When it comes to protecting riverbanks, bridges and coastal infrastructure, riprap has been a conventional and default choice for erosion control. Loose rock or concrete rubble is cheap, easy to source, and has been the standard across Australia for decades. But as this solution has real limitations, riprap alternatives are becoming the new standard.
When individual rocks shift and scatter under wave action or flood flows, gaps open up that let water undercut the structure, and repairs often mean bringing in heavy plants to redistribute or replace lost material.
These limitations weigh more heavily today than they once did. Extreme weather is intensifying across Australia, and the coastline is already showing it. Ex-Tropical Cyclone Alfred left sand cliffs up to six metres high along beaches in southern Queensland and northern NSW in 2025, showing how fast a single event can undo years of erosion protection.
Events like this call for protection built to hold under storm-scale conditions, and that’s where Bluemont Rock Bags™ come in. They are a flexible netted structure that resist displacement instead of scattering under load. This guide breaks riprap and Bluemont Rock Bags™ down side by side, including performance, cost, and sustainability.
Why Australia Needs Better Erosion Control Solutions
The Australian Actuaries Climate Index hit its highest recorded value in autumn 2025, driven largely by rising sea levels and a run of extreme rainfall events. Ex-Tropical Cyclone Alfred was a major contributor, cutting sand cliffs up to six metres high into Queensland and northern NSW beaches within days. A few months later, the Mid North Coast and Hunter floods pushed river levels at Taree to a record high, scouring banks and undercutting structures well away from any beach.
In both cases the damage wasn’t just water passing through. Floodwaters and storm surge strip the ground out from underneath riverbanks, bridge piers and coastal batters, undercutting the foundation of whatever sits above even when the structure looks intact from the surface. Left unaddressed, that undermining spreads: a section of eroded bank shifts more load onto the ground beside it, and the next event has less to hold onto than the last one did.
Erosion control now needs to keep pace with that kind of load, not just the conditions it was originally designed against.
What is Riprap?
Riprap is loose rock, sized and graded to suit the flow or wave conditions at a site. Riprap depends on the weight and mutual interlock of individual stones to resist movement, rather than any fixed structure holding them together. Rock size is calculated per site, based on the flow velocity or wave energy it needs to withstand. For example, larger, faster-moving water requires heavier stone.
In Australia, that sizing typically follows Austroads guidance for drainage structures, open channels and floodways, which sets out velocity-based methods for working out the rock size a given site needs. Engineers have used it on riverbanks, bridge abutments and coastlines for decades, and where local sources supply it, it’s a familiar and relatively affordable material.
Issues with Using Riprap for Erosion Control in Australia
Riprap’s problems tend to surface during sourcing, installation, in how it holds up under load, and in what it costs to maintain once something goes wrong.
High cost
Rock isn’t the only expense. Correctly graded armour rock has to be quarried, sized, and freighted to site. None of that is cheap before installation even starts. Placing riprap by hand or machine to the design profile takes time and labour that a pre-assembled unit doesn’t, and that installation cost sits on top of the material cost.
Sourcing and site constraints
Getting the right rock to site is a challenge in itself. A remote bridge, a rural council job, or a site off the main freight routes can face a long haul to the nearest quarry with rock in the right size and durability class for the flow conditions. Not every nearby quarry carries it. Installation is also labour-intensive and needs the right machinery. It can be very challenging in hard-to-reach sites. Riprap may also not be a practical option for erosion control in steep slopes.
How riprap fails under load
Well-designed riprap holds up for years. The problem shows up when a single rock in the layer gets moved, either because it was undersized for the site, hit by debris, or simply caught the full force of a flood peak. Once that happens, water gets underneath the layer next to it, and the ground beneath starts washing out. Left alone, erosion can worsen and affect the foundation of the structure.
Maintenance and repair costs
Maintenance guides for riprap describe the same pattern in practice. Routine upkeep after storm events usually starts with checking for displaced stone, and larger repairs typically need an excavator or loader on site to reposition or replace what’s lost, rather than a quick patch-up.
Ecological disruption
Placing riprap requires a lot of foundation work, including clearing, grading, and compacting ground that may have otherwise been left undisturbed. On sites where the surrounding vegetation or streambed is already fragile, that disturbance can add to the erosion problem the riprap was meant to solve.
