The optimal design of reactive covers for treating contaminants seeping from a riverbank to a stream requires a comprehensive understanding of the discharge of water and solute fluxes along the bank profile. This is particularly true following flood events that result in riverbank discharge rates back into the river during the recession period that are significantly higher than baseline conditions. Traditional modelling approaches used to describe surface water/groundwater interactions during flood events typically rely on simplified analytical solutions, or highly abstracted numerical solutions which simplify river geometry, and ignore bank soil heterogeneity.
To overcome these limitations a rigorous physics-based numerical model was developed using HydroGeoSphere. This numerical model incorporates channel geometry, heterogeneous bank soil distribution, and observed river stage data to drive the model. The model was used to simulate the interaction of surface water and groundwater within the riverbank soils during flood events, including in-channel and floodplain events.
The interaction between surface water and groundwater was computed by applying a conservative tracer to the surface water in order to track its migration into the bank soils on the rising limb of the flood hydrograph, and its return to the channel as the flood recedes. The results of this study include increased understanding into: the potential depth of penetration of surface water into the bank and the degree of mixing with groundwater; estimation of infiltration and exfiltration velocities and fluxes; timing of return flows after a flood event and their sensitivities to hydraulic conductivity; and, the identification of key discharge zones. All which are critical factors in reactive cover design.
The numerical simulations conducted as part of this study provided insights into the complex interactions between surface water and groundwater that occur during a flood event that would otherwise have been very difficult to glean from field data alone.