Important considerations for bridge design are safety and economy. Bridges on waterways are vulnerable to damage due to extreme hydrologic events that cause severe floods. In the USA, more than 70% of bridge damages are due to hydraulic parameters that cause scour of the river bed. Predicting the extent and depth of scour is critical to bridge foundation design to ensure structural stability. Current design practice recommends calculating the sum of all the interacting components of scour (pier, abutment, lateral contraction, and vertical contraction) to predict the total maximum scour depth. However, the interaction and simultaneous development of scour components results in considerably less scour than predicted. The main objective of this dissertation is to investigate the interactive scour of all the components of bridge scour under clear-water scour conditions for erodible, spill-through abutments and rectangular piers. A physical model-based study, which covered all individual and interactive scour conditions, was conducted on a typical compound channel river cross-section. A detailed collection of data for time development of scour, hydraulic parameters, and turbulence resulted in qualitative and quantitative observations, which assisted in the formulation of a realistic model. The suggested model for prediction of abutment and contraction scour, pier and vertical contraction scour, and interaction of all four components of scour captures the results within 10% of measured results. The application of the suggested model to field examples validated the findings. The results will not only improve economical bridge design but will also result in reliable hydraulic variables input. Another important finding of this research is the prediction of the location of the scour hole which will help improve the design safety, not only for the bridge structure by itself but also the design of downstream structures. The classification of long and short setback abutments has also been refined based on the location of the scour hole. An additional feature of this study is the preliminary investigation of interactive scour with wing-wall abutments and wall piers, which shows reasonable agreement with the suggested model of scour prediction for spill-through abutments and rectangular piers respectively. In summary, this study provides a comprehensive picture of the bridge scouring phenomenon and a methodology to predict its magnitude to produce more economical design of safe bridges.
Dr. Terry W. Sturm
Dr. Donald R. Webster, Dr. Philip J. Roberts, Dr. Hermann J. Fritz, Dr. James Wray (EAS)