Field measurements of coastal waves and currents in Portugal and Greece

The MAST II WAVEMOD Project aimed at improving stochastic modelling of ocean waves and currents in coastal waters. In order to supplement existing data, two measurement campaigns were carried out, one on the Atlantic coast of Portugal, and, secondly, on the northern coast of Crete in the Mediterranean. The Portuguese measurements covered 7 months during winter 1993–1994, while off Crete, measurements started early in February 1994 and continued for 10 months. In both sites, two directional Waveriders and one non-directional Waverider were deployed along a transect out from the coast in water depths between 10 and 100 m. In addition, the Mediterranean site also incorporated measurements of mean current velocities. Available satellite data were analysed to supplement the buoy data. In this paper, both measurement campaigns are described together with a presentation of some of the data collected.     

波流作用下海底边界层沉积物再悬浮与影响因素研究

海底沉积物再悬浮及其分布取决于海洋水动力、沉积物类型与床面形态之间复杂的相互作用,准确地理解和确定沉积物再悬浮过程对于沉积物输运的研究具有重要的意义。本文在祥云湾海洋牧场典型海域开展现场原位观测,获取研究区波浪、海流及悬浮沉积物浓度数据;分析了波、流作用下海底边界层悬浮沉积物垂向分布特征,并探究了海洋水动力和床面形态对悬浮沉积物垂向分布的影响。结果表明,研究区波流之间的相互作用不显著,沉积物再悬浮受控于风暴浪作用,风暴浪作用下底床切应力可以达到沉积物临界切应力的10～15倍,沉积物的再悬浮滞后于风暴浪作用2～3 h。在波浪荷载微小的情况下,悬浮沉积物垂向分布呈现"I"型,波浪荷载下,悬浮沉积物垂向分布呈现幂指函数分布,表现为"L"型;床面形态随波、流作用而演化,影响沉积物的再悬浮过程,u_?w/u_?c=1.00可作为波浪和海流起主导控制作用的床面形态的判别依据,纯波浪荷载作用下的u_?w/u_?c显著高于波浪主控作用下,但二者之间的界线随着波浪荷载的增加而升高。     

Tidal boundary layer measurements in the presence of waves

Measurements of tidal current and wave velocity made at 0.69 and 1.85 m above a rough seafloor exhibit large current gradients (boundary layer) in the water column. The logarithmic boundary layer flow model was fitted to the measurements, and thus roughness (z₀) and friction velocity (u^*) parameters were derived. The roughness parameter values were generally consistent with the observed upstream physical roughness. The values of both parameters for conditions in the rough turbulence flow regime are generally larger (much larger for ebb) than earlier published values for similar measurements of currents in the absence of significant waves but are comparable to values from recent measurements of currents in the presence of storm waves. The high parameter values here appear to relate more to the magnitude of the current and to the upstream physical bottom roughness than to the magnitude of the seastate. Large boundary layers in the flow at the seabed have a profound effect on the design of offshore structures such as offshore pipelines.     

Near-bed turbulence measurements,stress estimates and sediment mobility at the continental shelf edge

As a contribution to ocean-shelf exchange studies, the Sediment Transport and Boundary Layer Equipment STABLE has been deployed west of Spain near 42°40’N in 200m water depth, in December and August, and at three other continental shelf edge and slope locations off north-west Europe. Turbulence at 8Hz and acoustic back-scatter at 4Hz were measured at three levels between 0.3 and 1 m above the bed.Typically the speed profile shows little shear; the turbulence data are necessary to estimate near-bed stress. Reynolds stress and total turbulence energy approaches give consistent results; total turbulence results appear more robust but can be affected by long surface waves. Typical peak values of friction velocity u₁ per tide were 10 mm/s off Spain and 7–18 mm/s at the other locations, and they were asymmetric owing to along-slope flow. Record maxima were 30 mm/s, 11–48 mm/s respectively, sufficient to move local sediment except for a short ‘event-free’ record at Goban Spur. Acoustic backscatter (SPM) showed peaks matching peaks of u₁ or waves in the case of winter deployments off Scotland and Spain.Comparison of these STABLE measurement periods with contemporary current records of longer duration suggests that bed stresses in all these upper-slope locations are often sufficient to move the local sediment. Off Spain, shelf sediments are re-suspended and transported down-current. The sandy fraction settles soon, but the fine fraction (w_S < 0.1 mm/s) is progressively winnowed down-current to the shelf edge, suggesting export of the bound organic matter.     

