Effects of desert riparian vegetation roots on the riverbank retreat process in the Tarim River in China

Abstract

The desert riparian vegetation zone is an important part of the riparian ecosystem. It is significant for river evolution and to channel beds owing to the reinforcement effect of the vegetation roots on the bank and resistance to erosion. The quantitative effects on toe erosion and bank collapse were first investigated to determine how vegetation roots affect the continuous retreat process of riverbanks under different water levels. The case study is the Tarim river, the degrees of riverbank stability at eight sampling sites during the 2016 and 2017 hydrological years were calculated using the bank stability and toe erosion model (BSTEM). Furthermore, the effects of six vegetation species, Populus euphratica (PE), Tamarix ramosissima (TR), Alhagi sparsifolia (AS), Nitraria tangutorum (NT), Phragmites australis (PA), and Glycyrrhiza uralensis (GU), on riverbank stability were discussed. Taking the right bank of sampling site II (Alar New Bridge) as an example, the bank stability safety factor (Fs) and collapse process under the roots of six vegetation species with different root area ratios (RARs) were calculated. The following conclusions were drawn from the results: (a) Different vegetation roots can reduce toe erosion from 0.4 % to 68.4 %, with peak effects during the dry season and recession stages; (b) The presence of near-bank vegetation roots can strongly influence bank collapse, particularly during the recession stage when water level drops rapidly; and (c) Most of the calculated Fs with the roots of all six plant species studied were higher than those without roots during four stages. Moreover, it was greater than the critical value of 1.3 in most cases. The results also indicate that banks with PE roots had the highest Fs values. By contrast, banks with NT and GU roots had the smallest Fs. As the RAR increased, the Fs values in a hydrological year increased linearly for each vegetation specie. The rate of increase order was as follows: PE > TR > NT > AS > PA > GU under a particular range of RAR (0.9 % < RAR < 1.8 %).