Hurricane effects on surface gulf stream currents

A 3-dimensional model methodology for predicting the upper ocean currents under the combined influence of Gulf Stream and hurricane is described. Predicted currents are highly dependent on the turbulence closure scheme and nonlinear interactions. New field data will be required to determine the appropriate formulation for turbulent momentum transfer. However, the model points out the importance of nonlinear terms in the equations of motions for preserving the jet-like structure of the Gulf Stream and for properly accounting for interaction between the Gulf Stream and a hurricane. A potentially worst case condition is obtained for the so-called resonant hurricane.     

Numerical model of currents generated by sources and sinks in a circular rotating channel

This paper presents the results of numerical calculations using shallow water equations for the currents in the laboratory experiments with a rotating circular channel. An axial symmetric function of mass source is introduced into the equations for the depth of the layer to model experimental sources and sinks of fluid, which induces opposing zonal flows together with the Coriolis force. Different configurations and amplitudes of mass sources lead to the appearance of vortex motions in the channel with different circular motions in the vortices and azimuthal displacements of their centers along the channel. Diagrams of regimes are presented in the parameters of relative angular velocities of the mean zonal flow and vortex transport around the axis of the system rotation. The differences of the theory and real experiments with currents of finite depth in a channel are discussed.     

Welded slump-graded sand couplets: evidence for slide generated turbidity currents

Some massive channelized strata preserved in the rock record are characterized by a lower slump member which evolves upward to a turbidite. This merging is indicative of probable generation of sediment gravity flows from submarine sliding. Conditions essential for deposition of such sequences are short transport distance between point of failure and depositional site, and an environment likely to retain both facies. Fan valleys are a likely setting for welded couplets: flowing sand, initiated by the sliding event, comes to rest at nearly the same time and position as the slump mass deposited near the base of the valley wall and in the axis proper.     

Turbidity currents on steep slopes: Application of an avalanche-type numeric model for ocean thermal energy conversion design

To minimize cold water pipe lengths, the most favorable land or fixed platform based Ocean Thermal Energy Conversions (OTEC) sites have subbottom slopes greater than 5°. Observations at OTEC sites in Hawaii indicate that turbidity currents of an impulsive or episodic nature can occur with frontal speeds of several meters per second. Such speeds and the attendant potential for sediment transport and abrasion along routes containing OTEC installations indicate that the pertinent features of these flows are an important design criteria for OTEC or any other steep-slope marine installation. To satisfy this need, models of oceanic turbidity flows and similar flows have been examined. The model that addresses OTEC steep-slope conditions most succinctly was developed originally by Hopfinger and Tochon-Danguy (1977) for snow avalanches on land. This two-dimensional avalanche model is used to estimate the speed and growth characteristics of potential turbidity currents downslope for various postulated marine conditions of initial flow density, height, volume, and length at slopes from 5 to 60°. The areas of additional research required to increase reliability of the analyses are in the initiation and initial development of a turbidity plume, the mechanisms of sediment entrainment to and loss from the plume, and three-dimensional in addition to two-dimensional studies.     

Characterization of bedform morphology generated under combined flows and currents using wavelet analysis

The wavelet transform (WT) has been successfully implemented in many fields such as signal and image processing, communication theory, optics, numerical analysis, and fluid mechanics. However, the application of WT to describe bedform morphology in coastal areas, oceans, and rivers is rare. The present study demonstrates the capability of WT analysis to fully represent the space–frequency characteristics of signals describing bed topography generated in marine and river environments. In this study WT is used to examine the morphological characteristics of bedforms generated in two separate laboratory facilities: a wave tank and a meandering channel. In the wave tank a set of ripples superimposed upon large wave ripples were generated; while in the meandering channel, 2D and 3D migrating ripples and dunes were observed. The WT proved to be a useful tool in detecting the complex variability of the generated bedform structures. The size distribution of the bottom features such as ripples, large wave ripples and sandbars were first examined along a 2D bed profile. Later analysis studied the variability of features in the transverse direction by using the power Hovmöller. Experiments in the wave tank were conducted for a mobility number of ψ=(10, 28), and a Reynolds wave number of R_ew=(17,500, 83,500) which correspond to waves alone (WA) and to combined flow (CF) scenarios, respectively. Experiments in the meandering channel were conducted under a morphological regime that produced mainly migrating sandbars.     

