Rainfall Erosion Control of Granite Residual Soil Slopes Using Hydrophilic Polyurethane (W-OH)

Disturbed granite residual soil slopes are highly susceptible to rainfall-induced erosion because loose surface particles can be rapidly detached and transported by runoff. Polymer-film treatment and vegetation establishment are both used for surface protection, but the sediment-delivery behavior of a polymer–vegetation system during the establishment stage remains insufficiently clarified. This study examined the erosion-control response of granite residual soil slopes treated with modified hydrophilic polyurethane (W-OH), Cynodon dactylon seeding, and their combined application under simulated rainfall. Seed germination tests were first conducted to identify a suitable W-OH application range for vegetation establishment. Then, 60-min rainfall erosion tests were performed under three slope angles (33°, 45°, and 53°) and three rainfall intensities (90, 180, and 270 mm·24 h−1). Sediment-yield ratios, sediment transmission ratios, interaction indices, and within-event temporal slopes were used to evaluate the relationship between the W-OH film and vegetation establishment. The results showed that high W-OH concentration strongly inhibited seed emergence, while the 1–4% concentration range at 1.5 L·m−2 provided a more suitable basis for evaluating W-OH-assisted vegetation protection. Untreated slopes exhibited increasing sediment yield with slope angle and rainfall intensity, confirming the high erosion sensitivity of granite residual soil. Cynodon dactylon seeding reduced sediment delivery, but its protective effect remained dependent on rainfall and terrain conditions. In the combined treatment, sediment yield decreased as W-OH concentration increased, indicating improved surface stabilization relative to vegetation-only slopes. These findings provide experimental support for optimizing W-OH-assisted vegetation measures for early-stage sediment control on rainfall-eroded granite residual soil slopes.