Richard R. Simons

Wave-current interaction within and outside the bottom boundary layer

Abstract This paper reviews the state-of-the-art as perceived by the Wave-Current Interaction (WCI) group which forms part of the MAST G6M Coastal Morphodynamics project, and includes some new results arising out of that project. Those processes which affect the vertical profiles of current and wave kinematics, and the bed shear-stresses, are discussed, but “horizontal” processes such as refraction of waves by currents, and generation of longshore currents, are not included. Among the groups conclusions are recommendations for the calculation of wave-induced bottom orbital velocities with and without WCI, and direct parameterisations of the bed shear-stresses produced by WCI. The latter is the results of a comprehensive intercomparison of WCI boundary-layer models and data. The results are aimed at aiding the formulation of numerical models of coastal morphodynamics.

The interaction between waves and a turbulent current: waves propagating with the current

This paper describes an experimental programme carried out in a laboratory channel with rough and smooth beds, to investigate the interaction between gravity waves and a turbulent current. In particular, changes induced in the mean-velocity profiles, turbulent fluctuations, bed shear stresses and wave attenuation rates are considered for a range of wave heights, keeping the wave period constant. The smooth-boundary tests were carried out as a necessary preliminary to the more-realistic rough-boundary condition. A directionally sensitive laser anemometer was used to measure horizontal, vertical, and 45° velocity components in the oscillating fluid, and an on-line minicomputer was programmed to produce ensemble averages of velocities, Reynolds stresses and wave-elevation data. The cycle was sampled at 200 separate phase positions, with 180 observations at each position. Measurements were made at up to 30 points in the vertical. Preliminary tests were carried out on the unidirectional current and on the waves alone. These show that mean-velocity profiles and turbulence parameters of the current agree satisfactorily with previous experiments, and that the waves are approximated closely by Stokes’ second-order theory. For combined wave and current tests, mean-velocity profiles are generally found to differ from those suggested by a linear superposition of wave and current velocities, a change in boundary-layer thickness being indicated. However, shear stresses at the smooth boundary are found to be described by such a linear addition.

The interaction of waves and a turbulent current: waves propagating against the current

The results of an experimental study of the interaction between waves and a current propagating in the same direction, have been reported by Kemp & Simons (1982). This paper describes the second part of the study, and considers the case of waves propagating against the current. Tests were performed in a laboratory flume with smooth and rough beds, and velocity measurements were made with a directionally sensitive laser anemometer as described in the previous paper. Analysis, including ensemble averaging of velocities and surface elevation, was performed by an on-line computer. Results indicate that the rate of wave attenuation is greatly increased by the addition of an opposing current, and reduced by a following current. Wave profiles remain closely described by Stokes second-order theory; orbital velocities are also found to be in agreement with a second-order wave theory modified to take account of the presence of the current. Certain results described occur regardless of the relative directions of current and wave. Mean velocities in the upper flow increase in the direction of the wave generator for increasing wave height. This suggests that the current is enhancing the wave-induced mass transport. Near the bed the velocity profiles so change that above the rough bed the current is retarded by the wave motion. In the logarithmic layer over the smooth bed velocities are increased with increasing wave height. However, all changes to velocity profiles have to be carefully interpreted, as the sidewall boundary layer decreases in thickness with even the smallest wave superimposed on the current. Turbulence intensities and Reynolds stresses near the rough bed are increased by the presence of the waves, most strongly in a layer two roughness heights above bed level, where fluctuations are periodic and effected by vortices ejected from the roughness troughs. Above this level, and over the smooth bed, turbulence levels are similar to those for the currents alone.

Bottom friction in random waves plus current flow

Bottom friction in random waves plus current flow is presented. The model is an extension of the Myrhaug [Coastal Eng. 24 (1995) 259] approach for random waves alone. The effect of random waves on the bottom friction is studied by assuming the wave motion to be a stationary Gaussian narrow-band random process, and by using friction coefficient formulas for sinusoidal waves. The data used for comparison are obtained from statistical analysis of direct measurements of bottom shear stresses made in the UK Coastal Research Facility under combined random waves and orthogonal as well as near-orthogonal currents.