Vegetation can grow in the gaps between rocks, but as roots spread, they can dislodge the rocks from their positions.
Introducing Bluemont Rock Bags™: A Stronger Alternative
Bluemont Rock Bags™, also known as Kyowa Filter Units, take a different approach to the same problem. Instead of relying on individual stones to hold their ground, they use a flexible netted structure that keeps the whole unit intact under load. It’s one of the best riprap alternatives for erosion control in Australia.
What Are Bluemont Rock Bags™?
Simply put, a Bluemont Rock Bag™ is a durable, netted bag with a large number of small rocks. It can either be rock, gravel or crushed material. The synthetic net holds the fill together as one connected mass rather than a scatter of loose stones, so a single unit behaves more like a flexible mat than a pile of rocks.
How Are They Better than Riprap?
Bluemont Rock Bags™ act as a barrier that holds the soil layer beneath in place. Water can’t pluck a single rock out because the netted bag holds the rock together with every other rock in it. So, there’s no dislodging unlike with riprap. The net also flexes around uneven or already-eroded ground. There’s minimal to no foundation work needed before the units go in, even underwater or on a bank that’s already scoured out.
How Strong Are They?
Bluemont Rock Bags™ are designed to withstand sustained hydraulic pressure without shifting or losing shape. In fact, a 2-tonne bag can resist flow velocities up to 4.7 metres per second. A 4-tonne bag resists up to 5.3 metres per second. Bluemont Rock Bags™ can also be layered, one on top of another, to raise the defence and handle sites with greater flow or wave loading.
What Are They Made Of?
The net is of recycled material, and the fill is typically rock or gravel sourced locally to the site rather than trucked in from a distant quarry. That local sourcing is also where most of the environmental difference between riprap and Bluemont Rock Bag™ shows up. Because there’s no need for quarrying and long-haul transport, Bluemont Rock Bags™ avoid the ground disturbance and freight emissions that riprap sourcing typically brings.
How Are They Installed?
With a standard production frame and an excavator, a Bluemont Rock Bag™ can be filled, prepared, and stockpiled in around 5 minutes. A crew of two is typically enough to fill and stage them, compared with the larger crews and machinery needed to place and shape loose riprap by hand. Because a crane or even a helicopter can lift them into place, Bluemont Rock Bags™ reach hard-to-reach banks, steep slopes, or tight sites beneath structures without machinery ever needing to work the ground itself.
How Long Do They Last?
Another reason why Bluemont Rock Bags™ are one of the best riprap alternatives is its longevity. They carry a lifespan of up to 50 years, with the net rated for around 30 years of direct UV exposure before replacement. Because the whole unit moves as one mass rather than stone by stone, maintenance after a flow event is typically a visual check rather than a re-stoning job.
Riprap vs Bluemont Rock Bags™: A Quick Comparison
| Riprap | Bluemont Rock Bags™ | |
| Material source | Quarried rock, trucked to site | Local rock, gravel or crushed fill, sourced on or off-site |
| Install method | Placed and graded rock by rock | Filled on site or off site, craned or excavated into place as complete units |
| Foundation prep | Often needs a shaped, prepared base | Fits uneven or already-eroded ground with little to no shaping required |
| What happens under flow | Individual stones can be dislodged; the layer can undermine at the toe. | Rock is held inside a net, so the whole unit moves as one mass |
| Typical maintenance | Re-grading and replacing displaced stone after storm events | Visual check after events; the unit itself doesn’t scatter |
| Published lifespan | Design-dependent, varies with sizing and site conditions | Up to 50 years, the mesh net is rated for around 30 years of UV exposure |
Sustainability and Habitat Support
As extreme weather becomes more frequent, erosion control has to work with the surrounding environment, not against it. Only then can it hold up long term.
Riprap has a mixed record here. Placing it can disturb the bank, seabed or riverbed it’s meant to protect. And once it’s down, the gaps between stones let vegetation take root. Small plants can help bind the soil. But as larger roots grow, they can force their way into those same gaps and push stones out of position. That’s the same issue covered under maintenance above. When individual stones work loose, riprap can push scour toward the edges of the treated area instead of holding it in. This exposes the bank to erosion.
