Determination of Emergent Vegetation Effects on Manning’s Coefficient of Gradually Varied Flow

Based on mechanical derivation, a new formula showing the effects of emergent vegetation on Manning's coefficient was established for complex gradually varied flow affected by rigid unsubmerged vegetation in open channels. The new formula indicates that the Manning's coefficient is a quadratic sum of its vegetal and boundary components with coefficients related to vegetation parameters. The variation characteristics of the total Manning's coefficient and its vegetal component calculated by the new formula were investigated by a series of laboratory experiments under different vegetation coverage area ratios. It was found that the variations of both the total Manning's coefficient and its vegetal component followed a linear decreasing trend along the vegetation section. Moreover, linear and power empirical formulas were constructed for the average total Manning's coefficient and its vegetal component related to the vegetation coverage area ratio, respectively. The findings of the research may expand the theoretical connotations of the Manning's coefficient for open channel hydraulics and demonstrate practical significance to engineering applications of the Manning's coefficient for gradually varied flow in vegetated channels.