Consequences of the El-Asnam earthquakes: Turbidity currents and slumps on the Algerian margin (Western Mediterranean)

Seismicity due to the compressional motion in North Africa has an important effect on the stability of the sediments on the Algerian continental slope. The 1954 earthquake induced a large turbidity current that cut many telephone cables. Results of the 1980 El-Asnam earthquake on sliding of slope deposits arc not well documented, mainly because there were fewer cables than in the past. Nevertheless in the Bay of Alger, 200 km from the epicentre, a telephone cable was broken by a small turbidity current. This current reached a velocity of about 45 km/h.     

海水浊度测量传感器研究

利用光学散射原理,设计并研制了一种浊度传感器,发射器与接收器成90°,中心波长880 nm,使用Formazine标准液进行标定。通过试验表明,使用此种方法设计的浊度传感器,要准确快速地测量海水浊度。     

Wind-generated waves in Hurricane Juan

We present numerical simulations of the ocean surface waves generated by hurricane Juan in 2003 as it reached its mature stage (travelling from deep waters off Bermuda to Nova Scotia and making landfall near Halifax) using SWAN (v.40.31) nested within WAVEWATCH-III (v.2.22; denoted WW3) wave models, implemented on multiple-nested domains. As for all storm-wave simulations, spectral wave development is highly dependent on accurate simulations of storm winds during its life cycle. Due to Juan’s rapid translation speed (accelerating from 2.28 m s⁻¹ on 27 September, 1200 UTC to 20 m s⁻¹ on 29 September, 1200 UTC), an interpolation method is developed to blend observed hurricane winds with numerical weather prediction (NWP) model winds accurately. Wave model results are compared to in situ surface buoys and ADCP wave data along Juan’s track. At landfall, Juan’s maximum waves are mainly swell-dominated and peak waves lag the occurrence of the maximum winds. We explore the influence of surface waves on the wind and show that the accuracy of the wave simulation is enhanced by introducing swell and Stokes drift feedback mechanisms to modify the winds, and by limiting the peak drag coefficient under high wind conditions, in accordance with recent theoretical and experimental results.     

Sedimentation by non-tidal currents around northern Denmark

Side-scan sonar surveys of the shallow sea floor off northern Denmark have shown the presence of a number of bedforms of mobile sand which can be associated with nearby sand deposits. Both are of interest as they appear to have been formed largely by the strong, wind-induced, intermittent, coastal current, which converges from between west and south, and flows eastward past the north Danish coast.     

Sand ripples generated by regular oscillatory flow

The prediction of ripple geometry is a necessary precursor to the prediction of sand transport under waves for ripple regime conditions. The paper begins with a comparison of four existing methods for predicting the geometry of sand ripples generated by oscillatory flow. The comparison points to substantial differences between ripple dimensions predicted by the methods, especially for field-scale conditions. Ripple geometry experiments carried out in a large oscillatory flow tunnel are then described. The experiments involved a range of sand sizes and sinusoidal and asymmetric flows with periods and velocities typical of field conditions. Comparison of measured and predicted ripple geometries leads to the recommendation that the method of Mogridge, Davies and Willis be used to predict ripple geometry for field-scale oscillatory flows. The Nielsen method yields good predictions of ripple length, but the rapid fall-off in ripple steepness predicted by the Nielsen method at high mobility number is not supported by the measurements. The lengths and heights of symmetric ripples produced by sinusoidal flows are found to be similar to the lengths and heights of asymmetric ripples produced by “equivalent” asymmetric flows. Three-dimensional ripples occur with fine sand in long-period flows typical of field conditions. The dimensions of these ripples cannot be predicted using methods developed for two-dimensional ripples. Previously suggested criteria for predicting the occurrence of three-dimensional ripples fail when tested against a wide range of flow and sand conditions. The occurrence of three-dimensional ripples and the effects of ripple and flow history on ripple geometry require further research.     

