Experimental investigation of root mass density and vegetation discontinuity effects on riverbank erosion

Steep mountainous rivers in subtropical Taiwan Riverbank erosion is a critical issue in Taiwan’s steep river systems, where intense rainfall, high sediment loads, and frequent channel disturbances promote rapid bank instability. This study examines how riparian vegetation presence and spatial discontinuity influence riverbank erosion processes through controlled flume experiments. Five riverbank configurations were tested, including a non-vegetated bank, a fully vegetated bank, and intermittently vegetated banks with bare gaps of 10, 20, and 30 cm. Bank erosion ratio, retreat length, bank slope variation, vortex length, and root mass density (RMD) were used to quantify morphological and hydraulic responses under identical flow conditions. Fully vegetated riverbanks exhibited a mean erosion ratio 12.9 % lower than non-vegetated banks, indicating a clear stabilizing effect associated with root reinforcement. Across all vegetated configurations, bank erosion ratio decreased exponentially with increasing RMD (R² = 0.86), highlighting the importance of belowground biomass in resisting fluvial erosion. Vegetation discontinuity strongly altered near-bank flow structures: when bare gaps exceeded approximately one-third of the bank length, erosion ratios and vortex development approached those observed in non-vegetated banks. Narrow gaps promoted localized erosion through flow concentration, whereas wider gaps weakened root connectivity and reduced overall bank resistance. These results demonstrate that both vegetation presence and its spatial continuity play a key role in regulating bank erosion mechanisms in steep subtropical rivers. Riparian vegetation plays a critical role in regulating riverbank stability and erosion processes. This study employed controlled flume experiments to examine five vegetation configurations, including bare, fully vegetated, and intermittent vegetation with gaps of 10, 20, and 30 cm. The results demonstrated that full vegetation coverage reduced erosion by 12.9 % compared to bare banks. A strong inverse relationship was found between root mass density (R² = 0.86, p < 0.001) and erosion ratio, highlighting the role of root systems in reinforcing soil structure. Larger bare gaps led to erosion patterns and turbulence intensities similar to those observed in non-vegetated banks. Flow velocity reduction, sediment deposition, and slope modulation were also observed in vegetated zones. These findings emphasize the importance of vegetation continuity in mitigating erosion and promoting riverbank stability, offering valuable guidance for riparian restoration and nature-based solutions. Fig. 1 The relationship between vegetation configurations and riverbank characteristic factors. • Controlled flume experiments assessed five riparian vegetation configurations on bank erosion. • Fully vegetated banks reduced erosion by 12.9 % compared to non-vegetated controls. • Root mass density showed a strong negative correlation with erosion ratio (R² = 0.86). • Vegetation gaps ≥ 30 cm induced flow and erosion patterns similar to bare riverbanks. • Vegetation modulated slope formation, reduced vortex length, and enhanced sediment deposition.