Studies of fluvial geomorphology should consider the essential roles played by plant communities, in addition to the usual geological and hydrological factors. Mobile-bed flume experiments were undertaken to investigate the effects of vegetation roots on the protection of sandbars from erosion in fluvial channels. Loose sandbars (i.e., containing only sand) and sandbars covered with taproot and fibrous-root vegetation types were used to assess the influence of vegetation on residual sandbar volume and channel erosion in the case of emergent, partly submerged, and submerged sandbars. Results indicate that vegetation roots effectively increase soil cohesion, reducing flow scouring. Fibrous root systems form a root net around sandbars, preventing morphological damage caused by external erosion at low flow rates. Taproots develop solid erosion-inhibiting structures within sandbars through their strong primary and lateral roots, effectively preventing internal scouring at high flow rates. Relative to loose sandbars, vegetated sandbars were 24 %, 121 %, and 222 % more protected from sediment erosion under emergent, partly submerged, and submerged conditions, respectively. The ratio of effective erosion protection increased with increasing discharge, with vegetation roots playing a key role in stabilizing sandbars, particularly under submerged conditions.

Context Plant roots can increase soil shear strength and reinforce soil. However, wetting and drying alternation (WD) could lead to soil structure destruction, soil erosion and slope instability.Aims This study tried to explore the effects of wetting and drying alternation on shear mechanical properties of loess reinforced with root system.Methods Direct shear testing was conducted on alfalfa (Medicago sativa L.) root system-loess composites with three soil bulk densities (1.2 gcm-3, 1.3 gcm-3 and 1.4 gcm-3) under 0, 1, 2 and 3 cycles of wetting and drying alternation (WD0, WD1, WD2 and WD3).Key results The morphological integrity of the root-loess composites was obviously better than the non-rooted loess after WD. Under the three soil bulk densities, negative power-law relationships were observed between the shear strength, cohesion and internal friction angle and the cycles of WD. WD deteriorated the soil shear strength. The most obvious decrease in soil shear strength occurred under WD1, which was 13.00-22.86% for the non-rooted loess and 17.33-25.09% for the root-loess composites. The cohesion was decreased more than the internal friction angle by WD.Conclusions The most obvious damage to the soil was under WD1. The roots inhibited the deterioration effect of WD on the shear property of loess, and the inhibition by the roots decreased with the cycles of WD.Implications The results could provide new insights into the mechanical relationship between plant roots and loess under WD, and provide a scientific basis for the ecological construction in the loess areas. Wetting and drying alternation (WD) on the mechanical properties of root-soil composites is not clear at present, or if roots can inhibit the deterioration of soil under WD. This paper investigated the effect of WD on the shear strength of root-loess composites. WD was found to deteriorate soil shear strength and cohesion, while roots inhibited the deterioration of WD on the shear property of loess. The results provide a scientific basis for ecological construction in loess areas.