When does the design flood need to account for climate change? A regional analysis of the Climate Change Indicator for transportation infrastructure.

Abstract:

The design flood is a fundamental parameter for risk and vulnerability assessment of riverine transportation infrastructure. The design flood is traditionally calculated using standard hydrologic design techniques based on available observations. There is however growing recognition that design flood calculations based on historical data may misrepresent flood risk due to increasing climate non-stationarity. To determine if and when the design flood calculation should consider climate change, the U.S. Federal Highway Administration (FHWA) recommends calculating a Climate Change Indicator (CCI) at the proposed project site (Kilgore et al. 2016). The CCI is a dimensionless measure of the projected future change in the design flood relative to the uncertainty within the estimates of historic rainfall. When the CCI is greater than 0.8, the FWHA recommends using projected climate change scenarios to help determine the design flood. While the CCI can provide valuable design guidance, many design teams may be unfamiliar with the climate science and statistics needed for calculation. The goals of this study were to (1) calculate the CCI for a large portion of the continental United States and (2) identify and analyze major regional trends. The USGS Geo Data Portal was used to download downscaled LOCA climate change projections from 14 GCMS at 1/16^th degree resolution. The CCI with confidence bounds was calculated at each grid cell for a lower and higher emissions scenarios (RCP 4.5 and 8.5) and two future periods (mid- and late-century). Although data processing and analysis is ongoing, initial results indicate that CCI values are highest in late-century under the higher emissions scenario. While CCI values vary significantly across space and between GCMs, they are generally above the 0.8 threshold. Thus, for infrastructure expected to function until late-century in a higher emissions future, the preliminary findings suggest that design flood calculations should consider the effect of future climate change at most locations.