Author(s): Kate Bradbrook; Elizabeth Follett; Barry Hankin

Linked Author(s): Kate Bradbrook, Barry Hankin

Keywords: Natural flood management; Leaky barriers; Log jams; Porosity modelling

Abstract: Natural Flood Management relies on working-with-nature measures including leaky barriers (log jams) and other measures to provide flood mitigation benefits. G uidance highlights a need for improved representation of such structures across flow magnitude and spatial scale. This study presents a depth average modelling approach that represents leaky barriers as porous regions with specified loss coefficients, avoiding the overhead and instability of bespoke rating curves and the coarse locality of raised Manning's n. A canopy-drag formulation is adapted to derive theoretical quadratic loss coefficients expressed in terms of measurable barrier properties. Key implementation issues are addressed, and derived coefficients are tested against laboratory flume experiments using two depth -averaged hydrodynamic models (TUFLOW, HECRAS 2D) and two 3D CFD codes (FLOW3D, OpenFOAM). The method is then applied at catchment scale using the two depth- averaged models and compared with Manning's uplift and stage–discharge approaches. The porous region quadratic loss method is demonstrated to be a flexible, computationally efficient alternative for catchment modelling, with guidance on parameter selection and caveats for high -flow regimes and model sensitivity.

DOI: https://doi.org/10.64697/iahr.proc.ise2026.abs.163

Year: 2026