Towards integrated flood inundation modelling in groundwater-dominated catchments

•We present a computationally efficient method for modelling groundwater inundation.•A hydrodynamic model is coupled with a finite-difference groundwater model.•First groundwater flood inundation model of a Chalk aquifer with predictive capability. Traditionally in flood inundation modelling the contribution of groundwater is either neglected or highly simplified. Long-duration groundwater-induced events, such as those that occur across Chalk catchments of northern Europe, can, however, incur significant economic and social cost. We present a new methodology for integrated flood inundation modelling by coupling the 2D hydrodynamic model LISFLOOD-FP with the 3D finite-difference groundwater model ZOOMQ3D. We apply the model to two adjacent Chalk catchments in southern England, the Lambourn and Pang, over two flooding events, during the winters of 2000/01 and 2013/14. A dense network of monitoring boreholes reveals local-scale heterogeneities in the aquifer not captured by the model. However, we demonstrate through inundation extent and streamflow comparisons that, on a regional scale, groundwater levels are simulated sufficiently well to capture groundwater inundation extent. The role of the unsaturated zone is discussed and contrasted between the two events. Currently, predictive tools to simulate groundwater flood events are limited, and this new, computationally efficient methodology will help to fill this gap.