Evaluating bridge abutment scour models under varying contraction

The study conducts a comparative parametric and illustrative analysis of three bridge abutment scour models: Froehlich, HIRE, and NCHRP 24-20, to assess their model responses and design implications under varied contraction intensities. Using a systematic parametric analysis framework, we examined the evolution of the relative scour ratio ysp/ya as a function of the contraction ratio L/ya, explicitly focusing on the influence of median sediment size D50 and approach flow Froude number Fr. The results indicate a significant divergence in model trajectories: legacy empirical models (Froehlich and HIRE) maintain a continuous power-law increase regardless of sediment properties, whereas the physically based NCHRP 24-20 model provides a physically interpretable response related to flow redistribution and sediment-mobility conditions. A critical finding is the identification of a bounded model response, with the relative scour ratio approaching approximately 3.36 under live-bed conditions. Furthermore, the sensitivity analysis indicates that for coarser sediments, the NCHRP 24-20 model predicts lower scour depths under the adopted clear-water condition (V < Vc), reflecting the influence of sediment mobility on the computed scour response. Additionally, a dynamic transition zone was identified at L/ya ≈ 35-45, within the investigated Froude-number range, where the NCHRP 24-20 model’s predictions may surpass the traditionally conservative Froehlich envelope, highlighting a potential under-design risk for deep floodplain encroachments. These findings provide a design-oriented, screening-level interpretation for determining foundation depths and designing scour countermeasures in complex compound channels, subject to site-specific hydraulic, sediment, geometric, and geotechnical verification.