Rill erosion and controlling factors on highway side-slopes in the permafrost region

Soil erosion on highway side-slope has been recognized as a cause of environmental damage and a potential threat to road embankments in the high-altitude permafrost regions.To assess the risk to roads and to protect them effectively,it is crucial to clarify the mechanisms governing roadside erosion.However,the cold climate and extremely vulnerable environment under permafrost conditions may result in a unique process of roadside erosion,which differs from the results of current studies conducted at lower altitudes.In this study,a field survey was conducted to investigate side-slope rill erosion along the permafrost section of a highway on the Qinghai‒Tibet Plateau of China.Variations in erosion rates have been revealed,and intense erosion risks(with an average erosion rate of 13.05 kg/m^(2)/a)have been identified on the northern side of the Tanggula Mountains.In the case of individual rills,the detailed rill morphology data indicate that the rill heads are generally close to the slope top and that erosion predominantly occurs in the upper parts of highway slopes,as they are affected by road surface runoff.In the road segment scale,the Pearson correlation and principal component analysis results revealed that the protective effect of vegetation,which was influenced by precipitation,was greater than the erosive effect of precipitation on roadside erosion.A random forest model was then adopted to quantify the importance of influencing factors,and the slope gradient was identified as the most significant factor,with a value of 0.474.Accordingly,the integrated slope and slope length index(L0.5S2)proved to be a reliable predictor,and a comprehensive model was built for highway side-slope rill erosion prediction(model efficiency=0.802).These results could be helpful for highway side-slope conservation and ecological risk prediction in alpine permafrost areas.

Plot investigation on rill flow resistance due to path tortuosity

The path tortuosity t is an indicator of rill morphology accounting for the deviation of the thalweg from a straight alignment.The effect of t on flow resistance has been little investigated for rills.This paper reports the results of a plot investigation aimed to establish the suitable accuracy of the rill thalweg measurement to determine the tortuosity parameter and to test the reliability of a theoretical flow resistance law.Four rills were incised in clay soil(CS)and clay-loam soil(LS)and shaped by a clear flow discharge.The three-dimensional Digital Terrain Models were created by the Structure from Motion technique.For rills on LS,an approximate thalweg was tracked by photo-interpretation,and a specific calculation routine was applied to identify the cross sections with a constant spacing d.The actual rill thalweg was obtained as the line joining the lowest points of these cross-sections.Among the different tested d values,d=0.075 m was chosen to determine t.For both CS and LS,the Darcy-Weisbach friction factorffeatured a non-monotonic relation with t,which was explained as the result of three additive components due to bed roughness,sediment transport,and localized energy losses due to curves.The effect of the former two components onff contrasts that of the third,resulting in a linearly decreasing f-t relationship and constant flow velocity for the three lowest tortuosity values,and an increased friction factor and reduced flow velocity for the highest tortuosity value.The flow resistance law was positively tested,and the predicted friction factor was dependent on t.