海岸沙坝剖面和滩肩剖面特征研究

通过物理模型实验研究了海岸沙坝剖面和滩肩剖面的形成和演化过程,给出了稳定的沙坝剖面和滩肩剖面的几何特征。实验中考虑了两种初始坡度(1∶20和1∶10)和不同波高的规则波和不规则波,讨论了不同初始坡度海岸上破碎波空间分布特征。结果表明,沙坝产生后存在向岸和离岸两种运动形态,但最终将停留在稳定位置。稳定的沙坝剖面对应不同初始坡度和波浪存在不同的大沙坝和小沙坝分布。沙坝剖面由长时间小波高波浪序列作用后可转化为稳定滩肩剖面,该剖面不依赖于波浪和初始坡度。实验也给出了平衡剖面与理论曲线的对比以及剖面上泥沙粒径的分布。     

Preliminary analysis of the applicability of adiabatic modes to inverting synthetic acoustic data in shallow water over a sloping sea floor

The May 2001 Geoacoustic Inversion Techniques Workshop provided synthetic transmission loss (TL) data for four cases with range-dependent shallow-water all-liquid environments. In two of these cases ("0" and "1"), the sea floor has constant slope and the geoacoustic model (GAM) is range independent. Cost functions have been computed using a new adiabatic-mode TL algorithm (which uses an exact velocity boundary condition at the sloping sea floor), as one parameter in the GAM is varied. Two frequencies (80 and 220 Hz) were selected. In case 0, the sea-floor slope is 0.0183 and the GAM comprises an inhomogeneous layer over a basement. The sea-floor sound-speed was selected as the variable parameter. The resulting cost minima at 80 and 220 Hz are displaced from the actual sound speed by 2.3 and 3.4 m/s, respectively. In case 1, the sea-floor slope is 0.012 and the GAM comprises one homogenous layer, five inhomogeneous layers, and a basement. The selected parameter was the sound-speed in the homogeneous layer. The corresponding cost minima are displaced by -1.2 and +1.1 m/s. The relative values of these four errors indicate that mode coupling increases with sea-floor slope and that there may be a dependence on frequency at the greater slope.     

A Numerical Model for Edge Waves on A Compound Slope

An edge wave is a kind of surface gravity wave basically travelling along a shoaling beach. Based on the periodic assumption in the longshore direction, a second order ordinary differential equation is obtained for numerical simulation of the cross-shore surface elevation. Given parameters at the shoreline, a cross-shore elevation profile is obtained through integration with fourth-order Runge-Kutta technique. For a compound slope, a longshore wavenumber is obtained by following a geometrical approach and solving a transcendental equation with an asymptotic method. Numerical results on uniform and compound sloping beaches with different wave periods, slope angles, modes and turning point positions are presented. Some special scenarios, which cannot be predicted by analytical models are also discussed.     

The Levant Slumps and the Phoenician Structures: collapse features along the continental margin of the southeastern Mediterranean Sea

Two distinct series of slumps deform the upper part of the sedimentary sequence along the continental margin of the Levant. One series is found along the base of the continental slope, where it overlies the disrupted eastern edge of the Messinian evaporites. The second series of slumps transects the continental margin from the shelf break to the Levant Basin. It seemed that the two series were triggered by two unrelated, though contemporaneous, processes. The shore-parallel slumps were initiated by basinwards flow of the Messinian salt, that carried along the overlying Plio-Quaternary sediments. Seawater that percolated along the detachment faults dissolved the underlying salt to form distinctly disrupted structures. The slope-normal slumps are located on top of large canyons that cut into the pre-Messinian sedimentary rocks. A layer of salt is found in the canyons, and the Plio-Quaternary sediments were deposited on that layer. The slumps are bounded by large, NW-trending faults where post-Messinian faulted offset was measured. We presume that the flow of the salt in the canyons also drives the slope-normal slumps. Thus thin-skinned halokynetic processes generated the composite post-Tortonian structural patterns of the Levant margin. The Phoenician Structures are a prime example of the collapse of a distal continental margin due to the dissolution of a massive salt layer.     

Predictions on Irregular Wave run-up and Overtopping

A series of hydraulic model tests are carried out to investigate random wave run-up and overtopping on smooth, impermeable single slope and composite slope. Based on analysis of the influences of wave steepness, structure slope, incident wave angle, width of the berm and water depth on the berm and the wave run-up, empirical formulas for wave run-up on dike are proposed. Moreover, empirical formula on estimating the wave run-up on composite slope with multiple berms is presented for practical application of complex dike cross-section. The present study shows that the influence factors for wave overtopping are almost the same as those for wave run-up and the trend of the wave overtopping variation with main influence parameters is also similar to that for wave run-up. The trend of the wave overtopping discharge variations can be well described by two main factors, i.e. the wave run-up and the crest freeboard of the structure. A new prediction method for wave overtopping discharge is proposed for random waves. The proposed prediction formulas are applied to case study of over forty cases and the results show that the prediction methods are good enough for practical design purposes.     

