Delineating Flood-Induced Sediment Scour in Bridge-Contracted Channels: Processes, Patterns, and Scour Depth under Various Scour Regimes

Flooding water in extreme hydrometeorological events may be significantly contracted by bridge abutments and superstructures, leading to severe sediment scour and threatening structure stability. A series of large-scale laboratory experiments assessing flood-induced scour processes near bridge abutments are presented, with particular attention paid to the dependence of scour pattern on factors such as deck submergence (vertical contraction), abutment/embankment length (lateral contraction), abutment structure, and general bed mobility, which are analyzed based on post-scour transverse bed profiles and the general bed morphology. It is found that the scour hole's depth, geometry, location, and corresponding time dependence are intricately affected by the aforementioned factors and demonstrate very complicated behaviors. The current design criterion of HEC-23 is sufficient to prevent disastrous embankment breaching but cannot avoid channel erosion or abutment toe undercutting in severe floods. The location of the greatest scour may migrate with time during floods and is closely related to the abutment structure and length. Furthermore, the existing scour predictors are examined and compared with recent data sets; more improvements are needed to reflect the complex physical mechanism of scour processes. Finally, a new method for estimating sediment loss is proposed.