Flood history of the Danube tributaries Lech and Isar in the Alpine foreland of Germany

Abstract

The flood history of the German Alpine foreland has not previously been examined in detail, even though reliable historical data up to the 13th century are available. The rivers Isar and Lech were chosen for the current investigation because their neighbouring catchments are typical of the Alpine foreland. More than 85 historical floods were recorded for each river by means of written documents. Continuous water-level records from 1826 up to the present were made available by the Bavarian Environmental Authority. The water-level records were combined with the documentary flood record by means of statistics. The 31-year running frequencies of the historical floods on the Lech and Isar show similarities, but also unexpected differences. These differences can be explained by the regional distribution of rainfall and by different morphological features of the catchments. Most of the floods on the Lech and Isar occurred during the summer season due to heavy rainfall lasting a couple of days. Long-lasting orographic rainfall in the Alpine foreland is mostly due to a low pressure system, moving northeastward from the Adriatic to the Baltic region. Therefore, the flood history of the Alpine foreland of Germany can be seen as a history of this far-reaching pressure configuration, leading to a better understanding of the summer conditions in Central Europe.