CHAPTER 5

Summary of Findings

State DOTs recognize that management and maintenance practices for existing roadway and embankment culverts fundamentally influence culvert performance and have subsequent impacts on adjacent assets and overall transportation system performance. State DOTs have in common the need to reduce culvert degradation impacts and improve resilience, yet their approaches are diverse. Variability in climate, extreme weather events, seismic susceptibility, slope and embankment stability, watershed characteristics, and urban development and land uses creates system challenges. Documenting practices and remedial measures for culvert management and maintenance is an important step in planning for improved system operations, performance, and resilience. The objective of this synthesis has been to document management and maintenance practices used by state DOTs to enhance resiliency of existing roadway and embankment culverts. Information was gathered through a literature review, a survey of state DOTs, and follow-up interviews with selected DOTs for the development of case examples. Information gaps and suggestions for research to address those gaps have been identified and are presented in this chapter.

5.1 Key Findings

• While state DOTs are required by federal law to implement resilience strategies into their planning and completion of infrastructure projects, culverts are often not specifically addressed in these laws and regulations.
• Survey results showed 20 state DOTs out of 42 total DOTs (48%) lacked detailed information on culvert materials and shapes under state DOT inventory.
• Forty state DOTs (95% of respondents) provided an estimate of the culvert inventory owned and maintained by their DOT. 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 state DOT 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 state DOTs that replied, 37 (90%) have an expected design service life for culverts, and 10 of these DOTs (27%) specify a range of service life depending on the culvert material or the roadway classification under which the culvert is installed.
• Of the 37 state DOTs that replied, 17 (46%) answered that culvert material does affect the expected design service life, while 18 DOTs (49%) indicated that neither culvert material nor soil type affects the expected design service life of culverts.
• Of the 41 state DOTs that replied, 32 (78%) do not have a formal risk assessment methodology to evaluate culvert resiliency and durability.
• Twenty-nine state DOTs (69%) implement multiple practices such as routine maintenance, field inspections, and video/camera inspections to determine whether a culvert should be

• considered for replacement or rehabilitation. Life-cycle cost analysis, cost–benefit analysis, and risk assessment are all used to aid in making decisions regarding culvert replacement or rehabilitation.
• Thirty-two state DOTs (76%) have an asset management system in place that inventories a range of different culvert sizes. Ten (24%) indicated that certain culvert sizes are inventoried under the bridge division asset management system, although the size thresholds across answers varied. Ten state DOTs (24%) do not have an asset management system for culverts in place.
• Twenty-six state DOTs (62%) answered that culvert size affects inspection and maintenance practices, and seven (17%) indicated that culvert material affects these practices. Sixteen state DOTs (38%) answered that neither size nor material affect culvert inspection and maintenance.
• Practices ranked as very important for enhancing culvert resiliency are implemented every 5 years or more or not practiced at all. State DOTs answered that most inspection and maintenance practices are employed on an as-needed basis or a reactive maintenance schedule, even if they are extremely important or very important for enhancing culvert resiliency.
• Regarding documented failures related to deficiencies in resiliency, 18 states (43%) ranked debris clogging as extremely or very common, 16 states (38%) ranked premature degradation of the culvert material or system as extremely or very common, and 13 states (31%) ranked joint separation as extremely or very common. The least common failures reported were flotation and burning of the culvert, with 38 states (90%) and 34 states (81%) ranking these failures as less common or not at all common, respectively.
• Twenty-two state DOTs (52%) indicated that frequent maintenance, cleaning, and inspection of culverts are common practices to enhance culvert resiliency. Respondents agreed that cathodic protection was the least frequently implemented or not at all employed practice for enhancing culvert resilience.
• 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.
• Culvert failures due to resilience issues are often not recorded within state DOTs, with 14 respondents (33%) answering that these data are not tracked within their DOT.
• 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. Hurricanes and wildfires have also led to culvert replacements, but these events are more regional and have affected DOTs located along eastern coastlines and in the western half of the United States, respectively.
• 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. Thirty-nine state DOTs (93%) clean and remove debris from affected culverts after extreme events.
• The case examples demonstrated that asset management systems could improve culvert resilience by prioritizing those culverts in declining condition for maintenance, repair, and rehabilitation before costly replacements are required.
• The case examples showed that thorough inspections with clear condition rating criteria could result in choosing tailored repair and rehabilitation practices that address specific resiliency issues and prevent their recurrence.
• The case examples detailed DOT requirements for selecting appropriate culvert material, shape, and size for new culvert installation based on current and projected environmental and

• economic conditions. Consideration of these factors is important for ensuring that culverts meet expected design service life without major rehabilitation or replacement.

5.2 Potential Future Research

Based on the literature review, survey, and case example interviews, several areas for potential future research have been identified. Suggestions for research include the following:

• Defining and tracking culvert failures due to resilience challenges. Tracking culvert failure modes could lead to more strategic planning and preparation for extreme climate and extreme weather events, which are becoming more frequent due to the effects of climate change (NOAA NCEI 2025; Pluimer 2023).
• The development of a resilience optimization matrix for making decisions to improve culvert resilience. A design matrix evaluating materials, cost, decision methodologies, initial design, and emergency response could be useful at the culvert planning and design stage to optimize cost-effectiveness and resilience of the material for the application.
• Development of a formalized culvert risk assessment methodology to quantify and assess the risks of extreme weather events, as well as mitigation strategies to help minimize risk.
• Providing guidance on rehabilitation practices to enhance culvert resilience, focusing specifically on lining methods and developing documentation on rehabilitation practices with supporting research on effectiveness. A formalized rehabilitation guide providing standardized procedures with expected service life extensions could assist DOTs in decision-making for optimizing culvert resilience.
• Documenting major weather events to compare with asset management data to determine culvert vulnerability and risk exposure. This could allow for prioritization of culvert maintenance, repair, and rehabilitation based on assets exposed to the highest risk. Proactive maintenance could prevent premature failures of culverts due to resilience deficiencies.
• Conducting cost–benefit analyses regarding damages sustained from extreme weather events. Projecting costs associated with resilience-related failures could allow DOTs to compare mitigation alternatives and aid in making decisions about cost-effective and resilient culvert maintenance and management practices.
• Research related to more resilient materials for culverts, improved installation methods and backfill materials to potentially mitigate future resilience concerns for culverts, and rehabilitation methods to extend the service life of existing culverts in light of future resilience challenges.