Brian A. O'Connor

Mean bed friction of combined wave/current flow

Abstract Bijkers model for evaluating the wave-period-mean bed friction of combined wave/current flow has been modified to account for the current velocity reduction as well as the wave boundary layer near the sea bed. A method of estimating the bed frictional dissipation rate of the combined flow is also presented. The results obtained by the present approach are compared with several sets of experimental data, and confirm its accuracy and usefulness.

Offshore sand bank dynamics

Abstract The present paper reports some key results from field investigations and numerical modelling studies of the tide- and wind-induced hydrodynamics and sediment dynamics of Middelkerke Bank (MB) in the southern North Sea of Europe conducted during December 1992 to March 1993. Strong surface current refraction and acceleration effects were observed over MB using the HF radar system OSCR ( O cean S urface C urrent R adar). Results suggest that OSCR data may be used remotely to monitor large-scale bathymetry in shallow coastal environments. Spatial variation in tidal propagation characteristics and modification of shoreward propagating waves was not detected at locations around MB during the experiment. Observed residual currents were found to be correlated strongly with wind speed and direction during the period 26 February to 18 March 1993. However, in low wind stress condition, a three-dimensional numerical model (3D-Bank) indicated the presence of a clockwise residual circulation of water around MB consistent with theory. Spatial and temporal variation in the average total drag coefficient ( C d ) of MB were investigated and found to correlate strongly with tidal current speed. Fluorescent sand tracers, used to monitor net sediment transport pathways, revealed a net clockwise movement of sediments around MB consistent with predictions by 3D-Bank and with theory.

A three-dimensional model of suspended particulate sediment transport

Abstract Depth-averaged, one-dimensional and two-dimensional numerical models of bed and suspended particulate sediment transport provide unreliable answers for siltation and erosion quantities in situations dominated by three-dimensional flow patterns, as occur in harbour entrances due to flow separation and wind and density currents. Consequently, a numerical scheme has been developed which solves the complete three-dimensional diffusion-advection equation for suspended sediment concentration and thereby makes possible the study of siltation problems in complex, three-dimensional flows. The model analogue is based on a splitting technique and employs a mixed characteristics and finite difference approach. The accuracy and usefulness of the resulting scheme have been investigated by applying it to a number of hypothetical situations and to a laboratory situation involving the transport and dispersal of lightweight sediment. The results of the various tests show that the proposed approach works well and provides a useful basis for the study of practical problems.

Intercomparison of wave-driven current models

Abstract Seven numerical models which simulate waves and currents in the surf-zone are tested for the case of a reduced-scale detached breakwater subjected to the action of regular waves with normal incidence. The computed wave heights, water levels and velocities are compared with measurements collected in an experimental wave basin. The wave height decay in the surf-zone is predicted reasonably well. Set-up and currents appear to be less well predicted. This intercomparison exercise shows that radiation stresses are systematically overestimated by formulations used in the models, mean bottom shear stresses are not always co-linear with the mean bottom velocity vector in shallow water, and turbulence modelling in the surf-zone requires a sophisticated

MATHEMATICAL MODELLING OF WAVE-INDUCED NEARSHORE CIRCULATIONS

More than 150 tests have been analyzed in order to describe the dynamically stable profiles of rock slopes and gravel beaches under wave attack. Relationships between profile parameters and boundary conditions have been established. These relationships have been used to develop a computer program. This program is able to predict the profiles of slopes with an arbitrary shape under varying wave conditions, such as those found in storm surges and during the tidal period.This paper investigates the utility of winds obtainable from a numerical weather prediction model for driving a spectral ocean-wave model in an operational mode. Wind inputs for two operational spectral wave models were analyzed with respect to observed winds at three locations in the Canadian east coast offshore. Also, significant wave heights obtainable from the two spectral models were evaluated against measured wave data at these locations. Based on this analysis, the importance of appropriate wind specification for operational wave analysis and forecasting is demonstrated.

ESTUARY GEOMETRY AS A FUNCTION OF TIDAL RANGE

Tanah Lot Temple is situated in Tabanan Regency - Bali, on the coast of the Indonesian Ocean. Due to continuous wave attack, wind force, and weathering of the rock bank where the Temple stands, abrasion has occured which is more and more threatening the existence of the Temple. Considering that Tanah Lot Temple is a sacred place for the Hindu Balinese people and a place of high cultural value, and also an important tourism, steps to save the Temple are imperative. The Central as well as the Regional Authorities, and also the Bali nese community are very much interested in the effort to keep the Temple intact. Measures have been undertaken to protect both the seaside and land-side banks of the Temple rock bank. This paper only discusses counter measures of the sea —side bank of the Temple.

Changes in wave impact on the Flemish coast due to increased mean sea level

A numerical model has been used to study the effect of sea level rise on the wave climate along a stretch of the Belgian coast which lies adjacent to a linear sand bank system. Results of the model have been interpreted to permit predictions of total and local wave height and energy variation under the assumption that the banks do not respond quickly to rises in mean sea level. Results show that substantial variations in wave energy change occur along the coast under a uniform mean sea level rise. Average wave energy impacting the coast is predicted to increase by the order of 10% by 2130 under the wave conditions tested and the sea level rise scenario used.

A 2D MODEL OF WAVES AND UNDERTOW IN THE SURF ZONE

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.

NUMERICAL MODELLING OF FLOW OVER RIPPLES USING SOLA METHOD

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.

Hydrodynamic modelling of a dynamic inlet

This paper describes details of a 2D hydrodynamic model and its applications to a dynamic inlet. The model computes the depth-average tide and wave-driven currents, as well as wave characteristics. It forms part of a Q3D model which also includes a module to compute undertow velocity distribution through the water column at a point-in-plan in the surf zone. The 2D hydrodynamic module allows for the effects of wave diffraction and refraction, wave breaking and wave-current interaction to be included in dynamic simulations for combined tidal and wave situations. The 2D model was applied to a newly-opened dynamic inlet near Faro at southern Portugal, with conditions obtained during a four-month intensive field campaign under Project INDIA (Inlet Dynamics Initiative: Algarve) funded by EU MAST III Research Programme. The model results were compared with field data measured by a number of remote sensing instruments and showed good agreement.