An armor installation is only as good as its weakest boundary, and the boundaries are where the attention runs out.
The four terminations
The toe, at the bottom. Keyed below anticipated scour or extended as an apron. See toe protection and layouts, single, stacked and keyed.
The two flanks, at each end of the run. Keyed into the bank, terminating in ground that is not being attacked.
The crest, at the top. High enough that the design event does not get over it, or detailed to survive overtopping.
Three of the four are places where the protection stops and something unprotected begins. That discontinuity is what flow works at.
Flanking, and how it starts
The bank asks which layout details help prevent erosion from flanking the protected area, and flanking is the failure this whole page is about.
The sequence:
- Erosion continues just beyond the end of the protection
- The bed or bank there lowers
- The last units lose support beside them
- They slump into the developing hole
- The next units are exposed on a new face
- The failure travels inward along the run
The middle of the installation — the part everyone photographs — is the last to be affected and the first to be inspected.
The rule: terminate somewhere the flow is not attacking. In practice that means extending further than the visible problem, often considerably further, and it is the item most likely to be cut when a quantity is being trimmed.
The crest, and the mistake
The crest gets set to the water level someone saw on a site visit. The design event is higher.
Flow gets over the top, attacks the bank behind the armor, removes the material the armor bears against, and the installation fails from behind with its face intact.
Where overtopping is expected, the crest is turned back into the bank or the protection is carried over and down the landward side. Where it is not expected, the freeboard should be a stated number rather than an assumption.
Meeting another system
The bank covers how bagged units transition into existing riprap, concrete, or sheet pile protection. The questions to answer at any such joint:
Is there continuity of protection across the line? Not “do both systems reach it” but “is the line itself covered.” A butt joint with a seam running down it is a seam for flow to exploit.
Overlap rather than abut. Running one system into the other removes the straight-line seam.
Is there a filter discontinuity? If one side has a geotextile beneath and the other does not, the boundary between them is a route for fines. This is winnowing operating at a joint. See why scour protection fails.
Which side will move? If one settles and the other does not, the joint opens. Put the conforming system where movement is expected.
Key vertically where you can. A stepped joint is harder to exploit than one on a plane.
Combining rock bags with other systems covers the four combinations that recur.
Grade control and weirs
The bank covers how units are transitioned around a grade-control structure or low-head weir. These are hydraulic structures with a designed function, and the interface has to respect that function rather than only the erosion at it.
A drop structure concentrates energy at its downstream toe by design. Armor there is protecting against a mechanism the structure creates deliberately, and the detail belongs with whoever designed the structure.
Terminations at a bridge
Two constraints pull against each other, and it is the most awkward detailing case on this site.
Extend far enough that the termination is out of the attacked zone.
Do not narrow the waterway more than necessary. The contraction is part of the problem at a bridge, and material projecting into the opening makes it worse, as well as raising a no-rise question. See floodplain permitting and no-rise and bridge abutment scour.
The bank addresses protecting an abutment toe without unnecessarily narrowing the waterway, which is exactly this tension.
Recording them
Terminations are the details a later inspection most needs to know about, and the ones least visible once vegetation establishes.
The as-built should show where each termination is, at what elevation, and how it was keyed. Five years on, an inspector finding a step at the end of a run needs to know whether that is the designed termination or the start of a flank failure. See post-installation inspection.
Where to go next
- Revetment anatomy for the five parts
- Toe protection
- Interfacing with existing structures
- Combining rock bags with other systems
- The rock bag FAQ for the full layouts and transitions section
- The installation hub for the rest