Modeling the functional influence of vegetation type on streambank cohesion

Abstract

The important role of vegetation in adding cohesion and stabilizing streambanks has been widely recognized in several aspects of fluvial geomorphology, including stream restoration and studies of long‐term channel change. Changes in planform between braided, meandering, and anabranching forms have been attributed to the impacts of vegetation on hydraulic roughness and bank stability. However, these studies focus either on flume studies where analog vegetation is used, or case studies featuring one species, which is commonly invasive. We present functional differences of bank‐stabilizing root characteristics and added cohesion, with vegetation categorized as woody and non‐woody and by the vegetation groups of trees, shrubs, graminoids, and forbs. We analyzed root morphology and tensile strength of 14 species common to riparian areas in the southern Rocky Mountains, in field sites along streambanks in the montane and subalpine zones of the Colorado Front Range. Using the vegetation root component (RipRoot) of a physically‐based bank stability model (BSTEM), we estimated the added cohesion for various sediment textures with the addition of each of the 14 species. Significant differences exist between woody and non‐woody vegetation and between the four vegetation categories with respect to the coefficient of the root tensile strength curve, lateral root extent, and maximum root diameter. Woody vegetation (trees and shrubs) have higher values of all three parameters than non‐woody species. Tree roots add significantly more cohesion to streambanks than forb roots. Additionally, rhizomes may play an important role in determining the reach‐scale effects of roots on bank stabilization. Differences in root characteristics and added cohesion among vegetation categories have several important implications, including determining the likelihood of planform change, developing guidelines for the use of bank‐stabilizing vegetation, and linking the effect of vegetation to geomorphic structure that can benefit ecosystem functioning. Copyright © 2014 John Wiley & Sons, Ltd.

Spatial variability in bank resistance to erosion on a large meandering, mixed bedrock-alluvial river

“…These findings are consistent with recent work indicating that woody riparian vegetation substantially increases the erosional resistance of riverbanks (Polvi et al, 2014), which can strongly influence rates of bank erosion and modify patterns of channel migration on large meandering rivers.…”

“…Much attention has focused on whether trees act primarily to stabilize or destabilize riverbanks (Beeson and Doyle, 1995;Abernethy and Rutherfurd, 1998;Trimble, 2004;Gurnell and Petts, 2006;Eaton and Giles, 2009;Polvi et al, 2014). While results differ, it is generally agreed that the role of trees in stabilizing banks is dependent on the scaling between bank height and the depth of root penetration.…”

Estimation of soil reinforcement by the roots of four post-dam prevailing grass species in the riparian zone of Three Gorges Reservoir, China

“…De Baets et al (2007) attributed this result to the more efficiently of soil moisture uptake in the upper layer compare with the deeper depth. In addition, high RLD in the topsoil is a natural phenomenon that has been proven by many previous studies (Pollen and Simon 2005;De Baets et al 2008;Burylo et al 2011;Adhikari et al 2013;Polvi et al 2014). According to the root …”

“…As mentioned by many authors, this relationship indicates that thick roots are weak to withstand tension compared with thin roots (Mattia et al 2005;De Baets et al 2008;Burylo et al 2011;Adhikari et al 2013;Polvi et al 2014). The T R values herein for similar root diameters are mostly lower than the ones of several grass species from the Mediterranean environment in the Southeast Spain (De Baets et al 2008) and the Italian Alpine environment , but higher than the ones of Vetiveria zizanioides in semi-arid climate (Mickovski and Beek 2009) and Lotus corniculatus and Trifolium pretense from the Italian Alpine environment ).…”

Flow Hydrodynamics across Open Channel Flows with Riparian Zones: Implications for Riverbank Stability

“…This method allows for the establishment of a flow regime similar to the densely vegetated riverbank in our experiments (λ = 0.94 and 1.88 m −1 ), with reduced hydrodynamic stresses (by more than 50% on average), at the same region (e.g., for y > 1000 mm). This method is more effective to planting shootings alone, as young plants may not have established at a sufficiently high density or have developed a strong root system to sustain the hydrodynamic action of turbulent flows [48,49].…”