Authors: Hafiza Aisha Khalid, Kaisa Västilä & Juha Järvelä
Publication type: A1
Abstract
Riverine instream and floodplain vegetation often forms patches rather than being uniformly distributed, and these spatial patterns influence flow dynamics in distinct ways. Yet most existing river models do not account for patchiness, despite its role in governing key processes impacting river stage and sediment dynamics. This study addresses a major gap by developing a new flow resistance model that incorporates blockage factors to represent the hydraulic effects of woody flexible vegetation growing in form of patches. We evaluated the model using a combined dataset from flume-scale and field-scale experiments. In the flume experiments, we quantified patch reconfiguration (bending and streamlining) by measuring six variants of patch blockages: static and dynamic cross-sectional (Bx), planform (Bb,p), and volumetric (Bv) blockage factors. Among these, the incorporation of static Bx into the proposed model provided the most reliable predictions of flow resistance across scales under emergent conditions. Additionally, the static Bv showed better performance under low-blockage conditions, particularly for the field data. This suggests that Bv can also be reliably used, especially at low blockage. Overall, the new development (Eq. 3) significantly improved the prediction accuracy by extending the base model, designed for uniformly distributed vegetation, with a single term that accounts for non-linear blockage effects. The proposed model seamlessly applies to both uniform and patchy vegetation distributions and can be expressed as Darcy-Weisbach friction factor f″, Manning’s n and Chezy’s Cb. Its integration into hydrodynamic models enhances the description of vegetation-induced flow resistance, benefiting applications in river, floodplain, and watershed-scale hydrological modelling.