Slope is the third of the three rules in the published layout method, and it is the one that decides what the finished section looks like.
The rule
The method:
- Bag diameter sets linear coverage
- Protection depth sets the number of stacks
- Slope decides vertical stacking or stairstep
Applied by the calculator and covered in layouts, single, stacked and keyed.
Vertical stacking
Units placed more or less directly above one another, producing a steep face.
Suits: constrained sites with no horizontal room, situations where a defined near-vertical line is wanted, filling a scour hole against a structure, and working against an existing wall.
The trade:
- A steeper face reflects more energy than it absorbs, which is the mechanism that scours a wall’s own toe. See scour at bulkhead and sheet pile toes
- Stability depends more on the units’ bearing on each other and on what is behind them
- It loads the underlying slope more concentratedly
Stairstep and terracing
Units stepped back as they rise, each course bearing partly on the one below and partly on the slope behind.
Suits: flatter banks, sites with horizontal room, and anywhere a sloped, energy-absorbing face is preferable.
The advantages:
- Absorbs rather than reflects. A rough, stepped, permeable face dissipates energy, which is better for the bed in front of it and better for the neighbours. See rock bags vs a bulkhead on a lakefront
- More stable arrangement, with each course supported by the slope behind
- Better for vegetation, where soil can be placed on the steps and planted. See living shorelines and where armor fits
- Reads better with regulators, particularly in coastal programs that favour sloped protection over vertical. See state coastal zone approvals
The cost: horizontal space. A stepped section consumes run, and on a narrow lot or a constrained corridor that space may not exist. That constraint is what decides many residential designs. See lakefront erosion.
The check before either
Is the slope stable as a slope?
This is the question that has to come first, and it is the one this cluster keeps returning to.
If the bank is failing as a mass — a rotational or translational failure along a surface running behind the face — then armor on the face does not fix it, and placing weight on the upper part of a failing slope adds to the driving forces.
The signature is tension cracks behind the crest, arc-shaped scarps, movement continuing in dry weather, and trees tilted upslope. See how to read a failing bank.
If those are present, get a geotechnical opinion before designing a revetment. This is the most consequential thing on this page.
Slope and the underlying material
Beyond global stability:
Bearing. The slope has to carry the weight of the armor. On soft material, a heavy section on a steep slope is a bearing question.
Surface preparation. How much regrading is needed before placement, which is a cost and a disturbance. One of the advantages of unit placement is that it needs less of this than systems requiring a regular plane. See rock bags vs articulated concrete blocks.
Drainage. Water in the slope behind the armor needs a way out. A permeable armor face permits that, which is one of the arguments against an impermeable wall. See what the mesh is made of.
The filter question, decided by the bed and bank material rather than by the slope.
Where the slope is not a bank
The rule applies to bed work too, differently:
A flat bed takes a single layer or a shallow stack, and the arrangement question barely arises.
A scour hole has its own internal geometry, often steeper than the surrounding bed, and units placed into it are following that contour rather than a designed slope.
Against a structure, the arrangement is set by the structure’s face and by the available room, not by a natural slope. See interfacing with existing structures.
What to record
- Existing slope, measured rather than estimated
- Proposed arrangement and finished profile
- Whether global stability was assessed, and by whom
- Preparation required
- Drainage provision
- Available horizontal room, since it frequently governs