An experimental investigation of plunging breaker boundary layers in the transformation zone

Experimental investigation is made on the boundary layers of the transformation zone (i.e. the region between the last symmetrical wave profile depth and the breaking point) of plunging breakers propagating on a smooth beach with 1/12 uniform slope. Using a laser anemometer, the particle velocities are measured at four verticals along the transformation zone for three different steepnesses of waves within the plunging breaker range. The boundary layer flow in the transformation zone is found mostly of turbulent character and vertical distribution of particle velocities does not seem to conform to the classical law of the wall distribution given for steady-flow boundary layers. The results show that free-stream particle velocities, in the boundary layer of the breaker under the crest phase, increase considerably as the wave progresses towards the breaking point. The boundary layer thickness, defined as the velocity-affected region, remains constant throughout the transformation zone but it decreases with increasing deep-water wave steepness for the particular beach slope tested.     

Equilibrium beach profiles under breaking and non-breaking waves

Simple theoretical models to determine the equilibrium profile shape under breaking and non-breaking waves are presented. For the case of breaking waves, it is assumed that the seaward transport in the undertow is locally balanced by a net vertical sedimentation, so that no bottom changes occur at equilibrium. The parameterization of the water and sediment flux in the surf zone yields a power curve for the equilibrium profile with a power of 2/3, which is in agreement with previous field investigations on surf zone profile shapes. Three different models were developed to derive the profile shape under non-breaking waves, namely (1) a variational formulation where the wave energy dissipation in the bottom boundary layer is minimized over the part of the profile affected by non-breaking waves, (2) an integration of a small-scale sediment transport formula over a wave period where the slope conditions that yield zero net transport determine equilibrium, and (3) a conceptual formulation of mechanisms for onshore and offshore sediment transport where a balance between the mechanisms defines equilibrium conditions. All three models produced equilibrium profile shapes of power-type with the power typically in the range 0.15–0.30. Comparison with field data supported the results obtained indicating different powers for the equilibrium profile shape under breaking and non-breaking waves.     

Dynamics of Rossby waves in an ocean with inhomogeneous currents

A model for a two-layer ocean is applied to consider, in terms of the geometrical optics approximation, the effect of mean flows propagating within the upper layer upon the dynamics of Rossby waves. The case is theoretically analysed, with the depth of the ocean's upper layer much smaller than that of the underlying layer. In this case, the flow's impact upon the baroclinic mode of Rossby waves is ubiquitous, with the exception of synchronicity. Depending on the parameters, four types of wave packets' behaviour in the vicinity of synchronicity points are singled out, namely, the elimination of the peculiarity, shadowing, and convective/absolute instability. For the mean flow profile simulating cyclonic and anticyclonic gyres, we have obtained wave packet trajectories and have studied the wave packet's interaction with the current. Specifically, it has been demonstrated that, given some modulus of the wave packet, vigorous energy exchange between the wave vector and the flow takes place. Translated by Vladimir A. Puchkin.     

The importance of bioturbation to continental slope sediment structure and benthic processes off Cape Hatteras, North Carolina

Even though the continental slope off Cape Hatteras has sediment accumulation rates on the order of 1 cm/year, large areas of soft sediment are intensively reworked by infaunal organisms. Primary sedimentary structures have been completely replaced with biogenic structures. Surface sedimentary structures are dominated by the bioturbational activities of a deep burrowing infauna (to at least 30 cm). The layer actively mixed by the benthos, as evidenced by sediment profile and X-ray images, is estimated to range from 5 to 20 cm, with the residence time for particles within the surface mixed layer ranging from about 4.5 to 18 years. The biological mixing parameter (G) ranges from 0.4 to 5.5, which indicates moderate to strong biological mixing relative to accumulation and strata formation. Bioturbation contributes to the dynamic forces affecting the surface sediments by decreasing compaction of sediment layers and dilating sediment fabrics by sediment mixing, and introducing large water-filled burrows and voids to subsurface sediments. The sediment profile images captured numerous subsurface feeding voids, and worms in the process of making deep burrows, many of which extended below the 5 to 10 cm average depth of the apparent color redoxpotential discontinuity layer. High rates of accumulation of organic-rich sediment lead to high standing stocks of benthos and intensive feeding/burrowing activity that result in organic rich stratagraphic sequences that are thoroughly mixed. Cape Hatteras is a apparent focusing point for the transport of shallow water sediments to the deep sea. Sediments across other areas of the continental slope just 100's of kilometers south of Cape Hatteras are not as thoroughly mixed or biologically active.     

