Adequate Selection of Design Flood Frequencies for the Evaluation of Scour and Scour Countermeasures of Critical Bridges, and their Impact on the Puerto Rico Highway and Transportation Authority’s Bridge Program

Abstract

Federal regulations require all bridges, over waterways, to be designed for scour resistance and all existing bridges to be evaluated for scour vulnerability. Scour evaluations are typically based on the hydraulic design flood frequency of a 100- year flood event. Existing bridges determined to be unstable due to observed scour or assessed high potential for scour are deemed scour critical. When designing a new bridge or evaluating a scour critical bridge to determine the total scour depth, the selection of a hydraulic design flood frequency is one of the most important parameters. Various equations to evaluate scour are available, however many of them are considered conservative and leading to overestimation of the scour total depth. This overestimation could have an impact on the Puerto Rico Bridge Program, which has almost 500 scour critical bridges, all requiring flood monitoring and, consequently, greater resources. Key Terms – Bridge Scour, Hydraulic Design Flood Frequency, Scour Total depth, Scour Critical Bridges.