1 INTRODUCTION

In many European rivers groynes are used for the stabilisation of the banks. In general groynes are built as spur dikes almost perpendicular to the bank. A lot of variation is found in the exact lay out of the spur dike (Przedwojski et al., 1995; Nakel, 1970). In lowland rivers that have a natural tendency to be wide and shallow and to exhibit a dynamic meandering behaviour groynes are very efficient. With limited costs the river stream is confined and the main channel is deepened creating a navigation channel. This successful application has resulted in thousands of groyne structures in the greater parts of the large European rivers like the River Rhine and the River Elbe. The European groyne design has also been transferred from the Netherlands to Japan more than two centuries ago.

A by-product of groynes is the area between the groynes, the groyne fields. Here the flow is virtually stagnant and has a permanent open connection with the main stream. Hence there is a continuous exchange of mass and momentum with the main stream. In this exchange process dissolved and suspended matter will enter the groyne field and remain there for a certain time. For a solute it will be the retention time associated with the mean circulation. For sediment, algae and fish larvae the specific properties of the turbulent motion together with settling velocity or active locomotion determine the location...