A stage-partitioned model for algal-induced flow resistance in canals

ABSTRACT Diagram illustrating how periphytic algae growth stages affect canal roughness, with ecological regulation controlling algae development to manage roughness levels. Periphytic algal communities significantly alter flow resistance, yet the relationship between algal biomass (B) and Manning's roughness coefficient (n) remains difficult to quantify. To address this gap, a novel stage-partitioned dynamic roughness model was developed based on hydraulic relationships and a force balance, considering stage-specific mechanisms. Morphology-dependent intermediate variables were introduced to link B to n: equivalent roughness (ks) for stage 1 and curved algal height (ha) for stages 2 and 3. Experiments on Cyanobacteria-rich communities demonstrated n increased with B in stage-dependent patterns. Specifically, ks nonlinearly rose with B (the coefficient of determination (R2) = 0.67). ha was determined by the product of filament upright height (h0) and filament uprightness (δ). h0 increased nonlinearly with B (R2 = 0.70). lg δ decreased linearly with the logarithm of Froude number (lg Fr) (R2 = 0.93). Overall, the integrated model captured these relationships with high accuracy across stages (R2 = 0.91). These findings highlight how the impact of algal communities on flow resistance evolves with colonization stages, primarily due to changes in algal species composition and morphology. The proposed model improves hydraulic simulations by explicitly quantifying algal-induced roughness and supports practical canal management and ecological regulation.