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Culvert

A culvert is a closed conduit or tunnel-like structure embedded beneath roadways, railways, embankments, or trails to channel surface water from one side to the other, preventing flooding and erosion while providing structural support for the overlying infrastructure.^[1] These structures are distinguished from bridges by their smaller size, lack of extensive superstructure, and direct burial in soil, typically spanning less than 20 feet and handling flows from natural streams or drainage ditches.^[2] Unlike open channels, culverts maintain the integrity of the transportation route by allowing water passage without interrupting traffic.^[3] Culverts serve dual hydraulic and structural functions essential to civil engineering and infrastructure resilience. Hydraulically, they must accommodate peak flows to avoid upstream ponding or downstream scour, with design considering factors like inlet/outlet control, debris accumulation, and velocity to ensure efficient water conveyance.^[4] Structurally, they bear the weight of vehicles, soil overburden, and live loads without deformation or collapse, often requiring reinforcement to withstand long-term environmental stresses such as corrosion or settlement.^[5] In addition to drainage, well-designed culverts enhance ecological connectivity by facilitating fish passage and minimizing habitat fragmentation in streams, supporting biodiversity and water quality.^[6] Their failure can lead to road washouts, costly repairs, and environmental damage, underscoring their role in climate adaptation amid increasing extreme rainfall.^[7] Common types of culverts are classified by shape, material, and construction method to suit site-specific needs like span length, soil conditions, and flow volume. Pipe culverts, the most prevalent, feature circular or elliptical cross-sections for smaller spans and are often used in low-traffic areas.^[8] Box culverts provide rectangular openings for larger capacities and pedestrian or utility access, while arch and pipe-arch variants offer improved hydraulic performance with reduced sediment buildup.^[9] Open-bottom culverts, resembling small bridges, handle wider streams with open bottoms to mimic natural channels.^[10] Materials for culverts are selected based on durability, cost, corrosion resistance, and installation ease, with concrete and metal dominating applications. Reinforced concrete pipes and boxes excel in high-load environments due to their compressive strength and longevity, often lasting over 50 years.^[4] Corrugated steel or aluminum pipes provide flexibility for uneven terrain and are galvanized for rust protection, though they may require coatings in aggressive soils.^[11] Thermoplastic options like high-density polyethylene (HDPE) or PVC offer lightweight, corrosion-free alternatives for corrosive or low-load sites, increasingly used for environmental sustainability.^[12] Design standards, such as those from the American Association of State Highway and Transportation Officials (AASHTO), integrate these materials with hydraulic modeling to optimize performance and service life.

Fundamentals

Definition and Purpose

A culvert is a structure designed to convey surface water through a roadway embankment or away from the highway right-of-way, typically embedded in soil and surrounded by structural material around its perimeter.^[1] Unlike bridges, which span larger distances and carry roadways over waterways with elevated supports, culverts are shorter conduits or tunnels that pass water beneath roads, railroads, trails, or similar obstructions, often with the entire structure covered by embankment.^[13] In contrast to simple buried pipes, which lack additional structural elements and are primarily for subsurface drainage without supporting overlying loads, culverts incorporate features to handle both hydraulic flow and earth pressures from above.^[1] The primary purpose of a culvert is to manage stormwater runoff and prevent flooding by directing water under infrastructure, thereby protecting roadways from erosion and water damage.^[1] It also facilitates drainage for embankments, conveys floodwaters during high-flow events, and minimizes risks to traffic, property, and the environment through integrated hydraulic and structural performance.^[1] Where designed appropriately, culverts can additionally support ecological functions, such as providing safe passage for wildlife or pedestrians beneath barriers.^[14] Basic components of a culvert include the inlet, which controls water entry; the barrel or conduit, serving as the main flow passage; the outlet, where water discharges; and headwalls or end structures that stabilize the ends, reduce erosion, and improve hydraulic efficiency.^[15] These elements work together to ensure the structure withstands loads while efficiently transporting water without upstream ponding or downstream scour.

History and Evolution

The earliest known culverts date back to ancient civilizations, where they were essential for managing water flow under roads and in urban infrastructure. In ancient Rome, engineers constructed culverts using stone and timber to facilitate drainage beneath roads and aqueducts, allowing for the passage of streams and preventing flooding. Examples include stone-arch culverts under viae such as the Appian Way and large culverts beneath aqueducts like that of Nîmes, built around the 1st century CE.^[16] These early structures demonstrated advanced hydraulic principles, using gravity-fed channels often arched with stone blocks for durability.^[17] During the 19th century, the Industrial Revolution spurred innovations in culvert construction to support expanding railway networks. Cast iron and brick emerged as preferred materials for their strength and resistance to pressure from embankments and traffic loads. Brick culverts, often built as segmental arches or box sections, were commonly installed under rail lines to channel water efficiently, as seen in early British and American railways where they replaced less durable timber alternatives.^[18] Cast iron pipes and segments were also introduced for smaller crossings, offering corrosion resistance and ease of prefabrication, which accelerated construction during the rapid rail expansion of the era.^[19] The 20th century marked a shift toward more standardized and versatile materials, with precast concrete and corrugated metal becoming dominant after the 1920s due to their cost-effectiveness and ease of installation. Precast concrete pipes and boxes allowed for factory production and rapid on-site assembly, widely adopted for highway and rural drainage projects. Corrugated metal culverts, particularly steel, gained prominence as a key milestone; the first such designs were patented in 1896 by James H. Watson, with commercial production scaling up in the 1890s through innovations like those later advanced by Armco.^[20] Standardization efforts by the American Association of State Highway Officials (AASHTO), formed in 1914, culminated in the 1930s with unified specifications for culvert sizing, materials, and hydraulic performance, influencing national infrastructure guidelines. In recent decades up to 2025, culvert evolution has emphasized sustainability and environmental compliance, driven by regulations like the Clean Water Act of 1972, which mandates protections for water quality and aquatic habitats. This has led to eco-friendly designs such as fish-passable culverts that mimic natural stream profiles to support migration and reduce erosion, alongside the integration of sustainable materials like recycled plastics and geopolymer liners for rehabilitation.^[21]^[22] These advancements prioritize longevity and minimal ecological impact, reflecting broader shifts toward resilient infrastructure amid climate challenges.^[23]

