The layout method FES uses is published rather than proprietary, which means you can check the arithmetic.
The method
Three rules:
Bag diameter sets linear coverage. Roughly 85 to 90 percent of the bag diameter, allowing for how units bed against each other rather than sitting as perfect circles.
Protection depth sets the number of stacks. The requirement divided by the bag height.
Slope decides the arrangement. Vertical stacking on steeper sections, stairstep or terraced on flatter ones.
The calculator applies this, and the reasoning is set out there. Treat the output as a shortlist rather than a design — final selection belongs to the engineer of record.
Single layer or stacked
The bank asks when a project needs a single layer instead of a stacked or terraced layout.
A single layer suits shallow protection depths, bed armoring where the requirement is coverage rather than height, aprons below outfalls, and situations where the available depth or the available headroom does not allow more.
Stacking is needed where the protection has to reach a height — up a bank face, to a crest elevation, or to fill a scour hole of some depth.
Terracing or stairstepping on flatter slopes, where a vertical stack would be less stable than a stepped arrangement bearing back into the slope.
Keying the bottom row
The bank asks directly whether the bottom row needs to be keyed into a trench, and how much toe embedment is typically needed to reduce undermining.
The two approaches are the standard ones, covered in toe protection:
Keyed. Excavate and set the bottom row below the anticipated scour depth, so that after the design event the toe is still buried and supported. Requires knowing the anticipated scour depth and excavating in the wet, which is the expensive part.
Aproned. Extend units horizontally along the bed from the toe, with enough material that as scour develops at the outer edge, units settle into the hole and continue to protect.
The conforming behaviour of a unit-placed system is what makes the second option work: units follow the bed down and stay in contact rather than bridging a void. See rock bags vs concrete revetment.
Which one suits depends on whether you can excavate, and on how much scour is anticipated. The specific embedment for a given size and condition comes from the engineering team.
When a geotextile goes underneath
The bank asks when a geotextile underlay should be placed beneath the units, and this is one of the more consequential questions on the whole job.
The general position FES publishes is that the product does not require a geotextile underlayer — that came out of the August 2026 accuracy review and it is stated as the FES position.
That is not the same as saying no site ever needs one. The question is the interface, not the product: where the bed contains fines that flow can entrain, something has to stop those fines being drawn up through the armor, or the layer settles into the void it creates. That mechanism is winnowing, and it is covered in why scour protection fails.
So the honest framing: the units do not need an underlayer to function, and whether your bed needs a filter is a site question for the engineer of record, decided from the bed material. Raise it rather than assume it either way.
Accommodating settlement
The bank asks how a layout can accommodate expected settlement without opening large gaps.
Units bed in, the bed adjusts, and the layer moves. A layout that is dimensionally exact on day one and has no allowance for that will develop gaps as it settles.
Practically this means overlap and contact rather than precise abutment, and it is one reason the linear coverage rule uses 85 to 90 percent of diameter rather than 100.
Tying units together
The bank asks whether adjacent units need to be tied after placement. It is worth asking for your arrangement rather than assuming, because the answer differs by application, and a tied arrangement behaves differently from a free-placed one.
Aprons below outfalls
A specific case with its own geometry. The bank covers how apron length and width are established below a culvert or stormwater outfall.
The controlling factor is where the jet actually lands, which is downstream of the outlet rather than at it — see culvert outfall scour. An apron sized to the structure rather than to the impact zone leaves the real problem unprotected and its own downstream edge exposed.
Irregular geometry
Around bridge piers, piles, and headwalls, the layout adapts rather than following a pattern. The bank covers how layouts are adjusted for irregular piers, piles, and headwalls, and how the arrangement changes around the nose and tail of a pier.
This is the conditions-suit-the-method case: units placed individually into whatever geometry exists, rather than a system requiring a regular bearing plane. See interfacing with existing structures.
Where to go next
- Toe protection
- Transitions and terminations
- Placement sequence and coverage
- Rock bag size calculator
- The rock bag FAQ for the full layouts and transitions section
- The installation hub for the rest