Modelling the effects of groyne fields on the transport of dissolved matter within the Rhine Alarm-Model

The influence of artificial dead zones, such as groyne fields, on the transport of a spill in a river has been analysed. For this purpose, tracer experiments executed in the Dutch Rhine branches in the framework of the development of the Rhine Alarm-Model were used. Additionally for low water discharges with a suppressed flow in the Lower-Rhine River, measured concentration distributions of a few accidental spills were considered. In the Rhine Alarm-Model a lag coefficient applied to the convective transport of a spill represents the influence of the groyne fields. In the case of a complete exchange of the groyne field with the main stream, this coefficient is equal to the ratio between the cross-sectional areas of the groyne field and the main stream of the river. It is shown that this is correct to a certain extent for the Dutch Rhine branches Waal and Lower Rhine. However, in the case of low water discharges the influence of the groyne fields can reduce considerably. To explain these reductions, fundamental research of the mass exchange processes between groyne fields and main stream is needed.