Sensitivity analysis of gully sidewall expansion to topography and rainfall factors

Gully sidewall expansion is an essential process of gully development, and its trigger condition and dynamic process are subject to multifactor effects. This study consisted of 10 sets of experiments, and three factors were considered: three rainfalls (60 mm, 48mm and 24mm), two initial slope gradients (70° and 80°) and two slope heights (1 m and 1.5 m). Then the increase-rate-analysis method was used to evaluate the variations in the changes of slope gradients of sidewall and retreat rates of gully shoulder-line with respect to changes in other causal parameters of rainfall and topography. The results are shown as follows: (1) The area of land loss of the upper gentle slope was positively correlated with the volume of gravity erosion. The correlation coefficient between the volume of gravity erosion and the area of land loss of the upper gentle slope, r₁, is 0.91. The correlation coefficient between the volume of water erosion and the area of land loss of the upper gentle slope, r₂, is 0.59. This shows that mass failure was the main cause to induce the land loss of the upper gentle slope in the process of sidewall expansion. (2) The rainfall duration and initial slope gradient had significant influences on the change of slope gradient of landform in the experiments. The sensitivity coefficients of rainfall intensity, rainfall duration, initial slope gradient and slope height for the change of slope gradient of the sidewall were 0.3, 2.2, 1.3 and -0.2, respectively. The increased initial slope gradient and rainfall duration may have caused the increases of the volume and number of gravity erosion, ultimately resulting in a remarkable change of slope gradient of the sidewall. (3) The most significant factor affecting the retreat rate of gully shoulder-line were rainfall intensity and initial slope gradient. The sensitivity coefficients of the rainfall intensity, rainfall duration, initial slope gradient and slope height for the retreat rate of gully shoulder-line were 3.0, 1.5, 3.0 and -0.1. As a result, the retaining wall construction was preferable to control the gully sidewall erosion. The results of this study may be referred for analyzing the mechanism of sidewall expansion and controlling the loess sidewall expansion.