BOTTOM SHEAR STRESSES UNDER RANDOM WAVES WITH A CURRENT SUPERIMPOSED

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

KINEMATICS AND SHEAR STRESSES FROM COMBINED WAVES AND LONGSHORE CURRENTS IN THE UK COASTAL RESEARCH FACILITY

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

THE INFLUENCE OF CURRENTS ON WAVE ATTENUATION

The Labrador Sea Extreme Waves Experiment (LEWEX), is an international basic research programme concerned with full-scale measurements, analysis, modelling and simulation in test basins of 3-dimensional seas. The research is carried out in order to assess the significance of 3-dimensional sea states in engineering applications. The first phase of the programme full scale wave measurements in the North Atlantic Ocean was performed at a site and time that had a high probability of encountering severe sea states. The present publication shows examples of measured bi-modal directional sea spectra obtained with the WAVESCAN buoy and directional sea spectra measured with an airborne Synthetic Aperture Radar (SAR). Directional spectra of gravity waves are obtained with the SAR both in open waters and below an ice cover. Further work is needed in order to verify SAR-measurements with in-situ observations. In-situ measured directional spectra are also compared with hindcast spectra from the 3G-WAM model. Hindcast significant wave heights were found to be lower than the in-situ measurements.The littoral drift model developed at DHI and ISVA, see Deigaard et al. (1986b) has been extended to include the effects of the irregularity of the waves, of a coastal current and a wind acting on the surf zone. Further, a mathematical model to simulate the near-shore current pattern along a barred coast with rip channels has been developed. The influence on the littoral drift of the irregularity of waves, wind, coastal current, and rip channels is discussed. It is concluded that irregularity of waves and presence of rip channels must be considered while coastal current and wind action are of minor importance.At present, the Prodhoe Bay oil field in Alaska contributes a substantial amount of the domestic oil production of the United States. Oil is also expected to be present on the continental shelf of Alaska, and it is estimated that approximatedly 28 percent of the total U. S. reserve is located beneath the shallow ice covered seas of the Alaskan continental shelf. To expolre and to exploit these oil rich resources, engineers are confronted with hostile oceanographic conditions such as high tides, waves, strong currents and sea ice. The same area is also rich in fishery resources. Being one of the most productive fishing grounds in the nothern Pacific, the potential ecological impact due to an oil spill is of a major concern. This paper describes the methologies used for the development of a modeling system for the oil risk analysis. The system is designed with generality in mind so it can be used for other coastal areas. The development of three dimensional models used in the modeling system described here have been published in the earlier International Coastal Engineering Conferences (Liu and Leendertse, 1982, 1984, 1986) and a report published recently by RAND (Liu and Leendertse, 1987). In the oil-spill risk analysis, these three dimensional hydrodynamic models are coupled to a two-dimensional stochastic weather model and an oil weathering model.The two projects (LUBIATOWO 79 and LUBIATOWO 86) were aimed at study pore pressure behaviour in natural sand bed in the coastal zone of the Baltic sea under the action of storm waves. During both projects, the wave induced pore pressures at the various levels in the sea bed were measured. The collected data were used to verify the applicability of the various theoretical approaches. In the conclusion, the range of the application of the particular analytical method is given.The rapid recession of the shingle bank of Hurst Beach (up to 3.5m/yr) makes it an excellent natural laboratory for the study of the factors which influence the stability of shingle beaches. Studies have included: the significance of long period, high energy, swell waves the classification and quantification of overwash processes run-up and seepage characteristics the effect of settlement of the underlying strata and the implications for practices in shingle nourishment. The studies have revealed the distinctive character of shingle beaches as compared with the more fully researched sand beaches. More detailed research on shingle beaches is justified particularly in relation to (i) the run-up characteristics including its interaction with swash cusps and (ii) the influence of the subsidiary sand fraction on the beach characteristics.Environmental assessment, engineering studies and designs were completed for a new 26.5 m3/s seawater intake system in the Persian Gulf. The original intake facility consisted of a curved, 60m breakwater with one end attached to the shoreline, a settling basin immediately adjacent to the shoreline and dredged to a maximum depth of approximately 5m, and a pumphouse structure located on shore such that the seaward wall formed one side of the settling basin. The facility located on an island in the Gulf, which served multiple seawater uses, had experienced both structural and operational problems, the latter consisting principally of excessive ingestion of sediment and seaweed. These factors plus the requirement for additional demands for seawater beyond plant capacity caused the owner to initiate a study of alternative intake systems, produce a design for the most effective solution and construct the new intake system.A two-dimensional wave prediction model suitable for use on personal computers is described. The model requires the twodimensional time-dependent wind field as input. Output consists of wave height, wave period, and wave direction estimates at all grid points on a computational grid representing an enclosed or semi-closed basin. Model predictions compare favorably with observations from a wave research tower in Lake Erie. A formula is provided to estimate how long a model simulation would take on a personal computer given the surface area of the computational domain, the grid size, and the computer clock speed.Cullera Bay is a neritic ecosystem placed on the Spanish Mediterranean Littoral largely influenced by the Jucar River, that brings about lower salinities than surrounding waters, and broad variations of its values. An extensive research, with 9 samplings throughout the year, was carried out, measuring both physical and chemical parameters, and the planktonic communities. The trophic status of the ecosystem, the spatial and temporal variations of the nutrients and the planktonic communities were studied, evaluating the influence of the river loads and the littoral dynamics. Some essential basis to allow a suitable emplacement of waste waters disposals along the Valencian littoral are set up in order to minimize the gradual eutrophication of this coast.In the last two years a whole of studies was realized in order to determine precise solutions to the regeneration of Villajoyosas beach, in the Spanish mediterranean coast. Investigations were carried out to the surrounding coastal areas based in field investigations and laboratory analyses of the beaches materials.