Types and Classifications

By Shape and Structure

Culverts are classified by their shape and internal structure, which directly influence hydraulic performance, structural integrity, and suitability for specific site conditions. Common shapes include circular pipes, rectangular boxes, arches, and hybrid bridge-like configurations, each offering distinct advantages in managing water flow, minimizing material use, and accommodating environmental factors such as streambed preservation. Pipe culverts typically feature circular or elliptical cross-sections and are the most prevalent type due to their availability in various sizes and strengths from manufacturers. These shapes are particularly ideal for conveying low to moderate flows under roadways or embankments, as their smooth, rounded form promotes efficient hydraulic flow with minimal energy loss. The circular design facilitates ease of manufacturing and installation, often requiring limited cast-in-place concrete only for end treatments, which reduces construction complexity. Additionally, circular pipes exhibit self-cleansing velocities that help minimize sedimentation buildup, making them suitable for applications where sediment transport is a concern.^[24]^[25] Box culverts, characterized by rectangular cross-sections and often multi-cell configurations, are designed to handle higher flow volumes and are commonly deployed in urban or constrained settings where structural strength is paramount. Their flat soffit and vertical walls provide a low-profile option that can be adjusted in height and span to optimize capacity without excessive excavation, allowing multiple parallel barrels to manage large discharges efficiently. This shape offers superior resistance to external loads, such as overlying soil or traffic, and facilitates easier access for inspection and maintenance compared to enclosed pipes. Box culverts are particularly advantageous for rises exceeding 4 feet, where they provide cost benefits over alternative shapes due to their modular construction potential.^[24]^[26] Arch culverts employ semi-circular or elliptical arch profiles, often bottomless to utilize the natural streambed, which enables wider openings with reduced material volume and is beneficial in areas with low headroom or pedestrian crossings. These structures excel in preserving ecological features, such as maintaining the natural channel bottom to support fish passage and minimize habitat disruption, while their curved form distributes loads effectively over the span. Arch designs are suitable for sites prone to scour, though they require careful evaluation of foundation stability, and structural plate variants are limited to shallow cover depths. Compared to full enclosures, arches reduce the risk of velocity-related erosion at the inlet by allowing a more natural flow regime.^[24]^[27] Bridge culverts represent hybrid structures that blend culvert and bridge elements, typically for spans exceeding 20 feet where traditional enclosed culverts become impractical. These open-bottom designs, such as extended arch or box configurations without a full invert, function hydraulically like culverts but incorporate bridge-like supports to span wider waterways, offering advantages in reduced embankment disruption and enhanced flood capacity. They are selected when clear spans demand structural elements akin to bridges, ensuring minimal interference with high flows while providing durability against dynamic water forces.^[27]^[28] Selection of a culvert's shape is primarily driven by required flow capacity, with circular pipes favored for their ability to achieve high velocities that prevent sedimentation in smaller streams, whereas box and arch shapes better accommodate debris-laden flows through larger, less restrictive openings that reduce clogging risks. For instance, in debris-prone areas, the broader geometry of boxes or arches enhances passage without significant head loss, while site-specific factors like embankment height and upstream water levels further guide the choice to balance hydraulic efficiency and structural demands.^[25]^[3]

By Construction Method

Culverts are constructed using several methods tailored to site-specific conditions, such as terrain irregularity, accessibility, and required installation speed. These methods include cast-in-place concrete, precast assembly, corrugated metal fabrication, and jacking or tunneling techniques, each offering distinct advantages in adaptability, efficiency, and minimal disruption. Cast-in-place construction involves forming and pouring reinforced concrete directly at the project site to create the culvert structure. This method is ideal for irregular or challenging terrain where custom shaping is needed to fit unique site geometries, allowing for precise integration with surrounding earthworks. However, it is labor-intensive, requiring extensive formwork, reinforcement placement, and on-site curing, which can extend project timelines.^[29] Precast culverts consist of concrete units manufactured in a controlled factory environment and then transported to the site for assembly. This approach suits standardized designs and sites with good access for heavy equipment, enabling rapid on-site erection through joint connections and backfilling. It significantly reduces field construction time compared to cast-in-place methods, often completing installation in as little as one week for multi-barrel systems, due to minimized pouring and curing needs.^[30]^[31] Corrugated metal culverts are fabricated from galvanized steel or aluminum sheets formed into helical or annular corrugations, either riveted or lock-seamed, to produce flexible pipes or arches. These lightweight structures are particularly suitable for rural, low-traffic, or temporary applications where ease of transport and handling is prioritized over rigidity. Their flexibility accommodates minor ground shifts, making them appropriate for variable soil conditions, though they require protective coatings to prevent corrosion.^[32]^[33] Jacking or tunneling methods employ trenchless techniques to install culverts beneath existing roads, railways, or structures by excavating pilot paths and hydraulically pushing or pulling preassembled sections into place. These are essential for urban or constrained sites where open excavation would cause excessive disruption, such as under active transportation corridors. The process involves launching shafts and guided boring, suitable for stable soils but challenging in rocky or unstable ground, and it preserves surface integrity during installation.^[34]^[35] The selection of a construction method depends on factors like site access for materials and equipment, soil stability to support the structure, and overall project timeline. For instance, precast methods can substantially shorten on-site durations, making them preferable for time-sensitive projects, while jacking suits limited-access environments despite higher initial setup costs.^[36]

