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WP 3 Flash flood risk analysis

Last edited by Christina Maus

Introduction and background

Heavy rainfall and flash floods are increasing in intensity and frequency worldwide due to climate change. Damages to infrastructure up to losses of lives are always linked to such events. E.g. infrastructure damages have been enormous during the 2021 flash flood catastrophe in western Europe or the 2023 events in the Mediterranean. Losses of lives are striking particularly in countries that lack early warning systems and resources to measure rainfall and model flash flood events. Hazards of heavy rain and flash floods need to be studied and modelled to improve early warning. Also, modelling the effects of heavy rain and flash floods will allow to identify measures which are cost-effective to reduce flash flood risks.

The CapTain Rain work package 3 (WP 3) focuses on the approach of modelling flash flood hazards and risks, and illustrates how this approach was applied in Jordan within the research project (2021-2024). Looking at Jordan as one of the water scarcest countries of the world, challenges for modelling flash flood events were found both in data availability and in resources to measure and model flash flood hazards and risks. Nevertheless, an approach to model both heavy rain and flash flood hazards has been identified. Coupled with information on affected infrastructure and people, the risks of flash floods could be demonstrated for both the urban region of Amman and the rural area of Wadi Musa with the world heritage site of Petra.

Analysing flash flood hazards with hydrological and hydraulic models

To model flash flood hazards, i.e. water flows and water levels resulting from heavy rain, hydrological and hydraulic modelling is used. When the hydraulic models have been set up and run, the results are flood hazard maps. These maps are crucial information tools to inform administration and the public about the effects of heavy rainfall events. With such maps, early warning can be targeted to flooding areas. Elements of flash flood management can be improved from insurances and preparation of events up to object protection. The flash flood hazard maps are therefore very important climate services. Hydrological models provide hydrographs, which might be used in a real time early warning system because of very fast computation time. Also, first ideas about effects of potential changes in climate, land use, and adaptation measures can be simulated.

Both hydrologic and hydraulic models were set up by the CapTain Rain project for the city of Amman and for the rural area of Wadi Musa with the Petra world heritage site. Challenges include input data such as the digital elevation model (resolution, quality), information about infiltration rates based on land use and soil data, and availability and quality of rainfall data. Since no runoff gauge data exists for the modelled watersheds, model calibration was not possible, only a validation with comparing modelled water levels to real event documentation by pictures and videos. Model application is not limited to producing flash flood hazard maps: The models are the basis to analyse the risk of settlements to flash floods and their vulnerability. Also, the models allow to analyse the effects of measures to reduce such risks. Both steps were taken also in the CapTain Rain project for the two pilot areas of Amman and Petra. Hydrological and hydraulic models are important tools for decisions towards cost-effective and sustainable flash flood management.

Analysis of flash flood risk

Risk is a result of a hazard combined with a potentially vulnerable area. If the area is not vulnerable, even though there is a flash flood hazard, there will be no risk. As a first element of analysing this vulnerability, a standard procedure in Germany includes the assessment of damage potentials. Damage potential is high for densely built-up urban areas since floods can cause damage to many buildings. If buildings have basements, the damage potential increases. If people are less mobile and thus less able to leave flooded areas, the damage potential also increases. This is the case for hospitals or child care facilities. Then, damage potential is also high for critical infrastructure including emergency brigades or industrial and energy infrastructure. To produce damage potential maps requires again a solid data base, this time including building usages, detailed information about infrastructure (e.g. location of basements), and information about critical infrastructure.

When a damage potential map has been produced, the overlay with flood hazard maps produces the flash flood risk maps. These maps are again a critical information tool – in this case in particular for decision makers and planners. Also, emergency managers need to have such information to target their emergency measures to areas of highest risk first.
The CapTain Rain project has prepared both damage potential and risk maps for the two pilot areas of Amman and Petra.