Over 22,000 Bridges Are Vulnerable to Extreme Weather: How We Engineer for Climate Exposure
There are 623,218 bridges in the United States. Every day, more than 4.9 billion vehicle trips cross them. Most people don't give those crossings a second thought. Engineers do.
ASCE's 2025 Infrastructure Report Card puts 6.8 percent of U.S. bridges in poor condition and nearly half in fair condition, aging structures with shrinking safety margins. The number that should be keeping people up at night: 22,420 bridges are currently susceptible to extreme storm events. That's not a risk projection. That's today.
The average American bridge is 47 years old. It was designed using hydraulic assumptions, wind load maps, and flood frequency data the climate has already moved past. Wind events are growing more intense. And scour, the leading cause of bridge failure in this country, is getting worse as flow volumes rise.
This post breaks down what climate exposure actually means for U.S. bridges: the failure mechanisms, the standards that govern how we design and evaluate against them, how those standards have changed, and what rigorous climate-resilient bridge design looks like in practice.
1. What the Numbers Actually Mean
Bridges earned a C on the 2025 ASCE Report Card, the same grade they got in 2021. Holding that grade took a serious investment: the IIJA directed $27.5 billion to the Bridge Formula Program and another $12.5 billion to the Bridge Investment Program. And yet the rehabilitation backlog still sits at $191.3 billion, with a 10-year funding gap of $373 billion to get the national inventory into genuine good repair.
The grades tell you condition. They don't tell you trajectory. The 49 percent of bridges rated fair aren't safe and stable; they're aging, and where they land after the next major flood event is an open question. Fair-condition bridges carry less reserve capacity than good-condition ones. Their foundations may have experienced scour that hasn't compromised structural integrity yet but has already eroded the margin. Their superstructures may have corrosion or fatigue that reduces reliability under combined hydraulic and wind loading.
Key Figures to Know
FHWA estimates it would cost $69.7 billion to replace the nation's poor bridges and $47.4 billion to rehabilitate them. For the 22,420 bridges specifically flagged as susceptible to extreme storm events, the question isn't whether those structures will be challenged. It's whether their foundations were built to take it.
2. Scour: Why Bridges Fail
Scour is the erosion of streambed and bank material around a bridge's piers, abutments, and footings by flowing water. It doesn't announce itself. It happens below the waterline during a flood, as accelerating flow through a constricted bridge opening strips away the soil a foundation depends on. The result is a scour hole: a void that can leave piles or spread footings without adequate embedment. When that happens, the structure above can fail suddenly.
The 1987 I-90 Schoharie Creek collapse in New York and the Mianus River Bridge failure in Connecticut, both attributed to scour, led directly to the creation of the national bridge scour program. Decades later, scour is still the leading cause of bridge failure in the United States. Climate change is making it worse.
How a warming climate amplifies scour
Scour depth is driven by flow velocity and discharge. The faster and larger the flow through a bridge opening, the deeper the scour. As precipitation intensity rises, following the Clausius-Clapeyron relationship which predicts roughly a 7 percent increase in extreme precipitation per degree Celsius of warming, peak discharges at bridge crossings increase with it. Research published in the ASCE Journal of Bridge Engineering has shown that climate-adjusted temperature and precipitation directly increase long-term scour risk, particularly on rivers where snowmelt and rainfall combine.
Broader studies of the continental U.S. bridge inventory found that under multiple climate scenarios, tens of thousands of bridges could face materially higher scour vulnerability from projected increases in 100-year peak flows. Foundations sized for historical hydrology may no longer have adequate embedment for the flows that are coming.
The three types of scour engineers evaluate
HEC-18 defines the full scour picture as a sum of three components:
• Long-term aggradation and degradation: gradual bed elevation changes from sediment transport, channel migration, or upstream disturbances like dam construction or gravel mining
• Contraction scour: general erosion across the full channel width beneath the bridge, caused by flow acceleration through the constricted opening; calculated using hydraulic equations tied to flow depth, velocity, and bed material
• Local scour: the deepest and most structurally critical component, occurring right at pier and abutment faces as turbulent vortices excavate the bed; estimated using the CSU pier scour equation and related expressions in HEC-18
Total scour at any foundation element is the sum of all three, evaluated for both the design flood and the superflood. That's FHWA's term for a flood exceeding the 100-year event. New bridges must survive one without foundation movement that requires corrective action. That's the design requirement, not a conservative option.
