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1.
Flow transverse bedforms (ripples and dunes) are ubiquitous in rivers and coastal seas. Local hydrodynamics and transport conditions depend on the size and geometry of these bedforms, as they constitute roughness elements at the bed. Bedform influence on flow energy must be considered for the understanding of flow dynamics, and in the development and application of numerical models. Common estimations or predictors of form roughness (friction factors) are based mostly on data of steep bedforms (with angle-of-repose lee slopes), and described by highly simplified bedform dimensions (heights and lengths). However, natural bedforms often are not steep, and differ in form and hydraulic effect relative to idealised bedforms. Based on systematic numerical model experiments, this study shows how the hydraulic effect of bedforms depends on the flow structure behind bedforms, which is determined by the bedform lee side angle, aspect ratio and relative height. Simulations reveal that flow separation behind bedform crests and, thus, a hydraulic effect is induced at lee side angles steeper than 11 to 18° depending on relative height, and that a fully developed flow separation zone exists only over bedforms with a lee side angle steeper than 24°. Furthermore, the hydraulic effect of bedforms with varying lee side angle is evaluated and a reduction function to common friction factors is proposed. A function is also developed for the Nikuradse roughness (k s), and a new equation is proposed which directly relates k s to bedform relative height, aspect ratio and lee side angle. 相似文献
2.
Quantification of dune dynamics during a tidal cycle in an inlet channel of the Danish Wadden Sea 总被引：2，自引：3，他引：2
Verner B. Ernstsen Riko Noormets Christian Winter Dierk Hebbeln Alex Bartholomä Burg W. Flemming J. Bartholdy 《Geo-Marine Letters》2006,26(3):151-163
High-resolution swath bathymetry measurements at centimetre-scale precision conducted during a tidal cycle in the Grådyb tidal inlet channel in the Danish Wadden Sea reveal the short-term dynamics of a large, ebb-directed compound dune with superimposed small to medium dunes, all composed of medium sand. Dune dynamics were related to simultaneous measurements of flow using an acoustic Doppler current profiler. Spatially, dune crests displayed greater mobility than did dune troughs, due to higher flow velocities at the crests than in the troughs. Temporally, superimposed lower lee-side dunes migrated more during the flood than the ebb tide, due to higher near-bed trough flow velocities during the flood phase, resulting in varying exposure to flow. Net dune migration was flood-directed over the tidal cycle, despite annual net migration being ebb-directed. Hence, extrapolation of short-term migration rates is not possible in this case. The superimposed dunes reversed direction during each half tidal cycle whereas the compound dune only developed a flood cap during flood tide, i.e. the time required for complete reversal of the compound dune was much longer than that available in a half tidal cycle. Over the tidal cycle, the bed level was stable but significant erosion and accretion occurred during the tidal phases. During the ebb tide, bed material was brought into suspension with accelerating flow and settled with decelerating flow, resulting in an average erosion and accretion of the bed of ~7 cm in each case. During the flood tide, the bed of the compound dune was overall stable, although bed material was eroded from the exposed lower lee side, being partly transported to the crest in bedload and partly brought into suspension. In general, dune height fluctuated during the tidal cycle whereas dune length remained stable. The height of the compound dune responded to changes in water depth, which acts as a limiting factor to dune growth. By contrast, the height of the stoss-side dunes responded to flow velocity, i.e. the stoss-side dunes were water depth-independent. 相似文献
3.
Liu Zhenxi S. Berne Wang Kuiyang T. Marsset Xia Dongxing Tang Yuxiang J. F. Bourillet 《海洋学报(英文版)》1998,17(2):209-231
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4.
A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m3s−1). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging.Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions.The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations.A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume. 相似文献
5.
Self-burial processes of finite-length cylinders under oscillatory flows and waves plus currents were examined with the following two different experimental facilities: a large oscillating water-sediment tunnel (LOWST) and a large wave-current tank. More than 130 experiments, with different model cylinders, were conducted within both facilities. The burial mechanisms studied include burial due to local scour and bedform migration. Burial due to fluidization in the tunnel was also explored, but only in a qualitative way. In the case of experiments with LOWST, the equilibrium burial depth was found to be a power function of the shields parameter (thetas). In the wave-current tank, the equilibrium burial depth was also found to be a function of the Shields parameter, albeit with larger scatter. The experimental observations made in both facilities have similar trends but different magnitudes. For equivalent values of the Shields parameter, smaller equilibrium burial depths were observed in the wave flume when compared to the ones in LOWST. After burial induced by local scour takes place, bedform (ripples and sandwaves) formation and evolution play a strong and, in some cases, dominant role on the equilibrium burial depth of the cylinders. Depending on how the vertical dimensions of bedforms compare to the specimen's diameter, cyclical covering and uncovering of the object may take place due to the passage of the migrating sandwaves. In such case, burial depth Bd no longer coincides with the vertical displacement (Vd) of the object as in the case when the burial process is dominated by local scour. 相似文献
6.
