Fill selection for a rock bag is normally an engineering question — size, durability, gradation, density. See fill stone selection.
Placing it in a watercourse adds a second set of questions, and the answer bank has one on exactly this: what fill stone is environmentally acceptable.
The four checks
1. What is the source, and what is in it?
Natural quarried stone from a known source is the straightforward case. What raises questions:
Recycled concrete aggregate. Attractive on sustainability grounds and frequently problematic in water: it can raise pH in the immediate vicinity, it may carry reinforcement fragments, and its composition varies with the demolition stream it came from. Many agencies restrict it for in-water use. If it is proposed, it needs specific approval rather than an assumption.
Slag and industrial by-products. Some are inert and some leach. This is a testing question, not a judgement call.
Recovered or reused material from another site, which carries whatever that site had.
Anything from a contaminated site. Ruled out.
2. Does it leach?
Some materials release constituents into water — metals, alkalinity, sulphates — depending on their mineralogy and on the receiving water’s chemistry.
For most natural quarried stone this is a non-issue. Where the source is anything other than natural quarried aggregate, or the receiving water is sensitive, testing may be required and a reviewer may ask for it.
3. How clean is it?
Fines matter twice over.
Engineering: material below the mesh aperture migrates out over time, so the unit loses fill. See fill stone selection.
Environmental: fines placed in water are turbidity, released immediately on placement. Clean, screened stone is materially less turbid to place than material carrying a silt fraction, and on a site with a numeric turbidity limit that can be the difference between compliance and a stoppage. See construction turbidity control.
Washing or screening the fill is a legitimate line item where the limits are tight.
4. What is living on it?
The one most often forgotten.
Aggregate moved between watersheds can carry invasive species — mussel veligers, plant fragments, pathogens. Several jurisdictions have specific requirements about moving material between water bodies, and equipment decontamination protocols alongside them.
Where fill is sourced locally this largely resolves itself, which is one more argument for local supply. See when local stone changes the economics.
Getting it approved
The answer bank covers that locally sourced aggregate can be submitted for approval before construction begins, and doing that early is the difference between a smooth job and a variation.
For an in-water project, a submission generally benefits from:
- Source identification
- Gradation analysis
- Durability assessment for the environment
- Confirmation the material is natural quarried stone, or test data if it is not
- A sample
- Confirmation of any inter-basin transfer requirements
The material questions that are about the bag, not the fill
Worth separating, because they get merged in conversation:
Microplastics relate to the mesh polymer, not the stone, and no published standard measures in-service shedding from a geosynthetic, so no figure is published. See Microplastics and geosynthetics.
Service life relates to the mesh. See service life in water vs on land.
End of life applies to both, and the two components have different fates: recovered stone is aggregate, and the mesh is a polymer question. See end of service life and recovery.
What we will not do
Approve fill for you. We can say what the engineering requirements are and what the recurring environmental questions are. Whether a specific material from a specific quarry is acceptable in a specific water is a matter for the engineer of record and the regulator.
If you send the proposed source through the quote form, the engineering side can be assessed. The environmental side needs your permitting route.
Where to go next
- Fill stone selection and gradation for the engineering
- When local stone changes the economics
- Construction turbidity control
- End of service life and recovery
- Section 404
- The environment hub for the rest