FHWA Policy: 23 CFR 650.313 and Technical Advisory T 5140.23
New bridges over waterways with scourable beds must be designed to withstand a superflood without experiencing foundation movement requiring corrective action. This requirement governs all new federally funded bridges and forms the basis for scour evaluation of existing structures under the National Bridge Inspection Standards.
3. The Standards Stack: What Governs Climate-Exposed Bridge Design
Bridge design for climate exposure isn't governed by a single code. It's an interconnected set of federal standards, ASCE load criteria, AASHTO structural specifications, and FHWA hydraulic engineering circulars. Together they define how engineers quantify hazards, design foundations and superstructures to resist them, and evaluate existing bridges for vulnerability. The 2022 revision to ASCE 7 is the most significant update to this framework in a generation.
| Standard | Issuing Body | What It Governs for Climate-Exposed Bridges |
|---|---|---|
| ASCE 7-22 + Supplement 2 | ASCE/SEI | Risk-category flood return periods; updated wind maps; tornado loads; sea level change in design flood definition |
| AASHTO LRFD Bridge Design Specifications (9th Ed.) | AASHTO | Primary structural design standard for highway bridges; load combinations; scour treated as a foundation resistance condition |
| FHWA HEC-18 (5th Ed.) | FHWA | Scour design and evaluation: pier and abutment scour equations; risk-based evaluation; Plans of Action for scour-critical bridges |
| FHWA HEC-20 (4th Ed.) | FHWA | Stream stability at highway structures; channel migration; lateral erosion; geomorphic hazard assessment |
| FHWA HEC-23 (3rd Ed.) | FHWA | Scour countermeasure selection and design: riprap, articulated concrete mats, sheet piling, gabions, guide banks |
| FHWA HEC-25 (2nd Ed.) | FHWA | Scour at tidal bridges; coastal and estuarine hydraulics |
| NBIS, 2022 Update (23 CFR 650) | FHWA | Mandatory inspection program; 2022 revision added scour vulnerability and scour condition ratings to national inventory reporting |
| ASCE 24-24 | ASCE | Flood-resistant design; freeboard for Risk Category III/IV structures; dry and wet floodproofing criteria |
| 23 CFR 650, Subpart A | FHWA | Federal regulation for highways in floodplains; hydraulic study requirements for federally funded bridge projects |
4. ASCE 7-22 Supplement 2: The Flood Load Overhaul Engineers Need to Know
Before ASCE 7-22 Supplement 2, the flood load provisions hadn't changed in any meaningful way since 1998. For 25 years, flood design was pegged to the 100-year event regardless of what the structure was or what happened if it failed. Supplement 2, published in May 2023, changed that entirely.
Risk-category flood return periods
The revised Chapter 5 ties the design flood to the building's Risk Category using mean recurrence intervals that scale with consequence of failure. It's the same probabilistic logic the standard already applies to wind and seismic, now finally applied to floods too. Here's how it breaks down:
| Risk Category | Typical Use | Design Flood MRI |
|---|---|---|
| I | Low-occupancy agricultural or storage structures | 50 years |
| II | Standard commercial, residential, institutional | 500 years |
| III | Essential facilities, most major bridges | 1,000 years |
| IV | Critical infrastructure, emergency operations, hospitals | 2,000 years |
For bridges that serve major corridors, where failure would cut off emergency access or shut down commerce, we're now designing to a 1,000-year flood event or higher. The flood elevation, flow velocity, and scour depth associated with a 1,000-year return period are substantially larger than those tied to the old 100-year standard. That's not a minor adjustment. It changes foundation design, countermeasure sizing, and for some structures, whether the existing design is still adequate.
Sea level change is now part of the design flood definition
ASCE 7-22 Supplement 2 also changed how the design flood is defined for coastal and tidal crossings. The design flood now explicitly incorporates relative sea level change over the structure's service life. Engineers can't just pull a number from the current FIRM and call it done. The design flood elevation has to reflect where sea level will be at the 50-year mark of the bridge's intended life.
