72 / 2025-04-24 12:55:38

Predicting Abutment Scour Under Pressure Flow Conditions

Limited experimental data exists for vertical contraction abutment scour, which occurs under pressure flow conditions when flow inundates the low chord elevation of a bridge deck. Currently, the abutment scour methodology outlined in Hydraulic Engineering Circular No. 18 (HEC-18) does not account for altered flow dynamics and flow separation under the bridge deck during pressure flow conditions, thereby potentially underestimating the final scour depths. The objective of this study was to examine vertical contraction abutment scour with flume testing of a wing-wall abutment to simulate both free surface and pressure flow conditions under clear-water scenarios. A total of 27 trials, including 20 pressure flow and 7 free surface flow conditions, were conducted with two bridge deck thicknesses and multiple lateral contraction ratios (q-ratios). Results revealed that pressure flow conditions consistently resulted in deeper scour depths compared to free surface flow, and increased bridge deck thickness yielding even deeper scour depths. Reexamination of the contraction abutment scour equations provided in HEC-18 under pressure flow conditions demonstrated that the scour depth amplification factor (α_B) values, now normalized by the flow separation thickness and vertically contracted water depth, were lower for pressure flows than for free surface flows. Notably, the thicker bridge decks, while increasing scour depth, did not significantly alter the α_B value due to normalization by the altered flow separation thickness. To generate a design curve of the scour depth amplification factor versus q-ratio, various models were fitted to the data, with a second-order exponential model providing the best fit for both flow conditions. Recalculated α_B values for free surface flow were larger than previous findings but followed a similar trend. Statistical analysis using a reliability index ensured that the design curves enveloped the entire data set, leading to conservative estimates of scour depths. This study fills the gap of predicting abutment scour under pressure flow conditions.