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Summary Band 19

Heusch, K. (1997): Alluvial soils in fluvial system - shown in the Sieg river floodplain near Hennef.


The influencing control of the fluvial system on genesis and properties of alluvial soils is demonstrated in the Sieg river floodplain near Hennef/Northrhine-Westfalia. Therefore present sedimentation patterns and processes of the river as well as Holocene surface, sediment and soil development in the floodplain were examined.

The study of the noncalcareous flood loams was carried out with geomorphological, sedimentological and pedological methods. Four late Holocene floodplain terraces can be seperated and dated. On the Upper Floodplain Terrace an Auenparabraunerde-Auenbraunerde (Fluvi-Chromic Luvisol) of early Iron Age was found. The pedogenesis on the older Middle Floodplain Terrace lead to a Typische Auenbraunerde (Fluvi-Chromic Cambisol) of Latene Age. The younger part of this terrace is covered by a Braunauenboden (Fluvi-Eutric Cambisol) of late Middle Ages/modern times. The Lower Floodplain Terrace is the youngest formation, at most 200 years old. It is characterized by an Auengley-Auenregosol (Gleyi-Eutric Fluvisol). The Parabraunerde (Chromic Luvisol) on the Weichselian Low Terrace is supposed to be of Bronze Age. That anthropogenic flood loams bury traces of a presumably early Holocene soil on the Low Terrace.

Based on statistical data, the proofed soil chronosequence is related to both time and fluvial sedimentation. Assuming sediment transport by turbulent diffusion in floods, grain size distribution in the flood loams were simulated by a quantitative model of overbank deposition. The well predicted results indicate turbulent sediment diffusion as the dominant process of surface and sediment development of alluvial soils.

At present, this important mechanism of overbank sedimentation is nearly total prevented by construction of dykers for agriculture purposes As the Sieg river has embedded deeply itself, the present morphodynamics is concentrated on the bank area and the river bed with a predominance of matter transport, extreme outlet and soil erosion. However the latter phenomenon endangers the ecological potential of the alluvial soils as a integral part of fluvial systems.