FHWA’s HEC-23, Bridge Scour and Stream Instability Countermeasures, is the standard US reference for this, and it organises a large field into something navigable. What follows is the shape of that field rather than a substitute for the document.
Three families. Most real projects use more than one.
Family 1: Armor the bed
Put something on the bed that the flow cannot lift, so the material underneath stays put.
What sits here: riprap, bagged rock armor, articulated concrete block mats, gabions and gabion mattresses, grout bags, concrete revetment, cast armor units, partially grouted riprap.
Strengths: directly addresses the mechanism, works on existing structures where the foundation is already built, and is generally the cheapest retrofit.
Weaknesses: every armor layer has edges, and edges are where these systems fail. It also does nothing about the cause — if the bed is lowering across the reach, armor at one structure is protecting a spot in a channel that keeps degrading beneath it.
What decides within the family: whether the site can be dewatered, whether the bed is regular or irregular, whether local stone or graded armor stone is available, and how much geometric precision the finished surface needs. The comparison hub works through those trade-offs system by system.
The universal requirements, whatever you pick: a filter appropriate to the bed material, adequate plan extent, and keyed terminations at toe, flanks, and crest. Why scour protection fails covers what happens when any of those is missing.
Family 2: Modify the flow
Change the flow field so the excavating mechanism is weakened, rather than resisting it once it forms.
What sits here:
- Guide banks, leading floodplain flow smoothly into a bridge opening instead of letting it turn sharply around the embankment end. Particularly relevant to abutment scour.
- Spurs, groynes, and bendway weirs, projecting into the channel to move the high-velocity thread away from a bank.
- Vanes, submerged structures that reorganise secondary currents on a bend so the outer bank sees less attack.
- Collars and horizontal plates at a pier, interrupting the downflow that drives the horseshoe vortex.
- Sacrificial piles upstream of a pier, disturbing the approach flow before it arrives.
- Channel improvements and opening enlargement, treating a contraction problem at its cause.
Strengths: treats cause rather than symptom. A guide bank that fixes the approach flow reduces the problem at the abutment permanently rather than resisting it.
Weaknesses: more site-specific, harder to design with confidence, and it changes the hydraulics elsewhere — which can move a problem downstream or across to the opposite bank rather than removing it. Several of these need their own armoring.
Where it fits: where the mechanism has an identifiable cause you can address. Flow arriving badly at an opening is a flow problem, and a flow solution is more durable than armoring the consequence.
Family 3: Design past it
Found the structure deep enough that losing the material above the foundation does not matter.
What sits here: deeper piles, deeper spread footings, and foundation designs that assume the computed total scour has occurred.
Strengths: by far the most robust. Nothing to maintain, nothing to inspect, no edges to fail. If the foundation is below the scour depth, the scour is an inconvenience rather than a threat.
Weaknesses: available at design stage, and effectively unavailable afterwards. Retrofitting foundation depth into an existing bridge is possible and very expensive.
Why it matters to a retrofit conversation anyway: knowing where the foundation actually sits determines how much scour is tolerable, and therefore what the protection has to achieve. A bridge whose foundations are already deeper than the computed scour may need monitoring rather than armor. A bridge with unknown foundations is in a different category entirely — see scour critical bridges.
Monitoring, which is not quite a family
Instruments and inspection regimes do not prevent scour. They tell you it is happening.
What sits here: fixed sonar and magnetic sliding-collar instruments, periodic sounding surveys, underwater inspection programmes, and flood-triggered inspection protocols.
Where it fits: as a substitute for protection where a structure has adequate foundation depth and the risk is being managed rather than removed; as a supplement to protection, to confirm it is performing; and as an interim measure while a permanent fix is designed and funded.
Monitoring is frequently the honest answer for a structure where the protection budget does not exist yet. It is not a fix, and calling it one is how bridges get into trouble slowly.
How selection actually goes
- Establish which scour component dominates. Local, contraction, or degradation. This decides more than anything else, and it is covered in contraction, local, and degradation scour.
- Establish where the foundation is. Tolerable scour depth follows from it.
- If the cause is addressable, address it. Flow-altering measures and opening changes are more durable than armor.
- Armor what remains, sized and extended to the computed footprint, with a filter and keyed terminations.
- Monitor, whatever else you do.
The common error is starting at step 4.
Where to go next
- What is scour? for the process
- Bridge pier scour and abutment scour for the local mechanisms
- Why scour protection fails for what defeats armor
- How to read a scour evaluation for interpreting the assessment
- The comparison hub for choosing within the armor family
- When not to use rock bags for our own limits
We supply one product within one family. If the answer for your structure is a guide bank or a deeper foundation, that is the answer, and the quote form will get you a straight response either way.