A Simple Mechanistic Coupled Surface-groundwater Model to Evaluate Bank Storage and Flood Attenuation in Lowland Rivers

River–aquifer interactions play a significant role during floods, yet they are often simplified or neglected in hydrodynamic models. Horizontal infiltration across the riverbed and vertical infiltration through the floodplain jointly contribute to bank storage, delaying and attenuating flood waves. This study presents a conceptual numerical framework that couples 1D surface hydrodynamics, 2D groundwater flow, and a vertical infiltration module to represent these exchanges during overbank flooding. Applied to a simplified lowland river system, the model captures both lateral and vertical flux components and quantifies their contributions to flood wave attenuation. Our findings indicate that, in a 50-km river reach, vertical infiltration through the floodplain accounts for approximately 90% of total bank storage. Total bank storage can contribute up to 4.3% of flow attenuation and reduce peak river water levels by as much as 24 cm. These results highlight the substantial influence of bank storage on extreme flood events and underscore the importance of representing groundwater pathways in flood modelling. The proposed model provides a practical, process-based framework for representing bank storage through a two-way coupled surface water-groundwater formulation with a few physically based parameter set. By explicitly resolving the governing exchange processes, the model enables improved inclusion of bank storage dynamics in flood modelling, while avoiding both simplified loss-type formulations and the complexity of fully multidimensional coupled SW–GW models.