Impacts of plant roots on debris-flow bed erosion in laboratory experiments

Debris flows often increase in size due to bed erosion and entrainment, enhancing their hazardous potential. However, the effects of plant rooting on debris-flow erosion on ubiquitous vegetated slopes remain unknown, which hinders debris-flow hazard assessment. Here, we investigated the effects of roots on debris-flow bed erosion using scaled experiments in a 5 m long, 0.3 m wide laboratory flume with an erodible bed. Roots of fast-growing Sorghum bicolor (Sudan grass) seedlings were used as proxies for tree roots to quantify the effect of varying rooting characteristics on erosion. Our results indicate that erosion decreases non-linearly with increasing Root Length Density ( R L D ) and Root Area Ratio ( R A R ). Increases in either parameter enhance root–soil contact, thereby improving soil stability and reducing erosion. Among the two, R L D , and thus the combined effect of root length and root density, appears most influential, as R A R does not capture the three-dimensional structure of the root system. Our experimental results suggest that increasing root-soil contact at the debris-flow bed reduces erosion, decreasing or even preventing debris-flow volume growth. These findings imply that alterations in vegetation characteristics, such as those resulting from forest fires or reforestation, affect debris-flow erosion and open up possibilities for biogeomorphic scale experiments for slope processes. • Experiments show that rooting enhances soil stability and reduces debris-flow erosion. • Root Length Density has a strong non-linear effect in reducing bed erosion. • Seedlings offer potential for studying root effects on debris flows in experiments.