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Why scour threatens bridge foundations
Scour is the removal of streambed or bank material by flowing water. During a flood, higher velocity, turbulence around a pier, accelerating flow through a narrowed opening, debris, ice, or a shifting channel can lower the bed around foundation elements. A foundation that was adequately embedded when built may become exposed, undermined, or unable to carry the loads assumed in its design.
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Protection must therefore address both the hydraulic cause and the consequence. Armor placed around a pier can resist erosion locally, but it cannot compensate for an undersized foundation, an unstable channel, or a toe that is allowed to erode. HEC-23 treats countermeasures as part of a broader bridge-scour and stream-instability management system.
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Local pier scour
Flow separates and forms vortices at a pier, excavating a hole immediately around it. A countermeasure must cover the expected scour zone, remain stable under the design hydraulic loading, and continue to work if some settlement occurs.
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Abutment scour and bank erosion
Abutments constrict the channel and redirect flow. Erosion may progress along the approach bank, around the abutment toe, or behind an inadequate return. Protection often combines a revetment, a buried toe or launching apron, and channel-shaping measures.
Contraction scour
When a bridge opening or temporary obstruction narrows the effective flow area, the increased velocity can lower the bed across a broad portion of the opening. Local armor at one pier does not solve a contraction problem; the opening, overbanks, and channel geometry require assessment together.
Channel instability
Meander migration, bend erosion, aggradation, degradation, or a wandering channel can move the problem beyond the immediate foundation. Guide banks, spurs (also called stream barbs), bendway weirs, or other flow-alignment treatments may be appropriate where a hydraulic and geomorphic assessment supports them.
How to choose a countermeasure
There is no universal “best” scour countermeasure. The selection matrix in HEC-23 distinguishes where each system is suitable and what limitations apply. A designer should compare the following before specifying a material.
| Decision factor | Questions to answer |
|---|---|
| Scour mechanism | Is the problem local pier scour, abutment scour, contraction scour, bank erosion, or broader channel instability? |
| Hydraulic loading | What velocity, boundary shear, turbulence, debris, ice, and flood duration must the protection withstand? |
| Foundation geometry | Can the system reach the pier or abutment toe, and what happens if the surrounding bed degrades? |
| Flexibility | Is settlement likely? Flexible systems such as riprap, gabions, and articulated blocks can tolerate some deformation better than a rigid slab. |
| Filter and toe | What granular or geotextile filter, key, trench, apron, or toe reinforcement prevents soil loss and undermining? |
| Construction | Can equipment place the system accurately and safely in or beside the active channel, including during low-flow conditions? |
| Environment and permits | Will the work alter habitat, sediment transport, water quality, or flood conveyance? |
| Inspection and repair | How will displacement, exposure, settlement, and undermining be detected after floods, and can damaged areas be reached? |
Principal bridge-scour protection methods
Riprap armor
Riprap is quarried or otherwise suitable rock placed on the bed, banks, around piers, or at abutments. HEC-23 covers dumped and placed riprap as well as filled-trench, self-launching, partially grouted, and fully grouted arrangements. It is widely used because individual stones can accommodate some settlement and damaged areas can often be repaired with additional rock.
Riprap is not simply a layer of large stones dumped around a foundation. Stone size, gradation, layer thickness, placement limits, filter design, edge treatment, and toe behavior must be designed for the site’s hydraulics and underlying soil. A rock layer can fail when water carries the bed material through its voids, when stones are undersized, or when the outer edge is undermined.
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Articulated concrete blocks
Interlocking or cable-tied concrete blocks form a flexible revetment. The connected units provide a relatively uniform armor surface while allowing limited movement and settlement. They are used for banks, beds, and local pier or abutment scour protection where a manufactured system can be transported and installed to the required geometry.
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Gabions and gabion mattresses
Gabions are wire containers filled with rock; gabion mattresses are thinner units used as flexible armor over beds or banks. HEC-23 lists them for bankline, bed, pier, and local scour applications. Their flexibility helps on settling banks, while the wire baskets keep the fill distributed.
