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

Abstract

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.

A full‐scale study of Darcy‐Weisbach friction factor for channels vegetated by riparian species

“…Field measurements carried out by Chiaradia et al (2019) in a fullscale vegetated channel were used to assess the applicability of a theoretical flow resistance law proposed by Ferro (2019a) For the investigated vegetation species which are characterized by a condition with few leaves or leafless (AG1, SA1, SA2, SA3, SC2, SC3 and SC4), the scale factor resulted related to the bending stiffness and, in agreement with previous studies, the flow resistance (Equations 17 and 18) was able to estimate higher Darcy-Weisbach friction factors for more rigid vegetation species.…”

“…in which a s and b are coefficient to be determined by the available experimental data. In particular, when the measurements of NV session by Chiaradia et al (2019) due exclusively to the bed and the banks of the channel (grain resistance) without vegetation.…”

“…Rhee et al (2008) concluded that the resistance coefficients of dormant plants are usually smaller than those of green one and, as a consequence, the resistance coefficients are larger when the flow is affected by both the leaf and stem part of vegetation than when it is affected only by the stem part. and flow conditions near to the actual hydraulic behaviour occurring in field (Armanini et al, 2005;Chiaradia et al, 2019;Florineth, 1999Florineth, , 2007Florineth et al, 2003). (Ferro, 1999;Govers et al, 2007;Nouwakpo et al, 2016;Powell, 2014;Strohmeier et al, 2014), continues to be the most common method to estimate flow resistance in open channels.…”

“…Chiaradia et al (2019) studied the flow resistance law for three species of shrubs (Alnus glutinosa L., Salix alba L. and Salix caprea L.) using a real-scale channel under controlled flow conditions. The measured values of the Manning's coefficient due to vegetation, n v , varied in a wide range and resulted dependent on plant species, density and partially foliage condition.…”

“…in which a, b and c are coefficients to be determined from experimental measurements.2.2 | Full-scale experiments byChiaradia et al (2019) Chiaradia et al (2019) carried out some experiments in a channel obtained by adapting an existing ditch, which connects a large pumping station for land reclamation to the Adda river. The channel was 130 m long and was split into six different reaches (Figure 1A).The vegetation was placed in the central reach of the channel, which was 40 m long with a bed slope of 0.01, having a trapezoidal crosssection with a base width of 2 m and a side slope of 1.5 H:1 V (H = horizontal, V = vertical).…”