Validation of turbulence closure parameterisations for stably stratified flows using the PROVESS turbulence measurements in the North Sea

A number of parameterisations for the simulation of mixing processes in the thermocline are compared and tested against the microstructure data of the PROVESS campaigns, conducted in the northern part of the North Sea during the autumn of 1998. The transport term in the turbulent kinetic energy equation is parameterised via the introduction of a third stability function S_k for turbulent energy diffusion. The formulations are compared with a simpler scheme based upon limiting conditions for turbulence variables. Improved results are obtained with a new form of S_k. The best agreement is, however, found with the simpler limiting scheme. This is explained in terms of a turbulence length scale theory for stably stratified turbulence. In agreement with previous laboratory and ocean data it is found that the ratios of the Thorpe and Kolmogorov scales to the Ozmidov length scale approach critical limiting values in the thermocline. The first of these conditions is satisfied when limiting conditions are implemented into the scheme, providing the necessary minimum value for the dissipation rate, whereas the schemes without limiting conditions fail to produce this critical ratio. The basic reason for this failure is that the Thorpe scale is overestimated, which is shown to be connected to an even larger overprediction of the dissipation rate of temperature variance. To investigate the impact of non-resolved advective processes and salinity stratification on the turbulence predictions, additional numerical experiments were conducted using a simple scheme for data assimilation. The best agreement is found again with the limiting scheme, which is able to make reasonable predictions for the dissipation rate without knowing the detailed shape of the mean stratification profile. It is shown that advective transport due to tidally and wind-driven motions has a non-negligible impact on vertical mixing. This is seen in the data and the models by periodic enhancements of turbulent mixing inside the thermocline.     

Seasonal variability and the types of currents in the upper layer of the Black Sea

Spectral analysis of the climatic circulation in the upper 10 m layer of the Black Sea has been carried out. The current field was calculated from a three-dimensional baroclinic non-linear model. The mode composition of the circulation was derived. It is shown that the surface climatic circulation can be represented as the superposition of a finite number of elementary currents of two types:n- andm-modes. Only 4n-modes are shown to be energy-dominant throughout the year. A relation is established between the different sources of energy supply of currents and the modes which assimilate this energy. Problems concerning the nature of the classical Black Sea currents such as the main Black Sea current (MBSC) and the Knipovich spectacles are discussed. A scenario of the field large-scale experiment is suggested which allows the reconstruction of the amplitudes of the first 10n- and the first 10m-modes from mooring data.Translated by Mikhail M. Trufanov.     

Hydrophysical state of the Large Aral Sea in the autumn of 2013: Thermal structure,currents, and internal waves

This paper is focused on the hydrophysical state of the western basin of the Large Aral Sea in its present conditions of ecological crisis. The investigation is based on the direct observations carried out during a field survey in the Aral Sea in October of 2013. The analysis of the distribution of the temperature values across the western basin is given. Time series of the water currents variability were recorded at four mooring stations. The pattern of the basin-scale circulation response to the wind forcing was described. Based on the in situ measurements of the time variability of the temperature fields, the principal characteristics of the internal waves were specified for the present-day Aral Sea for the first time.     

Model analysis of the currents and wind waves in the Vistula Lagoon of the Baltic Sea

This paper considers the results of the numerical simulation of the steady currents and waves field structures in the Vistula Lagoon under constant wind forcing. The currents?? structure is essentially 3-dimensional. The direction of the near-shore drift is determined by the wind and shore orientation, but the currents involve two layers in the deeper part of the basin: the upper layer downwind current and the upwind compensative current in the underlying layer. The wind waves depend upon the wind speed, fetch, and depth until the wind speed is less than 6?C8 m/s. As the wind increases, the fetch??s dependence disappears. The wind height depends only on the wind??s magnitude, and, under stormy conditions (when the wind??s speed exceeds 15 m/s), the waves are limited by the water depth alone.     