A critical test of the concept of submarine equilibrium profile

The existence of a slope equilibrium profile has been widely used to account for erosional and depositional processes on submarine slopes and turbidite systems. Profiles out-of-equilibrium are commonly observed in actively deforming areas where channels seem to be deflected or diverted by seafloor structures. In this study the concept of the submarine equilibrium profile is tested in an area of extensive surface faulting to examine whether channels adopt an equilibrium-type profile through time. The study area is on the slope of the Nile Delta, which is disrupted by a number of surface-rupturing normal faults. Prior to fault linkage, several submarine channels flowed down the slope and either utilised relay ramps or flowed through fault scarps of the fault array. Where a relay ramp had been utilised, post fault linkage, the channels of the area either avulsed or converged into one major channel in response to a change in the deformed slope profile to a more concave shape. The thalweg of the post fault linkage channel and two slope profiles either side of it are measured in the area of the fault array, to understand how the channel evolved in response to the active faulting. When fault displacement is relatively small the combination of channel erosion and aggradation results in a channel thalweg profile near-equilibrium with predictable modifications of channel dimensions (depth and width) even if sediment supply was infrequent and episodic. It is concluded that turbidite channels can conform to the concept of equilibrium and submarine base level if it is the most energy efficient route for submarine gravity flows downslope. The most energy efficient route will be one where flows bypass the slope without eroding or depositing and move in a direct downslope course towards base level.     

Path and volume transport of the Kuroshio current in Sagami Bay and their relationship to cold water masses near Izu Peninsula

The path of the Kuroshio in Sagami Bay was surveyed through drifter tracking from Oshima-West Channel to Oshima-East Channel. A subsurface drifter with a drogue at 300 m depth flowed around Oshima from Oshima-West Channel to Oshima-East Channel. A difference in flow directions between the upper and lower layers was apparent in the northwest of Oshima. Flow directions there were shown to change from north in the surface layer to east in the bottom layer, and this was confirmed with moored currentmeters.A profile of northward current velocity was estimated from measurements in six layers with currentmeters deployed in the Oshima-West Channel. The profile shows a core of northward flow along the eastern bottom slope and a weak southward flow along the western bottom slope. Volume transport of the Kuroshio into Sagami Bay was estimated to be 1.8×10⁶m³sec^–1 from the profile.Long-term current measurement showed that southward flows were observed in Oshima-West Channel in July 1977, May 1978 and April 1979. Cold or warm water masses appearing south of the Izu Peninsula are suggested to have caused the changes.Displacement of the cold water mass in July 1977 is discussed on the basis of current measurements and offshore oceanographic conditions.     

海上风的边界层模式

由于航海、海上开发作业等对海洋上风和海浪的预报提出越来越高的要求,而海浪、风暴潮等海洋水文要素的数值计算和预报,迫切需要解决海洋上风场的精确计算。但是,复杂的海面结构,大气稳定度的影响以及风、浪之间动量的交换等,使海上风的理论计算遇到很多困难,至今大部分工作是依靠统计方法。利用天气预报的形势场计算地转风或梯度风,以及它与海面摩擦、大气稳定度的经验订正关系。     

Depositional architecture and sequence stratigraphy of the Karoo basin floor to shelf edge succession, Laingsburg depocentre, South Africa

