You may be familiar with the story of the boiling frog. Whether or not it’s scientifically accurate, it offers a timely analogy for recognition of climate change impacts and the need for adaptation and resilience. The story goes that if you drop a frog into a pot of boiling water, it will immediately jump out. However, if you put the frog in a pot of cool water and slowly increase the temperature of the water until it is boiling, the frog will remain in the water until it… you get the idea.
Earlier this fall, two major hurricanes made landfall along the west coast of Florida causing catastrophic impacts. Hurricane Helene landed 40 miles southeast of Tallahassee as a Category 4 storm and proceeded to track inland through the Tennessee valley. Over 230 deaths and approximately $225 billion in damages have been attributed to Helene to date, much of which was caused by the inland rainfall and subsequent flooding in western Carolinas and Virginia, and eastern Tennessee.
In western North Carolina alone, over 500 roads are closed or have only partial access as of today, including sections of Interstates 26 and 40 (see NCDOT Detours Map as of 10-10-2024). Two weeks later, Hurricane Milton slammed into Siesta Key as a Category 3 storm, cutting a path across central Florida. Early damage estimates from Milton have reached up to $160 billion, with 17 confirmed deaths caused by the storm. Among the many grim reminders, these storms reinforce how fragile our transportation network can be. In terms of the boiling frog analogy, these events are sudden boiling pots from which the frog jumps!
These hurricanes represent acute impacts and disruptions that have hours’ long lag time before they’re felt. The more routine climate change-induced flooding and sea level rise (SLR) impacting much of the United States has gradually increased over time, measured in terms of decades, not days, and offer reminders of the gradually warming pot in the frog story.
According to global insurer Swiss Re’s preliminary estimates, global insured flood losses are above average for the first half of 2024. Insured losses for all natural catastrophes for the period were up to $60 billion—62% above the 10-year average—driven by a higher frequency of small-to-medium events. Thunderstorms, particularly in the U.S., accounted for 70% of the overall insured losses, though that ratio may drop once Helene and Milton are included. Even so, while catastrophic events get more publicity, smaller isolated events create more frequent damage claims.
What Is at Risk from Rainfall and Sea Level Rise?
Transportation agencies and authorities across the country manage and maintain a tremendous amount of hard infrastructure. Many transportation elements in use today, including runways, roads, bridges, ports, and the drainage features that support them (ditches, pipes, culverts, detention ponds, etc.) were designed to rainfall standards established decades ago. For example, in its 2023 State Highway System Resilience Action Plan, the Florida Department of Transportation provides the following statistics depicted in the graphic below.
The centerline miles numbers are mild when compared to the noted bridges at risk, approximately 45% in a 100-year floodplain, 28% in a Category 3 storm surge zone, and 20% in areas impacted by a two-foot sea level rise by 2070.
Management Metrics – Rainfall and Sea Level Rise
In the U.S., the National Oceanographic and Atmospheric Administration (NOAA) ATLAS data sets (the most recent being ATLAS 14) provide the rainfall numbers used to build the recurrence intervals that govern many state and local design standards. Importantly, ATLAS data currently only relies on historical events to frame the recurrence intervals we use today. However, as we experience more Intense rainfall events, storms with differing Durations, and more Frequent rain events, the so-called “IDF” curve numbers are shifting while the models built on ATLAS 14 data remain static. This leaves the currently implemented design standards wanting.
Transportation agencies must also account for climate-based sea level rise (SLR). Coastal areas already experience so-called “sunny day floods,” occurring primarily due to SLR, land subsidence, and regional oceanographic effects (think El Nino/La Nina). In its most recent Annual High Tide Flooding Outlook, NOAA noted a nationwide prediction of four to eight high tide flooding days in 2024-25, while also noting that today, the U.S. experiences a median of 5 more flood days per year since 2000, a nearly 200% increase. Several regions around the Gulf Coast and Mid-Atlantic are projected to experience between 14 to 20 high-tide flooding days in 2024-25.
For transportation infrastructure, SLR threatens existing roads and bridges, port infrastructure, and other appurtenant coastal zone development. In extremely low-lying coastal areas, the combination of increased rainfall and storm sewer networks that drain out—or outfall—to tidal waters creates an additional challenge. Rain events and tide cycles enhanced by SLR more frequently create a “compound flooding” condition, which occurs when storm sewer networks cannot discharge collected rainfall to outfalls that may be below sea level because of a now higher high tide. This compound flooding condition can be difficult to overcome, because even if the storm sewer network has been designed for the enhanced rainfall, it may not have the detention capacity to wait out a multi-hour tide cycle to release detained runoff.
