Flow resistance of partially flexible vegetation: A full-scale study with natural plants

Riparian vegetation plays a crucial role in riverine ecosystems, providing many types of benefits to nature and humanity. However, a high vegetation density can reduce the conveyance capacity of a watercourse, particularly in the case of shrubs, which are very common within riverbeds and widely used in river and channel restoration works. In this paper, we study the influence of three species of shrubs (white and goat willows and black alder) on the hydraulic resistance factor of a real-scale channel under controlled flow conditions. A system for the anchorage of shrubs to the channel bed allowed us to carry out repeated experiments with the three plant species and with varying plant densities and flow rates. The experimental results provided a range of values for the additional contribution of the vegetation to the hydraulic resistance factor from 0.004 to 0.071 m^–1/3s, in terms of Manning’s coefficient. This variability is related to the vegetation setup (plant species and density) but also to the increasingly hydrodynamic configuration assumed by plants at higher flow velocities and submergence ratios. We found that these factors can be summarised quite effectively by the product of elasticity (E), plant density (M), and plant area index (PAI). At small (E∙M∙PAI) values (<10⁸) the resistance coefficient is less than 0.01, while it increases of up to one order of magnitude when (E∙M∙PAI) exceeds 10¹⁰. Furthermore, our results show a distinct two-stage trend of the value of the additional contribution to the n coefficient of a given vegetation setup at varying velocities and submergence levels, with values decreasing when a threshold of velocity and submergence ratio is exceeded. The position of this threshold point appears to be related to the geometrical and mechanical characteristics of the plants. Although our experiments do not provide enough data to identify a functional relationship between n and specific characteristics of the plants and of the flow, they show that the effect of shrubs on hydraulic resistance is highly variable with the flow conditions and that the conveyance capacity may be significantly larger than expected.