Materials

Common Materials

Culverts are constructed using a variety of materials selected for their structural integrity, durability, and suitability to environmental conditions. The most prevalent modern materials are concrete, corrugated metal, and plastic, each offering distinct advantages in strength, corrosion resistance, and ease of installation. Composite materials represent an emerging option for specialized applications, while brick and stone persist in historical contexts. Concrete remains one of the most widely used materials for culvert construction due to its exceptional compressive strength and longevity. It is available in precast forms, which are factory-manufactured for precise dimensions and rapid on-site assembly, or cast-in-place configurations, allowing customization to specific site geometries. Typical compressive strengths for reinforced concrete culverts range from 4,000 to 5,000 psi, enabling them to withstand heavy loads and soil pressures effectively. Additionally, concrete exhibits high durability in corrosive environments, such as those with acidic soils or saline exposure, owing to its low permeability and chemical stability.^[37]^[38] Corrugated metal, primarily steel or aluminum, provides a lightweight alternative with superior tensile strength for spanning wider openings. Steel culverts are often galvanized, featuring a zinc coating that enhances corrosion resistance by sacrificial protection, or aluminized with a metallized aluminum layer for improved longevity in aggressive soils. Aluminum variants are inherently lightweight and highly resistant to corrosion without additional coatings, making them suitable for coastal or humid regions. The corrugated profile increases structural rigidity while keeping the material flexible enough for installation in uneven terrain, with minimum tensile strengths of 45,000 psi (310 MPa) for standard steel grades, and higher in advanced alloys.^[39]^[40]^[41]^[42] Plastic materials, such as high-density polyethylene (HDPE) and polyvinyl chloride (PVC), are favored for their flexibility and complete resistance to corrosion, eliminating the need for protective coatings. HDPE culverts offer high impact resistance and elasticity, allowing them to deform under load without cracking, which is ideal for seismic areas or frost-prone regions. PVC provides similar corrosion-proof properties but with smoother interiors for improved hydraulic flow. These materials are commonly applied in diameters up to 60 inches (1,500 mm) and various load conditions, including highway drainage under adequate cover and installation guidelines, due to their lightweight nature and ease of handling.^[43]^[44] Recent trends as of 2025 emphasize sustainability, with increasing adoption of recycled HDPE for environmentally friendly culverts and advanced polymer coatings on metal pipes to extend service life in challenging conditions.^[45] Composite materials, particularly fiber-reinforced polymers (FRP) like glass or carbon fiber embedded in resin matrices, have emerged since the early 2000s as innovative solutions for extreme conditions where traditional materials degrade rapidly. These composites combine high tensile strength (often 100,000 psi or more for carbon fibers) with exceptional corrosion and abrasion resistance, enabling service lives exceeding 100 years in harsh environments. FRP culverts are lightweight and non-conductive, reducing transportation costs and eliminating electrolytic corrosion risks, though their use remains specialized due to higher initial costs.^[46]^[47] Historically, brick and stone were primary culvert materials from the 18th to early 20th centuries, valued for their availability and compressive strength in masonry arches. Brick offered modular construction with good hydraulic performance in barrel vaults, while stone provided superior durability in load-bearing applications, often sourced locally for cost efficiency. Today, these materials are rare in new installations but are preserved or replicated in heritage restorations to maintain structural and aesthetic integrity.^[48]^[49]

Material Selection Factors

Material selection for culverts is guided by several key criteria that ensure long-term performance under site-specific conditions. Durability is a primary consideration, encompassing resistance to abrasion from high-velocity water flows carrying debris, corrosion from electrochemical reactions in soil and water, and chemical attack from aggressive elements such as sulfates in alkaline soils. For instance, in environments with sulfate-rich soils, engineers typically specify sulfate-resistant concrete to prevent degradation of standard Portland cement mixes, as outlined in guidelines from state departments of transportation. Corrugated metal pipes may require aluminized coatings or polymeric liners to mitigate corrosion, particularly where soil resistivity is low (below 1,500 ohm-cm) or pH levels are extreme (less than 5 or greater than 9).^[50]^[51]^[52] Load-bearing capacity is another critical factor, requiring materials to support overburden from earth fill, vehicular traffic, and dynamic forces without excessive deflection or failure. Culverts under highways must generally withstand HS-20 loading, equivalent to a 32,000-pound axle load per AASHTO standards, which simulates heavy truck traffic. Reinforced concrete pipes excel in high-load scenarios due to their rigidity, while thermoplastic pipes like HDPE are suitable for lower covers but may need additional encasement in deep fills to distribute loads effectively.^[53] Cost and lifecycle economics influence choices by balancing initial installation expenses against long-term maintenance and replacement needs. High-density polyethylene (HDPE) pipes often present lower upfront costs due to lightweight construction and ease of handling, but their service life may be shorter in abrasive conditions compared to concrete, potentially increasing lifecycle costs over 50-75 years. Lifecycle cost analysis (LCCA) tools, as recommended by AASHTO, incorporate factors like repair frequency and environmental degradation to evaluate alternatives, often favoring durable materials in high-traffic areas despite higher initial investment.^[51]^[54]^[55] Regulatory compliance ensures materials meet established performance thresholds through standardized testing and specifications. ASTM and AASHTO standards, such as ASTM C76 for reinforced concrete pipe and AASHTO M294 for HDPE culverts, mandate tests for hydraulic efficiency, structural integrity, and environmental impact, including recyclability to align with sustainability goals. Compliance also addresses broader regulations, like those from the U.S. Army Corps of Engineers, which prioritize materials that minimize ecological disruption while adhering to federal procurement rules.^[56]^[57] Site-specific conditions further tailor selections to local geotechnical and environmental variables. Soil pH and resistivity dictate corrosion potential, with acidic or low-resistivity soils favoring non-metallic options like concrete or plastics; for example, pH below 5.0 often necessitates HDPE to avoid metal pipe deterioration. Water velocity influences abrasion resistance, where velocities exceeding 10 ft/s may require concrete with hardened inverts or lined metal pipes. In seismic zones, flexible materials such as HDPE or elastomeric jointed concrete are preferred to accommodate ground movement without brittle failure, as per seismic design provisions in AASHTO LRFD specifications.^[58]^[59]^[60]^[61]

