July 6, 2018
When engineers design a bridge, they design to a specific standard to ensure the bridge can handle the expected weight of the vehicles driving over it, throughout the many decades the bridge will be in operation. But what about stormwater sewer systems? These are the roadside grates and pipes responsible for collecting rainwater off of the road, and many of these systems have been in place for a 100 years or more. Lots of the systems we have now were designed decades ago, and lots of the systems we design today will be in place decades from now. As greenhouse gases from human activities increased in the last few decades, the warmer air can now hold more water. Many cities and communities are experiencing heavier rainfall, and stormwater infrastructure can’t seem to keep up — which can lead to dangerous flash flooding and infrastructure failures. In a new research paper, we looked at the design standards used by states for stormwater infrastructure and we found many of these systems are under-designed to deal with the rainfall changes we’re already seeing, not to mention as things get worse under climate change.
Stormwater Engineering Design in the United States
Designing stormwater infrastructure that reduces the risk of street flooding while balancing infrastructure costs requires understanding the extreme precipitation events at a regional and local level. While there are many studies that have analyzed changes in precipitation patterns in the last decades, few provide guidance on the potential ways to climate-proof infrastructure that that was designed based on historical precipitation information. In our research, we reviewed the engineering design manuals for the each of the State Departments of Transportation (DOT) in the continental U.S. to assess the minimum stormwater design standards of each state.
A state DOT’s manual outlines, for example, if storm drains in that state should be designed to handle a 50-year storm, or a 100-year storm. A system designed for a 100-year storm can carry more water, but is bigger and more expensive. We found a wide variation in state standards, even within the same region. This means states are locking in different levels of flooding risks into infrastructure that will last for decades. We then developed a normalized index that ranks states on how stringent their standards are compared to others in the same climate region.
Comparing Historical Rainfall Data Used
Engineers look up the local expected rainfall volume (for example for a 50-year storm) using standardized precipitation frequency documents from NOAA to size stormwater infrastructure. But up until the mid-2000s, the last official precipitation document released in U.S was the Technical Paper 40 (TP40), which was released in 1961 using data from across several decades before the report was published. This means that storm drains designed before the mid-2000s were designed using rainfall data from the early part of the 20th century. NOAA released an update, Atlas 14, in multiple volumes for some states starting in 2006. We set out to estimate if there has been a change in the rainfall volume that an engineer would design for if using the earlier document or the latest NOAA document. We found that in more than 90% of the study area, changes in the rainfall volume were significantly different. This implies that infrastructure designed to comply with a standard when TP-40 was in practice might be under- or over-sized for present conditions.
Who Should Update Their Standards?
We assigned a high priority for updating stormwater standards to states that experienced rainfall increases higher than 10% between the two precipitation documents we examined, haven’t updated their state standards since the latest precipitation document was released, and have standards were below average for their climate region. We found many states in the Northeast and upper Midwest as well as Louisiana, Florida, and Utah should update their standards to ensure new stormwater infrastructure performs under current and projected precipitation levels. These states should also examine whether additional measures such as green infrastructure are required to serve as adaptive capacity to manage the additional rainfall they’re experiencing now.
Designing infrastructure that performs as intended throughout its long service life becomes a challenging task if our understanding of future extreme events is subject to deep uncertainty. Quantifying rainfall volume changes based solely on historical records won’t be representative of future climate conditions, so to understand how priority levels would change in future climate conditions we used the National Climate Assessment RCP 4.5 and RCP 8.5 scenarios. Because the National Climate Assessments projects an increase for all states, the priority to revise stormwater standards for future climate change increases across all states.
Engineering for a Future Climate
We envision our results can serve as a preliminary screening that can initiate action as part of each state’s resilience plan to adapt stormwater infrastructure assets to changing climate conditions. Although local economic analyses and climate risk assessments are needed to quantify and compare costs of higher standards versus expected costs of future damages, revising state standards for at least the observed changes and adding a climate factor of safety for critical roadways represents low-regret actions states can take. In addition, reducing stormwater with green infrastructure or other measures reduces the stress on our existing under-designed stormwater infrastructure. Engineers have an ethical responsibility to “hold safety paramount” and “comply with the principles of sustainable development”. If we are designing stormwater infrastructure for the past instead of the future, we are not living up to that responsibility. We can be doing better, and we should.
Lopez-Cantu, T., & Samaras, C. (2018). Temporal and spatial evaluation of stormwater engineering standards reveals risks and priorities across the United States. Environmental Research Letters. 13 074006 [Main paper and Supplemental Info are free to read]
Third National Climate Assessment, Precipitation Changes, U.S. Global Change Research Program, 2014.