Risk Assessment of Rainfall-Induced Debris Flow Based on HEC-RAS and GIS Technologies

This paper investigates hazard assessment and mitigation measures for rainfall-induced debris flow at a highway tunnel portal in Guangdong, China. The tunnel is situated at the outlet of a steep gully with a channel length of 2.38 km, an elevation difference of 682 m, and a gradient of 23.4%. Under extreme rainfall conditions, the portal faces severe risks of scouring and inundation that threaten the structural safety and operational stability of the highway. A high-resolution digital elevation model was established via UAV oblique photogrammetry, and debris flow processes were simulated using HEC-RAS 6.4. The hydrologic behavior under 20-year, 50-year, and 100-year recurrence intervals is first investigated in the research area. After that, the Bingham flow model is used for the debris flow simulation. Results show that as the return period increases from 20 to 100 years, the maximum flow depth at the tunnel portal rises from 1.92 to 2.21 m and the maximum flow velocity rises from 5.8 to 7.8 m/s, indicating that flow velocity is more sensitive to rainfall intensity than flow depth. These simulated flow depths far exceed the 0.5 m flood level stipulated in the Chinese highway tunnel design code, indicating a serious threat to tunnel safety. Based on a quantitative comparison between single-dam and multi-dam schemes, a multi-dam combination with different heights at four positions along the gully is proposed for segmented interception. This multi-dam scheme successfully reduces the flow depth at the tunnel portal to zero without requiring any individual dam to exceed 10 m in height, demonstrating a favorable balance between engineering feasibility and disaster mitigation effectiveness.