On low-lying coastal crossings, the difference between today's base flood elevation and the climate-adjusted design flood elevation can exceed a foot. That's enough to change scour calculations, alter hydraulic load demands, and shift whether an existing foundation can be kept or needs to be rebuilt.
What This Means for Existing Bridge Evaluations
Supplement 2 applies to new construction and major reconstruction, but its return periods and climate-adjusted flood elevations are also reshaping how engineers approach existing bridge vulnerability assessments. When developing Plans of Action for scour-critical bridges under NBIS, best practice now means running hydraulic models against climate-adjusted hydrology, not just historical flood records. A bridge that passed its last HEC-18 evaluation under the old 100-year standard may not pass under 1,000-year conditions.
5. Wind Loads: What ASCE 7-22 Changed and Why It Matters for Bridges
Bridges aren't buildings, but ASCE 7-22 wind provisions feed directly into AASHTO LRFD bridge design and govern wind demands on superstructures, deck systems, railings, and sign structures. A few key changes in ASCE 7-22 affect bridge wind design directly.
Updated wind speed maps
ASCE 7-22 provides separate basic wind speed maps for each Risk Category, built from updated meteorological data, improved hurricane modeling, and revised extreme wind statistics. For Risk Category III, where most major bridges sit, the design wind speed corresponds to a 1,700-year mean recurrence interval. Along portions of the Gulf and Atlantic coastlines, the updated maps carry higher design wind speeds than previous editions. Engineers must pull site-specific values from the ASCE 7 Hazard Tool rather than interpolating from printed maps, which isn't acceptable under ASCE 7-22.
Tornado loads, now in a U.S. design standard for the first time
ASCE 7-22 introduced the first-ever tornado design criteria in a U.S. structural design standard. Not every structure and not every region, but for bridges in tornado-prone areas of the central and southern United States, engineers in designated hazard zones now need to evaluate whether major bridge structures can resist tornado-induced wind loads. That geography overlaps significantly with states like Iowa, Missouri, Oklahoma, and Kansas, which also carry some of the country's highest concentrations of aging, vulnerable bridges.
6. Scour Countermeasures: Engineering the Defense
Knowing how deep scour will go tells you half the story. The other half is what you do about it. HEC-23 is the governing document for scour countermeasure selection and design, and it covers a wide range of options. The right choice depends on scour type and severity, channel geomorphology, material availability, and the owner's long-term maintenance capacity.
Riprap
Riprap is angular crushed rock placed around piers and abutments, and it's the most widely used scour countermeasure in the country. It's cost-effective under standard conditions and well understood. HEC-18 and HEC-23 both provide sizing guidance based on design flow velocity, stone specific gravity, and required layer thickness. The catch: riprap that was adequately sized under historical hydrology may not be adequate anymore. Climate-adjusted hydraulics means higher design velocities, which means larger stones and deeper placement. Any riprap design on a climate-exposed crossing needs to be checked against updated hydraulic analysis.
Articulated concrete mats
Where flow velocities exceed riprap design limits, or where placing loose rock is difficult, articulated concrete block mats (ACBs) are a better option. They're interlocking concrete blocks linked by cable or geotextile fabric, and they resist flow-induced displacement more reliably than riprap under high-velocity conditions. HEC-23 covers sizing and installation. They're commonly used where maintenance access is limited or where countermeasure failure carries severe consequences.
Sheet piling and concrete encasement
For severe scour or for existing bridges where HEC-18 analysis shows foundation embedment is insufficient, sheet piling and concrete pier encasement are structural solutions. Sheet pile cutoff walls installed around a pier group intercept scour development and effectively extend the foundation into deeper material. Concrete encasement of an existing spread footing does the same. These cost more than surface armouring, but they're used when the scour calculations leave no room for softer approaches.
Guide banks
Where abutment scour is driven by flow concentration at the ends of bridge approaches, guide banks, earth embankments aligned to redirect flow through the bridge opening, reduce scour severity significantly. HEC-23 has design equations for guide bank geometry based on bridge opening length, approach flow conditions, and channel characteristics. On climate-exposed crossings where increasing flood flows are overtopping approach embankments and attacking abutments from angles the original design didn't anticipate, guide bank geometry needs to be rechecked against the full range of likely future conditions.
