Scenario’s for flood risk management

To find out how an area may flood, when a dike breaches for example, scenarios are developed. This can be done with a computer simulation model. The topographical map of an area is first divided into squares of, for instance, 100 by 100 meters. This map is fed into the model. The simulation model calculates how the water spreads over the area on the basis of the height of the terrain and the obstacles the water encounters. The simulation shows how the water spreads and how high the water will be in the end. Some models also make it possible to see other aspects of the flood like the velocity of the incoming water.

In this film you see how the water from a river spreads through an area of the Netherlands, called Land van Maas en Waal, when there is a breach in the dike near Weurt. You can see the height the water reaches in the course of time. This is not likely to happen, but it is a good example of what this kind of modelling can show. You can see that nearly the whole area will get flooded. The west side is lower than the east and therefore you see the water flowing in that direction first. In this film the water can continue to enter for days. In reality measures will be taken to prevent this. This film has been kindly provided by D. Alkema of the ITC and University of Utrecht.

Different flood aspects.

Source: D. Alkema, 2003.

Other things can be calculated with this model too. The next illustration shows several other aspects of the flood:

• the depth the water can reach
• the highest speed for each location (square)
• the maximum impulse (“force” or “strength” of the water: mass multiplied by speed)
• the maximum speed with which the water rises
• the duration of the flood (time that the land will remain flooded)
• the relative erosion or deposition (will in the end soil be lost from or be brought to a location)
• the progress of the flood (which areas are flooded first)

These aspects are important for disaster preparation. For a disaster plan it is necessary to know which areas will flood first, but also which area will become most dangerous. The dangerous places may not be those locations that are flooded first, but those that experience the highest impulse of where the water will reach the highest level.

This information can also be used when decisions need to be made on spatial planning: where can you build and where you can’t.