Regulators increasingly distinguish between kinds of hard armor rather than treating it as one category. Knowing the criteria they apply is useful whichever system you end up specifying.
The five criteria that recur
1. Reflection versus absorption
A hard vertical face reflects wave and current energy. Much of that reflected energy goes downward into the bed at the base of the structure, scouring its own toe, and outward toward neighbouring property. See scour at bulkhead and sheet pile toes.
A sloped, rough, permeable face absorbs and dissipates. The bed in front is not being attacked by the structure’s own presence, and the ends do not project energy sideways in the same way.
This is the criterion behind much of the policy shift against new vertical walls. See state coastal zone approvals.
2. The intertidal and shallow margin
A vertical wall eliminates the shallow-water margin between land and channel. That zone is disproportionately productive, and replacing it with a plane at a fixed line removes it.
A sloped section retains a graded transition through the tidal or seasonal range.
3. Permeability and connection
An impermeable face seals the bank hydraulically. Groundwater that used to seep out has to find another route, pressure builds behind, and the bank margin is disconnected from the channel.
A permeable layer lets that exchange continue. See how permeable armor changes drainage.
4. Surface complexity
A plane offers one surface condition. A rough, three-dimensional, permeable arrangement offers interstitial spaces, varied flow refuges, and a range of conditions.
Physically more complex. What colonises it and how much better that is, we cannot quantify — see what colonises an installation.
5. Reversibility
A concrete structure is demolished at end of life, producing waste and a substantial disturbance. Discrete units are recovered by the plant that placed them. See end of service life and recovery.
For a programme thinking about shoreline change over decades, a treatment that can be removed is a different proposition from one that cannot.
Construction phase
Frequently the larger environmental difference, and the one most often left out.
Concrete generally needs a dry working area: a coffer dam, dewatering, discharge treatment, fish salvage, then re-watering — with the installation and removal phases the most disturbing parts of the whole job. See what dewatering really adds.
Placement in flowing water avoids all of it. On the avoid-minimise-mitigate test a reviewer applies, that is a substantive minimisation argument and worth stating explicitly. See what an agency reviewer wants to see.
Where concrete is environmentally defensible
Not one-sided:
- Founded on rock, where there was no soft margin to lose
- Existing urban waterfront, where the shoreline is already hardened and the alternative changes nothing
- A designed hydraulic structure — a spillway, a stilling basin — where the shape is the function. See supercritical flow and spillways
- Where a longer service life genuinely reduces intervention frequency over the asset’s life
- Where a smaller footprint achieves the same protection, since footprint is itself an impact
That last one matters. A compact concrete section may disturb less area than a sloped armor section achieving the same thing, and area disturbed is a criterion too.
The claim we do not make
Rock bags are not the environmentally friendly option. They are hard armor. Placing them converts a natural substrate to an armored one over the area they occupy, and where a programme’s preference is for a non-structural approach, that preference applies to us as much as to anyone.
What is defensible is the comparison within hard armor: permeable and sloped performs better on most of the criteria above than impermeable and vertical. That is a narrower claim than “environmentally friendly” and it is the one the reasoning supports.
The genuinely soft options are covered in living shorelines and where armor fits, including the cases where a hard toe is still needed and the cases where nothing structural is.