Design and Engineering

Hydraulic Principles

Culvert hydraulics govern the conveyance of water through the structure, ensuring adequate capacity to handle design flows while minimizing upstream flooding and downstream erosion. Flow in culverts can occur under two primary regimes: free surface flow and pressurized flow. Free surface flow, also known as open-channel flow, predominates when the culvert barrel is not fully submerged, allowing the water surface to be exposed to atmospheric pressure; this is common in unsubmerged or partially submerged conditions at the inlet. Pressurized flow, conversely, arises when the culvert is fully filled, creating hydrostatic pressure within the barrel, typically under outlet control with high tailwater or steep slopes leading to full flow. The performance of a culvert is determined by either inlet control or outlet control, whichever imposes the greater hydraulic restriction. Under inlet control, the flow is limited by the geometry and configuration of the culvert entrance, where the inlet acts as a weir for unsubmerged conditions (headwater depth less than the culvert rise) or an orifice for submerged conditions (headwater depth greater than the culvert rise plus about 1.2 times the diameter for pipes). This control is independent of downstream conditions and focuses on the energy required to enter the culvert. Outlet control, on the other hand, occurs when downstream conditions, including tailwater elevation and barrel friction, dominate; here, the flow may be partly full with free surface conditions or full and pressurized if the tailwater submerges the outlet. Engineers must evaluate both controls to select the governing headwater elevation for design. Culvert capacity under outlet control is commonly calculated using Manning's equation, which estimates the discharge based on channel geometry, roughness, and slope:

Q = \frac{1}{n} A R^{2/3} S^{1/2}

where Q is the discharge (m³/s), n is Manning's roughness coefficient, A is the cross-sectional area of flow (m²), R is the hydraulic radius (m), and S is the slope of the energy grade line (dimensionless). This empirical formula applies to both full and partly full flow in the barrel, with adjustments for entrance losses and tailwater effects to compute the total headwater required. For instance, in a concrete pipe culvert with n = 0.012, the equation helps determine if the structure can pass peak flows without excessive ponding. Headwater, the depth of water upstream of the culvert inlet, and tailwater, the depth downstream at the outlet, significantly influence hydraulic performance and submergence risks. Headwater calculations under inlet control use dimensionless nomographs relating headwater depth to culvert diameter (HW/D) ratios, ensuring the structure passes the design discharge without overtopping the roadway; allowable HW/D is typically limited to 1.2–1.5 to prevent flooding. Tailwater effects are critical under outlet control, where submergence (tailwater depth exceeding critical depth) can increase upstream headwater by backwatering the flow, potentially requiring larger culverts or energy dissipators. Submergence calculations involve comparing computed tailwater to critical depth, with full outlet submergence occurring when tailwater exceeds the culvert crown, shifting to pressurized flow. Debris and sedimentation pose operational challenges that affect long-term hydraulic efficiency. Debris accumulation at inlets can reduce effective opening area, increasing headwater and flood risk, while sedimentation within the barrel diminishes capacity through silt buildup. To mitigate sedimentation, minimum flow velocities exceeding 0.9 m/s (3 ft/s) are recommended, as lower velocities allow fine sediments to deposit; for example, velocities below this threshold in low-gradient culverts often necessitate steeper slopes or larger diameters to self-clean. Debris management involves site-specific assessments for potential blockage, with trash racks or debris basins used where woody debris is prevalent, though these must balance hydraulic losses against blockage prevention.^[3]^[62] Culvert sizing integrates these principles to match structure dimensions to peak design flows, typically the 50- or 100-year storm event. Traditional methods rely on nomographs from FHWA guidelines, which plot headwater, discharge, and slope against culvert size for both inlet and outlet control, allowing iterative selection of diameter and length. Modern approaches employ software such as HY-8, developed by the FHWA, which automates these calculations by incorporating Manning's equation, inlet loss coefficients, and backwater profiles to optimize sizing while accounting for site-specific hydrology and geometry. For representative cases, a 1-m diameter pipe might be sized for a 5 m³/s peak flow on a 1% slope, ensuring velocities between 1–3 m/s for stability.^[63]

Structural Considerations

Culverts are engineered to resist a range of mechanical and geotechnical forces that ensure structural integrity over their service life. Primary load types include dead loads from soil overburden and the culvert's self-weight, live loads from vehicular traffic, and uplift forces arising from buoyancy in saturated conditions. These loads are systematically addressed in the AASHTO LRFD Bridge Design Specifications, where dead loads (DC, DW, EV) and earth loads (EH, ES, DD) represent permanent components, while live loads (LL, PL) capture dynamic vehicular effects; buoyancy uplift is calculated as the product of water density, gravity, and submerged volume to counteract potential flotation.^[64]^[65] Design for moment and shear capacities follows the AASHTO LRFD Bridge Design Specifications, treating culvert components such as box sections as simple beams under distributed loads for preliminary analysis. For instance, the maximum bending moment in a simply supported beam configuration is given by M = \frac{w L^2}{8}, where w is the uniform load per unit length and L is the span length, with shear forces evaluated at supports using V = \frac{w L}{2}; load factors and resistance factors are applied to achieve safety margins at ultimate limit states. This approach ensures the structure can handle combined axial, flexural, and shear demands without exceeding material strengths.^[66]^[64] Foundation and bedding design is critical for transferring loads to the supporting soil, requiring assessment of soil bearing capacity to prevent settlement or failure. Allowable bearing pressures, typically ranging from 2 to 5 tons per square foot depending on soil type, must support the superimposed fill height and structure weight; in weak soils with low bearing capacity (e.g., below 1 ton per square foot), geogrid reinforcement is employed to distribute loads and enhance stability by interlocking with aggregate fill. Bedding materials, such as compacted granular soils, provide uniform support and minimize differential settlement.^[66]^[36]^[67] Joints between culvert sections must maintain watertight integrity to prevent soil migration and leakage, utilizing flexible gaskets or rubber seals compliant with AASHTO M 198 standards for circular concrete pipes. Expansion joints accommodate movements from settlement, construction tolerances, and minor thermal fluctuations in buried conditions, using compressible fillers or modular systems to avoid cracking or misalignment.^[68] Seismic protection incorporates AASHTO LRFD provisions for dynamic loading in high-risk zones, including ductility detailing and foundation anchoring to resist lateral accelerations. Scour protection at inlets and outlets employs energy-dissipating features such as riprap aprons and cutoff walls; riprap, sized according to local velocity and flow depth (e.g., D50 median stone diameter of 6-12 inches), extends at least 10 feet beyond the structure toe and is embedded 2 feet into the bed to mitigate erosion from hydraulic forces.^[69]^[66]^[70]

