Eng Soon Chan

Laboratory study of deep-water breaking waves

Abstract In an attempt to elucidate the mechanics of deep-water wave breaking, a variety of breaking waves, including spilling and plunging waves, of different length scales and geometries was studied. The waves were generated through wave-wave interactions using wave packets with constant-steepness components, constant-amplitude components, and also components following the Pierson–Moskowitz distribution. Wave steepening prior to breaking were found to cause an increase in the high frequency spectral slope of the wave spectrum. The slopes were correlated to the type of breaking and the intensity of the breaking. The energy loss through breaking varied with the spectral characteristics of the wave packet. On the other hand, it was also noted that, irrespective of the wave packet, the losses were from the higher frequency end of the first harmonics.

Mechanics of deep water plunging-wave impacts on vertical structures

Abstract The pressures resulting from a critical case of plunging-wave impact on a vertical wall are examined. A deep-water wave breaking condition, with the crest elevation about a fifth of the mean water depth, has been considered. Two distinct scales of the pressure time histories, a slowly varying (in the range of the wave period) component in the order of stagnation pressure and a transient impact component with much higher pressures, have been identified and correlated to the incident wave kinematics. It is found that impact pressures can be approximately decomposed into a primary component associated with the overall wave evolution and a component influenced by the trapped air dynamics. These characteristics are comparable to those observed in shallow water-wave impact studies. The loads at the zone of impact are also found to be comparable if the span of impact and the local incident wave kinematics are comparable.

Laboratory study of plunging wave impacts on vertical cylinders

The characteristics of pressures associated with plunging wave impacts on a vertical cylinder are presented. Despite a high variability in the peak pressures, spatial distributions of the impact pressure time histories, both in the vertical direction and around the seaward front of the cylindrical surface, are found to vary systematically for a range of cylinder locations in the wave plunging region. Through correlations between the incident wave kinematics and the impact pressures, the mechanics of the impact process have been identified and elucidated. These results serve as a useful reference for future numerical and theoretical modelling of extreme wave loads on surface-piercing vertical cylinders.

The Physical Oceanography of Singapore Coastal Waters and Its Implications for Oil Spills

The physical oceanography of the Singapore Strait and ambient seas is governed mainly by the tides together with a seasonal net circulation. The interaction of tidal streams from the Malacca Strait, South China Sea and straits linking the Malacca and Singapore Straits to the Java Sea makes the overall flow in the Singapore Strait rather complex. Being a node at the confluence of these interacting water bodies, any discharges in the domain, such as oil spills, would affect the surrounding seas, although the concentration would be diluted through mixing and transport. In addition to physical processes, the ultimate fate of a pollution event would depend on the coupled interactions with biology and chemistry of the water body, including food-chain interactions and biodegradation. For the more persistent organics in the marine environment, baseline studies showed that concentrations of PAHs measured in Singapore’s coastal waters were generally higher than levels reported elsewhere, whereas OCPs and PCBS were generally lower than reported levels for other Asian countries, but higher than some levels reported elsewhere in the world. Overall, the prevalence of POPs in Singapore’s coastal waters suggest the need for continued monitoring and evaluation of their transport and biological impact in the marine environment.

Modeling of the Turbulence in the Water Column under Breaking Wind Waves

Past studies have shown that there is a wave-enhanced, near-surface mixed-layer in which the dissipation rate is greater than that derived from the “law of the wall”. In this study, turbulence in water columns under wind breaking waves is investigated numerically and analytically. Improved estimations of dissipation rate are parameterized as surface source of turbulent kinetic energy (TKE) for a more accurate modelling of vertical profile of velocity and TKE in the water column. The simulation results have been compared with the experimental results obtained by Cheung and Street (1988) and Kitaigorodskii et al. (1983), with good agreement. The results show that the numerical full model can well simulate the near-surface wave-enhanced layer and suggest that the vertical diffusive coefficients are highly empirical and related to the TKE diffusion, the shear production and the dissipation. Analytical solutions of TKE are also derived for near surface layer and in deep water respectively. Near the surface layer, the dissipation rate is assumed to be balanced by the TKE diffusion to obtain the analytical solution; however, the balance between the dissipation and the shear production is applied at the deep layer. The analytical results in various layers are compared with that of the full numerical model, which confirms that the wave-enhanced layer near the surface is a diffusion-dominated region. The influence of the wave energy factor is also examined, which increases the surface TKE flux with the wave development. Under this region, the water behavior transits to satisfy the classic law of the wall. Below the transition depth, the shear production dominantly balances the dissipation.

