Most channel and revetment design assumes subcritical flow: relatively deep, relatively slow, controlled from downstream. On steep slopes the flow regime changes, and so do the design rules.
What changes
Supercritical flow is fast and shallow, with a Froude number above one. It is controlled from upstream rather than downstream, and disturbances cannot propagate back up it.
Practical differences:
High velocity, low depth. The near-bed velocity is high relative to the depth of water available.
Standing waves and cross-waves. Supercritical flow in a channel with any curvature or irregularity develops standing wave patterns, which produce locally elevated water surfaces and asymmetric loading.
Sensitivity to roughness and geometry. Small irregularities generate large disturbances.
Air entrainment on steep chutes, which changes the flow depth and the loading.
The hydraulic jump
The condition that governs most of these sites.
Where supercritical flow transitions back to subcritical — at the base of a chute, downstream of a spillway, below a drop structure — it does so through a hydraulic jump: an abrupt rise in water surface accompanied by intense turbulence.
The jump dissipates a large amount of energy in a short distance. That is exactly what it is for on a designed structure, and it is exactly why the location of the jump is the most hydraulically aggressive point in the system.
Two consequences:
The jump location moves with discharge. A stilling basin designed to contain it at the design flow may not contain it at a lower or higher one, and a jump that sweeps out of the basin lands on whatever is beyond.
Conditions in the jump are far beyond what average velocity suggests. Turbulence, fluctuating pressures, and reverse flow within the roller act on the bed and on anything placed there in ways a velocity check does not represent. It is the same reason the plunge pool below a culvert is more aggressive than the outlet velocity implies. See culvert outfall scour.
Why this is a hard-structure case
Being straightforward about where the product does not fit.
A designed hydraulic surface — a spillway profile, a stilling basin, a chute — is doing a hydraulic job through its shape. The geometry is the function. A conforming armor layer with a rough, irregular surface does not deliver a designed profile, and shape substitution is not a detail. See rock bags vs concrete revetment.
High-abrasion, high-velocity duty on a spillway apron suits a hard monolithic surface.
Fluctuating pressures in a jump are a loading regime that armor design methods for channel and bank protection were not calibrated against.
Our own data does not cover it. We publish which method backs which claim and hold no physical model test results for these conditions. See when not to use rock bags.
So on a spillway, a chute, or an engineered stilling basin, the answer is usually a designed concrete structure, and we would say so.
Where bagged armor does have a role
Around these structures rather than as them:
Downstream of a stilling basin, where flow has returned to subcritical and the duty becomes ordinary channel or bank protection. This is a common requirement and it is a normal armor job.
Repairing scour that has developed below a structure, where a hole already exists and the alternative would need dewatering to fix. See culvert outfall scour.
Protecting the transition where a structure ends and a natural channel begins. The bank covers how units are transitioned around a grade-control structure or low-head weir.
Grade control structures themselves, which are a designed hydraulic feature and whose design belongs to the engineer of record rather than to a supplier’s layout method.
Emergency stabilisation at a damaged structure, as an interim measure. See emergency scour repair.
The questions that decide it
- Is the flow supercritical at the location, and at what discharges?
- Where does the hydraulic jump form, and does it move with discharge?
- Is a designed hydraulic profile required, or is the requirement erosion protection?
- Is the location within the energy dissipation zone, or downstream of it?
- What does the specification require the surface to do?
If the answers put you inside the dissipation zone with a required profile, that is a concrete structure. If they put you downstream of it protecting a channel, that is an armor job.