Construction and Maintenance

Installation Processes

The installation of culverts begins with thorough site preparation to ensure proper alignment and stability. This involves clearing vegetation and excavating a trench to the specified line, grade, and width, typically allowing sufficient space for bedding and backfill while avoiding over-excavation that could lead to settlement. Dewatering is essential in wet conditions to maintain a dry working area and prevent instability during placement, often using pumps or sumps as needed.^[33]^[71] Foundation work follows, focusing on creating a stable base to distribute loads evenly. A compacted bedding layer, usually 6 to 12 inches thick of granular material such as gravel or crushed stone, is placed and compacted to provide uniform support along the culvert's length, with adjustments for the pipe's shape to avoid point loading. This bedding must conform to the culvert's invert elevation and slope as per design specifications.^[71]^[72] Assembly and placement of the culvert occur next, tailored to the construction method such as precast or on-site fabrication. For precast units, sections are joined using gaskets, bands, or mortar according to manufacturer guidelines, starting from the downstream end to ensure proper flow. The culvert is then lowered into the trench using equipment like backhoes for smaller diameters or cranes for larger units exceeding 48 inches, ensuring precise alignment to grade without damage.^[71] Backfilling proceeds in controlled layers to secure the culvert and restore the embankment. Material is placed in lifts of 6 to 12 inches on both sides and above the pipe, compacted to at least 95% of standard Proctor density using vibratory rollers or plate compactors to minimize voids and settlement. Initial haunching around the haunches provides lateral support before full backfill.^[71]^[73] Inlet and outlet features are installed to protect against erosion and enhance hydraulic performance. Headwalls and wingwalls, often precast concrete or gabion structures, are anchored at the ends to prevent scour and provide structural transition. Energy dissipators, such as riprap aprons or stilling basins, are placed downstream to dissipate flow velocity, with riprap sized based on expected discharge.^[33]^[74] Safety and quality control measures are integral throughout to ensure compliance and durability. Trenches deeper than 5 feet require shoring or sloping per OSHA standards to prevent cave-ins, with personal protective equipment mandatory for workers. Post-installation, the culvert is inspected for alignment using levels or lasers, and leak testing—such as low-pressure air or water exfiltration—is conducted to verify joint integrity before final backfill.^[71]

Inspection and Upkeep

Routine inspections of culverts are essential to identify potential issues early and ensure hydraulic and structural integrity. According to Federal Highway Administration (FHWA) guidelines under the National Bridge Inspection Standards (NBIS), culverts with spans greater than 20 feet that function as bridges must be inspected at intervals not exceeding 24 months, while smaller culverts may follow state-specific schedules, often annually or biennially based on condition and risk assessments.^[75] Visual inspections typically involve checking for cracks, corrosion, deformation, joint separation, and debris accumulation at inlets, outlets, and along the barrel, performed during low-flow periods by trained teams using ladders, boats, or remote tools for safety.^[76] Maintenance techniques focus on preventive and corrective actions to extend service life. Cleaning methods commonly include high-pressure water jetting or vacuum excavation to remove sediment, debris, and vegetation that could impede flow, with frequency determined by site-specific accumulation rates.^[77] Joint repairs often employ sealants or grout to address leaks and prevent further deterioration, while corrosion in metal culverts can be mitigated through slip-lining with HDPE or PVC liners or applying protective coatings.^[78]^[79] Advanced monitoring tools enhance inspection accuracy and efficiency. Closed-circuit television (CCTV) systems, deployed via crawlers or push-rod cameras, allow non-invasive internal assessments to detect blockages, structural defects, and erosion without entry.^[80] Flow gauges, such as acoustic Doppler devices like the SonTek-IQ, measure water velocity and discharge to evaluate hydraulic performance and identify flow restrictions. Lifecycle planning involves assessing material-specific durability to schedule replacements proactively. For instance, corrugated metal culverts typically last 50-75 years with proper coatings and maintenance, concrete variants 75-100 years, and thermoplastic pipes over 50 years, guiding inventory management and budgeting through condition rating systems.^[81]^[82] Upkeep costs for culverts cover inspections, cleaning, and minor repairs, though this varies by size, material, and environmental exposure as outlined in life-cycle cost analyses.

Environmental and Ecological Aspects

Impacts on Ecosystems

Culverts significantly alter natural hydrological processes by channeling stream flows through confined structures, often leading to increased water velocity and concentrated discharge downstream. This acceleration can exacerbate erosion at culvert outlets, forming scour pools and destabilizing stream banks, which in turn contributes to downstream channel incision and habitat degradation.^[83] Additionally, by routing surface water more rapidly across impervious road surfaces, culverts may reduce opportunities for infiltration, potentially diminishing groundwater recharge rates in surrounding aquifers and altering baseflow contributions to streams.^[84] One of the primary ecological impacts of culverts is habitat fragmentation, as they frequently act as barriers to aquatic organism migration, particularly for fish species requiring access to upstream spawning or rearing areas. Perched or undersized culverts create velocity barriers or insufficient water depths, isolating stream segments and reducing overall habitat connectivity, which can lower fish abundance and diversity by limiting gene flow and access to essential resources.^[6] For instance, in regions like the Pacific Northwest, a significant portion of culverts on federal lands in fish-bearing streams have been identified as partial or complete barriers, with assessments showing rates up to 72% in some areas, contributing to population declines in salmonids and other species.^[85] Culverts also influence water quality through their interaction with sediments and pollutants. By constricting flow, they can trap fine sediments upstream during low flows, potentially improving localized water clarity but leading to sediment buildup that degrades benthic habitats. During high-flow events, however, scour within and downstream of culverts may resuspend and release accumulated sediments or associated contaminants, such as road-derived metals, into the stream, temporarily elevating turbidity and harming aquatic biota.^[86] Beyond direct stream effects, culverts contribute to broader disruptions in wetlands and riparian zones by modifying flow regimes that support these areas. Altered hydrology can lead to reduced inundation in adjacent wetlands, stressing vegetation and wildlife dependent on periodic flooding, while downstream erosion may encroach on riparian buffers, diminishing their role in stabilizing banks and filtering nutrients. In the United States, culverts are estimated to account for a substantial portion of anthropogenic stream blockages, with regional assessments indicating they obstruct a substantial portion of potential fish habitat in many watersheds, with some areas showing blockage rates around 50%.^[83] Culvert installations and replacements must comply with federal regulations such as the National Environmental Policy Act (NEPA) and the Endangered Species Act (ESA), which mandate environmental assessments to evaluate potential ecosystem impacts, including effects on listed species and critical habitats. These frameworks require agencies to analyze hydrological changes, fragmentation risks, and water quality alterations prior to approval, ensuring that projects do not unduly harm protected resources.^[87]

