Physical effects of load fluctuations in rivers

Abstract

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.