Temporal and spatial evaluation of stormwater engineering standards reveals risks and priorities across the United States

Abstract

Stormwater infrastructure in the United States is designed using governmental precipitation frequency documents and informed by State Departments of Transportation (DOT) guidelines that balance risks and costs. However, both governmental precipitation documents and State DOT guidelines are updated infrequently, which enhances risks in areas where precipitation patterns have changed over time. This study reviewed State DOT design manuals from the 48 contiguous US states and the District of Columbia and found wide variation in design return period standards recommended for similar roadways and infrastructure types. Precipitation differences between successive US precipitation documents for 43 states over the period of 1961–2000 were found to be statistically significant in more than 90% of the study area. These differences indicate that stormwater infrastructure installed prior to the latest update of precipitation frequency documents could be under-designed for present and future climate conditions. Comparing State DOT design storm values for each roadway and infrastructure type, an index for each climate region was developed to assess the relative stringency of each state’s requirements. Using these index values, the observed change in precipitation frequency estimates, and each state’s design manual publication date, this research identified the states that need to prioritize revision of their stormwater standards to maintain the originally intended design performance over time. Eight out of 43 states were found to have the highest priority for immediately revising their stormwater standards. In addition, these states should assess whether existing infrastructure requires additional adaptive capacity to manage observed precipitation increases. The priority increased for all states under both the RCP 4.5 and RCP 8.5 emissions scenarios for 2050. While local assessments comparing infrastructure costs of increasing the stringency of standards versus the expected costs of future damages under climate change remain necessary, a no-regret action is revising stormwater standards to incorporate observed precipitation increases.