A Practical Note on Countermeasure Adequacy
A countermeasure designed for the historical 100-year flood may not be adequate for a climate-adjusted 100-year flood with higher peak discharge. When engineers develop Plans of Action for scour-critical bridges under NBIS requirements, hydraulic models should be run against both historical and climate-adjusted frequencies. The goal is to know whether existing countermeasures will still hold, not to find out during the next major flood event that they don't.
7. The Updated NBIS: Better Data on a Problem We Can't Afford to Undercount
The National Bridge Inspection Standards haven't had a major update in decades. The 2022 revision changed that. It's the first significant overhaul of the SNBI (the Specifications for the National Bridge Inventory) since 1995, and two of the key changes go directly to climate exposure.
Inspectors now have to record both a scour vulnerability rating and a scour condition rating for all applicable bridges. The vulnerability rating captures whether a bridge is scour-critical based on hydraulic evaluation. The condition rating reflects what inspectors actually find underwater: observed scour damage, measured hole geometry. Together, they give bridge owners and state DOTs a clearer picture of actual scour risk than the old single scour-critical designation ever provided.
What this means in practice: scour evaluations have to be documented rigorously enough to support both ratings. Underwater inspection, using divers or sonar to measure scour hole geometry, has to be part of a systematic program for bridges over scourable beds. Bridges that come up scour-critical need a Plan of Action with interim protective measures, a countermeasure installation timeline, or a foundation remediation plan. That's not optional under NBIS. And for bridges where climate-adjusted hydrology now produces scour depths exceeding design embedment depth, the POA process is how that risk gets managed until the structure can be upgraded.
8. The $373 Billion Gap and What It Actually Represents
ASCE's Bridging the Gap report puts the 10-year bridge funding shortfall at $373 billion. That number covers rehabilitation of poor-condition structures, replacement of structurally deficient crossings, and the upgrades needed to bring aging foundations into compliance with current scour, hydraulic, and wind design standards. Not future standards. Current ones.
A significant portion of that gap is driven by climate exposure. Bridges that were adequate under historical hydrology may need deeper piles, extended footings, or enlarged countermeasures to remain adequate under the design conditions ASCE 7-22 Supplement 2 now requires. The cost of proactive foundation remediation is a fraction of the cost of post-failure replacement, to say nothing of what a bridge closure costs the communities that depend on it. The Fern Hollow Bridge collapse in Pittsburgh in 2022, which ASCE's 2025 Report Card cites by name, is a reminder that deferred maintenance eventually stops being deferred.
The IIJA's $40 billion for bridge programs is the largest single federal bridge investment in history. Using it well means prioritizing climate vulnerability alongside structural condition. That means running updated hydraulic analyses rather than relying on 30-year-old flood maps. It means incorporating climate-adjusted hydrology into scour evaluations from the start. And it means designing countermeasures to the return periods ASCE 7-22 Supplement 2 actually requires, not the weaker thresholds of previous decades that helped create this situation.
Conclusion: Design for the Floods That Are Coming
The 22,420 bridges flagged as susceptible to extreme storm events aren't a fixed population. As precipitation intensities rise and storm frequencies increase, more structures will cross the threshold from manageable exposure to genuine vulnerability. The standards have responded. ASCE 7-22 Supplement 2, the updated NBIS, HEC-18's risk-based evaluation framework. The code environment is more capable of capturing climate risk than it's ever been.
What's needed now is consistent application. Not designing to the minimum code floor, but to the actual exposure these structures will face across their service lives. That means using climate-adjusted hydrology in every scour evaluation. It means pulling current wind and flood parameters from the ASCE 7 Hazard Tool rather than relying on outdated maps. It means designing countermeasures that'll still be adequate at the 50-year mark, not just the day the project closes out.
America's bridges have held up under enormous pressure for generations. With the right engineering and sustained investment, they can hold up for generations more. But that requires designing for the floods that are coming, not the ones we've already measured.
