Effect of Climate Change on Reliability of Bridges under Threat from Flood-Related Scour

Abstract:

Flood-induced scour poses a significant threat to infrastructure since 55% of bridge failures in the U.S. are caused by such hydraulic events. The risks are amplified when bridges that serve as critical access points to mega cities and major communities fail. It is currently well-documented that the increased frequency of natural hazards and floods in many regions in the US is a "new normal." However, work that associates changes in flood frequencies and intensities to the reliability of bridges has only begun to generate interest.

In this study, we assess the effect of increasing flood frequency and intensity on the reliability of bridges at risk from flood-induced scour. This can translate to present value costs that flood-induced scour poses on bridges, and can develop a starting point to what kind of safety factors should be included in bridge design. A bridge over the Schoharie Creek at Prattsville, NY, with 100+ years of peak flow data, was used as a case-study. Trends and change points in the peak streamflow data was first analyzed using the Mann–Kendall trend test and Pettitt's change-point detection test, respectively. Peak flow data was modeled using a generalized extreme value distribution to derive the expected maximum event in different return periods along with their uncertainty intervals. The corresponding local scour depths are predicted using HEC-18 scour standards. Taking into consideration the variability of actual scour depth as observed in the field, a fragility analysis was then conducted to assess the safety of the bridge under various changing scour depths. This then paved the way for determining which beta factors represented different levels of safety conditions. Changes in floods are then related to risk of failure for the remaining expected design life of the bridges, and ultimately, that risk is related to present value cost of damages and how that cost has changed in recent years.

The reliability and risk analysis accounts for full uncertainty in estimating floods and the bridge's structural, foundation, and scour characteristics. There is currently limited research that links changes in climate to bridge reliability analysis that sets a strong standard for scour safety for the future. Our work provides an end-to-end framework for relating climate changes to urban infrastructure designs.