Rain related flash flooding that occurs away from permanent watercourses (pluvial flash flooding) is a topic that has gained a lot of attention over the last years. Not only urban areas with a high percentage of sealed surfaces, but also rural areas have been adversely affected by these phenomena in the recent past. Besides causing a substantial amount of material damage and monetary losses, these pluvial flash floods can also present a threat to the lives of people in the affected areas. Previous studies indicate that pre-alpine areas (in Austria, Germany,...) are especially susceptible to this type of flooding.
Consequently government authorities on different levels of administration are currently working on strategies to manage the risks associated with pluvial flash floods. Amongst others the production of state-wide hazard indicator maps for pluvial flash floods and the development of guidelines for pluvial flash flood impact studies on local and municipal scale are currently topics of ongoing discussion.
Within this context it is important to assess the availability and usability of data sets from national and pan-European climate-services for pluvial flash flood hazard and risk assessment on different spatial scales (regional to local).
Decision support to client
For pluvial flash flood hazard assessment the definition of representative scenarios is an important topic. This includes the choice of appropriate precipitation-scenarios and their spatial distribution as well as initial conditions (e.g. antecedent soil moisture content, disposition of different areas of land use and soil types to produce surface runoff). Depending on the scale of a study the choice of scenarios can be either pragmatic (e.g. regional scale hazard indicator maps) or has to be more elaborate (e.g. design of mitigation structures). In either case information about the availability and quality of respective datasets is indispensable for the involved stakeholders.
Regardless of scale also possible future developments concerning the intensity and frequency of potential triggering events are of interest for developing sustainable coping and adaption-strategies.
Currently in Austria design-precipitation sums are available in approx. 6kmx6km resolution for return-periods between 1 and 100 years and durations ranging from 5 min up to 6 days. On national level currently no future projections of climate change are included in theprovided datasets. However some pan-European climate services provide projected change rates that might be utilized.
Temporal and spatial scale
Pluvial Flash Floods are often triggered by convective rainfall events featuring heavy rainfall with short-durations. Thus, the affected areas are in many cases confined to a few km² and their typical event-durations range from around 15min to only a few hours.
Thus, process scales of pluvial flash flooding are usually small with regard to spatial and temporal extends. In contrast, scales and responsibilities for decision making processes are varying and take place at different scales ranging from national and provincial (national and federal government authorities) to municipal and local level (municipalities and property owners).
Pan-European and local indicators
The most important climate indicators for pluvial flash flood assessment are statistical values characterizing extreme precipitation sums for short-duration precipitation events (“design-rainfall” for short-duration events). These indicators are available on national level in Austria based on extreme-value analysis of time-series from meteorological stations.
Projections of future developments for short-duration rain events are not available on the national level, but might be supplemented with data from pan-European climate-services.
However, given the local nature of convective storm-cells, there are well-known challenges in reliably measuring peak precipitation intensities. Therefore the usability and quality of the data from both national and pan-European climate-services with respect to pluvial flash flood impact studies should be carefully evaluated.
Case study workflow
Importance and Relevance of Adaptation
Recent years have shown that pluvial flash floods pose a serious threat to lives and properties in pre-alpine regions (of Austria, Germany,...). The assessment of potential pluvial flash flood impacts (endangered areas, potential flow heights and velocities) serves as an important information base for the development of mitigation strategies for existing infrastructure. Also sustainable strategies for spatial and settlement development rely on information about the flash flood hazard on currently undeveloped areas.
Climate-service data (e.g. design-precipitation, future precipitation trends) is a crucial ingredient for reliably assessing pluvial flash flood hazards. Therefore it is relevant to evaluate the potential and limitations of datasets currently offered by different climate-services for pluvial flash flood impact studies on different scales. Communication and discussion of the status-quo with involved stakeholders also presents an opportunity to bring up ideas for the improvement of existing climate services.
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Consultancy or Knowledge Purveyors
University of Innsbruck, Unit of Hydraulic Engineering - Institute for Infrastructure Engineering (UIBK); Austria
University of Natural Resources and Life Sciences, Vienna; Institute of Meteorology (BOKU-MET); Austria
Federal government of Upper Austria, Section of surface water bodies; Austria
Municipality of Schwertberg, Upper Austria; Austria
Assoc. Prof. DI Dr. Stefan Achleitner, Unit of Environmental Engineering, University of Innsbruck
Assoc. Prof. Dr. Herberg Formayer, Institute of Meteorology, University of Natural Resources and Life Sciences, Vienna