Wave overtopping hazards can be identified based on the strength of overtopping flows. Direct impact of wave overtopping on people can cause him/her to fall, resulting in injury or being carried into the sea by backwash flows. Therefore, the top concern is whether a person can maintain balance during a wave overtopping event. Less severe hazards, e.g. getting wet or being frightened by the threat of inundation, need to be considered only for very special situations. Apparently, practical guidelines must be developed based on a good understanding of flow-human interaction and a rigorous risk-analysis methodology. Therefore, the overall objective will be realized by achieving the following specific objectives.

1. Develop a numerical model for wave overtopping and overtopping-human interaction. The whole hydrodynamics process from waves arriving at the toe of seawalls to wave slamming a human body must be accurately modeled, which is the basis for fully understanding how wave overtopping produces risk. Thus, the core of this project is to develop a robust and accurate numerical model for the hydrodynamic process.
2. Develop a conceptual model for human instability. To predict the risk occurrence, a conceptual human instability model, which assesses whether the overtopping forces on human body can destabilize him/her, will be established. The model development will be based on examining two typical failure modes, e.g. sliding and overturning, and the model parameters will be calibrated against existing laboratory or field measurements, including some real-human experiments.
3. Develop a quantitative framework for assessing pedestrian’s risk under wave overtopping. Probability is the most common method for assessing and managing risks. Therefore, a probabilistic risk-assessment framework will be established based on the numerical hydrodynamic model and the conceptual human-instability model developed in this study. This methodology allows us to assess the risk level as functions of the offshore wave climate (wave heights and period), the seawall design (slope of the seawall and surface condition) and other relevant variables, e.g. location on the seawall crest.
4. Propose guidelines for seawall design and risk management. With the risk-assessment framework and a large dataset generated by numerical and physical models, new guidelines for seawall design will be proposed to make the local risk level on the seawall crest comparable to that of some common activity, e.g. driving. In addition, case studies will be conducted, in which the risk-assessment framework will be applied to selected local seawalls. Risk-mitigation strategies, e.g. changing seawall design, will also be recommended.

Singapore is the quintessential land-scarce island city state, of which a large portion of land is reclaimed from the sea and protected by seawalls (commonly 1:3 slope revetments). As a result, seawalls cover most (63.3%) of Singapore’s coastline, and the coverage percentage may even increase due to the future land reclamations. Public access to seawall crests is allowed in many locations, since these areas are of high recreational values. However, the associated public risk is not immaterial, as evidenced by the number of severe accidents in other countries, e.g., approximately 60 people have been killed by wave overtopping in Italy over the last 20 years (Allsop, 2004). Furthermore, the future risk level is believed to be higher due to the harsher wave conditions and the rising sea level associated with the global climate changes. On the other hand, it is desirable to reduce conservatism in the design of seawalls due to high construction and maintenance costs. With the proposed research, we seek to understand whether the existing seawalls can protect us against the foreseeable future risks, and also provide guidelines for better designing new coastal defense structures. Therefore, this work can enhance Singapore’s coastal resilience and make Singapore a safer city. Beside the local translational values, the research outcomes are readily transferrable to other countries, and therefore can be globally impactful.