Characteristics and mechanics of tall vegetation for resistance to flow

Characteristics and mechanical properties of vegetation have a considerable effect on the resistance to flow for both water in vegetation zone of rivers and air in forest canopies. For tall vegetation, these properties include leaf density, shape and the general flexibility of tree species. To estimate the resistance coefficients (i.e., coefficient of drag, the Darcy-Weisbach friction factor, f, or Manning's n value) of flow inside tall and flexible vegetation, an index is required to account for the effects of vegetation type and properties. In this study, an index is proposed to characterize physical properties of tree species. The index is derived based on the resonance frequency of the first mode of vibration of the trees and a fundamental relationship for the homogeneous beams. The estimated indexes for four species of coniferous trees were used in a mathematical model for estimation of the friction factor (f), and were able to account for the differences due to the leaf density, shape and rigidity of the tree species.