On the measurements and analysis of ocean waves and currents on the Norwegian continental shelf

Environmental data, particularly wave and current data, are of vital importance in offshore engineering. The needs for such data are discussed with reference to the influence of various environmental parameters on the loads of offshore structures. Data collected until now from the Norwegian continental shelf are reviewed. Furthermore, a planned data collection program is presented and discussed with respect to experience with instrumentation techniques, data recovery, and representativity, as well as interpretation and analysis gained from the present activity in this field.     

Surface layer variability in the Ross Sea,Antarctica as assessed by in situ fluorescence measurements

Phytoplankton fluorescence, temperature and salinity were measured from December through February using in situ instruments deployed at two locations in the southern Ross Sea, Antarctica during the austral summers of three consecutive years (2003–2004, 2004–2005, and 2005–2006) to assess the short-term, seasonal and interannual variations in phytoplankton biomass and oceanographic conditions. The seasonal climatologies of physical forcing variables were also determined from satellite measurements, and the data from the two sites compared to the 2000–2009 mean. In situ fluorometers were deployed at three depths at 77°S, 172.7°E and 77.5°S, 180°. Significant differences between the two sites were consistently observed, confirming the anticipated high level of spatial and temporal heterogeneity. Chlorophyll fluorescence was maximal in late December, and generally decreased rapidly to modest levels in January and February. However, during 1 year (2003–2004) a secondary bloom was found, with summer maxima being similar to those observed during spring. Fluorescence displayed a strong diel cycle, with strong quenching during periods of maximum irradiance. The magnitude of this reduction was large (the minimum average fluorescence was 25% of the daily mean) and decreased with depth. Fluorescence varied interannually, with the absolute levels and temporal patterns being different among years. The two sites had different temperature/salinity properties as measured at 24 m, and both variables changed with time. During 2004–2005 we were able to continuously measure the photosynthetic quantum efficiency of PSII (F_v/F_m) at 11 m, which revealed a minimum in December, and an increase in January, whereas the absolute fluorescence (F_o) decreased simultaneously. We suggest that this reflected a mixing event, whereby available irradiance increased, allowing a short period of growth in a more favorable optical environment. While substantial variations from the mean physical forcing were observed, the linkage of these physical variations with fluorescence was not always clear. Short-term (over 24-h) changes in fluorescence occurred, and were likely related to advective events. Wind events altered fluorescence in the surface layer, and these redistributed phytoplankton in the surface. The variability in chlorophyll fluorescence and physical forcing over a variety of scales in the Ross Sea provides insights into temporal–spatial coupling of phytoplankton.     

Laboratory study of the dynamics of a layer of turbulence excited by standing surface waves

The evolution of a turbulent layer excited by standing surface parametric waves is studied experimentally. The time dependence of the depth of the turbulized region is obtained. It is found that the thikness of the layer is described by a nonlinear dependence as distinguished from the previously obtained theoretical results. The dynamics of turbulent-layer embedding is described on the basis of a semiempirical theory and the assumption that the outer scale of turbulence is constant. Satisfactory agreement with experimental data is obtained. In the context of the proposed model, a relationship is found between the quantities characterizing the turbulence and surface waves. The dependence of the steepness of surface waves on the acceleration of the bottom is obtained experimentally. The data obtained in this study can be used for estimation of turbulence generation during a submarine earthquake.     

Duality of turbulence and wave in wind waves

The three-seconds power law for wind waves of simple spectra, already derived byToba (1972 and 1973), may also be derived by introducing surface-wave properties into the form of the rate of energy dissipation in the theory of turbulence. The universal constantB, which was formerly determined empirically as 0.062 is here obtained asB=(2)^–3/2=0.0635. Thus wind waves have the duality of turbulence and wave.     

Interaction mechanisms among waves,currents and a submerged plate

Wave-induced loads on a submerged plate, representative of submerged breakwater, coastal-bridge deck and a certain type of wave energy converter, in a uniform current are investigated in this study using fully nonlinear numerical wave tanks (NWTs) based on potential flow theory. The coupling effect of wave and current is explored, and the underlying interaction mechanisms of the hydrodynamic forces are described. The presence of a background current modifies the frequency dispersion. It produces changes of the water-surface elevation, and also has an effect on wave-induced loads. Depending on the nonlinearity, higher harmonic wave components are generated above the submerged plate. These contribute to the wave forces. It is found that the horizontal and the vertical force, hence the moment, are affected in the opposite way by the currents. The Doppler shifted effect dominates the vertical force and the moment on the plate. Whereas, the Doppler shifted effect and the generation of higher wave harmonics play opposite roles on the horizontal forces. The contribution of 2^nd order harmonics is found to be up to 30% of the linear component. The current-induced drag force, represented by the advection term ρU∂φ/∂x in the pressure equation, is found to lead to a decrease in the moment for the most range of wavelengths considered, and an increase in the moment for a small range of longer waves.     