Framing the Alternatives
To understand the value of the alternatives available to build climate resilience into our transportation infrastructure, we must remember that the impacts of the climate change we’re now seeing took decades to emerge. Even gradual increases in temperature allow the atmosphere to hold more water vapor, which leads to the increase in the IDF precipitation characteristics noted above.
When evaluating the tools available to address the challenge, one glaring deficiency stands out. To inform the hydrologic and hydraulic models that transportation agencies, states, and localities use to establish design standards, modelers have only had historical rainfall to establish the precipitation metrics. Data such as ATLAS 14 rely on historical rainfall events to establish the extremes for weather events and the subsequent design storm confidence intervals. As a result, the current climate dynamics impacting today’s rainstorms are missing from the historical rain event data used to establish storm recurrence intervals.
New Data to the Rescue?
Among the factors needed for long-term solutions, more forward-looking and consistently updated data is a critical component. On this front, some helpful tools are on the horizon. NOAA precipitation frequency estimates in ATLAS 14 have not only been based solely on historical data, they have also been inconsistently funded by congress and partnering states. Recently enacted federal legislation has provided NOAA with direct, federal funding to update the NOAA ATLAS 14 precipitation frequency standard while accounting for climate change in the ATLAS 15 update. Another recent federal law authorizes NOAA to establish and run a program to “compile, analyze, and communicate the frequency of precipitation in the U.S. and update these precipitation frequency estimates no less than once every 10 years.”
Planning for Adaptation and Resilience in Transportation
With the new ATLAS 15 data, transportation infrastructure programs will have access to data that incorporates both historical data and climate change-based metrics for rainfall. However, the ATLAS 15 updates will not be widely available until 2026. The timing urgency is only heightened by the needs that transportation agencies already recognize and the open funding windows currently available. Funding programs through the Federal Highway Administration (PROTECT grant), NOAA, the Federal Emergency Management Agency, and others offer significant flood resilience grant assistance today. Transportation agencies understand the need to implement resilience and adaptation planning now.
With the needed data lagging, where do we start? We can begin with a longer planning horizon to deal with SLR and IDF rainfall change, like the time horizons for these phenomena, is entirely appropriate. Near term, intermediate term, and long-term time horizons for this type of effort may be 10, 40, and 70 years (or more), respectively, with climate change SLR and rainfall estimates often using a planning horizon out to 2100.
Overcome Near Term Data Gaps
While we wait for ATLAS 15, transportation agencies have the option to fine-tune their own design criteria when needed to adjust to experienced conditions. Mitigation measures can include horizontal setbacks from existing. Regulatory floodplains can incorporate additional vertical freeboard. The addition of a “measure of safety” (MOS) for existing design criteria (i.e. include an extra 20% design for stormwater conveyance capacity and post-construction stormwater management to account for enhanced rainfall events) is another near-term design adjustment already being employed around the country. Where feasible to implement, each can provide an extra layer of flood mitigation to existing design standards.
How to Prioritize and Plan
Planning for climate change-induced impacts to hard infrastructure requires practitioners to take a long-term view of the solution menu while attacking the obvious issues. Problem areas today are likely to become more problematic over time in frequency or scale or both.
Against that backdrop, as transportation agencies prepare plans to enhance infrastructure resilience, a logical asset evaluation process offers a roadmap to defensible prioritization metrics. For road and bridge infrastructure, examples of metrics to evaluate may include:
- Roadway classification
- Average daily traffic counts
- Trade/commerce movement routes
- Connectivity to other critical infrastructure
- Degree of redundancy
- Transit routes
- Bridge deck elevations above waterways.
While each plan will need to address the specific infrastructure inventory and observed conditions agencies feel the need to prioritize, utilizing these metrics can help clarify the path forward.
Also, long-term resilience plans need to be revisited on a regular schedule to account for needed updates, better data incorporation, and demographic shifts that may move the focus of future infrastructure investments.
The changes in atmospheric and oceanic conditions—leading to more intense, longer duration, and more frequent rainfall and rising sea levels—warrant serious resilience planning.
The time to get hopping is now. The water in our pot is only getting warmer.
Related reading: It’s about chance: Statistics of 100-year floods »