Abstract Sediment transport at the Tauranga Entrance was studied in relation to tidal currents and waves. Bedforms resulting from tidal flow were investigated with scuba divers and echo‐soundings. The alignment and scale of bedforms indicated the direction and approximate rate of sediment transport. Sediment transport was measured directly using sediment traps, and results were compared with rates calculated by another method. Maximum sediment transport rates of 20 000–30 000 g.m?1 per half tidal cycle occur near the inlet gorge, but rates vary considerably in time and space, depending mainly upon power of tidal currents. A model of sediment transport for this inlet has been evolved based on tidal flow streamlines, bedform features, and the measured and calculated rates of sediment transport. 相似文献
7.
The evolution of energy, energy flux and modal structure of the internal tides(ITs) in the northeastern South China Sea is examined using the measurements at two moorings along a cross-slope section from the deep continental slope to the shallow continental shelf. The energy of both diurnal and semidiurnal ITs clearly shows a～14-day spring-neap cycle, but their phases lag that of barotropic tides, indicating that ITs are not generated on the continental slope. Observations of internal tidal energy flux suggest that they may be generated at the Luzon Strait and propagate west-northwest to the continental slope in the northwestern SCS. Because the continental slope is critical-supercritical with respect to diurnal ITs, about 4.6 kJ/m~2 of the incident energy and 8.7 kW/m of energy flux of diurnal ITs are reduced from the continental slope to the continental shelf. In contrast, the semidiurnal internal tides enter the shelf because of the sub-critical topography with respect to semidiurnal ITs.From the continental slope to the shelf, the vertical structure of diurnal ITs shows significant variation, with dominant Mode 1 on the deep slope and dominant higher modes on the shelf. On the contrary, the vertical structure of the semidiurnal ITs is stable, with dominant Mode 1. 相似文献
8.
Traykovski P. Richardson M.D. Mayer L.A. Irish J.D. 《Oceanic Engineering, IEEE Journal of》2007,32(1):150-166
Several experiments to measure postimpact burial of seafloor mines by scour and fill have been conducted near the Woods Hole Oceanographic Institution's Martha's Vineyard Coastal Observatory (MVCO, Edgartown, MA). The sedimentary environment at MVCO consists of a series of rippled scour depressions (RSDs), which are large scale bedforms with alternating areas of coarse and fine sand. This allows simultaneous mine burial experiments in both coarse and fine sand under almost identical hydrodynamic forcing conditions. Two preliminary sets of mine scour burial experiments were conducted during winters 2001-2002 in fine sand and 2002-2003 in coarse sand with a single optically instrumented mine in the field of view of a rotary sidescan sonar. From October 2003 to April of 2004, ten instrumented mines were deployed along with several sonar systems to image mine behavior and to characterize bedform and oceanographic processes. In fine sand, the sonar imagery of the mines revealed that large scour pits form around the mines during energetic wave events. Mines fell into their own scour pits, aligned with the dominant wave crests and became level with the ambient seafloor after several energetic wave events. In quiescent periods, after the energetic wave events, the scour pits episodically infilled with mud. After several scour and infilling events, the scour pits were completely filled and a layer of fine sand covered both the mines and the scour pits, leaving no visible evidence of the mines. In the coarse sand, mines were observed to bury until the exposed height above the ripple crests was approximately the same as the large wave orbital ripple height (wavelengths of 50-125 cm and heights of 10-20 cm). A hypothesis for the physical mechanism responsible for this partial burial in the presence of large bedforms is that the mines bury until they present roughly the same hydrodynamic roughness as the orbital-scale bedforms present in coarse sand. 相似文献
9.
10.
