Karl-Friedrich Wetzel

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.

Impact of forest fires on geomorphic processes in the Tyrolean Limestone Alps

Sass, O., Haas, F., Schimmer, C., Heel, M., Bremer, M., Stöger, F. and Wetzel, K.‐F. 2012. Impact of forest fires on geomorphic processes in the Tyrolean Limestone Alps. Geografiska Annaler: Series A, Physical Geography, 94, 117–133. doi:10.1111/j.1468‐0459.2012.00452.x ABSTRACT We investigated geomorphic processes on two slopes (Arnspitze and Bettelwurf) burned by wildfire in the region north of Innsbruck. Both burned in 1946 and both are still characterized by severe vegetation destruction. Sparsely grass‐covered rock and debris slopes have developed replacing the former dwarf pine (Pinus mugo) shrub stands. Our aim was to establish disturbed and undisturbed erosion rates and to decide whether recent debris flows can be assigned to these historical wildfires or not. We measured fluvial erosion by means of collectors, estimated the amount of post‐fire erosion from stratigraphic exposures in the adjacent talus, modelled bedload discharge with a statistical model developed in a nearby study area and quantified recent debris flow activity by combining airborne and terrestrial laser scans. We measured erosion rates of 3–30 gm−2a−1, which is roughly ten times higher than the undisturbed sediment yields. Slopewash was higher than linear fluvial transport in the four years of our investigation. Surplus material was removed from the channels by avalanches and debris flows; both being more important for the sediment budget than fluvial action. The modelling approach allowed measured sediment yields to be transferred to larger slope parts and to calculate scenarios of pre‐fire conditions. The concordance of measured and modelled yields was reasonably good; deviations may be explained by differing amounts of precipitation. Our results support the impression that current debris flow activity at the Bettelwurf was enhanced by the aftermath of the 1946 wildfire.

Abfluß- und Niederschlagsmessung

In alpinen Wildbachen wird besonders nach Starkregenereignissen eine grose Menge Feststoff transportiert. Das Gefahrenpotential eines Wildbaches wird durch das Relief, den geologischen Untergrund und die Vegetationsbedeckung bestimmt. Das Angebot leicht erodierbarer Lockersedimente im Einzugsgebiet bestimmt die Intensitat der Sedimentfuhrung der Gewasser wahrend eines Hochwasserereignisses. In den Kalkalpen finden sich grose pleistozane Sedimentdeponien in Moranenablagerungen und Schutthalden, die wichtige Feststoffherde fur die Wildbache darstellen.