The challenge of restoring dynamics by river engineering: where to find the truth about river flow - in the computer, in the lab or in the field?

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Rivers are probably the most complex features which we can be found in the environment. They have been and are still the driving force for shaping landscapes and biotopes on our planet. The behavior of river systems is a result of the complex interaction between flow, sediments, morphology and habitats. “Nature is not to be trifled with, she is always true, always earnest, always severe; she is always right, and the faults and errors are always those of man.” (Goethe) Hydraulic engineers responsible for the planning of river training and restoration works as well as scientists doing river research should bear well in mind this quotation from Goethe. Reliable and sustainable protection against floods can only be assured by hydraulic structures that preserve the natural character of a watercourse during flood events which is often difficult to identify. Accordingly, modern river engineering must take a river's natural forces and behavior into account. For example, attempts to straighten a meander that has developed naturally may be dangerous: in extreme flood conditions, if not earlier, the apparently tamed river will recall and reoccupy the space to which it is entitled [1]. “The world is like a river, running along in its bed, this way and that, forming sand bars by chance and then being forced by these to take a different course. Whereas this all proceeds smoothly and easily and gradually, the river engineers have great difficulties when they seek to counteract this natural behavior.” (Goethe)