Small-scale sediment transport patterns and bedform morphodynamics: new insights from high-resolution multibeam bathymetry 总被引：2，自引：1，他引：1
New multibeam echosounder and processing technologies yield sub-meter-scale bathymetric resolution, revealing striking details
of bedform morphology that are shaped by complex boundary-layer flow dynamics at a range of spatial and temporal scales. An
inertially aided post processed kinematic (IAPPK) technique generates a smoothed best estimate trajectory (SBET) solution
to tie the vessel motion-related effects of each sounding directly to the ellipsoid, significantly reducing artifacts commonly
found in multibeam data, increasing point density, and sharpening seafloor features. The new technique was applied to a large
bedform field in 20–30 m water depths in central San Francisco Bay, California (USA), revealing bedforms that suggest boundary-layer
flow deflection by the crests where 12-m-wavelength, 0.2-m-amplitude bedforms are superimposed on 60-m-wavelength, 1-m-amplitude
bedforms, with crests that often were strongly oblique (approaching 90°) to the larger features on the lee side, and near-parallel
on the stoss side. During one survey in April 2008, superimposed bedform crests were continuous between the crests of the
larger features, indicating that flow detachment in the lee of the larger bedforms is not always a dominant process. Assessment
of bedform crest peakedness, asymmetry, and small-scale bedform evolution between surveys indicates the impact of different
flow regimes on the entire bedform field. This paper presents unique fine-scale imagery of compound and superimposed bedforms,
which is used to (1) assess the physical forcing and evolution of a bedform field in San Francisco Bay, and (2) in conjunction
with numerical modeling, gain a better fundamental understanding of boundary-layer flow dynamics that result in the observed
superimposed bedform orientation. 相似文献
11.
12.
The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat,Jiangsu,in 2009,and in the Jiulong Estuary,Xiamen,in 2010,respectively.The "multi-threshold value" method was utilized to reveal the morphological undulations along which bedforms were present.Analyses of the datasets obtained show that:(1) sand ripples can have irregular shapes,and(2) changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles.Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness.As current speed increases,seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures.Furthermore,the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures. 相似文献
13.
14.
Arnold H. Bouma Monty A. Hampton Robert C. Orlando 《Marine Georesources & Geotechnology》2013,31(1-4):291-308
Abstract Lower Cook Inlet in Alaska has high‐ tidal currents that average 3–4 knots and normally reach a peak of 6–8 knots. The bottom has an average depth of about 60–70 m in the central part of the inlet that deepens toward the south. Several types of bedforms, such as sand waves, dunes, ripples, sand ribbons, and lag deposits form a microtopography on the otherwise smooth seafloor. Each bedform type covers a small field, normally a few hundred to a few thousand meters wide, and usually several kilometers long parallel to the tidal flow. High‐resolution seismic systems, side‐scan sonar and bottom television were used to study these bedforms. Large sand waves with wavelengths over 300 m and wave heights up to 10 m were observed. Fields of ebb‐oriented or flood‐oriented asymmetric bedforms commonly grade into more symmetric shapes. Several orders of smaller sand waves and dunes cover the flanks of the very large bedforms. The crest directions of both size groups are normally parallel, but deviations of up to 90° have been observed; local deviations may occur where smaller forms approach the crests of the larger sand waves. Bottom television observations demonstrated active bedload transport in a northerly direction on crests and midflanks of southward asymmetric large sand waves, but not in their troughs. Movement of bedload occurs in the form of small ripples. Although the asymmetry of the large bedforms suggests that migration has taken place in the ebb or flood directions, the very low surface angles (2.5°‐8°) of these bedforms do not indicate regular movements. The large bedforms are probably relict features, or they migrate only under severe conditions, whereas active sand transport by ripples and smaller sand waves and dunes moves bedload back and forth with the tides. An understanding of such movements is essential for determining design criteria for offshore installations and in benthic‐faunal studies. 相似文献
15.
Shiho Kobayashi Eisuke Hashimoto Masayuki Nagao Yoshio Takasugi 《Journal of Oceanography》2009,65(5):657-664
We observed tidal currents, turbulent energy dissipation and water column stratification at the entrance of a narrow strait
(Neko Seto) in the Seto Inland Sea, Japan, using a free-falling turbulent microstructure profiler (TurboMAP) and acoustic
Doppler current profiler (ADCP). The variation in turbulent energy dissipation at the entrance of the strait was not at quarter-diurnal
frequency but at semi-diurnal frequency; turbulent energy dissipation was enhanced during the ebb tide, although it was moderate
during the flood tide. This result is consistent with the results of Takasugi (1993), which showed the asymmetry of tidal
energy loss during a semidiurnal tidal cycle using control volume analysis. It is suggested that significant turbulent energy
dissipation is generated in the strait, which influences the properties of water outside the strait when tidal currents flow
out from the strait. 相似文献
16.
