Flow resistance law in channels with fully submerged and rigid vegetation

Abstract

The estimate of flow resistance in vegetated channels is a challenging topic for programming riparian vegetation management, controlling channel conveyance and flooding propensity, for designing soil bioengineering practices. In this paper, measurements collected by Gualtieri et al. (2018), in a flume where rigid cylinders were set in two arrangements (staggered, aligned) at high submergence ratios (ratio between the water depth and the vegetation height greater than 5), were used to study the effect of rigid submerged vegetation on estimating flow resistance. The theoretical flow resistance equation, obtained by integrating the power flow velocity distribution, was first summarized. Then, this flow resistance equation was calibrated and tested by measurements of Gualtieri et al. (2018). In particular, a relationship between the Γ function of the power velocity distribution, the channel slope, the flow Froude number, and the submergence ratio was established by using the available measurements carried out for the two arrangements with different stem concentrations. The calibration of this relationship was carried out by (i) distinguishing measurements corresponding to different vegetation arrangements (staggered, aligned), (ii) joining all available data, and (iii) using only a scale factor representing the effect of vegetation arrangements. For the cases (ii) and (iii), the analysis demonstrated that the theoretical flow resistance equation allows an accurate estimate of the Darcy–Weisbach friction factor, which is characterized by errors that are always less than 5% and less than or equal to 2.5% for 88% of the investigated cases.