NEW METHOD FOR 'UNCERTAIN' FLOOD MAPPING ADOPTED BY NATIONAL RISK WEBSITE
NEWS
In everyday life, people are used to dealing with uncertainties. On a cloudy day, we are more inclined to carry an umbrella because the expectation of getting wet is higher than on a sunny day. Apparently, a decision is made under uncertain conditions (in this case the likelihood of rainfall), based on information perceived.
Sometimes, we also weigh up the possible damage of a decision under uncertain circumstances. If you have an important appointment at 9 a.m. and there is a high likelihood of traffic jams en route, it is logical to allow yourself more time to reach your destination. However, if you are just heading to your office for 9 a.m. on a regular working day, you are not likely to take extra measures, because the acceptance of being late is viewed differently, as the impact is less damaging to your career.
Uncertainties in flood crises
So why not consider uncertainties in decision making for flood crises? Research over the past years has revealed vital information regarding flood forecasting, flood modelling and the strength of flood protection structures. This information also includes insight into the uncertainties that are related to the above. In decision-making procedures, uncertainty information can be used to reach a more informed and optimized decision in flood crisis management, for instance where to evacuate people first, where to evacuate to, what routes to be used and so on. In particular, when means are scarce, evacuation resources are limited, temporary flood protection measurements such as sand bags are in use, an optimized decision should take uncertainties into account.
Introducing uncertainties in a flood map (or other form, such as a table of water levels) sounds good, but how can we present these uncertainties? Experience shows that a “simple” flood map already gives rise to discussion, especially regarding the underlying assumptions and under which conditions the flood scenario will occur. Additional information in the legend or title is often needed and a variety of colours is used, to illustrate flood depth. The discussion becomes even more complicated when information on the severity of the event is added, for instance by showing a red colour in those areas that are regarded as “severely” flooded. However, whereas 30 centimetres of water might be regarded as “non-threatening” by a hydrologist, for a crisis manager, 4 centimetres of water might already form a problem.
New tool for visualizing uncertainties in flood maps
In the Flood Control 2015 research project “Uncertainties in Flood Scenarios” these questions were studies and partly solved. The following questions were addressed:
- Which uncertainties must be included in flood scenarios?
- How can these impact on the flooded area?
- How can we present these uncertainties in a flood map, which will improve decision making, during crises situations?
The project developed a tool for visualizing uncertainties in maps, in which the user can see what the impact of these uncertainties is. The project has also provided a first step to a better presentation of uncertainties for a wide group of users. The results of this ongoing research will be integrated in the www.risicokaart.nl website.
Above, you can see the impact of the new method. The figure on the left shows the probability of flooding, without consideration for uncertainties. The figure on the right does consider uncertainties, resulting in a more pronounced flood map.
For more information on this research, please contact Joost Lansen.