Modelling of the interaction between river and groundwater: Examples from the Zürich region
In horizontal two-dimensional aquifer models the interaction between rivers and aquifers is usually taken into account by a leakage concept. Such approaches are based on various assumptions and simplifications like Dupuit’s assumption for the representation of the flow conditions between river and aquifer. The leakage concept is primarily based on the observation of an additional hydraulic resistance caused by a clogging of the riverbed, which may be strongly spatially variable. Moreover, the resistance can be time-dependent due to various processes. Furthermore, it might be of importance whether infiltration or exfiltration conditions prevail. Highly important are also the hydrogeological conditions below or close to the river, which may be strongly non-homogeneous. The prevailing infiltration or exfiltration rate furthermore depends on the geometry of the riverbed, which may vary over time due to variable flow regimes. In cases where the groundwater table is below the river bottom the infiltration rate depends on the conditions in the capillary zone. Near-shore vegetation can exhibit a further influence.
Various concepts exist to model the interaction between river and groundwater. Often a linear relationship between the specific leakage rate and the difference between river head and groundwater head is assumed. The resulting leakage parameter of extended river reaches cannot be measured directly. The usual way is therefore to estimate it by model calibration using field data. However, how successful are such parameters when modelling uncalibrated periods? Are the leakage parameters constant over time? Is the relationship linear? What is the impact of impounded sections of the river?
The problem of the interaction between river and groundwater has been investigated based on field data for two examples in Switzerland (River Rhine at Rheinau, upper Limmat valley at Zurich). It could be shown that the leakage coefficient can be time dependent due to sedimentation and flood events. Moreover, seasonal fluctuations of the water temperature lead to temperature-dependent infiltration conditions. Furthermore, the leakage coefficient can be non-linear and can depend on the river head by exhibiting increased bank infiltration due to an increase of the wetted perimeter of the river bed. The transient behaviour of the leakage coefficient has to be taken into account in groundwater modelling. Therefore a periodic checking and, if required, re-calibration of the leakage coefficient is needed.