横向流 (垂直于海湾主轴方向的流动) 对横向动量以及物质分布具有重要影响。已往研究表明,潮汐应变对横向流的垂向空间结构具有重要的调控作用。但这种认识仅局限于强层化海区,弱层化条件下潮汐应变对于横向流空间结构的影响仍未可知。为此,本文以象山港为例,基于实测数据阐释了弱层化条件下潮汐应变对横向流及其余环流垂向空间结构的调控作用。结果显示,象山港内湾横向流的垂向空间结构随大小潮呈现出明显的变化规律。大潮时,潮汐应变现象明显,涨潮时较强的垂向混合使得横向流在高潮阶段呈现出一层结构;落潮时垂向混合较弱,横向流在低潮时呈现两层结构。小潮时,潮汐应变受到抑制,垂向混合在涨落潮时均较弱,因此横向流在高低潮阶段均呈现出两层结构。经过潮时均进一步得到的横向余环流呈现出上层向南、下层向北的两层结构。由于潮汐应变的大小潮变化,横向余环流的反转深度反转点自大潮到小潮呈现出上升的趋势。 相似文献
17.
David E. Cartwright 《Progress in Oceanography》1997,40(1-4)
The spring-neap cycle of global energy dissipation by ocean tides is calculated with a view to trying to explain an observed anomaly in the tidal fluctuations in the length of day. Calculations are performed in three ways: dissipation by friction linearly proportional to semidiurnal tide velocity, by quadratic friction, and by the torques of the lunar and solar tidal forces on the solar and lunar ocean tides, respectively. All methods give comparable results equivalent to an amplitude of about 0.1 μs change in the length of day with a small phase lag. These are inadequate to explain the observed anomaly of about 3 μs and 0.1 rad phase lag. Further investigations, to determine the generation of a non-equilibrium global MSf wave of equatorial amplitude 0.9 mm by nonlinear interactions in shallow seas using global tide models and observations, are suggested. 相似文献
18.
为减轻海上风电单桩基础周围局部冲刷对其结构安全的影响,提出了一种新型旋转减冲装置。在波流水槽中开展物理模型试验,改变波流条件、装置安装高度、安装距离,记录桩周冲刷发展历时,运用激光地形仪扫描冲刷坑形态,分析各工况下冲刷坑形态差异,验证装置不同安装距离、安装高度下的冲刷防护效果,提出了不同安装位置下的防护效率公式。结果表明：新型旋转减冲装置具有较好的冲刷防护效果,本试验工况下,桩周最大冲刷深度可减小44%左右。装置安装距离对冲刷防护效果影响较小,波流作用下的冲刷防护效果受装置安装高度影响显著,冲刷防护效果随装置安装高度的增加而减弱。 相似文献
19.
应用MIKE数值模拟软件,采用无结构三角形网格,建立一套计算区域包括整个渤海、黄海、东海以及东海大陆架和琉球群岛的高分辨率数值模型,考虑了实际水深和岸线,外海开边界采用西北太平洋大模型结果的潮位提供,模拟了东中国海潮波的波动过程,对潮波垂直运动过程进行调和分析,得到了渤海、黄海、东海的M2,S2,K1,O1以及N2,K2,P1,Q1八个主要分潮的传播和分布特征。利用中国沿海14个潮位站的调和常数对模型结果进行了验证,验证结果显示模型较为准确可靠。研究结果表明:4个主要半日潮(全日潮)在渤、黄、东海的传播情形基本相似,即潮波在渤海、黄海、东海沿岸的传播性质上类似沿岸开尔文波的传播形态,并且成功再现了计算海域的4个半日分潮无潮点和2个全日分潮无潮点。全日潮振幅各无潮点附近振幅最小,而海湾的波腹区振幅最大,东海潮差呈现近岸方向振幅大、离岸方向振幅小,浙闽沿海振幅也较大,黄海振幅相对较小,渤海振幅在辽东湾和渤海湾顶最大,两个无潮点周边振幅较小。 相似文献
20.
Boundary roughness and bedforms in the surf zone 总被引：1，自引：0，他引：1
Hydrodynamical models of the nearshore system frequently assume that a single friction coefficient is sufficient to represent flow conditions at a point in the surf zone. Furthermore, models attempting to relate bed configuration to surf zone flows have relied primarily upon the wave orbital velocity as an indicator of potential bedforms, and thus as the control on boundary roughness. The data presented here point out potential errors arising from either of these approaches. The results of a field experiment conducted at Wendake Beach, Ontario, show that at a single location in an active surf zone, the Darcy-Weisbach friction coefficient, f, varied by approximately 250% (in this case between 0.016 and 0.041).
It is also shown that existing bedform models, based upon primary wave motions alone, do not accurately predict conditions at this study site. For a relatively constant wave orbital velocity and velocity asymmetry, it is found that changes in bed roughness, as a result of bedform development, are reflected mainly in the vertical profile of the long-shore current velocity. A sequence of bedforms, from oscillatory ripples through flat bed, is inferred from the data, and found to be supported by diver observations and preserved primary sedimentary structures. 相似文献