Heinz Bungartz

Turbulence structure in a river reach with sand bed

Measurements and analysis of the three-dimensional turbulence structure in a straight lowland river reach are presented. Accurate measurements of velocity profiles were taken with an acoustic Doppier velocimeter (ADV) and a multichannel micropropeller-based system. It is shown that analytical expressions derived from investigations of laboratory open-channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three-dimensional. At the same time, clear differences of empirical parameters between river and laboratory flows are detected. It was established that river turbulence is isotropic for spatial scales smaller than the river depth. The data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents. Special attention is paid to the investigation of coherent structures. Their spatial scales were evaluated applying the conditional-average procedure based on uw quadrant analysis. On average the scales of ejection and sweep events are as large as 1.5 times the flow depth.

Turbulence structure in an ice-covered, sand-bed river

Although many high-latitude rivers are characterized by seasonal ice cover, few empirical studies have examined the effect of surface ice on the turbulent properties of the underlying flowing water. This study presents three-dimensional turbulence measurements obtained with an acoustic doppler velocimeter (ADV) in a straight reach of an ice-covered, sand-bed river. The statistical properties of three-dimensional turbulence were analyzed. It is shown that the spatial structure of the flow was nonuniform due to nonuniformity of the ice cover in the cross-stream direction and due to the riverbed geometry. These factors are responsible for the complex spatial structure of the flow field which limits the predictability of available physical models. Observed distributions over depth of Reynolds stresses and mean streamwise velocities generally conform with distributions derived from piecewise solutions of a two-dimensional flow model at least in the central part of the flow, but observed and expected values of some parameters differ substantially. We have hypothetically addressed these differences to the effect of flow stratification, and the values of a stratification parameter have been estimated. The velocity measurements allowed to analyze the coherent structures near the bed and beneath the ice cover. Their spatial scales were evaluated to be as large as the river depth for ejections and 0.7–0.8 times the river depth for sweeps, whereas low-frequency variations of the streamwise velocity were observed to have scales of two to four river depths.

Simulation räumlich verteilter Stoffausbreitung in einem Spree-altarm

Mit Hilfe eines dynamischen Schwebstoffmodells fur eutrophe Fliesgewasser (Programmsystem PAMIR, Prochnow et al. 1998) soll hier ein Absetzprozes im sudostlich von Berlin gelegenen Freienbrinker Altarm der Muggelspree simuliert werden, um an einem komplexen Szenario der Schweb- und Schadstoffausbreitung die Arbeitsweise der wesentlichen Modellteile in ihrem gekoppelten Zusammenwirken zu demonstrieren. Dabei wird eine im naturlichen Gewasser detailliert vermessene Transportsituation vom Mai 1996 realistisch widergespiegelt.