In case of flooding many authorities and organizations become involved and there is a problem to take in the whole situation and have a common picture when many incidents happen at the same time. There is also a lack of efficient tools showing critical buildings and constructions in combination with actual and forecasted water-levels. When handling critical situations we are facing challenges of complexity, uncertainty, and unpredictability. Such management- and decision activities are normally supported by some kind of models and support tools. However, complexity is normally not explicitly addressed in such models and tools. In this paper we analyse and discuss different kinds of complexity, which are a challenge in critical situations caused by flooding.