The Laingsburg depocentre of the SW Karoo Basin, South Africa preserves a well-exposed 1200 m thick succession of upper Permian strata that record the early filling of a basin during an icehouse climate. Uniformly fine-grained sandstones were derived from far-field granitic sources, possibly in Patagonia, although the coeval staging and delivery systems are not preserved. Early condensed shallow marine deposits are overlain by distal basin plain siltstone-prone turbidites and volcanic ashes. An order of magnitude increase in siliciclastic input to the basin plain is represented by up to 270 m of siltstone with thin sandstone turbidites (Vischkuil Formation). The upper Vischkuil Formation comprises three depositional sequences, each bounded by a regionally developed zone of soft sediment deformation and associated 20-45 m thick debrite that represent the initiation of a major sand delivery system. The overlying 300 m thick sandy basin-floor fan system (Unit A) is divisible into three composite sequences arranged in a progradational-aggradational-retrogradational stacking pattern, followed by up to 40 m of basin-wide hemipelagic claystone. This claystone contains Interfan A/B, a distributive lobe system that lies 10 m beneath Unit B, a sandstone-dominated succession that averages 150 m thickness and is interpreted to represent a toe of slope channelized lobe system. Unit B and the A/B interfan together comprise 4 depositional sequences in a composite sequence with an overall basinward-stepping stacking pattern, overlain by 30 m of hemipelagic claystone. The overlying 400 m thick submarine slope succession (Fort Brown Formation) is characterized by 10-120 m thick sand-prone to heterolithic packages separated by 30-70 m thick claystone units. On the largest scale the slope stratigraphy is defined by two major cycles interpreted as composite sequence sets. The lower cycle comprises lithostratigraphic Units B/C, C and D while the upper cycle includes lithostratigraphic Units D/E, E and F. In each case a sandy basal composite sequence is represented by an intraslope lobe (Units B/C and D/E respectively). The second composite sequence in each cycle (Units C and E respectively) is characterized by slope channel-levee systems with distributive lobes 20-30 km down dip. The uppermost composite sequence in each cycle (Units D and F respectively) are characterised by deeply entrenched slope valley systems. Most composite sequences comprise three sequences separated by thin (<5 m thick) claystones. Architectural style is similar at individual sequence scale for comparable positions within each composite sequence set and each composite sequence. The main control on stratigraphic development is interpreted as late icehouse glacio-eustasy but along-strike changes associated with changing shelf edge delivery systems and variable bathymetry due to differential substrate compaction complicate the resultant stratigraphy.     

沿岸流流速垂向分布的实验研究

进行了沿岸流模型实验,测量沿岸流流速的垂向分布.采用Faria等(1996)的方法对实验结果进行了分析,将水深分为上层和下层两部分:下层采用对数分布来表达沿岸流的垂向分布;上层考虑了波浪引起的自由表面的波动,使流速分布包括了修正的对数分布和质量输移速度两部分.对实验结果得到的相对粗糙度与sleath(1991)的经验公式结果进行了对比,也对摩阻流速结果与已有经验公式计算结果的进行了对比.     

Modelling the cross-shore profiles of sand beaches using artificial neural networks

Abstract

Artificial neural networks (ANN) have been widely used successfully to solve coastal engineering problems. In this article, they are used to model the cross-shore profile of sandy beaches taking into account the possible effect of marine vegetation (Posidonia oceanica). Sixty ANNs were generated by modifying both the inputs and the number of neurons in the hidden layer. The best results were obtained with the following inputs: wave height perpendicular to the coast and the associated period and probability of occurrence, median sediment size, profile slope, and energy reduction factor due to P. oceanica. With these inputs and 10 neurons in the hidden layer, a mean absolute error of 0.22?m during training and 0.21?m during the test was obtained, which represents an improvement of 81.2% and 55.5% compared to models without and with P. oceanica.     

Geometrical properties of perfect breaking waves on composite breakwaters

The experimental results have so far shown that when a wave breaks on a vertical wall with an almost vertical front face at the instant of impact that is called perfect breaking or perfect impact, the greatest impact forces are produced on the wall. Therefore, the configuration of breaking waves is important in the design considerations of coastal structures. The present study is concerned with determining the geometrical properties of oscillatory waves that break perfectly on the vertical wall of composite-type breakwaters. The laboratory tests for perfect breaking waves on composite breakwaters are conducted with base slopes of 1/2, 1/4 and 1/6, and with berm widths of 0.00, 0.10, 0.20, 0.30 and 0.40 m. The shape and the dimensions of waves at the instant of perfect breaking on the wall are determined using a video camera. The experimental results for the geometrical properties of the breakers are presented non-dimensionally. Within the range of present experimental conditions, it is found that the dimensionless breaker crest height, h_b/d_w, and dimensionless breaker height, H_b/d_w, decrease; and, dimensionless breaker depth, d_w/H₀, increases with increasing relative berm width, B/D. The breaker height index, H_b/H₀, is almost unaffected by B/D. The deep-water wave steepness and the base slope of the breakwater do not seem to influence the geometrical properties of the breakers at wall systematically.     

