If there is one recurring theme across this site, it is the toe. Bulkheads fail at the toe. Concrete revetments are undermined at the toe. Turf reinforcement mats slide because the toe went. Riprap blankets unravel from the toe.
There is a reason, and there is a reason it keeps happening.
Why the toe carries the load
It is the highest-energy zone. The toe is wetted permanently or almost permanently, and it is where velocity near the bed is highest. The slope above is wetted only in larger events.
It is the base of everything above. Every treatment on the slope depends on the material at its base staying there. Remove that and the treatment above has nothing to bear on.
It is where scour concentrates. At a structure, at a bend, at a wall face, the mechanisms that excavate a bed all act at the bottom.
It is where bedload abrades. Sediment moving along the bed works at whatever is in contact with it.
Why it gets skipped
Three reasons, and none of them is that engineers do not know it matters.
You cannot see it. The toe is underwater. On a site visit at normal flow, the visible problem is the slumped face. The toe is a description in a report rather than something anyone looked at.
It is expensive to work on. Reaching the toe means working in water, or dewatering, or reaching from a distance. It is the most difficult part of the job to build, so it is the part most likely to be trimmed when a budget tightens.
The design event is not today’s water. Toe protection has to be designed for the bed elevation the design event will produce, not the one you can measure. That requires a computation, and where none has been done the default is to protect what is visible.
How far down
The question that decides whether a treatment survives, and there are two accepted answers.
Key it below the anticipated scour depth. Excavate down and build the toe below the level the bed is expected to reach, so that even after the design event the toe remains buried and supported.
Requires knowing the anticipated scour depth, which requires the hydraulic work described in design and sizing, and requires excavation in the wet, which is the expensive part.
Provide a launching apron. Extend the armor horizontally out from the toe along the bed, with enough material that when scour develops at the outer edge, the apron material settles or “launches” into the developing hole and continues to protect the toe.
This is the standard approach where excavating a keyed toe is impractical, and it is the reason riprap designs specify apron thickness and width rather than only a blanket on the slope.
Bagged armor can be used either way — units keyed into an excavated trench, or extended as an apron that follows the bed down as it lowers. The conforming behaviour is what makes the second approach work: the units settle into the developing depression and keep covering it, rather than bridging it. See rock bags vs concrete revetment for the rigid comparison.
The three ways toe design goes wrong
1. Protection stops at the water line. The most common. Whatever is visible at the time of the survey becomes the bottom of the treatment. Everything below it is unprotected, and it is the part under attack.
2. The toe is protected but not keyed or aproned. Armor placed on the bed surface, with no depth and no lateral extension. The first event lowers the bed just outside it, the edge is undermined, and it slumps. This is the edge failure in why scour protection fails.
3. The toe is designed for the current bed, not the design bed. The computation was never done, so the anticipated scour depth is unknown and the toe was set at a level that seemed reasonable.
The interaction with everything else
Toe protection is why so many designs are hybrids. The energy at the toe justifies armor; the slope above frequently does not need it.
Armor at the toe with vegetation, mat, or lighter treatment above is the standard section for a vegetated bank, and it is covered in combining rock bags with other systems and rock bags vs turf reinforcement mats.
It is also the reason toe armor belongs with a new wall of any kind. A bulkhead, sheet pile wall, or concrete revetment reflects energy into the bed at its base, and the toe treatment removes the mechanism that ends most of them. See scour at bulkhead and sheet pile toes.
Where bagged armor suits toe work
The conditions at a toe are the ones unit placement handles:
- Permanently wet, and dewatering is the expensive alternative
- Irregular, particularly where a hole already exists
- Deep relative to the access, so placement is from above at a distance
- Progressive, since more units can be added if the bed lowers further
What to check on an existing treatment
- At low water, look at the bottom edge. Is there a step down from the armor to the bed?
- Is the bottom row exposed or undercut?
- Has the bed level changed since the as-built or the last survey?
- Is there fresh material at the base, suggesting the treatment is shedding?
- Sound or probe for voids beneath a rigid treatment
A treatment that has settled uniformly looks fine and has lost that much protection. Only a level comparison finds it.
Where to go next
- How to read a failing bank for the mechanism first
- Revetment anatomy for the other three parts
- Why scour protection fails for edge failure
- Design and sizing for the scour depth computation
- Rivers and streambanks
- The erosion control hub for the rest