Gravity waves interacting with a narrow jet‐like current

[1] A unique experiment to investigate the transformation of near-linear gravity waves propagating across a narrow horizontally sheared jet-like current, typical of those found in the nearshore, coastal, and ocean regions, is described. A single wave condition was studied, propagating across the current orthogonally and at oblique incidence in both a following and opposing sense to the current. The length scale of the current shear layers was comparable to the incident wavelength. The experiment is the first attempt to assess the kinematics and dynamics of the interaction of regular waves and currents in three dimensions at a physically realistic scale. The resulting data set provides direct quantitative measurements of the spatial variation of the primary flow variables. Negligible reflection of the incident wave at the current shear layers was observed. Typical refraction behavior was observed: A following wave is refracted to a more current-parallel direction with an increased wavelength and reduced wave height as it moves onto the current, while the opposing wave becomes more current normal with a shortened wavelength and enhanced wave height. The experimental data are compared with predictions of a wave ray model, assuming a depth-averaged current and slowly varying horizontal shear, and a new model that incorporates the influence of vertical shear.

Scour development around truncated cylindrical structures

2-6 July 2007 This paper reports results from experiments carried out in a laboratory flume to investigate the process of seabed scour around cylinders of finite length under the action of currents close to the threshold of motion for the bed sediment. The conditions relate to the design of large caisson foundations for offshore oil and gas installations and wind turbines. Results show that depth and extent of scour are much reduced for cylinders that stand less than one diameter above the seabed compared to tests with a surface-piercing cylinder. When truncated cylinders stand on shallow foundations, scour patterns are not sensitive to skirt depth, even when the cylinder has been undermined. For the present tests, maximum scour depth around truncated cylinders in a tidally reversing current was found to be less than 50% of equilibrium scour depth for an equivalent unidirectional flow.

Effect on turbulence production due to sudden change in flow regimes

Flow regimes were investigated in the cavity formed between the roughness elements in a laboratory water flume. Further tests were carried out on similar aspect ratio of the cavity to study the effect on turbulence production because of change in local bed geometry. The idealized 2-D bed roughness has been aligned perpendicular to the flow direction, and velocity measurements were made using a laser Doppler velocimeter. Skimming and transition regime has been identified between the roughness elements with the help of flow visualization. Turbulence production was investigated at test section with and without the presence of urban approach roughness. The effect on turbulence production in the boundary layer was significantly influenced by the upstream roughness and height of the roughness element.