Dynamics of Turbidity in the Tweed Estuary

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30 ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (<30) although the relationships varied throughout a tidal cycle. At the highest salinities, in the lower 2·5 km of estuary, significantly enhanced turbidities occurred during strong, onshore swell-wave conditions. Near-mouth turbidities were very low (<5 ppm) at high-water (HW) when the swell-wave height was small (<0·3 m). There was no correlation between near-mouth turbidity at HW and tidal range, whereas the correlation between near-mouth turbidity and swell-wave height explained over 90% of the variance in near-mouth turbidity. The temporal trends in freshwater turbidity and freshwater inflow during the fieldwork period were fairly similar. A statistically significant relationship existed between these variables when freshwater turbidity was correlated against inflow 30 h earlier. A ‘ model ’ of turbidity, based on 5 days of inflows and used to hindcast turbidity from 14 days of inflows between 16–29 September, explained over 80% of the variance in freshwater turbidity. Therefore, inflow exerted a significant control on the fluvial turbidity. It appears that very fine-grained particles were responsible for the observed turbidity in the central and upper reaches of the Tweed. During the flood, in the presence of strong wave activity in the coastal zone, larger sediment was rapidly winnowed in the lower reaches, close to the mouth, whereas fine silt particles remained in suspension and reached the limit of saline intrusion.     

Buried relic seawall mitigates Hurricane Sandy's impacts

Field observations to quantify damage, overwash, sediment deposition, and breaching in the aftermath of Hurricane Sandy revealed clear differences in patterns of the impact between two neighboring boroughs along the New Jersey shore: Bay Head and Mantoloking. Field data and observations gathered immediately after the storm indicate that a relic seawall in Bay Head appeared to lessen the wave-driven effects of Hurricane Sandy as compared to its southern neighbor Mantoloking. Complimentary detailed numerical simulations demonstrate that this relic seawall reduced the wave-induced forces on ocean front structures, indicated by wave-averaged momentum flux, by a factor of two. The difference in impact of Hurricane Sandy between Bay Head and Mantoloking underscores the urgent need for sustainable multi-level protection against natural hazards, in order to create resilient coastal communities.     

Interface wave generated by an electromagnetic induction source

Interface waves such as the Scholte wave are a useful tool to study geoacoustic properties and can be conventionally generated by an explosive or a pneumatic source on/above the seafloor. This type of source, however, generates strong compressional waves in the water and sediment at the same time; these waves then disturb an observation of interface waves and leads to difficulty in processing. These sources are also relatively hard to control at sea from a viewpoint of repeatability and stability of interface waves to be generated. In addition, environmental problems caused by those sources is a concern. In this paper, an electromagnetic induction source whose vibrator plate hits the seafloor directly and excites interface waves is described. The capability of this source was evaluated both in a water tank and at seashore. The pulsed Scholte waves excited both by several types of electromagnetic induction source having a different shape of vibrator plate and by dropping weight were transmitted in sediment and received using geophones. As a result of comparison of measured signals, the pulse signal propagating from the source demonstrates a sharper rise time than that from dropping weight     

环形障碍物-轴棱锥产生局域空心光束

提出一种产生局域空心光束(bottle beam)的新方法,它是在传统的轴棱锥底面上放置一个环形障碍物.平面波入射到带有环形障碍物的轴棱锥底面时,将在轴棱锥产生的最大无衍射距离内形成bottle beam,而在bottle beam的前后仍然保持无衍射贝塞尔光.通过变化障碍物大小及轴棱锥底角实现bottle beam尺寸的控制.利用衍射理论描述了平面波经带有环形障碍物的轴棱锥产生bottle beam的原理,分析和模拟了传输过程中不同位置的截面光强分布及整个过程的三维光强分布,并给出相关的实验结果.研究结     

Surfzone currents

Investigations of surfzone currents including longshore currents, rip currents and nearshore circulations have been conducted worldwide for over sixty years. The motivation is understanding and prediction of sediment transport and coastal pollution processes. This article summarizes a recent state-of-the-art review of theory and experiments conducted since 1967 (Basco, 1982, with Volume II including over 350 annotated bibliographic entries). Emphasis is on the now generally accepted, time-averaged theory based upon radiation stress principles. The newly emerging Boussinesq-type theory, which follows the instantaneous water surface and current variations within each wave period, is also briefly considered. Both approaches assume uniform currents with depth, rely upon empirical wave breaking and surfzone energy empiricism, and suffer from a limited data base for verification. Emphasis throughout is on needed areas for further research.