Welcome to this module on the risk of natural hazards.

My name is Pieter van Gelder,

and I work at the Safety and Security Science Section of

Delft University of Technology in the Netherlands.

In this module, I will tell you about the methods

used to quantify the risks of natural hazards

and about the factors which influence the perception of

the general public who are exposed to these hazards.

Let's begin by asking ourselves how do we analyze the risks related to natural hazards?

To analyze the risks related to natural hazards,

experts usually adopt a two stage approach.

Firstly, we estimate the likelihood that extreme natural events will take place.

And secondly, we estimate the consequences to society of these natural events.

Let's start with the first step.

One of the standard ways to estimate the likelihood of

an extreme natural event is through statistical data analysis of historical observations.

Over the course of many years,

meteorological as well as geophysical data has been recorded and is available

for detailed statistical analyses on both temporal and spatial scales.

Let me give you an example.

What you are seeing now is a figure of extreme water levels which have occurred at

a location Hook of Holland along Dutch coastline over the past 120 years.

These extreme water levels have occurred due to strong and persistent winds over

the North Sea in combination with high astronomical tide levels.

Each yellow circle represents the water level in centimeters above mean sea level

and the corresponding likelihood of that particular water level occurring per year.

If we extrapolate these observations on logarithmic paper,

we can estimate the height of the North Sea for very small chances.

For example, a chance of a storm event where

the water level has exceeded is 10 to the power of -4.

This means that it occurs on average only once in 10,000 years.

The figure shows that this event corresponds to

a water level of approximately 460 centimeters,

although it should be noted that the uncertainty is quite large,

roughly from 400 to 520 centimeters.

In the second step, we estimate the consequences of a natural hazard.

We can do so by using advanced models and simulations.

For example, for flooding,

hydrodynamic calculations can be made to simulate a flood.

This will allow us to see which land areas will be submerged.

For one type of natural hazards, such as floods,

this exercise may go better than for other types of natural hazards, such as earthquakes.

This is because models to simulate the effect of an earthquake

to non-earthquake resistant buildings are still quite unreliable.

These two steps can quantify the risks of

natural hazards from an objective or expert point of view.

However, do these align with the perceived risks of the public?

The answer is negative.

While the public may be exposed to the risk of natural hazards,

they tend to forget about this risk

if no events have happened over a large number of years.

For example, many Dutch people are not aware of the risks of flooding any more.

This is because the last catastrophic flood experienced

in the Netherlands is already more than 50 years ago.

On the other hand, if natural disaster has struck,

people perceived the natural risk much higher than

the objective risk level as calculated via the expert models.

As a consequence of this bias,

there can be a significant discrepancy between how experts see the risk of

natural hazards and how it's perceived by the public.

So what have we learned?

First, we have seen that the likelihood of

natural hazards can be estimated through analysis of

historical data and the impact can be estimated by advanced models and simulations.

Secondly, we have seen that there is a difference between objective and perceived risk.

When it comes to natural hazards,

the public tends to underestimate the risks of these hazards if

they have not observed them in their close neighborhoods over a long enough time.

If they have observed them,

they tend to overestimate the risk.

In the next video, we will talk about some of the strategies that

are used to deal with the risk of natural hazards.