Mitigation and Design Solutions

To mitigate the environmental impacts of culverts on aquatic ecosystems, nature-based solutions such as roughened channels and baffles are employed to replicate natural stream conditions within and around the structure. Roughened channels involve adding boulders, rocks, or other natural materials to the culvert invert to reduce flow velocities, dissipate energy, and create resting areas for fish and invertebrates, thereby facilitating upstream and downstream migration while minimizing scour.^[88] Baffles, typically made of concrete or rock, are installed along the culvert bottom to further slow water, maintain adequate depths, and promote sediment transport similar to undisturbed streams; these features have been shown to substantially improve passage success for juvenile salmonids in retrofitted culverts.^[89] Such approaches prioritize hydraulic roughness to mimic riffle-pool sequences, enhancing habitat connectivity without relying solely on structural alterations.^[90] Permeable designs, particularly open-bottom culverts, address habitat fragmentation by spanning over the natural streambed rather than embedding in it, allowing gravel, sediment, and large woody debris to move freely and maintaining geomorphic processes. These structures, often using precast arch or box spans supported by footings, preserve the streambed's natural substrate and reduce perching that can block fish passage; for instance, in gravel-bed rivers, they prevent aggradation upstream and degradation downstream by permitting unimpeded bedload transport.^[91] By avoiding full burial, open-bottom culverts also minimize groundwater interception and support riparian vegetation continuity, contributing to overall stream health.^[92] This design is especially effective in low-gradient streams where sediment dynamics are critical for aquatic organism passage.^[93] Ecological sizing strategies involve designing culverts larger than the minimum hydraulic requirements to accommodate projected increases in flow from climate change and to provide space for wildlife migration corridors. Guidelines recommend oversizing culverts beyond minimum hydraulic requirements, such as spanning wider than bankfull width, to ensure resilience to higher peak flows and reduced low-flow depths that could otherwise exacerbate barriers for species like salmon.^[22] This approach, often using a geomorphic analog method, accounts for future precipitation variability while allowing for natural channel widening during floods, thereby supporting long-term ecosystem adaptability.^[94] Best management practices for culvert installations include establishing vegetation buffers and implementing erosion control measures to protect surrounding habitats from sediment runoff and thermal pollution. Riparian buffers of native plants along streambanks filter pollutants, stabilize soils, and provide shade to maintain cooler water temperatures essential for sensitive species.^[95] Bioengineering techniques, such as live staking with willow or other woody cuttings driven into erodible banks, promote rapid root establishment to bind soil and substantially reduce erosion in high-risk areas near culvert outlets.^[96] These practices are integrated during construction to enhance site stability and biodiversity without chemical interventions.^[97] Policy drivers, such as the U.S. Fish and Wildlife Service's National Fish Passage Program, emphasize upgrading legacy culverts to ecological standards to restore connectivity for migratory fish across thousands of barriers, with ongoing federal funding support.^[98] These guidelines promote the adoption of the aforementioned designs in federal and state projects, prioritizing funding for replacements that incorporate nature-based elements to meet Endangered Species Act obligations.^[22] As of 2025, federal programs like the National Fish Passage Program have awarded grants under the Bipartisan Infrastructure Law to fix or remove hundreds of culverts, supporting fish passage and habitat restoration nationwide.^[99]

Specialized Applications

Fish Passage Designs

Traditional culverts pose significant barriers to upstream fish migration, primarily through high water velocities exceeding 4 ft/s during peak flows and perched outlets that elevate the culvert above the downstream channel bed, forcing fish to expend excessive energy to ascend. These conditions particularly impede anadromous species such as Pacific salmon (Oncorhynchus spp.), which rely on accessible streams for spawning and juvenile rearing, often resulting in blocked access to critical habitats.^[100]^[101]^[102] To mitigate these barriers, fish passage designs incorporate features like sloped inverts to gradually reduce the overall gradient, submerged weirs to create resting pools, and rock baffles to increase hydraulic roughness and dissipate energy. The Alaska Steep Pass design, a modular baffled system adapted for steep-gradient culverts, uses offset wooden or concrete baffles to maintain low velocities and provide vertical slots for fish to navigate, enabling passage on slopes up to 10%. These modifications transform conventional culverts into fish-friendly structures by simulating natural stream conditions.^[103]^[104]^[105] Performance targets for these designs emphasize maintaining average velocities below 1 ft/s for juvenile salmon to ensure safe passage without exhaustion, with baffles typically reducing flow speeds by 50-70% compared to unmodified culverts. Studies have demonstrated passage success rates improving to 80-90% with baffle installations, compared to less than 30% in smooth culverts, allowing weak-swimming juveniles to traverse longer distances effectively.^[106]^[107]^[108] A prominent case is the Washington State culvert replacement program, initiated by a 2013 federal court injunction mandating the removal or modification of over 400 state-owned barriers to restore salmon habitat access. As of June 2025, the program had corrected 176 such culverts, improving access to approximately 655 miles of stream habitat and enhancing migration for species like chinook and coho salmon.^[109]^[110]^[109] Monitoring passage efficiency in these designs often employs passive integrated transponder (PIT) tags implanted in fish, combined with antenna arrays at culvert inlets and outlets to track individual movement and survival rates. This telemetry method has revealed passage efficiencies exceeding 80% in retrofitted culverts, providing data to refine future installations and verify ecological benefits.^[111]^[112]^[113]

