Banks carry water. Rain infiltrates, groundwater moves through, and after a flood the material is saturated and draining back to the channel. Whatever you put on the face has to account for that.
What an impermeable face does
Put a solid surface against a bank and the water behind it has to find another route.
Hydrostatic pressure builds. Water standing behind an impermeable face exerts pressure on it. That load is a design case for retaining structures and a common cause of distress in ones where it was under-estimated.
Rapid drawdown becomes a problem. When a river or reservoir falls faster than the bank can drain, the water level inside the bank is higher than outside it. The pressure difference acts outward on the face, and in the soil it reduces effective stress. Slumping after drawdown is a recognisable failure and it is a stability mechanism rather than an erosion one. See how to read a failing bank.
Seepage finds another path. Water that cannot exit through the face exits around it — at the ends, under the toe, or through joints. Each of those routes carries fines with it.
That last point is why concrete revetments and bulkheads need weep holes, drainage layers, and filters behind the face. Those are not optional extras; they are what stops the structure becoming a dam. See the hidden costs of concrete revetment.
What a permeable layer does instead
Water moves through it. The bank drains roughly as it did before, pressure does not accumulate behind the armor, and drawdown does not produce a differential the structure has to carry.
For a rock bag installation the permeability comes from two places: the open knitted mesh, and the voids between the units. See raschel weave explained.
Practical consequences:
- No hydrostatic design case on the armor itself in the way a wall has one
- No weep-hole detailing
- Rapid drawdown is not a loading condition for the armor
- Seepage exits through the face rather than seeking a route around it
The question permeability does not answer
Being clear, because this is where the argument is often over-stated.
Permeable armor still needs the interface resolved. Water moving through the layer can entrain fines from the bank behind and carry them out. That is winnowing, the bank loses material from underneath the armor, and the layer settles into the void it created.
Permeability is what makes the mechanism possible. A filter — granular or geotextile, specified against the bank material — is what stops it. See why scour protection fails.
The published FES position is that the product does not require a geotextile underlayer. That is a statement about the units, not about your bed. Whether your bank material needs a filter is a site question for the engineer of record, decided from the soil. Raise it rather than assume it either way. See layouts, single, stacked and keyed.
Where permeability does not help
Piping and seepage erosion. Where groundwater is already exiting the face and carrying fines with it, permeable armor lets that continue. The treatment is drainage and filtration designed for the seepage, not armor placed over it. Armor over a seeping face without a filter can make things worse by concentrating the exit.
A cut-off requirement. If the design needs to stop water passing through — a seepage barrier, a containment — permeability is the opposite of what you want, and the answer is a sheet pile or a lined structure. See rock bags vs sheet pile.
Deep-seated stability. Drainage helps stability, but a slope failing along a surface well behind the face is a geotechnical problem, and no facing treatment fixes it.
The environmental side
Permeability matters ecologically as well as hydraulically, and it is one of the more defensible environmental arguments for this kind of armor:
- The interstitial spaces are habitat, in a way a smooth impermeable face is not
- The bank margin stays hydraulically connected to the channel rather than being sealed
- Water exchange between bank and channel continues
What we do not have is data quantifying any of that. What colonises an installation is covered in what colonises an installation at the level our evidence supports, which is qualitative.
Where to go next
- What colonises an installation
- Hard armor vs a wall, environmentally
- Why scour protection fails for winnowing
- How to read a failing bank for seepage signatures
- Rock bags vs concrete revetment
- The environment hub for the rest