The crest is the least interesting of a revetment’s boundaries and it produces a disproportionate share of the failures, for a simple reason: it gets set to the water level someone saw.
What freeboard is
The height of the crest above the design water level. Some margin above the level the design event produces, to account for wave run-up, uncertainty in the level, and the consequences of being wrong.
Where the crest is set below what an event delivers, flow goes over it.
What overtopping does
The failure is not on the face. It is behind it:
- Flow passes over the crest
- It lands on the bank behind the armor and erodes it
- The material the armor was bearing against is removed
- Voids develop behind and beneath the upper courses
- The upper units lose support and drop into the void
- The failure works downward from the top
Meanwhile the armored face has not been touched. An installation can fail this way with every unit on its front intact and correctly placed, which is why it gets misdiagnosed.
It is the crest version of the same principle that runs through why scour protection fails: the boundaries fail, not the middle.
Why the crest gets set too low
The site visit level. Someone stands at the bank at normal flow and the crest is set relative to what they can see. The design event is higher, sometimes much higher.
Seasonal and operating range. On a reservoir or a managed water body, the level moves. A crest set to summer level is undersized for the operating high.
Wave run-up not counted. On a lake, coast, or any site with wave action, water runs up the face well above the still-water level. A crest at the still-water design level will be overtopped by run-up. See lakefront erosion and combined wave and current loading.
Budget. Height is quantity. When a scope is trimmed, the crest comes down, and it is one of the two places — with the toe — where trimming does the most damage.
Designing a crest that can be overtopped
Sometimes overtopping is unavoidable or acceptable, and then the crest is detailed to survive it rather than to prevent it:
Turn the protection back into the bank at the crest, so there is armored material behind the top course rather than bare soil.
Carry it over and down. Where overtopping is expected regularly, protecting the landward side as well converts the crest from a termination into a continuous surface.
Key the top course into the bank rather than resting it on the surface.
Treat the landward slope. Vegetation, turf reinforcement, or lighter armor on the back face where flow lands. See rock bags vs turf reinforcement mats.
Provide drainage. Water that gets behind the armor needs somewhere to go, or it builds pressure. A permeable armor face helps, but the bank behind still needs a route.
The interaction with the floodplain
A taller crest is a larger encroachment, and in a regulatory floodway that has to be shown not to raise the base flood elevation.
So there can be a genuine tension: freeboard wants a higher crest, no-rise wants less material in the channel. Resolving it usually means keeping the section close to the existing bank line rather than projecting outward, so the height comes without additional encroachment. See floodplain permitting and no-rise.
What to record
- Design water level, and its source
- Wave run-up allowance, where it applies
- Freeboard adopted, and why
- Crest elevation
- Crest detail — keyed, turned back, or carried over
- Whether overtopping is expected, and what happens if it occurs
That last item is the one most often absent. A design that has not asked what happens when the crest is overtopped has not designed the crest.
Where to go next
- Revetment anatomy for the four terminations
- Transitions and terminations
- Choosing a design storm
- Combined wave and current loading
- Floodplain permitting and no-rise
- The design hub for the rest