Surface film characteristics of ultra-wide bridge deck drainage pavement based on fluent simulation: a case study

Ultra-wide highways (eight lanes or more) increase capacity, but their enlarged deck width raises drainage demand during heavy rainfall. Bridge deck drainage involves surface runoff and seepage through deck and pavement layers. In ultra-wide cross-sections, the longer transverse flow path makes surface water film accumulation and discharge central to drainage performance. This study investigates water film dynamics on ultra-wide bridge decks using a CFD model in ANSYS Fluent that couples the Discrete Phase Model (DPM) with the Eulerian Wall Film (EWF) model. The Chengdu-Mianyang section of the G5 Beijing-Kunming Expressway is used to assess the effects of longitudinal slope, cross slope, and inlet configuration on water film distribution. The results show that increasing the longitudinal slope from 3% to 5% reduces the peak water-film thickness by about 6%, while increasing the cross slope from 1.5% to 2.5% decreases roadside film thickness by 8.3%, improving lateral drainage. Rectangular inlets (40 × 30 cm) and trench drains (30 cm wide) improve drainage performance, with efficiency reaching up to 50% of the theoretical maximum. Furthermore, the combination of rectangular inlets at 5 m spacing and trench drains gives the best performance. The study provides a numerical basis for performance-oriented drainage design of ultra-wide bridge decks.