Changes to Bridge Flood Risk under Climate Change

Bridge designs are commonly based on a criterion to withstand the n-year flood event. For example, a highway bridge might be designed to pass the 100-year flood. Failure may occur if the structure faces an event larger than this. Climate change may necessitate different design criteria because of changes to flood frequency behavior. This study examines the consequences for bridge design for flooding under the influence of climate change. In this study, climate change is quantified simply as a change in the frequency of a given precipitation or flood event. Flood discharges for current conditions are estimated from the applicable U.S. Geological Survey regression equations. Natural Resources Conservation Service methods are used to inverse calculate the causal precipitation for such floods. Return frequency for this causal precipitation is determined from both the current national precipitation frequency source and future climate intensity-duration-frequency curves. This study specifically looks at the freeboard criteria for bridges of different road classifications and how the margin of safety associated with a designed freeboard is reduced under future climate conditions. The results demonstrate that all categories of roadways are vulnerable to climate change and that the magnitude of bridge vulnerability to future climate change is variable depending on which climate model projection is used. The results show that increases in impervious area upstream of a bridge crossing increase the susceptibility of the bridge to failure. Holding other watershed characteristics constant, increased impervious cover leads to a greater loss in the margin of safety represented by the designed freeboard, suggesting that limiting the development to control imperviousness can bolster resilience of bridge infrastructure to climate change. A sensitivity analysis shows that uncertainty in both flood and precipitation frequency estimates is at least of comparable magnitude to projected changes in flood risk posed by climate change. (C) 2016 American Society of Civil Engineers.