Flood risk emerges from the interaction of flood hazard, exposed assets and their vulnerabilities. Flood hazard can be described by the probability and intensity of high river flows, and the exposed assets are the economic, ecologic and social elements that are threatened by floods. The vulnerability of the element-at-risk decides to which degree a flood-affected element is damaged. All three components of flood risk are changing in time, and increasingly faster changes are observed.
Given the currently observed climate change, variations in the frequency and intensity of floods have to be expected. Although the concrete impacts of climate change on the regional flood situation are uncertain for most regions, process-based reasoning and observations point to increasing flood risks in many regions. Whereas climate change occurs on the time scale of decades, changes in assets and in vulnerability may even be faster and may have a more profound consequence on risk. Modern societies depend to an increasing degree on infrastructures (energy supply, communication, transportation, water, etc.) and rapidly develop new vulnerabilities if their functionality is harmed by extreme events.
This global change increases the uncertainty with which water resources authorities and decision makers are faced. Mitigation options have to be chosen in view of an unclear future. Given large uncertainty bounds, flexible mitigation solutions and no-regret-policies are preferable. For example, resilience strategies, such as early warning systems, may be more appropriate than traditionally preferred resistance strategies, such as large-scale flood retention basins. Capacities for crisis intervention have to be developed in case that the future flood risk is underestimated.