End of August 2005 the flood defences of New Orleans were hit by hurricane Katrina. It quickly became apparent that they could not withstand this force of nature. The three bowls of the city were flooded. Over a thousand people lost their lives and the total damage exceeded $20 billion US. What can we learn from this disaster? Can the process of understanding be supported by mathematics? Is it possible to draw conclusions with the help of mathematics that can help to avoid a repeat of this tragedy?

Two years after the disaster no decision has been taken about the required level of protection. This is a mathematical decision problem where the increasing cost of protection is equated with the reduced risk (probability × consequence) of flooding. Where the sum of the cost of protection and the present value of the risk reaches a minimum, the optimal level of protection is found. Along this line of reasoning the level of flood protection of the Netherlands was decided in 1960. However today some think that an insurance against the consequences of flooding is to be preferred over spending money on a flood defence system that will never be absolutely safe. Others judge it necessary to prepare the evacuation in case of a flood because perfect safety by flood protection is unattainable. Mathematics shows that both options are probably no alternative to optimal prevention.