Low-Energy Flow Culverts

Low-energy flow culverts represent an advanced class of hydraulic structures engineered to minimize energy losses during water conveyance under roadways, thereby reducing headwater buildup and turbulence for efficient performance across a range of discharges. These designs incorporate expanded inlet sections and gradual transitions that contract and expand the flow smoothly, maintaining nearly constant total head in accordance with Bernoulli's principle and promoting critical flow conditions with minimal velocity head dissipation. By prioritizing low-friction materials and geometries, such culverts achieve superior hydraulic efficiency compared to conventional configurations, particularly in scenarios demanding stable low-head operations.^[114] Central to their design are features like inlet flares and tapered throats, which significantly lower entrance losses, alongside smooth barrel alignments and outlet energy dissipators to curb turbulence and exit velocity impacts. For instance, rounded or beveled inlet edges can reduce the entrance loss coefficient (Ke) to 0.2 or less, in contrast to 0.5 for standard square-edged pipes, while integrated dissipators such as riprap aprons further attenuate downstream energy. These elements collectively ensure low overall headloss, often keeping the structure in partial or full flow with controlled velocities that align with natural channel dynamics.^[115]^[116] The advantages of low-energy flow culverts include substantially reduced scour potential at inlets and outlets due to diminished turbulence and velocity gradients, leading to lower long-term maintenance needs from decreased erosion and sediment issues. With energy loss coefficients typically below 0.2, these structures exhibit enhanced durability in erosive environments and support more economical sizing by optimizing flow conveyance without excessive headwater elevations.^[115]^[114] Such culverts find primary applications in high-velocity streams susceptible to scour and debris-laden watersheds where smooth flow paths mitigate blockages and erosion. Originating from FHWA research and guidelines in Hydraulic Design Series No. 5 (HDS-5, 2001), they are widely used for irrigation canal crossings, flood-prone roadways, and sites requiring minimal upstream ponding to protect adjacent infrastructure.^[117] Representative examples encompass FHWA-endorsed tapered-inlet box culverts and stream simulation profiles, which demonstrate 20-30% greater flow capacity relative to standard pipe culverts under equivalent headwater constraints, enabling more compact installations without compromising hydraulic performance.^[118]^[114]

Failures and Analysis

Common Failure Mechanisms

Culverts can fail through a variety of mechanisms, primarily involving material degradation, hydraulic overload, structural instability, and flow obstruction, each compromising their ability to convey water safely under roadways. These failures often result from environmental exposure, design limitations, or installation deficiencies, leading to costly repairs or replacements.^[119] Corrosion and abrasion represent major degradation processes for metal culverts, particularly corrugated steel pipe (CSP), where internal corrosion at the invert—accelerated by acidic waters with pH below 5—thins the metal walls and weakens structural integrity. Abrasion from sediment-laden flows exacerbates this, eroding protective coatings and exposing bare metal to further electrolytic and galvanic corrosion mechanisms. In harsh conditions, such as coastal or industrial areas with high acidity and salinity, CSP culverts may have a service life of 10 to 35 years before perforation and potential collapse occurs.^[120]^[121] Hydraulic failures frequently stem from undersizing, where culvert capacity cannot accommodate peak flows, resulting in overflow that erodes embankment soils and causes road washouts. At the outlet, high-velocity discharges lead to scour, undermining foundations and accelerating structural compromise through headwall and riprap displacement. These issues are prevalent during intense storms, where even moderate undersizing can increase upstream water levels and downstream erosion rates.^[122]^[123] Structural issues arise from differential settlement due to poor bedding and backfill compaction, causing uneven load distribution and joint separation in concrete or rigid culverts. Under vehicle overload or soil movement, this manifests as longitudinal cracking exceeding 0.1 inches in width, indicating shear or flexural failure and potential infiltration of fines that further destabilize the system. Medium-diameter pipes (24-36 inches) are particularly vulnerable, with joint separation noted as the most common failure mode in concrete culverts.^[124]^[125] Debris blockage occurs when organic matter, sediment, or woody material accumulates at inlets, especially in vegetated or forested watersheds, reducing hydraulic capacity by up to 50% and diverting flows to overtop the roadway. This partial obstruction heightens pressure on the culvert walls and promotes upstream ponding, amplifying risks during subsequent events.^[126] In the United States, culvert failures contribute significantly to flood-related road closures, with debris clogging identified as the leading resiliency deficiency by 43% of state departments of transportation and material degradation by 38%. Surveys indicate that joint separation alone affects approximately 18% of inspected concrete culverts, underscoring the prevalence of these mechanisms nationwide.^[127]^[124]

Case Studies and Lessons

During the 1986 flood in Edmonton, Canada, heavy upstream rainfall caused the North Saskatchewan River to rise 12 meters above normal levels, resulting in widespread inundation of homes and infrastructure with total economic damages estimated at approximately $12.8 million across north-central Alberta. This incident highlighted the critical need to integrate long-term climate projections into infrastructure sizing to account for increasing precipitation intensities and avoid similar vulnerabilities in future designs.^[128]^[129] In 2017, Hurricane Harvey brought record-breaking rainfall to Texas, causing extensive infrastructure damage estimated at $125 billion statewide, including multiple culvert failures due to high-velocity floodwaters eroding foundations and leading to road washouts. These failures were primarily attributed to hydraulic overload rather than isolated corrosion, though the event exposed underlying material degradation in aging systems that inspections could have identified earlier. The disaster underscored the importance of routine structural assessments to detect potential weaknesses, enabling proactive maintenance and reducing the risk of catastrophic breaches during extreme weather.^[130] Fish-friendly culvert retrofits, incorporating baffles and natural stream profiles, have demonstrated benefits in improving hydraulic efficiency and ecological connectivity while minimizing debris accumulation and flow-related issues.^[131]^[132] For example, during Hurricane Ida in 2021, intense rainfall led to widespread culvert failures and road washouts in the Northeastern United States, with damages exceeding $65 billion and highlighting vulnerabilities in aging infrastructure to increasingly frequent extreme events.^[133] Key lessons from these cases include the adoption of structured risk assessment frameworks, such as the Federal Highway Administration's (FHWA) Vulnerability Assessment and Adaptation Framework, which evaluates culvert exposure to climate stressors like flooding and guides prioritization of upgrades based on sensitivity and adaptive capacity. Cost-benefit analyses of retrofits often show long-term savings, with investments in resilient designs recovering costs through avoided damages within 10-20 years in high-risk areas. Looking ahead, climate change projections indicate a substantial rise in culvert failure risks due to intensified storms and sea-level rise, emphasizing the urgency of widespread adaptations to maintain infrastructure integrity.^[134]^[135]

