Effects of typical desert vegetation roots on the erosion process of cohesive riverbank

Abstract: Desert vegetation roots of various species can contribute to the soil shear strength, and thereby to effectively stabilize the riverbank slope, particularly in the Tarim River Basin with the serious soil erosion. Tamarix ramosissima (TR) and Populus euphratica (PE) are the dominant plant species in typical shrubs and trees, respectively, where both have significant positive effects on the riverbank erosion. In this study, taking the vegetation roots of TR and PE in the Tarim River Basin as the study objects, the influence of desert vegetation roots on the erosion process of cohesive riverbank was investigated systematically via a generalized laboratory experiment. In the experimental design, the roots were mainly arranged in two forms of vertical and V-shaped with the angles of 90° and 60°, respectively. Subsequently, the roots were equally spaced along the section with an average spacing of 4cm. The results showed that: 1) the capacity of soil consolidation in the cohesive riverbank was significantly improved under the action of the roots network. Specifically, the erosion resistance of riverbank with roots increased by 30% to 56%, compared with that without roots. The roots of TR increased by 19.6% for the vertical layout, and 28.1% for the V-shaped, compared with that of PE, indicating that the capacity of soil consolidation varied in types of roots. The roots of TR and PE for the V-shaped increased by 27.1% and 18.6%, respectively, compared with that for the vertical layout, indicating that the capacity of soil consolidation varied in the layout methods of roots. Generally, the effect of roots network of TR increased by 24.2% on average, compared with that of PE. 2) There was a significant protection of roots retaining effect on the riverbank, mainly aiming to change the near-shore flow field, and protect the bank toe. The experiment results show that the vertical average velocity and turbulent energy of flow near the riverbank with roots were reduced to 45%-67% and 75%-91%, respectively, compared with that without roots. 3) The roots pulling effect was mainly explored by the impact on the accumulation and decomposition of the collapsed root-soil blocks and the deposition in riverbed. Consequently, the experimental results demonstrated that the roots pulling effect can be much stronger, as the increase in the volume of collapsed block accumulation. Specifically, the TR root with the average value of accumulation, 0.33, has more collapsed blocks than that of PE with the value of 0.24, indicating that the adhesion strength of root-soil blocks for the TR was stronger than that of PE. Furthermore, the roots pulling effect can be enhanced, as the increase in the time for further decomposition of the root-soil blocks. Two relationships of exponential functions were also obtained between the volume of collapsed blocks and scouring time for the TR and PE roots. The decomposed rate of root-soil blocks for the TR was lower than that for PE, whereas, the pulling effect of TR was greater than that of PE. In addition, the roots length density was positively correlated with the cumulative deposition rate of the riverbed, reflecting the interpenetration and winding ability of the roots. The contribution value of roots pulling effect to riverbed protection increased by about 20%, according to the cumulative deposition rate. These data can be used to reveal the effects of different vegetation roots on the erosion processes of cohesive riverbank, including the network, retaining and pulling effects. As such, this findings can also provide a sound technical support to the soil reinforcement and riverbank protection by the vegetation roots in arid desert areas.