Objective In central Yunnan, engineering excavation has led to the exposure of the eluvial and parent material layers in subalpine meadows, resulting in slope soil erosion. Currently, there is a lack of research on the erosion mechanisms of these two soil layers under different slope gradients after engineering disturbance, which hinders erosion prevention and control in this region.

Methods Soil samples from the eluvial and parent material layers of typical subalpine meadows in Xundian county, Yunnan, were collected. Indoor simulated rainfall experiments were conducted under a rainfall intensity of 90 mm/h and three slope gradients (8°, 15°, and 25°). The runoff characteristics, erosion rate (D_r), and hydrodynamic parameters such as Reynolds number (Re), Froude number (Fr), resistance coefficient (f), runoff shear stress (τ), and runoff power (ω) of the two soil layers were monitored and analyzed.

Results 1) Regarding runoff and Dr, the runoff yield of the parent material layer was 6.53% to 21.74% higher than that of the eluvial layer at the same slope gradients. The Dr of the eluvial layer increased linearly with slope, increasing by 257% (from 8.44 to 30.34 g/(m²·min)) from 8° to 25°. At 8°, the Dr of the parent material layer (10.4 g/(m²·min)) was slightly higher than that of the eluvial layer; however, the rate of increase slowed significantly after the slope exceeded 15°, with an overall increase of only 104% (from 10.4 to 21.28 g/(m²·min)) from 8° to 25°. 2) Regarding hydrodynamic parameters, the Re of the parent material layer was 33.76% and 20.87% lower than that of the eluvial layer at 8° and 25°, respectively. The Fr of the parent material layer was 68.62% lower at 8° but increased by 17.74% at 25°. The f of the eluvial layer exhibited a "V"-shaped trend across the three slopes, reaching its minimum at 15°, whereas the parent material layer exhibited an "∧"-shaped pattern, peaking at 15°. The increase in τ with slope for the eluvial layer (2.75 times) was significantly higher than that for the parent material layer (1.65 times). ω at 25° was 3.76 times higher than that at 8°, and the fitting degree of ω with Dr (R² = 0.39−0.85) was superior to that of the parent material layer (R² = 0.43−0.69).

Conclusions The eluvial layer is more sensitive to external hydrodynamic conditions, with significantly intensified erosion at slope gradients greater than 15°. The parent material layer shows a decelerating trend in erosion increase as the slope gradient exceeds 15°. Therefore, attention should be focused on runoff concentration and gully erosion occurrence within the slope gradient range around 15°, and surface roughening and drainage management should be strengthened in this slope zone.