Effects of riparian plant roots on the unconsolidated bank stability of meandering channels in the Tarim River, China

Abstract

Meandering channels with highly sinuous and distorted bends have developed along the middle and lower reaches of the Tarim River in northwestern China. Riverbanks composed of unconsolidated materials have much lower bank strengths and are susceptible to fluvial erosion and bank collapse, particularly under high flow conditions. Thus, the formation and development of meander bends in such environments appear unintuitive. It is logical to speculate that riparian plant roots in unconsolidated banks have significant effects on bank stability and bend development in the absence of cohesive sediments, but this has not been verified. To determine the influence of root cohesion on the bank stability of meandering channels in the Tarim River, in situ measurements and sampling of roots from locally dominant riparian vegetation, i.e., Populus euphratica (PE), Tamarix ramosissima (TR), and Phragmites australis (PA), were conducted on the meandering bends to obtain root parameters, including diameter and root area ratio. The radius of curvature of local bends corresponding to sampling sites was also calculated using remote sensing imagery. We used the Bank Stability and Toe Erosion Model (BSTEM) to quantitatively estimate the effects of different root conditions on channel bank strength. The results confirmed that roots (and sand–root composites) provide effective reinforcement to unconsolidated banks, control the bank erosion, and hence enhance bank stability. However, the effects were highly variable under different root conditions. PA roots generally improved bank strength more than those of PE or TR, with PA, PE, and TR improving bank strength by 88.2%, 73.2%, and 63.6%, respectively. Riparian vegetation in this extremely arid and unconsolidated sediment environment therefore appears to be a prerequisite for sustaining the meandering channel.