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Research

The main focus of these workings lies upon the reconstruction of river floods. Thereby extreme events since the Pleistocene are investigated, for example outbursts of natural dam-lakes. Numerous historical floods are documented in respect of water levels; in order to interpret these floods for risk assessment and disaster prevention water levels have to be converted into peak discharges. Floods often leave specific small-scale morphological features in the landscape, which can be interpreted hydraulically by using methods of process-orientated research. Here these methods primarily comprise experiments in laboratory flumes.

Natural dam-lakes

Natural dam-lakes can develop through blockages of stream courses by landslides, volcanic eruptions, glaciers or moraines. Often these dams are not stable enough to persistently retain the dammed water. As a consequence unexpected, abrupt outbursts are likely to occur. These extreme events can exceed the magnitude of usual river floods by far. In the Pleistocene ice-dammed lakes in North-America and Russia existed, which were hundreds of meters deep and covered vast regions. When the glaciers withdrew and the natural dams became instable flood waves with peak discharges of several million cubic metres per second gushed and flooded adjacent landscapes. Referring to the earth's history these floods belong to the most extreme flood events ever. Beside reconstructions of former dam-lake outbursts investigations on recent natural dam-lakes are being carried out in order to survey the stability of the dams and to estimate potential flood scenarios.

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In the middle-reach of river Katun (Altai-Mountains, S-Siberia) huge terraces with heights up to 200m can be found at the valley-flanks. The terraces mainly consist of gravel which was transported as suspension load and finally was deposited during a dam-break flood 20.000 years ago. Their height indicates a certain paleo water level which has been associated with a discharge of 10x106 m3/s.

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These two natural dam-lakes of Ikhnach are located in the upper catchment of river Pskem (NE Tashkent, Uzbekistan) in the western foothills of the Tien Shan Mountains. The upper lake results from a rockslide of the adjacent mountain flank, that built up a natural dam, while the lower lake is dammed by a Pleistocene end moraine. A spillway on the rockslide-dam only becomes active in spring when water level in the upper lake is at its highest stage. This spillway seriously endangers the stability of the dam, though.

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The outburst flood waves from the Pleistocene ice-dammed lake Missoula have cut several channels into the basalt-bedrock in the western part of Washington (USA). To the west of the Palouse-Falls in the SW region of the state the former water level is marked by erosion of the overlying loess cover.

Historical Floods

Historical flood marks along numerous rivers indicate water levels of past floods. These are complemented by textual deliverances and descriptions. Taking these archives as a basis the climate and flood frequencies in historical times can be deduced. Up to now rivers have been significantly influenced by human activities - as a consequence most of the historical water levels do not correspond to present-day water levels. In consideration of historical floodplain topographies it is possible to estimate maximum discharges of past floods, though. In respect of climate changes, these quantifications of historical floods can thus help to assess current flood risks more precisely.

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Flood water levels of the Main on the city gate of Eibelstadt.

 

 

 

 

 

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From historical sources it is reported that during a flood of the river Rhine in 1374, people were able to pass the city wall of Cologne by boat. This extraordinary water level can be correlated with a maximum discharge of 23.000 m3/s, which is more than twice as high in comparison to the floods from 1994 and 1996 in Cologne (Fig.: H. Meurs).

 

 

 

 

Process-orientated Research

Depending on hydraulic conditions specific small-scale fluvial bedforms develop in rivers. Even without direct measurements bedform-morphologies indicate flow conditions during the process of formation. In order to investigate the influence of individual physical parameters on these processes systematically, experiments in laboratory flumes are conducted.

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In the Chuja-Basin (Altai-Mountains, S-Siberia) a giant fluvial obstacle mark of 50m height has developed at a bedrock-hill during the drainage of the Pleistocene ice-dammed lake. The morphology of this bedform cannot be explained by corresponding textbooks. So as to gain a better understanding of the genesis the systematic use of physical scale models proves to be inevitable here.

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Fluvial obstacle mark in a laboratory flume.

 

 

 

 

 

 

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Fluvial gravel dunes in the Kuraj-Basin, Altai-Mountains/ Siberia.

Laboratory Flumes

In Geomorphology laboratory flumes are used for analytical studies of hydraulic and sedimentary processes in rivers. In order to investigate formative processes systematically and under controlled boundary conditions laboratory flumes prove to be helpful devices in terms of process-orientated research.

Principally flumes are physical models of natural streams, in which water is conducted through an artificial channel that is fixed on a massive frame. These channels are usually rectangular or trapezoid in shape and often made of glass or Plexiglas. Depending on the type of flume studies on sediment movement can be made. Specific flow conditions are normally regulated via pump-power, slope and gates in the channel.

This working group is in possession of two laboratory flumes which are used for research as well as for teaching purposes. Both channels are made of Plexiglas and mounted on steel-frames.
The smaller (and older) flume has got a working section of 450 cm (L) x 45 cm (H) x 4,5 cm (W) and is filled with immobile pebbles. The larger, newer flume has got a working section of 500 cm (L) x 27 cm (H) x 32 cm (W). Here sedimentary studies are generally possible. Right now experimental series on the formation of fluvial obstacle marks in sandy rivers are being carried out.Seitenansicht_Kanaele.jpg    Kanalauslass.jpg

Lateral view on both flumes (left). In front the newer, in the background the older flume. View over the tailgate of the new flume (right).

 

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