Rogger, M., Pirkl, H., Viglione, A., Komma, J., Kohl, B., Kirnbauer, R., Merz, R. and Blöschl, G. 2012. Step changes in the flood frequency curve: Process controls. Water Resources Research, 48, W05544, doi:10.1029/2011WR011187.
Doctoral Programme on Water Resource Systems
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Step changes – a step towards understanding big floods
Understanding how floods occur is essential for their prediction. Considering catchment storage and critical thresholds will help overcome the inaccuracy of statistical forecast models.
Information on the magnitude and frequency of flood events is critical to ensure that infrastructure such as dams, bridges and flood protection measures are built to cope with high water levels. Engineers need to know how big a flood will be, and how often it will occur. Hydrologists traditionally provide this information using the flood frequency curve that describes how often water levels of a certain magnitude occur (e.g. every 5, 50 or 100 years). Smaller magnitude events occur more frequently than larger magnitude events. Hydrologists often fit a statistical model to available river flow data to predict how often a large flood event might occur. One problem with this is that often the available data can be very limited, perhaps only covering the last 5 or 10 years. A second problem is that for some river catchments, the flood record suggests that large events seem to occur more often than a statistical model would predict.
Big floods occur when critical thresholds in catchment storage occur
Magdalena Rogger and her co-authors were keen to identify why some catchments seem to experience a jump or “step-change” in the frequency of big floods. They hypothesised that big floods might occur when a critical threshold in the catchment is exceeded. The threshold could relate to the amount of rainfall occurring, and the water storage capacity of the catchment. To test their hypothesis, the authors collected information on the geology and water storage capacity of two catchments in the Austrian Alps. They used this information to create a rainfall-runoff model to simulate flood events in the catchments. By running statistical simulations of runoff, they were able to see that large floods occur when storage compartments in the catchment are exceeded. The excess water flows rapidly over the land surface, giving rise to high river levels. These findings mean that statistical models might underestimate the probability of a large flood occurring. They suggest that hydrologists and engineers may be better able to predict floods if they also consider the catchment storage and the critical thresholds that lead to step-changes.