Projektbericht

Abstract

The floods of 2013 and 2016 were rather different in their characteristics, but inflicted comparable damage in Bavaria. Both were triggered by a low pressure system over central Europe but in a different specificity. In 2016 a stationary weather system caused recurrent extreme flash floods, and in 2013 a nearly Vb-track (pathway of low pressure from the Mediterranean Sea to the Northeast) caused long-lasting continuous rainfall (Fig. 1 and 4). These resulted in 2016 in extreme floods in very small river catchments, whereas in 2013 especially the big rivers Danube and Inn had extreme flood events which affected the cities close to those rivers, in particular the city of Passau. In 2016 one of the two biggest events took place in the area around Ansbach and the worst of these in the area around Simbach, where a flood considerably more rare than once in ten thousand years was observed (Fig. 2). The duration of high intensity rainfall in 2016 from 2 to 6 hours is astonishingly long for flash floods, in the case of Simbach it was furthermore imbedded in long-lasting precipitation of 36 hours. This extreme rainfall caused not only high saturation overflow but also accelerated the saturation overflow due to three aspects (Fig. 3). The high intensity of rainfall over a long time period created a new surface watercourse. In combination with the pre-rainfall event all small rills were filled, so that there was no more retention, and small retention structures like street dams were overflown. The resulting discharge was so large that even in the very small rivers the floodplain forest was swept away, which could have retarded the flood peak. The question is how to deal with different flood risks like those of 2013 and 2016. In addition to being a challenge for flood management, flash floods like that of 2016 are completely different when compared to river flooding. For large rivers, flood prevention can technically be established by building river dams or using mobile flood protection elements, and precautions can be taken by using flood risk maps and flood forecasting. Since flash floods occur faster, local protection as well as self-provision at the own building and through insurance are more helpful than mobile elements. A flood warning is less effective because there is only a short time for measurements between rainfall and the first flooding. A flood forecast delaying on gauges may be inadequate. In Simbach, the first damages were caused upstream the gauge (Fig. 5). A forecast based on precipitation can't calculate accurately the place and the amount of local flash floods. In Bavaria this year some new research projects will start to improve the knowledge of flash floods and in particular the understanding of the hydrological processes during flash floods and how to calculate the local risk. Planning is underway to develop a guideline for a state of the art model combining hydrological and hydrodynamic aspects for modeling flash floods. This is necessary for calculating and setting up local concepts against flash flooding in order to improve flood protection in Bavaria.