长江口及其邻近海区环流和温、盐结构动力学研究III 温度结构

数值模拟结果表明: 冬季长江口及其邻近海区温度分布为近岸低, 外海高; 近岸和海底地形变化缓慢区温度呈垂直均匀分布, 海底地形变化显著的陡坡区生成温度锋; 外海深水区的中上层温度低且呈垂直均匀分布, 底层温度高并形成弱的分层。春季, 近岸温度高、外海低; 近岸温度大致呈垂直均匀分布, 外海出现明显分层; 长江口以北温度表层低、底层高; 长江口及其以南表层和底层温度低, 而中层高; 陡坡区至外海生成温度锋, 随着温度锋自陡坡至外海的下移,锋面以上温度逐渐变为垂直均匀分布, 而锋面以下温度却大致呈水平均匀分布。夏季, 海区的温度分布和春季一样, 为近岸高、外海低; 长江口及其以南近岸浅水区温度呈垂直均匀分布; 长江口以北和长江口及其以南的外海温度自表层至底层由高变低且大致呈水平均匀分布, 并在表层至次表层生成强温跃层, 跃层强度随水深增加迅速减弱, 深底层温度几乎呈均匀分布且保持低温特征。秋季, 海区的温度分布与冬季相同, 也为近岸低, 外海高; 在长江口以北, 近岸温度为表层高, 底层低; 外海底层温度低且大致呈水平均匀分布, 而底层以上温度高且大致呈垂直均匀分布; 长江口及其以南, 近岸温度呈垂直均匀分布, 陡坡至外海的表层至底层生成弱的温度锋,随温度锋自陡坡至外海的下移, 锋面以上温度逐渐变为垂直均匀分布, 长江口以南陡坡区的底层温度几乎呈均匀分布。     

TOPEX/Poseidon and Jason Equatorial Sea Surface Slope Anomaly in the Atlantic in 2002: Comparison with Wind and Current Measurements at 23W

A time series of velocity profile in the upper 150 m of the equatorial Atlantic was gathered at 23W in 2002 within the PIRATA program. It constitutes the first time series of near surface current measurements simultaneous with altimetric data in the equatorial Atlantic. The surface slope anomaly along the equator is computed from satellite altimetry, and, as a proxy for the pressure gradient along the equator, compared with the wind and near surface current data. In a first step, a time series of the surface slope anomaly along the equator in the Atlantic is computed from the 10-year-long TOPEX/Poseidon sea level anomalies. A sensitivity study establishes the robustness of the calculation. Apart from a 15 cm bias, the equatorial sea surface slope anomalies estimated either from TOPEX/Poseidon or from Jason over the 6-month overlap (Feb.–Aug. 2002) do not reveal drastic differences. We produce two sea surface slope anomaly composite time series for 2002 (one with T/P data, the other with Jason data during the commissioning phase) and compare them to the wind and velocity data at 23W. As expected, the near surface velocity and depth of the upper limit of the equatorial undercurrent (EUC) are extremely well correlated with the surface pressure gradient anomaly. 10-year-long time series of altimetry-derived zonal sea surface slope anomaly and ECMWF ERA40 wind stress are also well correlated. They exhibit similar spectral content and similar anomalous years. This is a first step towards a full analysis of the EUC dynamics using altimetry, PIRATA data (near surface current and subsurface thermohaline structure) and model. These initial comparisons reinforce the utility of Jason measurements for continuing the 10-year and highly accurate TOPEX/Poseidon time series for study of equatorial signals.     

TOPEX/Poseidon and Jason Equatorial Sea Surface Slope Anomaly in the Atlantic in 2002: Comparison with Wind and Current Measurements at 23W

A time series of velocity profile in the upper 150 m of the equatorial Atlantic was gathered at 23W in 2002 within the PIRATA program. It constitutes the first time series of near surface current measurements simultaneous with altimetric data in the equatorial Atlantic. The surface slope anomaly along the equator is computed from satellite altimetry, and, as a proxy for the pressure gradient along the equator, compared with the wind and near surface current data. In a first step, a time series of the surface slope anomaly along the equator in the Atlantic is computed from the 10-year-long TOPEX/Poseidon sea level anomalies. A sensitivity study establishes the robustness of the calculation. Apart from a 15 cm bias, the equatorial sea surface slope anomalies estimated either from TOPEX/Poseidon or from Jason over the 6-month overlap (Feb.-Aug. 2002) do not reveal drastic differences. We produce two sea surface slope anomaly composite time series for 2002 (one with T/P data, the other with Jason data during the commissioning phase) and compare them to the wind and velocity data at 23W. As expected, the near surface velocity and depth of the upper limit of the equatorial undercurrent (EUC) are extremely well correlated with the surface pressure gradient anomaly. 10-year-long time series of altimetry-derived zonal sea surface slope anomaly and ECMWF ERA40 wind stress are also well correlated. They exhibit similar spectral content and similar anomalous years. This is a first step towards a full analysis of the EUC dynamics using altimetry, PIRATA data (near surface current and subsurface thermohaline structure) and model. These initial comparisons reinforce the utility of Jason measurements for continuing the 10-year and highly accurate TOPEX/Poseidon time series for study of equatorial signals.