The role of groundwater in the inundation of a river-connected floodplain : case study of the river Silverån in southeast Sweden

Abstract: Fluvial flooding has long been recognized as one of the most frequently occurring natural disasters worldwide, with consequences as large economic losses from damages on infrastructure and agriculture, as well as severe impacts on human health. A less known and explored type of flooding is groundwater flooding. A flood type that for instance can arise in river-connected floodplains when groundwater levels rise to the ground surface due to increased river stages in the watercourse. Although groundwater flooding in general is a poorly understood phenomenon, it has become more recognized since its inclusion in the European Floods Directive (2007/60/EC) in 2007. Sweden has however excluded pure groundwater flooding as a separate flood type in its interpretation of the directive, but recognizes groundwater as a component which together with soil water and river water can influence the appearance of a flood event. One of the difficulties regarding groundwater floods that occur in connection to a river is that they typically are hard to differentiate from inundations of fluvial or pluvial origin. It is however important to address the role of groundwater in the inundation of these settings, since traditional flood protection strategies like levees might be circumvented by flows through the subsurface. The aim of this study has been to investigate the role of groundwater in the flooding of a river-connected floodplain by setting up a groundwater model in the integrated hydrological modeling tool MIKE SHE and couple it to an existing MIKE 11 river model, developed by DHI. The study area is a floodplain located along the river Silverån, a tributary to the river Emån, located in the south eastern part of Sweden. By running the model using four different sub-scenarios, regarding initial groundwater level and amount of precipitation, flood extent and contribution of groundwater to the inundation, in relation to other flood sources, has been investigated for different river discharges. A scenario with artificial levees constructed along parts of the river was also examined as levees have been found to have little effect on groundwater floods. As the model provides a simplified and generalized representation of reality it possesses several uncertainties, and so does the results. In summary, the results are in line with what is stated in the Swedish interpretation of the European Floods directive. It has not been possible to demonstrate pure groundwater flooding, but the results suggest that an elevated groundwater level in the beginning of a flood event will increase the extent of the inundation and result in a larger contribution of groundwater to the total amount of flood water. This suggests that there, in some cases, might be a value in integrating groundwater processes in flood risk mapping. Something that is not included in the conventional hydraulic 1D and 2D models, which traditionally are used in flood mapping. As could be expected, the results indicate that groundwater only accounts for a minor part of the flood water added to the total floodplain, while the major sources are river water and surface runoff. A delimited floodplain section that was investigated more in detail, as an increased flow from groundwater to overland water was detected along it, did however show larger contributions from groundwater. This river reach was less vulnerable to fluvial flooding, which in total resulted in a less severe flood, but also enabled a larger amount of groundwater to seep up to the floodplain surface. These conditions did also result in that the river section experienced a worsened inundation at the sub-scenario of high precipitation and high initial groundwater level, as levees were constructed along the river. Most likely because a lot of surface runoff, otherwise able to drain to the river along this section, got trapped outside the levees since it was unable to drain both to the river and to the saturated ground. These results support the theory that levees have little impact on groundwater flooding and stresses the importance 0f surveying and understanding the governing processes in the inundation of a floodplain when planning which type of flood protection scheme to use.