近岸沿岸流及污染物运动的数值模拟

基于双曲型缓坡方程和近岸浅水方程对近岸波浪斜向入射破碎所生成的沿岸流及污染物在沿岸波流作用下的运动进行了数值模拟,并对数值模拟结果进行了验证分析。数值模拟结果表明,在相近工况参数下,随着入射波高的增大,沿岸流流速和平均水面升高值均明显增大;随着岸坡坡度的增加,沿岸流流速和平均水面升高值明显增大;随着入射波浪周期的增大,平均水面升高值明显增大。在沿岸缓坡区域,由斜向入射波浪破碎所产生的沿岸流对污染物的运动起着不可忽略的影响。     

Combined numerical model of currents, waves, and sediment transport in Lake Donuzlav

We present a numerical model of the dynamics of Lake Donuzlav, which enables one to perform simultaneous numerical analyses of the currents, sea level, waves, and sediment transport. The model is based on the hydrodynamic block and the spectral wave model. For typical storm situations, we study the specific features of the integral circulation of waters and the three-dimensional structure of currents, investigate the wind-induced wave fields, and evaluate the flows of sediments and deformations of the bottom. The presence of intense eddy structures is revealed in the field of currents caused by the bottom topography. A significant intensification of waves in the south part of the lake is established in the case of penetration of storm waves through the strait. It is shown that the account of waves leads to qualitative changes in the structure of circulation in the lake and to the formation of well-pronounced areas of wave-induced elevations and lowerings of the sea level. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 43–65, March–April, 2006.     

Intense near-bed sediment motions in waves and currents

In the past few years, two-phase models have been developed to describe the detail behaviour of fluid/sediment interactions and transport under the sheet flow conditions. Due to the complexity of the governing equations and uncertainties in the formulations of various stress terms, few complete solutions of these equations are known and the validations are thus far limited to only a few experimental data. In this paper, the numerical predictions of the behaviour of sheet flows using an improved version of an earlier two-phase flow model [Coastal Eng. 36(2) (1999) 87] are described. Although the general structure of the model was retained, a number of improvements had been made to give better account the underlying physics of the flow in areas very close to the stationary bed. All key flow parameters have been predicted and analysed in order to gain insight into the processes. Calculated time-dependent as well as time-averaged concentrations are compared with experimental data from purely oscillatory flows and oscillatory flow plus a current. Good qualitative agreements between predictions and measurements were achieved for the time-dependent concentrations while the time-averaged concentrations are quantitatively accurate as well.     

Moored fish cage dynamics in waves and currents

Recent work in the area of open ocean aquaculture system dynamics has focused separately upon either the response of fish cages in waves or the steady drag response due to ocean currents. In reality, however, forcing on these open ocean structures is a nonlinear, multidirectional combination of both wave and current profiles. At the University of New Hampshire-operated Open Ocean Aquaculture site, data were collected from a wave measurement buoy and a downward-looking Acoustic Doppler Current Profiler to characterize the surface elevation and water velocity profiles during an extreme northeast storm event. In addition to waves and currents, fish cage motion response in heave, surge, and pitch was inferred from accelerometer measurements during the same storm. The environmental data sets obtained during the peak of the storm were processed, analyzed, and used as input to a dynamic finite-element model. Simulations were performed using three load case scenarios: 1) in both waves and currents; 2) in waves only; and 3) in currents only. Model motion response results in both the time and frequency domain were compared with data obtained in situ . In addition to the motion response tests, the wave and current forcing influencing the mooring line tension response was also investigated. Analysis shows that in this case, the currents do not severely influence the oscillatory motion response, but do cause the cage to tilt, layback, and sink. The wave and current interaction effect did, however, influence the anchor line loads with a portion being attributed to nonlinear effects.