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[110]
[PDF] Fish Passage Monitoring Methods
Monitoring the effectiveness of culvert fish passage restoration. CDFG ... Passage was evaluated with the use of PIT tags and antenna arrays. Passage ...
[111]
PIT‐Tag Detection System for Large‐Diameter Juvenile Fish Bypass ...
The system exceeded the intended goal of 95% tag‐reading efficiency and yielded reading efficiencies near 100% for the four antennas combined. In tests using ...
[112]
[PDF] Hydraulic Design of Highway Culverts - PDH Online
analysis of this type of flow is the same as for free surface flow. 2. Types of Flow Control. Inlet and outlet control are the two basic types of flow control.Missing: pressurized | Show results with:pressurized
[113]
Effects of Inlet Geometry on Hydraulic Performance of Box Culverts
Based on difficulties measuring low flow depths accurately, the FHWA Hydraulics Laboratory is developing an optical pressure measurement (OPM) system, which ...
[114]
Entrance Loss Coefficient - Hydrologic Engineering Center
Table 6-3 indicates that values of the entrance loss coefficient range from 0.2 to about 0.9 for pipe-arch and pipe culverts. As shown in Table 6-4, entrance ...
[115]
Improvements to the Hydraulic Performance of Culverts under Inlet ...
May 30, 2024 · ... 1.5 times the culvert height. Harrison [41] states that bevelled edges increase culvert capacity by 5% to 20%, while side-tapered inlets ...
[116]
[PDF] Culverts: A Hidden Risk - Caltrans
Some common causes for culvert failures are clogs, pipe damage, washouts, rusted or failed inverts, cracked concrete, exposed or corroded reinforcing steel, ...Missing: mechanisms | Show results with:mechanisms
[117]
[PDF] corrugated steel culvert pipe deterioration - NJ.gov
a life expectancy of 10 years to about 35 years before ... Acidic environments and exposure to salt water exacerbate corrosion in steel culverts.
[118]
[PDF] CORRUGATED STEEL CULVERT PIPE DETERIORATION - ROSA P
CSCP failures by wall thinning initiated at the inverts are mostly due to erosion and internal corrosion. There are many kinds of corrosion mechanisms: • Stress ...
[119]
[PDF] Implementation and Effectiveness Monitoring of Hydraulic Projects
Undersized culverts create flow restrictions at the culvert inlet that increases velocities through the structure causing downstream scour to occur. This ...
[120]
What Causes Flooding? | Federal Judicial Center
Undersized culverts become hydraulic pinch-points during storm events, resulting in local flooding upstream, overtopped roadways, and downstream erosion due to ...
[121]
[PDF] A Research Plan and Report on Factors Affecting Culvert Pipe ...
AASHTO (2007) Chapter 14 cites the following causes of joint separation: uneven bedding, poor compaction, and unexpected settling. Chapter 14 claims that ...
[122]
[PDF] Post Installation Evaluation and Repair of Installed Reinforced ...
Longitudinal cracking in excess of 0.1 inch in width may indicate overloading or poor bedding. If the pipe is placed on hard mate- rial and backfill is not ...
[123]
[PDF] Methods for Inventory and Environmental Risk Assessment of Road ...
Plugging of culverts by organic debris is a common failure mechanism. Debris lodged at the culvert inlet reduces hydraulic capacity and promotes further ...
[124]
Summary | Practices to Enhance Resiliency of Existing Roadway ...
Of the documented failures related to deficiencies in resiliency, debris clogging (18 DOTs, 43%), premature degradation of the culvert material or system (16 ...
[125]
July 19, 1986: 12-metre river swell turns Edmonton streets into canals
Jul 20, 2017 · In July 1986, heavy rainfall upstream of Edmonton caused the North Saskatchewan river to swell to 12 metres higher than normal.
[126]
North-central Alberta - Canadian Disaster Database
North-central Alberta, July 13-24, 1986. Very heavy precipitation caused the McLeod, Pembina, the Athabasca rivers and the North Saskatchewan River and its ...
[127]
Impact of flooding events on buried infrastructures: a review - Frontiers
Apr 16, 2024 · This review delves into the profound implications of flooding events on buried infrastructures, specifically pipelines, tunnels, and culverts.
[128]
[PDF] Fish Friendly Culverts
Setting the culvert bottom at least 6 inches (or 10-20% of the culvert diameter, whichever is greater) below the stream bed elevation will allow for better fish ...
[129]
[PDF] FISH PASSAGE THROUGH RETROFITTED CULVERTS Final Report
The long term results showed that the baffle equipped culverts do in fact allow fish passage, even though the fish in the study areas did not appear to move a ...
[130]
[PDF] Vulnerability Assessment and Adaptation Framework
They used VAST to evaluate bridges and large culverts and they developed a Hazard Vulnera- bility Index for roads. A workshop with agency engineers to match ...
[131]
Climate effects on US infrastructure: the economics of adaptation for ...
Aug 19, 2021 · Changes in temperature, precipitation, sea level, and coastal storms will likely increase the vulnerability of infrastructure across the USA.