Incorporating blockage effect of flexible woody vegetation patches in flow resistance analyses

Abstract Riparian vegetation often grows in patches, impacting the hydraulic characteristics differently from uniformly distributed vegetation. Although the impact of the latter is well documented, there is a significant knowledge gap regarding patchy distributions. This experimental study is novel in quantifying vegetative coverage through dynamic patch blockage factors and in analyzing the relationship between reach scale flow resistance and patch characteristics of flexible vegetation made of woody plants with understory grass. The experiments were performed under low relative submergences, where the density and shape of flexible nature-like patches were altered. The results revealed that vegetative friction factor for patchy vegetation was not linearly correlated with the reach scale LAI, as opposed to uniformly distributed vegetation. Among the three investigated patch blockage factors, planform ( B b,p ) and volumetric ( B v ) blockage factors showed equally strong correlations with the vegetative friction factors. The application of existing leaf area index (LAI)-based models under emergent conditions suggested that LAI alone was insufficient in explaining the hydraulic resistance. Detailed statistical analyses were conducted to identify the optimal combinations of vegetative properties for predicting the vegetative friction factors. Under emergent conditions, the cross-sectional ( B x ) blockage factor combined with the nonlinear influence of LAI can improve the accuracy of existing flow resistance formulas. With the increase in submergence level, the effect of patch patterns became less pronounced, and the only parameter required to explain the variation in hydraulic resistance was LAI. Overall, accurate description of flow resistance associated with flexible woody patches under emergent conditions can be improved by properly integrating the patch blockage factor into flow resistance models.\