Flood risk arises where a flood hazard meets people and assets that are exposed and vulnerable to it. The application combines hazard, exposure and vulnerability into a relative risk index and identifies the areas where mitigation and preparedness measures are most needed.
OutputIndex map per category and a combined flood risk map
DecisionPrioritisation of areas for flood mitigation and preparedness measures
ScaleMunicipality to river basin
Built forA defined flood type and event scenario
Default categories
The suggested criteria follow the disaster-risk concepts of the United Nations Office for Disaster Risk Reduction (UNDRR) and fall into three categories, each with its own index map. The selection is a starting point, not a prescribed risk index. These default categories can be renamed or removed by the user, and new categories can be added.
Flood hazard
Depth, velocity, duration and probability of flooding, or terrain and runoff proxies where no flood model exists.
Exposure
People, buildings and critical facilities located in the area that floods.
Vulnerability and capacity
Susceptibility of buildings and population to damage, and the capacity to evacuate and respond.
The criteria library is maintained continuously. In each project, criteria can be added, removed or renamed.
Flood type and scenario
Hazard criteria refer to one flood type and one event scenario, for example a flood with a 100-year return period. All hazard layers of a project need to describe the same event.
- Fluvial
- Pluvial
- Coastal
- Flash flood
- Groundwater flooding
Modelled hazard layers are preferred where available. Terrain proxies, such as height above the nearest drainage, are input variables of flood models; combining them with modelled hazard counts the same effect twice.
Constraints
Risk maps usually cover the whole study area. Constraints serve to mask permanent water bodies or areas outside the flood model domain; the application suggests none.
From criteria to risk
- CriteriaStandardised so that high scores consistently mean high risk
- Category mapsWeights within each category give the hazard, exposure and vulnerability maps
- Risk mapWeights between the categories combine the three maps
- Priority areasContiguous areas of very high risk in which no criterion falls below a minimum value
The direction of each criterion usually follows from its role in the risk: greater flood depth and duration, a higher density of people and assets and a lower capacity to evacuate increase the risk, while a greater height above the drainage network decreases it. Weights come from the analytic hierarchy process, ranking, rating, the multi-influencing factor method or an anonymous stakeholder survey. Stakeholder weighting is common in flood risk studies, where residents, authorities and emergency services value exposure and vulnerability differently.
Results
The result classes rank the locations within the study area by their relative risk. The index does not state a flood probability or an expected monetary loss.
Sensitivity analysis shows which areas keep their class when the weights change. Validation against observed flood extents and damage records will follow in a later version.
Typical questions
- Where do high flood hazard and dense settlement coincide?
- Which critical facilities, such as hospitals or schools, lie in areas of high risk?
- Where would early warning and evacuation planning reduce risk most?
- How does the risk pattern change between residents’ and authorities’ weights?
Case stories
No case stories for this application yet.
References
- de Brito, M.M., Evers, M. (2016). Multi-criteria decision-making for flood risk management: a survey of the current state of the art. Natural Hazards and Earth System Sciences 16, 1019–1033. doi:10.5194/nhess-16-1019-2016
- European Union (2007). Directive 2007/60/EC of the European Parliament and of the Council of 23 October 2007 on the assessment and management of flood risks. Official Journal of the European Union L 288, 27–34.
- Kron, W. (2005). Flood risk = hazard • values • vulnerability. Water International 30(1), 58–68.
- UNDRR (2017). Terminology on disaster risk reduction: disaster risk. United Nations Office for Disaster Risk Reduction. undrr.org
Start a flood risk project
Select “Flood risk assessment” when creating a project. The suggested criteria appear in step 2.