Abstract
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.
Highlights
The importance of groundwater flooding has only been widely acknowledged in the past two decades, subsequent to widespread groundwater flooding events in the UK and France in the winter of 2000/01 (Marsh and Dale, 2002; Pinault et al, 2005)
Combined with above-normal antecedent groundwater levels, due to two preceding wet winters, this rainfall caused groundwater flooding that started in December 2000 and in many areas lasted until June of 2001 (Finch et al, 2004; Hughes et al, 2011)
It was originally developed as a water resources model to study the sustainability of groundwater ab stractions (Jackson et al, 2006a, 2006b). It was calibrated based on mean baseflow and mean seasonal peak and low flows, and it was run with monthly stress periods and time steps. We reduced both to daily because Jackson et al (2011) showed that shorter time steps and stress periods were appropriate for high groundwater levels in this Chalk aquifer
Summary
The importance of groundwater flooding has only been widely acknowledged in the past two decades, subsequent to widespread groundwater flooding events in the UK and France in the winter of 2000/01 (Marsh and Dale, 2002; Pinault et al, 2005). Major groundwater flood events occurred on the Chalk outcrop in the southern UK and northern France in the winters of 2000/01 (Marsh and Dale, 2002; Pinault et al, 2005) and 2013/14 (Ascott et al, 2017). In contrast to 2000/01, antecedent groundwater levels in 2013/14 were close to or below normal (Muchan et al, 2015) During both events, baseflow to streams substantially increased resulting in downstream fluvial flood ing, and groundwater discharges into normally dry valleys caused the flooding of property, land and transport routes down-gradient (Ascott et al, 2017; Morris et al, 2018). High groundwater levels severely compromised the sewerage network, causing overflows of contaminated water onto road surfaces
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