SUMMARY

Practices to Enhance Resiliency of Existing Roadway and Embankment Culverts

Culverts are buried drainage structures underneath roadways or embankments that are open at both ends and used to convey and transport water. Culverts were frequently identified as vulnerable components to extreme weather events and climate change during a 2013–2015 pilot study (FHWA-HEP-16-079, ICF International 2016), and many culverts under the jurisdiction of state departments of transportation (DOTs) were designed and installed well before the effects of climate change were understood or realized. Existing culverts must be able to handle increasingly frequent and severe weather events [National Oceanic and Atmospheric Administration (NOAA) National Centers for Environmental Information (NCEI) 2025; Pluimer 2023], which can accelerate failure mechanisms and shorten culvert lifetimes from expected design lifetimes. Accelerated deterioration of culverts can lead to a suite of unplanned repairs, rehabilitation projects, or full replacements, all of which are financially burdensome and present state DOTs with issues of staffing or sourcing of work.

State DOTs have in common the need to improve resilience, but practices are diverse due to variability in climate, weather events, seismic susceptibility, slope and embankment stability, watershed characteristics, and challenges from urban development and land usage. This synthesis provides a summary of culvert management and maintenance practices employed by state DOTs to enhance the resiliency of their existing roadway and embankment culverts with a span (inside diameter or width) of less than 20 feet. It also documents common failure modes of culverts and challenges in implementing resilience strategies. Documenting proactive and sustainable maintenance and management practices to enhance culvert resilience can mitigate the potential costs associated with the effects of extreme weather events, seismic events, and other threats to roadway and embankment culverts.

FHWA Order 5520 defines resilience as “. . . the ability to anticipate, prepare for, and adapt to changing conditions and withstand, respond to, and recover rapidly from disruptions” (FHWA 2014). The 2017 AASHTO Resiliency Peer Exchange on Extreme Weather and Climate Impacts brought together U.S. transportation officials to discuss the topic of resilience and the challenges and successes state DOTs have experienced in preparing for and responding to extreme weather events. The exchange was illustrative in highlighting resilience needs at the DOT level. In roundtable discussions with state DOT representatives focused on the budgetary, planning, engineering design, and operations and maintenance functions of DOTs, the following characteristics were identified as key elements of transportation resilience:

• Determining a return on investment of resilience projects
• Implementing consistent framework for assessing asset vulnerability
• Updating design manuals and utilizing AASHTO and FHWA guidance documents
• Incorporating risk assessment and life-cycle costs in project designs
• Prioritization of maintenance based on risk.

Common culvert materials include reinforced concrete pipe (RCP), corrugated metal pipe (CMP), and thermoplastic pipe. Culverts are manufactured or constructed in various shapes, including circular, elliptical, arched, and rectangular. Many resilience issues are universal and not material specific. Common resilience issues are:

• Overtopping,
• Infiltration or piping of backfill materials through joints or the embankment,
• Flotation or buoyancy of the culvert,
• Erosion of the embankment materials,
• Upstream and downstream channel stability (scour),
• Structural degradation of the culvert,
• Joint separation,
• Impact damage from debris,
• Deformation or cracking from excessive live load, and
• Corrosive damage due to local environment.

Some resilience issues and failure modes are specific to different culvert types and materials. Material-specific resilience issues associated with concrete culverts include joint separation, cracking, and spalling. Metal culverts can experience buckling, corrosion, perforation, abrasion, and deflection. Thermoplastic culverts can experience buckling, abrasion, cracking, and deflection. Additional failure avenues present themselves in the type and quality of the backfill material and in culvert hydraulic capacity.

Culvert failures due to inadequate resilience are costly to the environment as well as to the traveling public that uses these assets. Closure of a roadway due to culvert failure can result in long detour routes, which puts stress on the traveling public. Cost considerations include not only the cost of the repair, rehabilitation, or replacement of the culvert, but also the cost to the public, including travel time delays, number of affected vehicles, and additional mileage from road detours.

Different geographic regions present different climate challenges and specific extreme weather events. Floods and severe storms can overwhelm the hydraulic capacity of culverts, leading to piping of backfill materials. The backfill material loss from piping can lead to catastrophic failure of the surface above the culvert. Seismic activity and tornados can disrupt the backfill material surrounding the culvert, crack the culvert pipe or separate joints through excessive movement, damage end treatments applied to the structure, and clog culverts with excessive debris. Wildfires can lead to structural degradation of all culvert materials and can be particularly concerning for flammable materials such as thermoplastic pipes with exposed end sections that are not protected.

This synthesis was developed through a literature review, a survey sent to 52 DOTs, including all 50 state DOTs and those of the District of Columbia and Puerto Rico, and case example interviews with DOT staff from selected DOTs. The literature review is discussed in Chapter 2 and includes information from federal legislation and regulations, relevant research, and selected state DOT publications. The summary of results is presented in Chapter 3. Case examples from five states are discussed in Chapter 4. An overall summary of findings is presented in Chapter 5.

