New formulation for the effective relative roughness height of open channel flows with submerged vegetation

The friction factor f is a crucial parameter for scaling friction resistance and energy loss in open channel flows [11,16]. We propose an auxiliary bed for the distinctive flow structure arising from mutual effects of flow and vegetation. Using the auxiliary bed, a two-layer dynamic model with meaningful physical significance is established to modify equations for channelized flows with vegetation. This new approach divides the vegetated flow into two parts: a basal layer and a suspension layer. A parameter named 'effective relative roughness height', analogous to the relative roughness height of unvegetated open channels, is proposed to predict the friction factor of the suspension layer. By analyzing hundreds of datasets, a series of formulae are proposed based on the application of the effective relative roughness height. Comparisons show that the new formulae yield more precise estimates of the flow rate, Manning coefficient and Chezy coefficient than previously published formulae on the subject.