Breaking wind waves as a source of ambient noise

A theoretical model for the prediction of ambient noise level due to collective oscillations of air bubbles under breaking wind waves is presented. The model uses a budget of the energy flux from the breaking waves to quantify acoustic power radiation by a bubble cloud. A shift of the noise spectra to lower frequency due to collective bubble oscillation is assumed. The model derives good estimates of the magnitude, slope, and frequency range of the noise spectra using the wind speed or height of breaking waves.

Sensitivity studies with the three-dimensional multi-level model for tidal motion

Abstract A three-dimensional multi-level hydrodynamic model has recently been developed and applied to tidal motion in Singapore’s coastal waters. This paper describes a series of numerical experiments to evaluate the sensitivity of the tidal currents and elevations to model parameters. The results show that the predicted tidal elevations are insensitive to three model parameters: horizontal eddy viscosity coefficient (Smagorinsky constant, c h ), bottom friction coefficient ( c b ) and internal friction coefficient ( c v ), whereas the effects of these parameters are quite different for tidal current velocities. The velocities are slightly reduced with an increase in c h and c b . The bottom friction effects on velocity profiles increase with water depth. The effect of c v might be significant for the tidal velocities at all levels. The velocities at upper layers of the water column decrease with the increase in c v , whereas the velocities at the bottom layer show the reverse trend. The effects of three model parameters on the magnitude and phase of the simulated currents are in the order (from strong to weak) of c v , c b and c h .

Wind Wave Effects on Hydrodynamic Modeling of Ocean Circulation in the South China Sea

Wind, wave and current interactions control the boundary fluxes, momentum and energy exchange between the atmosphere and the ocean, and within the water column. The wind wave effect on the circulation is investigated in a three- dimensional time-dependant ocean circulation model. This POM (Princeton Ocean Model) based model is implemented with realistic coastlines in South China Sea and emphasizes the simulation of physical parameters in the water column. Taking account of the wind waves, an increase in air-sea drag coefficient, reflecting an enhanced sea surface roughness due to increased wave heights, is shown to improve the simulated surface current and the sea surface elevation. It is also found that developing waves with smaller peak periods influenced the surface circulation more significantly. The inclu- sion of the wind wave parameterization also affects the current near the seabed in the shallow water. The model is validated against current, temperature and salinity data measured in the Asian Seas International Acoustics Experiment (ASIAEX). The simulation results in the period of April - May 2001 show that wave-induced surface stress increases the magnitude of currents both at the surface and near the seabed. On the other hand, wave-induced bottom stress retards the near bottom currents in shallow water. Therefore the net effect of wind waves on circulation depends on the significance of current and elevation changes due to wind waves through both the surface and the bottom.

EXPERIMENTAL STUDY OF TURBULENT OSCILLATORY BOUNDARY LAYERS IN A NEW OSCILLATORY WATER TUNNEL

A new oscillatory water tunnel has been built in the Civil and Environmental Engineering Department’s Hydraulic Laboratory at the National University of Singapore. It can accurately produce oscillatory flows that correspond to full-scale sea waves. Tests including pure sinusoidal waves and combined wave-current flows over smooth and rough bottoms have been performed. High quality measurements of the boundary layer flow fields are obtained using a PIV system. The PIV measured flow field is phase and spatially averaged to give a mean vertical velocity profile. It is found that the logarithmic profile can accurately approximate the near-bottom first-harmonic amplitude of sinusoidal waves and give highly accurate determinations of the hydrodynamic roughness and the theoretical bottom location. The bottom shear stress obtained from momentum integral is in general agreement with results from log-profile fitting. The current profiles of combined wave-current flows indicate a two-log-profile structure as suggested by simple combined wave-current flow theory. The difference between the two current shear velocities obtained from combined wave-current flows, as well as a small but meaningful third harmonic embedded in a pure sinusoidal wave, suggest the existence of a time-varying turbulent eddy viscosity.

TSUNAMI PROPAGATION MODELING AND FORECASTING FOR EARLY WARNING SYSTEM

After the devastating Indian Ocean 2004 Tsunami, coastal economies around the Indian Ocean have been reminded of the necessity to make well-coordinated efforts to deal with the tsunami problem. An integrated socio-technological infrastructure has to be built, with key tasks including advanced sensors, reliable communication networks, fast predictive algorithms, early warning systems, and educational outreach. This paper highlights the key features of a prediction system under development in Singapore in support of the early warning system being developed in the region.