Design must account for wire durability, abrasion, corrosion exposure, vandalism, debris impact, and the ability to replace damaged baskets. A gabion that loses its wire confinement can behave very differently from the designed system.
Grout-filled and fabric-formed mattresses
These systems use a fabric form or mattress that is filled with grout or another cementitious material to create flexible or rigid armor. They can provide a continuous surface in locations where placing individual rock is difficult. Their suitability depends on access, subgrade preparation, joint and edge detailing, and the consequences of cracking or local undermining.
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These locally established systems can be adapted to particular sites, but published guidance is more limited. HEC-23 presents them as possible options and cautions that some agency procedures have not been evaluated or endorsed by the Federal Highway Administration. The owner should require a documented design basis, installation specification, inspection criteria, and maintenance plan rather than treating local practice as proof of universal performance.
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Concrete armor units
Specialized concrete armor units may be considered where project scale, hydraulics, and construction conditions justify them. They require careful attention to unit stability, interlocking or placement pattern, filter compatibility, and repair logistics.
Sheet piles and cofferdam-related measures
Sheet piles, cutoff elements, and cofferdam-related measures can protect a toe or isolate work during construction. They are not interchangeable with a permanent armor layer: the design must address corrosion, structural loads, seepage, end-around flow, removal or abandonment, and how the measure interfaces with the streambed and foundation.
Flow-alignment and channel-stability measures
Guide banks, spurs or stream barbs, bendway weirs, and related channel treatments redirect or distribute flow rather than merely armoring a surface. They may reduce an adverse attack angle or stabilize an eroding bend, but they can transfer erosion to another reach. Hydraulic and geomorphic analysis, including downstream effects and permitting, is essential.
What goes under riprap?
The answer is normally an engineered filter, not bare soil. FHWA states in HEC-23 Volume 2: An appropriate granular or geotextile filter is essential for any countermeasure requiring a protective armor layer.
The filter prevents the underlying soil from washing through the voids between stones, blocks, or mattress elements while allowing water to pass.
Granular filters
A graded layer of smaller aggregate can transition between the native bed or bank soil and the armor. The gradation must be compatible with both materials; a single generic “bedding stone” specification is not adequate for every soil and rock combination.
Geotextile filters
A geotextile can provide separation and filtration where it can be placed continuously and protected from tearing, puncture, ultraviolet exposure, and construction damage. Seams, overlaps, wrinkles, anchors, and penetrations around a pier or abutment must be detailed. The fabric is not a substitute for a stable toe or sufficient armor thickness.
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Keys, trenches, aprons, and toes
Edges are common initiation points for failure. A buried key or filled trench can lock armor into the bank. A riprap apron or self-launching arrangement can provide reserve material if the bed scours. Toe reinforcement limits the ability of the current to get beneath the armor. HEC-23 treats these details separately because an exposed or undermined toe can defeat an otherwise adequate surface.
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For an existing bridge
- Inspect and document the site. Establish foundation elevations, existing bed levels, prior repairs, visible voids, exposed piles, bank condition, debris accumulation, and the condition of any existing armor.
- Define the hydraulic and geomorphic problem. Evaluate flood flows, velocity, turbulence, debris and ice potential, opening geometry, channel alignment, bed material, and evidence of long-term degradation or migration.
- Separate the scour components. Analyze local pier or abutment scour, contraction effects, and channel instability rather than combining them into an unexplained single depth.
- Select a countermeasure and its filter. Use the HEC-23 application guidance, then design armor, filter, edge, and toe as one system.
- Plan construction around the active channel. Set out limits, access, temporary works, turbidity controls, dewatering or diversion needs, placement tolerances, and worker safety controls.
- Define acceptance and maintenance criteria. Record the installed footprint and elevations so later inspections can identify displacement, settlement, exposure, or loss of filter material.
For a new bridge
Countermeasures may protect the bed and banks, but they do not permit a designer to reduce the required foundation embedment. HEC-23 Table 2.1 states: Piers at new bridges cannot rely on countermeasures to reduce the design depths of foundation elements.
The foundation must be designed for the governing scour condition; armor is supplemental protection and part of the management plan.
