On the role of bank storage in attenuating lowland river floods

Ferraz, Gadadhara de Figueiredo and Krámer, Tamás (2026) On the role of bank storage in attenuating lowland river floods. HIDROLÓGIAI KÖZLÖNY, 106 (EN1). pp. 17-29. ISSN 0018-1323

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Abstract

Overbank river floods generate a positive hydraulic gradient toward adjacent aquifers, while floodplain soils become saturated through vertical infiltration during the overbank phase of the flood. These processes contribute to temporary water storage and attenuate flood waves, yet their role in flood dynamics remains unrepresented or only partially represented in many models of river hydraulics. Here, we quantify river-aquifer exchange and evaluate the contribution of bank storage to flood attenuation at the scale of the Danube and Tisza Rivers in Hungary using a mechanistic model. We developed a coupled 1D surface water–2D groundwater model to simulate 100-year flood events in an idealised, prismatic geometry. Results, integrated over a 50 km reach, indicate that bank storage can account for 26–37% of the total (surface+bank) storage and can reduce peak flow by 0.7–0.8 percentage points in addition to the attenuation by surface storage. This translates to an average peak water level reduction of about 4 cm. The attenuation effect was most pronounced in systems with high soil hydraulic conductivity. Despite its role in mitigating flood risk, bank storage is frequently simplified to a conceptual level or omitted altogether in hydraulic river models. Since surface flow roughness parameters are routinely calibrated primarily against peak water levels, this omission is partially offset – even without identifying the source of error. However, compensating for the lack of groundwater storage within the timescale of surface storage is not only imperfect but also leads to inaccuracies when extrapolated to floods significantly larger than previously observed ones. Our findings underscore the benefits of integrating groundwater-surface water interactions into flood modelling to improve flood forecasts and risk assessments.

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