Two FHWA circulars do most of the work on bridge scour in the United States, and they do different jobs.
HEC-18: how much scour
Evaluating Scour at Bridges. It is the reference for computing scour: separating long-term degradation, contraction scour, and local scour, computing each, and adding them to a total at each foundation.
That total, compared against the foundation elevation, is what determines whether a bridge is scour critical. See how to read a scour evaluation and contraction, local, and degradation scour.
For an armor design, HEC-18 produces the input that matters most after velocity: the depth the bed is expected to reach. That drives toe treatment and stack count.
HEC-23: what to do about it
Bridge Scour and Stream Instability Countermeasures. It is the reference for selecting and designing countermeasures, organised into families: armoring, flow-altering measures, and structural or monitoring approaches. See the scour countermeasure families.
It contains design guidance for specific countermeasures, including riprap at piers and abutments, articulated concrete blocks, gabions, grout-filled mattresses, guide banks, and spurs — each with its own design procedure.
Where bagged armor sits
Plainly: there is no bagged rock armor chapter.
The countermeasure families in HEC-23 were written around the systems in common use, and a bagged system placed unit by unit is not one of them. That is a fact about the guidance rather than a judgement about the product, and pretending otherwise does not survive a reviewer’s first question.
What follows practically:
The evaluation side transfers fully. HEC-18’s scour computation is about the site and the structure, not about what you place. Contraction scour, local scour, and degradation are the same numbers whatever the countermeasure. That work is directly usable.
The design principles transfer. Plan extent expressed relative to pier width, filter requirements, keyed or aproned toe treatment, edge and flank detailing — these are general armor principles that HEC-23 sets out clearly, and they apply regardless of the unit. They are also the things that actually decide whether an installation lasts. See why scour protection fails.
The unit sizing procedure does not transfer. A riprap sizing equation produces a stone D50 from hydraulic conditions. It does not produce a bag size, because the element resisting the flow is different — the unit rather than the individual stone. See translating a specified riprap D50.
What this means on an agency project
This is the procedural obstacle rather than a technical one, and it is covered in getting a non-standard product approved.
A specification written from HEC-23 will name systems HEC-23 covers. A system it does not cover is not rejected on merit; it is unaddressed, and unaddressed is functionally the same as unavailable to someone writing a biddable specification.
The routes through are performance specification, an equivalency submission, or getting onto an approved products list. All three go better when you lead with what the guidance does and does not cover rather than implying coverage that does not exist.
What we can and cannot supply toward it
We can supply: the test programme and which method backs which claim, the laboratories, the published layout method, product dimensions and rated conditions, and a written statement of limitations.
We cannot supply: measured test values, certifications, a P.E. stamp, or physical model test results for wave stability. See which test standards apply and when not to use rock bags.
A reviewer who requires the design to follow a HEC-23 procedure for the specific countermeasure is a reviewer we cannot fully satisfy, and week one is the right time for that to surface.
Using the framework anyway
Even without a chapter, the framework is the right structure for the work:
- Compute scour to HEC-18, or use the existing evaluation
- Establish which component dominates, because it decides whether armor is the answer at all
- Establish foundation elevation and therefore tolerable scour
- If armor is the answer, apply HEC-23’s general principles for extent, filter, and terminations
- Size the units from the hydraulic conditions using the supplier’s published method and the engineer of record’s judgement
- Document the basis
Step 5 is where the deviation sits, and stating it as a deviation is better than burying it.