee‐side eddy on the backshore or transfer sediment from the dune and berm crest to the water are evaluated, as are differences in wind speed and sediment transport on the dune crest, berm crest and a pedestrian access gap. The dune is 18–20 m wide near the base and has a crest 4.5 m above backshore elevation. Two sand‐trapping fences facilitate accretion. Data were obtained from wind vanes on the crest and lee of the dune and anemometers and sand traps placed across the dune, on the beach berm crest and in the access gap. Mean wind direction above the dune crest varied from 11 to 3 deg from shore normal. No persistent recirculation eddy occurred on the 12 deg seaward slope. Wind speed on the berm crest was 85–89% of speed at the dune crest, but rates of sediment transport were 2.27 times greater during the strongest winds, indicating that a wide beach overcomes the transport limitation of a dune barrier. Limited transport on the seaward dune ramp indicates that losses to the water are mostly from the backshore, not the dune. The seaward slope gains sand from the landward slope and dune crest. Sand fences causing accretion on the dune ramp during onshore winds lower the seaward slope and reduce the likelihood of detached flows during offshore winds. Transport rates are higher in access gaps than on the dune crest despite lower wind speeds because of flatter slopes and absence of vegetation. Transport rates across dunes and through gaps can be reduced using vegetation and raised walkover structures. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
13.
The on–offshore (cross-shore) transport of sand on beaches is highly time-variable, which has made it difficult to model or predict. In this paper, simple energetics modelling is used to compare velocity moment predictions with field observations of suspended sand transport rates. Separate consideration is given to transport associated with the three main frequency-dependent cross-shore transport processes: that associated with the short (incident) waves, that due to the long (infragravity) waves, and transport associated with the mean flow. Direct comparison between the depth-averaged model predictions, and the in-situ point measurements was facilitated by making the first order assumption that the time-averaged suspension profile is exponential and the wave velocity profile is vertically uniform. An appropriate rippled bed roughness was used to provide the drag coefficient in the energetics model and the vertical length scale of the exponential suspension profile. Despite these simple assumptions, comparison of the velocity moment predictions with the field observations of suspended sand fluxes reveals that this approach has the capacity to predict transport magnitudes due to short wave, long wave, and mean flow components to within about one order of magnitude. However, owing to the limitations of the model, the transport direction of the short wave component could not, on occasion, be correctly determined, probably due to ‘reverse’ transport over ripples. © 1998 John Wiley & Sons, Ltd. 相似文献
14.
This paper concerns new field measurements of wave height and crest elevation probability distributions as measured in the North Sea during a storm in December 2012. The water surface elevation was recorded by Saab WaveRadar REX instruments mounted on eight fixed-jacket platforms in addition to a Datawell Directional Waverider buoy. The storm generated an easterly sea state which peaked well in excess of the 100-year wave height for that direction in the region. Furthermore, 19 freak waves occurred during the storm according to the definition as reported by Haver (2000). The present study demonstrates that the significant steepness and spectral bandwidth during the storm remain almost constant. Consequently, there is little change in the commonly applied design wave height and crest elevation probability distributions throughout the storm. Whilst the bulk of the recorded data was in good agreement with the theoretical distributions, it was demonstrated that when the wind speed was larger than 25 m/s, the measured crest elevation lies above the second-order Forristall distribution. 相似文献
15.
Wang Guangqian 《国际泥沙研究》1994,(3)
ＡＮＡＬＹＳＩＳＯＮＴＨＥＤＥＢＲＩＳＦＬＯＷＳＵＲＧＥＳ￥ＷａｎｇＧｕａｎｇｑｉａｎ（Ｐｒｏｆｅｓｓｏｒ，Ｄｅｐｔ．ｏｆＨｙｄｒａｕｌｉｃＥｎｇｉｎｅｅｒｉｎｇ，ＴｓｉｎｇｈｕａＵｎｉｖｅｒｓｉｔｙ）Ａｂｓｔｒａｃｔ：Ｂａｓｅｄｏｎｔｈｅｆｕｎｄａ... 相似文献
16.
主要研究在役结构的抗震性能评估方法。首先,在前人研究的基础上,给出了在役结构的力学模型;其次,通过对2次小震的动力时程分析,得出结构的最不利方向,并在此方向上利用三维分析程序CANNY 99,对结构进行Pushover分析,将得出的结构能力曲线与由实际地震波得到的需求谱叠合,求出结构性能点,进而评估结构的抗震性能。通过一算例具体说明该方法的应用步骤,并由程序输出结构在到达性能点时的梁柱破坏情况。本文从钢筋锈蚀和混凝土老化这两个方面考虑使用年限对结构的影响;同时,从结构的最不利方向对结构进行Pushover分析,考虑了结构在地震作用下柱子双向受弯的特点,更加符合实际情况。 相似文献
17.
