Physical effects of load fluctuations in rivers
Doctoral thesis
Permanent lenke
http://hdl.handle.net/11250/279013Utgivelsesdato
2014Metadata
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Sammendrag
The present dissertation describes the consequences of rapid discharge variations
on gravel bed rivers. It consists of a thesis part and eight attached
research papers.
A novel technique to artificially reproduce the surface structure in a
stream-bed is introduced and basic research experiments are performed on
an artificial static armor layer. These investigations, conducted under unsteady
flow, focus on (i) bed-shear stress, (ii) dynamic lift on the streambed
and (iii) spatial fluctuations in the near-bed velocity field. They form the core
of the present study and lead to the following results: Bed-shear stress during
unsteady flow could be very well predicted with the St. Venant equation
and showed no significant dynamic e↵ects as it consistently increased with
increasing discharge. The dynamic lift acting on a patch of the streambed,
however, showed remarkable variations in form of three distinct peaks during
increasing flow. The following hypothesis regarding spatial velocity fluctuations
in the near-bed flow field has been proposed: In uniform flow conditions
the form-induced stresses, being an indicator for spatial velocity fluctuations,
are independent from the discharge. In non-uniform flow conditions the
magnitude of form-induced stresses close to the streambed increases with
increasing discharge, while the qualitative shape of the form-induced stress
distributions is independent from the discharge and unsteadiness of the flow.
Further experimental and numerical studies included in this dissertation
discuss (i) how these basic research findings might a↵ect armor layer stability
and pore water exchange in the hyporheic zone, (ii) how sediments move as
bed load over a fully developed armor layer, (iii) how the flow in naturally
bent rivers might imply additional physical e↵ects of flow fluctuations and
finally, (iv) how mitigation measures could be designed.