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Wave overtopping risk for pedestrians

In urban coastal areas, people can freely access the crest area of coastal protection structurs (e.g. revetment or seawalls) for various recreational activities. Wave overtopping can be a potential threat to these pedestrians. In the context of global climate change, we expect to see rising sea level and harsher wave conditions in Singapore coastal regions, so it is unclear whether the existing seawalls will be sufficiently safe for allowing public accesses in the future. We also need to know how new seawalls can be better designed to achieve a balance between ensuring safety and reducing construction cost.

drawing Our group studied the allowable wave overtopping conditions regarding people’s safety through quantitative model predictions. Here the risk is defined as people losing balance under flow impact, and the research work is targeted on typical seawalls in Singapore, e.g. with a 1:3 frontal slope. Under this project, we used a numerical model that can precisely simulate the process from wave shoaling to overtopping-human interaction. In addition, physical model tests were conducted using the state-of-the-art facilities at the hydraulic lab of CEE @ NUS. This model enables developing a probabilistic risk-analysis framework for people’s safety under overtopping flows. A risk framework is proposed to quantify the probability of risk occurrence for different people groups (e.g. children) and different locations on the seawall (distance to the seafront edge). With this risk-analysis framework and the projected future sea levels and wave conditions, we can investigate how climate change affects wave overtopping and propose risk mitigation strategies (for instance, new designs of seawall or restricting people’s access during certain wave conditions).

Hydrodynamics and sediment transport for wave-induced sand ripples

Sand ripples generated by water waves are commonly observed in the coastal area, and their existence complicates the local sediment transport, but very few detailed full-scale experiments on this topic is available in the literature. We used oscillatory water tunnels to conduct a variety of full-scale experiments, and investigated the following:

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  • Development and geometry of sand ripples

  • Flow resistance due to sand ripples

  • Net sediment transport rate in the vortex ripple regime

  • Coherent vortex produced by asymmetrical oscillatory flow over vortex ripples

In addition, we also used Direct Numerical Simulation (DNS) and large Eddy Simulations (LES) to reveal the detailed flow process, e.g., turbulence characteristics. The research findings on boundary layer flow and sediment transport are used to develop simple models for predicting ripple- and time-averaged net sediment transport rate.

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Coastal sheet-flow sediment transport

Sediment transport under very intense flow conditions usually occurs in a layer very close to the bed with a thickness of several times the typical particle diameter, which is defined as the sheet-flow sediment transport. Sheet flow occurs in shallow coastal waters under storm waves, and therefore is an important research topic in coastal engineering.

drawing We combined experimental and numerical techniques to investigate the net wave sheet-flow sediment transport due to a variety of factors: (1) wave-current interaction, (2) bottom slope, (3) wave nonlinearity, and their combinations. All experiments were conducted using an oscillatory water tunnel for achieving full-scale conditions. An interesting finding is that bottom slope is equally important as wave nonlinearity in producing net transport rate. In addition, we developed a RANS-based one-phase model, which include a novel method for parameterizing the sediment motion in the lower part of sheet layer. Finally, a semi-analytical model was developed for quick estimating net sheet-flow sediment transport rate under wave-current flows.

Coastal turbulent boundary layers

drawing In the coastal environment, surface waves and currents are always simultaneously present and nonlinearly interact with each other in the near-bottom region, which leads to a wave-current boundary layer. A thorough understanding of this combined wave-current boundary layer is a prerequisite for the prediction of currents, i.e. circulation, as well as sediment transport in coastal waters. We conducted a variety of experiments of turbulent wave-current boundary layer flows using the oscillatory water tunnel at NUS for flow generations and a Particle Image Velocimetry system for velocity measurements. Meanwhile, a semi-analytical model which adopts a rigorous way to account for a time-varying turbulent eddy viscosity is developed to interpolate experimental results. Many interesting features of turbulent wave-current boundary layer is discovered and neatly interpreted by the theoretical model. We manage to show that turbulence asymmetry streaming is an important factor for wave-current interaction, which is not considered in many models we know. We also investigate the effect of wave irregularity on wave-current boundary layer. Through comparative experiments, we confirmed the validity of the assumption of an equivalent regular wave for irregular-wave-current interaction. We also showed that the wave-by-wave approach is mostly valid for studying boundary layers under individual waves.

WCS