Physical and Numerical Experiments to Investigate the Influence of Dead-Water Zones on the Dispersive Mass Transport in Rivers

The prediction of transport velocities, maximum concentrations and skewness of a cross sectional averaged pollutant cloud leads to strong uncertainties, because, the influence of morphological heterogeneities on the transport characteristics is not completely understood. An important type of heterogeneities are dead-water zones, such as groin fields that are intensively used in various rivers to stabilize the main channel at a certain position, to increase the water depth in the main channel for shipping purposes in low water periods and to prevent bank erosion. In order to implement new Alarm-Models like the River-Rhine-Alarm-Model, to predict the mass transport in case of pollutant spill scenarios, expensive field studies are needed to calibrate these models. Hence, the predictions in cases of different hydrological conditions is very limited and inhibits the transfer of such a model to different river systems. In the present study laboratory experiments have been performed, in order to analyze the flow and mass transport characteristics in the presence of groin fields. Large-Scale-PIV measurements to analyze the mean flow field and the turbulence characteristics at the water surface have been performed as well as concentration measurements at single dead-water zones to determine the mass exchange. Experiments with different groin field aspect ratio and inclination angles as well as varied groin field volume have been conducted. The results show, that the groin field geometry, that can be expressed with a certain shape factor, has an influence on the mass exchange properties. With the help of a Lagrangian-Particle-Tracking-Method (LPTM), based on a random walk simulation, the experimental results obtained locally at a single groin field could be translated into the transport characteristics in the far field, comprising the effect of many groin fields, by analyzing the statistics of the virtual released particles during such a simulation. This transport model has been verified with the help of analytical solutions of the 1-D advection diffusion equation. The influence of dead-water zones has been parameterized in the LPTM using a transient-adhesion-boundary that leads to a certain retardation of particles that cross this boundary. The retardation effect is related to the mean residence time of tracer in a dead-water zone. In a further step the influence of changing boundary conditions along the flow on the persistence of the skewness of the cross-sectional averaged tracer distribution in longitudinal direction has been analyzed.