Tsunami hazard and risk

Tsunamis are rare events that are often accompanied by advanced warnings. It is not possible to predict where and when the subsequent Tsunami might occur. However, tsunami warning centers know which events could likely generate tsunamis and issue warnings. This is done through networks of the ocean and coastal sea-level observation systems designed to detect tsunamis. These early-warning systems facilitate evacuation to areas of high ground outside of the tsunami inundation regions.

Often when tsunamis are triggered by distant events, which allows for longer evacuation times. Thus, the coastal communities can be evacuated. When this is triggered by local events, there might not be sufficient time for a horizontal evacuation. In such scenarios, a potential solution is a vertical evacuation above the rising waters into high-rise buildings and other structures with the strength and resilience necessary to resist the effects of tsunami waves. Tsunami risk assessment and management can include four major stages:

• Survey: In the first stage, a survey of existing structural heights is conducted. Further on, based on historical records and preliminary simulations, the necessary heights are obtained for the existing structures.

• Off-shore simulation: Here, the possible tsunamis and storm surge cases are simulations, and the resulting wave heights are studied.

• Inland simulation: Here, the resulting flood is simulated to calculate the possible and projected flood depths, velocity, time-series of a flood, etc.

• Time requirement: Finally, the time needed for evacuation and possible loss of life in the event of a tsunami are calculated.

Tsunami measurements

One of the essential aspects is related to the measurement of Tsunami. Three definitions are considered for the water level. The first is the “Inundation height.” The inundation height is the height from the average water level to the top of the flooded water level.

The second measure is the “Inundation depth.” This is the distance from the ground surface to the top of the flooded water level.

The last measure is known as the “Run-up height.” This is the height of the average water level to the water level where the Tsunami stops. This is important to gauge the overall tsunami energy.

Hazard vs. risk

Hazard is generally defined as the potential for an event to occur, while the risk is related to the consequences, given the event’s occurrence. Hazard is often characterized by the potential magnitude of the site-specific effects like an inundation, height of runup, flow depth/velocity. Risk is measured in terms of damage, loss of function, injury, and loss of life. The risk depends on many factors, including the vulnerability of the built environment, population density, warning times, ability to evacuate, etc.