Tracer experiments in the Rhine Basin: evaluation of the skewness of observed concentration distributions

Field studies reporting on the propagation of a pollution wave travelling down a river mostly show persistence of the temporal skewness. As a result, in the Rhine Alarm-Model a constant skewness coefficient (equal to 1) has been applied. The appropriateness of this assumption has been proven by tracer experiments. This finding seems to be in conflict with the solution of the transient storage equations of the one-dimensional Fickian-type diffusion equations, the so-called dead-zone model, showing a continuous decrease of the skewness with the distance. On the other hand, based on these equations as an initial-boundary value problem for the transport of a spill in a river with dead zones Schmid [Schmid, B.H., 2002. Persistence of skewness in longitudinal dispersion data: can the dead zone model explain it after all?. Journal of Hydraulic Engineering 128 (9), 848-854, September 1, ASCE], showed that the skewness can locally increase, if there are river reaches with different values of the mass-transfer coefficient between the main stream and the dead zone, or due to changing topography. This paper shows that by applying Schmid's [Schmid, B.H., 2002. Persistence of skewness in longitudinal dispersion data: can the dead zone model explain it after all?. Journal of Hydraulic Engineering 128 (9), 848-854, September 1, ASCE] approach to the River Rhine and its tributaries Mosel (Germany) and Aare (Switzerland), the observed persistence of the skewness can be reproduced, taking into account the changes in the river topography. Moreover, it is demonstrated that irregularities of the riverbed and banks, and vegetation along the river borders, resulting in 'natural dead zones', contribute to the persistence of the skewness. In addition, the physical processes behind the observed mass-transfer coefficient have been analysed.