Influence of one-side wart-type roughness on flow dynamics in open channels: experimental and statistical analysis

Accurate estimation of Manning’s roughness coefficient ( n ) is critical for modeling open-channel flow dynamics, particularly in steep-gradient streams where conventional empirical methods often fail to represent complex resistance mechanisms. This study enhances the understanding of hydraulic resistance through controlled experiments conducted in a 7-m Plexiglas flume at the Ujigawa Hydraulic Laboratory, Kyoto University. Two experimental conditions were examined: a smooth channel, serving as a reference case, and a channel with one-sided wart-type roughness installed at 20-cm intervals. Measurements included velocity profiles and energy dissipation patterns, with emphasis on the cumulative downstream effects of roughness. The rough-bed condition exhibited skimming flow and localized turbulence, captured through three-dimensional velocity measurements and heatmap analyses. This is the first controlled study of one-sided (asymmetric) hemispherical wart-type roughness in steep open channels ( S / K ≈ 5.71, wake-interference regime). Unlike symmetric roughness, the unilateral arrangement generates lateral secondary currents and a cumulative downstream increase of Manning’s n by up to 40 %. Statistical testing confirmed that roughness-induced differences were highly significant ( p = 0.000741), while sensitivity analysis demonstrated the strong dependence of n on roughness density. The findings underscore the necessity of accounting for spatially variable roughness in hydraulic models.