Causes of Flood
Is it assumed that there are three main causes favoring the emergence of flood disasters: extreme precipitation, settling in flood-prone regions and sealed landscape areas.
Speaking about the disastrous August 2002 flood events, it is easy to confirm the extremely high precipitation levels on the basis of measurements. Especially on August 6th and 7th , the Mühlviertel and Weinviertel regions were struck by precipitation levels occurring, statistically speaking, only every 500 years. In some places like, e.g., Lichtenau, Weikertschlag or Schenkenfeld, 220-250 mm of rain fell within only two days. A precipitation level of 1 mm corresponds to 1 liter of rain water per square meter. At the Schenkenfeld gauging point, the average monthly amount in August is 87 mm, meaning that the place was, within only two days, struck by two and a half times more rainfall than would usually go down in a whole month. If rainfall is that heavy, other factors such as river training or soil sealing play a minor role in the emergence of flood disasters.Flood control management aims at providing protection of residential areas from floods which have a once-in-a-century event probability. Accordingly, risk zone plans for residential areas have been devised for floods occurring statistically every 100 years. What happened, however, in August 2002 in the River Naarn and River Kamp catchment areas, is deemed to occur, statistically speaking, only every 500 to 1,000 years. So, the chosen safeguards were, in many places, no longer sufficient. Furthermore, residential areas were afflicted by severe damage also because, in the past, areas situated in risk zones were labeled building land and flood-prone land was, thus, built up.
12.12.2005, Lebensministerium VII/5