Construction and quality-control priorities
- Verify materials. Confirm rock durability, grading, block or mattress specifications, wire properties, grout properties, and geotextile type before placement.
- Prepare the subgrade. Remove loose debris and unsuitable material, preserve the designed bed grade, and avoid tearing or folding filters.
- Control placement. Dumping from excessive height, pushing rock with a blade, or placing blocks out of sequence can segregate the material and damage the filter.
- Protect edges first. Check keys, trenches, returns, upstream and downstream terminations, and the pier interface before accepting the central field.
- Record as-built conditions. Survey armor limits and elevations, photograph concealed layers before covering, and document deviations for the inspection baseline.
- Account for underwater uncertainty. Where visibility is poor, use suitable survey or inspection methods to confirm that the filter and toe reached the intended location.
Monitoring scour-critical bridges after floods
Protection is not complete when construction ends. FHWA describes Plans of Action for scour-critical bridges, including procedures for inspection, decision thresholds, and actions during or after a flood. Monitoring may use portable instruments deployed during a high-flow event or fixed instrumentation that records bed movement or foundation exposure.
Build a Plan of Action
- Identify the bridge, responsible officials, emergency contacts, alternate routes, and access constraints.
- Define the flood stage, rainfall, forecast, or observed condition that triggers a watch, inspection, closure, or other action.
- Specify what inspectors must look for: new bed lowering, exposed piles or footing, displaced rock or blocks, settlement, bank retreat, debris accumulation, cracking, wire damage, and flow bypassing the protection.
- Set out documentation requirements, including photographs, measured elevations, instrument records, and comparison with the as-built baseline.
- State who can reopen the bridge and what temporary repairs or engineering review are required.
Portable versus fixed instrumentation
Portable equipment can be moved between bridges and used when a forecast or inspection indicates elevated risk. Fixed sensors can provide continuous or event-triggered records at a high-priority site. The choice depends on bridge criticality, accessibility, expected warning time, instrumentation survivability, data communication, and the agency’s ability to maintain and interpret the system.
USGS field assessments published for the 2014–2016 period demonstrate that condition assessment of installed countermeasures is a practical part of the protection lifecycle. An inspection should evaluate the whole system, not just whether stones remain visible: loss of filter support, toe undermining, hidden voids, and channel migration may be developing beneath an apparently intact surface.
Common failure modes and corrective responses
Riprap disappears at the edge
This usually indicates an unprotected or poorly keyed toe, an insufficient apron, or flow attacking the termination. Repair should address the cause and rebuild the edge detail, not merely add stones to the eroded hole.
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Fine soil washes through the armor
Cloudy discharge, sinkholes, settlement, or voids can indicate filter incompatibility, missing overlaps, or a torn geotextile. The remedy is to expose and redesign the failed transition where necessary, then replace the filter and armor as a system.
Armor settles unevenly
Settlement may be tolerable for a flexible system if the filter remains continuous and the foundation is not exposed. Concentrated settlement, open gaps, or a depressed toe require survey and engineering review before placing additional material.
Flow is redirected to another bank
Guide banks, spurs, weirs, and hard armor can transfer energy. Inspect adjacent and downstream reaches, and modify the channel treatment only after confirming the new flow pattern and its consequences.
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Debris or ice damages the surface
Debris impact can displace blocks, break wire, or remove individual stones. Designs in debris- or ice-prone rivers should provide robust edges, accessible repair zones, and post-event inspection triggers.
Standards and responsible use of guidance
HEC-23 Third Edition (Federal Highway Administration, 2009) consolidates countermeasure experience, selection, and design in two volumes and incorporates National Cooperative Highway Research Program work on riprap, abutment protection, pier protection, and environmentally sensitive measures. Volume 2 provides design guidance for riprap, articulated concrete blocks, grout-filled mattresses, gabion mattresses, filters, and other systems.
The guidance is a design reference, not a license to copy a detail without checking the site. Hydraulic loading, soil, foundation geometry, construction access, environmental requirements, and inspection capability determine whether a particular countermeasure is appropriate. The published material does not establish a universal success rate; performance varies with design, hydraulics, installation, and maintenance.
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