Mathijs van Ledden Zheng-Bing Wang Han Winterwerp Huib de Vriend 《Ocean Dynamics》2004,54(3-4):385-391
The influence of sand and mud transport on the morphological behaviour of a short tidal basin is investigated in this paper. For this purpose, a morphological model is applied in which sand and mud transport are included and the temporal and spatial bed composition variations are taken into account. Initially, the morphological development shows a sand wave near the entrance of the basin and a mud deposition wave more landward. A quasi equilibrium bed level profile is found after a long period (order century) with a sandy bed surface over almost the entire basin and only a small muddy area near the landward end. The dimensionless ratio between the deposition and erosion flux turns out to be a crucial parameter for the understanding of the observed behaviour. Comparison with previous studies on short tidal basins for sand indicates only that the presence of mud in a combined sand mud model does not change the equilibrium bed level profile considerably for the applied parameter settings herein, but drastically decreases the morphological time scale. Comparison between model results and field data of the Wadden Sea suggests that the obtained bed level and bed composition profile are realistic, indicating that the process-based sand mud model is a first step towards a better understanding of sand mud distributions in tidal basins.Responsible Editor: Jens Kappenberg 相似文献
18.
Based upon the long-term observation of field data, a two-dimensional numerical model is applied to simulating the tidal flow covering from the neap tide to spring tide in the radial sandbank area in the southern Yellow Sea. From the development of tidal current ridges under the hydrodynamic action, multi-purpose analysis and study are carried out, which include the propagation process of tidal wave, the distributions of tidal wave energy rate and tidal range, the tidal ellipses and traces. It is shown that the tidal current is the major dynamic factor for the formation and development of the radial sandbanks, and the differences of tidal wave energy rate and current strength determine the distinct plane shapes of ridges and troughs in this region. 相似文献
19.
A.G. Davies 《Continental Shelf Research》1985,4(5):553-580
The results of two field experiments are described, both of which were carried out at Blackpool Sands, Start Bay. In the first experiment in 1978, observations were made of the near-bed flow, and of the movement of coarse sand on the bed, beneath progressive swell waves in shallow water. In the second experiment in 1980, similar observations were made, but for a bed comprising medium to fine sand, and for a more varied range of wave periods. In addition, a number of observations were made of the formation of ripples on an initially flat sand bed. For the naturally rippled beds, critical conditions for the onset of vortex formation and shedding have been established, and reasonable agreement with previous laboratory results has been found. In particular, it has been shown that vortex formation occurs above the lee slopes of ripples only if the near-bed orbital excursion exceeds the ripple wavelength. Prior to certain experimental runs, the area of the seabed in the vicinity of the bottom rig was flattened by divers, and an (equilibrium) ripple pattern was allowed to develop. The wavelengths of the ripples which formed have been found to be in close agreement with the field results of previous workers. To examine in detail some of the properties of separating flow above a rippled bed, an irrotational standing vortex model is presented. 相似文献
20.
Understanding fracture orientations is important for optimal field development of fractured reservoirs because fractures can act as conduits for fluid flow. This is especially true for unconventional reservoirs (e.g., tight gas sands and shale gas). Using walkaround Vertical Seismic Profiling (VSP) technology presents a unique opportunity to identify seismic azimuthal anisotropy for use in mapping potential fracture zones and their orientation around a borehole. Saudi Aramco recently completed the acquisition, processing and analysis of a walkaround VSP survey through an unconventional tight gas sand reservoir to help characterize fractures. In this paper, we present the results of the seismic azimuthal anisotropy analysis using seismic traveltime, shear‐wave splitting and amplitude attenuation. The azimuthal anisotropy results are compared to the fracture orientations derived from dipole sonic and image logs. The image log interpretation suggests that an orthorhombic fracture system is present. VSP data show that the P‐wave traveltime anisotropy direction is NE to SW. This is consistent with the cemented fractures from the image log interpretation. The seismic amplitude attenuation anisotropy direction is NW to SE. This is consistent with one of the two orientations obtained using transverse to radial amplitude ratio analysis, with the dipole sonic and with open fracture directions interpreted from image log data. 相似文献