Bluemont Rock Bags™ take a different approach on both counts. The mesh net is made from durable recycled material. It holds the rock fill together as one connected mass. So there’s no individual stone for water or vegetation to work loose. Plants can still root and grow in the spaces between a rock bag’s fill. Underwater, those same small spaces between the fill can provide habitat for marine life. You can fill the bags off site and lower them into place by crane or helicopter. The deployment itself disturbs less of the surrounding site than placing and grading riprap by hand.
Proven on Australian Bridges, Creeks and Coastlines
Bluemont Rock Bags™ have been used as riprap alternatives on a wide range of scour protection and erosion control projects across Australia.
In Gilbert River Bridge, Far North Queensland, strong, seasonally variable currents were washing sediment out from around the bridge piles on the Burke Developmental Road. Bags placed directly around the piles held the sediment against that current, and the same fix has since been used at other Queensland road and rail bridges, including Coondoo Creek, where piers have stayed stable under ongoing scour.
Bluemont Rock Bags™ are also used to protect Wamberal Beach in NSW Central Coast following a heavy storm and property damage. As the site needed an immediate measure, crews filled Bluemont Rock Bags™ in as little as three minutes each and placed them along the shoreline. A total of 2,120 tonnes of armour was used to build a defence long enough for a permanent solution to be designed.
At Clare Weir on the Burdekin River in North Queensland, where flood flows bring high sediment volumes downstream, routine inspections picked up early-stage scour at the weir’s toe. Using helicopters, the team installed two hundred rock bags in a single day, at the base of the weir where the current hits hardest, holding that ground together as one connected mass instead of loose stone that the river could pick apart.
When Riprap Still Makes Sense
Riprap is not a failed mechanism for erosion control. Correctly graded riprap can be a solid option where a quarry sits close to site. Also, it’s less likely to fail if executed well. Very large individual armour units, well beyond what a standard Bluemont Rock Bags™ carries, still have a place on high-energy coastal structures where sheer mass is what’s needed. An open riprap batter can also let vegetation establish through the gaps between individual stones over time.
Riprap’s performance depends heavily on correct sizing, a properly designed base layer, and freight economics that favour a nearby quarry.
Combining Riprap and Bluemont Rock Bags™
In some cases, erosion control works better when it combines solutions. Riprap can armour the main batter or slope, where mass and a natural finish matter. Bluemont Rock Bags™ handle the areas where scour risk runs highest and need a connected, netted mass to resist undermining.
A coastal rail project at Hundalee, on New Zealand’s South Island, shows this layered approach in practice. Wave action was eroding the foreshore directly beside an active rail corridor, close enough to threaten the track formation itself. The project used large basalt boulders for the lower layers to absorb direct wave impact, then stacked Bluemont Rock Bags™ on top to reinstate the section above a culvert headwall. The boulders absorbed the heaviest impact. The Bluemont Rock Bags™ above them reinforced the defence against long-term wave action.
Choosing the Right Protection for Your Site
If your site has quarried rock nearby, a stable bank profile, and tolerance for periodic re-stoning, correctly sized riprap remains a sound, well-understood option. If your site is remote, your timeline is tight, or your bank has already shown the stone displacement and toe scour that riprap is prone to, Bluemont Rock Bags™ solve the same problem with locally sourced fill and a structure that doesn’t rely on individual stones staying put.
Bluemont has supplied rock bags across Australian bridge, riverbank and coastal projects for 20 years, with local stock and local teams. To get product specifications or discuss a site-specific application, get in touch with us by calling +61 2 9091 0360 or emailing us at info@bluemont.com.au.
Sources:
- https://www.bom.gov.au/weather-and-climate/past-weather-and-climate/state-of-the-climate-2024/report-at-a-glance
- https://www.actuaries.asn.au/research-analysis/autumn-2025-sees-highest-ever-climate-index
- https://www.mdpi.com/2073-4441/13/24/3606
- https://austroads.gov.au/publications/road-design/agrd05b
- https://www.sciencedirect.com/science/article/pii/S2590123025012265