Exchange processes between a river and its groyne fields: Model experiments

The exchange of dissolved matter between a groyne field and a main stream influences the transport and distribution of a pollutant cloud in a river. In forecasting models, groyne fields are represented as dead zones with effective properties like exchange coefficients and exchanging volume. Despite its relevance for such practical applications, little research has been done on the exchange process between a groyne field and the main stream itself. Therefore, this study is aimed at examining this exchange process and validating the dead-zone prediction model, which treats the exchange process as a first order system. A schematized physical model of a river with groynes was built in a laboratory flume. The exchange process was visualized quantitatively with dye in, adjacent groyne fields. In order to couple the exchange process to the velocity field, particle tracking velocimetry measurements were performed. Two different types of exchange were observed. First, exchange takes place via the mixing layer that is formed at the river-groyne-field interface. The large eddies formed in the mixing layer are the major cause of this exchange. Second, under certain conditions, even larger eddies are shed from the upstream groyne tip. Distortions in the flow field caused by such intermittent structures cause a much larger exchange than that by the mixing layer alone. The occurrence of large shed eddies depends on the presence of a sufficiently large, stationary, secondary gyre located at the upstream corner of the groyne field. The overall exchange of matter could be characterized as a first-order process, in accordance with the dead-zone-theory. The corresponding exchange coefficients agreed reasonably well with the results of earlier experiments and the effective coefficients as found in experiments in real river flows.