Fine sediment transport under tidal action

The behavior of fine-grained sediment transport under tidal action is reviewed. Erosion/sediment cycles are the characteristic features, indicating the suspended sediment concentration is primarily governed by local phenomena. The long-term transports cannot be understood without insight into the strongly variable short-term transports. The distribution of fine-grained sediment deposits within an estuary and the properties of the top layer of the bed are essential. The amount of fine-grained sediment in the bottom, potentially available for resuspension under specific conditions, can vary enormously within an estuary. Accurate fine-grained sediment transport predictions often fail because of insufficient knowledge of this factor.     

Some aspects of coronal streamer dynamics

Observations of coronal streamers suggest that these configurations are stationary in terms of the convective time scale, but not with respect to the diffusive processes. In this paper, the diffusive time scale is estimated and the thickness of a stationary streamer where convective and diffusive processes are balanced turns out to be very small.Instead of balancing the terms in the magnetic equation to obtain a zero time derivative, we suggest that the diffusive part is negligible and therefore, the convective term must be zero in the largest part of the streamer. This can be obtained by introducing flux transport through the streamer.We show that the arising convective model is to be understood from evolutionary arguments; the upward flux transport leads to a time scale for the life of a streamer, related to the evolution of the underlying magnetic field. Slight differences occur between stationary models and the present model; these differences are shown not to be in conflict with radio observations.     

On the correspondence between morphological and hydrodynamical patterns of groyne fields

This paper examines a variety of recirculation flow patterns that develop in the groyne fields on rivers. A comprehensive data set was obtained from flume experiments at Delft University of Technology and field measurements performed on the Elbe River in Germany. The analysis focuses on patterns of velocity, scour and deposition, and corresponding change of riverbed morphology. The results show that velocity patterns in the groyne fields depend mainly on the aspect ratio between groyne length and length of groyne field. When the aspect ratio is greater than 0·5, a one‐gyre pattern of recirculation develops, while at groyne fields with aspect ratios less than 0·5 a two‐gyre recirculation pattern emerges. The spatial distribution of fine‐sediment deposition between the groynes coincides with the locations of gyres. Moreover, the thickness of the fine‐sediment layer decreases toward the gyre margins where recirculation velocities are greatest. Although the total concentration of suspended sediment in the river does not change considerably as the flow moves through the groyne field, the biological and gravimetrical composition of the suspended material changes substantially within the groyne field. These changes are due to preferential deposition of coarse mineral particles and the replacement of those minerals with finer organic material. Copyright © 2002 John Wiley & Sons, Ltd.     

Planktonic diatom communities in temperate South-Central Chilean lakes with a focus on Asterionella formosa and the genus Aulacoseira

Journal of Paleolimnology - Paleolimnological records from temperate South-Central Chilean lakes revealed a recent and marked increase in the relative abundances of the key-planktonic diatom...