The survey was completed by 42 DOTs (the DOTs of 41 states and Puerto Rico), resulting in an 81% response rate. Case example interviews were conducted with five state DOTs representing geographic diversity and all four AASHTO regions, wide representation of culvert types and materials installed and specified in their states, and a variety of potential threats that could affect resiliency, including floods, hurricanes, severe storms, and wildfires.

Findings from the survey results, completed by 42 DOTs (except where noted for individual questions), include:

• Forty state DOTs (95% of respondents) provided an estimate of the culvert inventory owned and maintained by their DOTs. Based on these reported numbers, the total culvert inventory owned and maintained by these 40 states was estimated to be 2.57 ± 0.69 million.
• Thirty-six state DOTs (86%) provided information related to the estimated percentage breakdown of culvert materials and shapes in their inventory. Of these, circular reinforced concrete culverts made up approximately 35% of the total reported culvert inventory, and circular galvanized corrugated metal culverts made up approximately 30%.
• Of the 41 that responded to the question, 37 state DOTs (90%) have an expected design service life for culverts, and of these 37, 17 state DOTs (46%) answered that culvert material does affect the expected design service life.
• Of the 41 that responded, 32 state DOTs (78%) do not have a formal risk assessment methodology to evaluate culvert resiliency and durability.
• Twenty-nine state DOTs (69%) implement multiple practices to determine if a culvert should be considered for replacement or rehabilitation. Commonly employed practices include field inspections and routine maintenance.
• Thirty-two state DOTs (76%) have an asset management system in place that inventories a range of different culvert sizes.
• Twenty-six state DOTs (62%) stated that culvert size affects inspection and maintenance practices, and 7 DOTs (17%) indicated that culvert material also affects these practices. Sixteen state DOTs (38%) answered that neither size nor material affect culvert inspection and maintenance. DOT representatives stated that most inspection and maintenance practices are employed on an as-needed basis or a reactive maintenance schedule.
• Of the documented failures related to deficiencies in resiliency, debris clogging (18 DOTs, 43%), premature degradation of the culvert material or system (16 DOTs, 38%), and joint separation (13 DOTs, 31%) were ranked as extremely common or very common. Failures ranked as less common or not at all common included flotation (38 DOTs, 90%) and burning of the culvert (34 DOTs, 81%).
• Twenty-two state DOTs (52%) indicated that frequent maintenance, cleaning, and inspection of culverts are common practices to enhance culvert resiliency.
• Regarding practices to mitigate piping at the outlets and soil loss through joints, 27 DOTs (64%) reported sliplining culverts with a more resilient and watertight material, 20 DOTs (48%) reported specifying watertight joints on their culverts, and 20 DOTs (48%) reported specifying backfill materials that are less prone to piping and soil loss through joints.
• Thirty-eight state DOTs (90%) ranked culvert hydraulic capacity or size as a very important attribute to consider when replacing culverts to enhance resiliency, and 30 DOTs (71%) ranked culvert material corrosion resistance as very important. The culvert material’s flame resistance ranked as the least important attribute to consider when replacing culverts to enhance resiliency, with 28 DOTs (67%) ranking that attribute as less important.
• All 42 participating state DOTs (100%) answered that flood events and severe storms have led to the replacement of culverts.
• Thirty-eight participating state DOTs (90%) employ multiple post-event response practices to maintain, manage, and resume culvert function and performance after extreme weather events, including cleaning and removal of debris from affected culverts.

The literature review, survey, and case examples identified several areas for improvement as well as for potential future research. Suggestions for research include the following:

• Defining and tracking culvert failures that occur because of resilience deficiencies. Causes of culvert failures are not typically tracked within DOTs. Tracking culvert failure modes

• could lead to more strategic planning and preparation for weather events by informing decisions made on culvert material, inspection and maintenance practices, and repair or rehabilitation techniques.
• The development of a resilience optimization matrix to compare decisions on culvert resilience improvements. A design matrix consisting of evaluation of materials, cost, decision methodologies, initial design, and emergency response would be useful in the planning and design stage of culvert replacement or construction to identify a cost-effective and resilient material for application. A centralized matrix could be useful and time-efficient for DOTs when they need to make decisions quickly.
• Formalization of rehabilitation practices to enhance culvert resilience, focusing specifically on lining methods. There is a lack of research supporting various rehabilitation practices, with many methodologies using rule-of-thumb approaches and lacking supporting evidence about their effectiveness. A formalized rehabilitation guide providing standardized procedures with estimated costs of expected service life extensions would assist DOTs in decision-making to optimize culvert resilience.
• Documenting major weather events to compare with asset management data to determine culvert vulnerability and risk exposure. This would allow for prioritization of culvert maintenance, repair, and rehabilitation based on assets exposed to the highest risk. Proactive maintenance would prevent premature failures of culverts that occurred because of resilience deficiencies.
• Conducting cost–benefit analyses regarding damage sustained from extreme weather events. Projecting costs associated with resilience-related failures would allow DOTs to compare mitigation alternatives and would aid in making decisions about cost-effective and resilient culvert maintenance and management practices.