Influence mechanism of herbaceous plants on debris flow bank erosion

Abstract

The importance of implementing vegetation measures to mitigate debris-flow disasters is widely recognized. However, the specific role of herbaceous plants, especially those flourishing on the banks of debris-flow channels, in mitigating lateral erosion remains insufficiently understood. This study aims to elucidate the impact of herbaceous plants on bank erosion during debris-flow surges. In the experiment, Cymbopogon citratus (DC.) Stapf growing on the channel banks was cultivated for 20 to 100 days. Accordingly, the leaf area index (LAI) of plants ranged from 0.10 to 3.21, and the root length density (RLD) in the topsoil varied from 2.5 to 135.5 km/m3. In addition, the banks with plants were subjected to scouring by multiple debris-flow surges in a specially designed flume. The findings demonstrated a significant reduction in the erosion efficiency of debris flow on grassland banks, with reductions ranging from 70.84 % to 92.49 %, in comparison to bare soil banks. This reduction is attributed to the combined influence of two parts: leaves and roots. On one hand, the leaves partially shielded the shear stress of the debris flows. Under the impact of the debris flows, plants quickly fell down, with their leaves covering the soil. Lodging plants absorbed a portion of the shear stress from debris flow and transmitting it to the deeper soil through stems and roots. This process mitigated erosion on the topsoil. The erosion efficiency exhibited an exponential decline with increasing LAI. On the other hand, the roots altered the pattern of the soil erosion. Debris flows eroded soil from the gaps in the root systems with rarely cutting roots off, which means the layered shear failure of soil was suppressed. Additionally, the erosion efficiency exponentially decreased with the increase in RLD.