Loose rock has been a standard fix for scouring riverbanks or coastlines in New Zealand for decades. It’s cheap where a quarry sits nearby, and it’s simple to specify. What it doesn’t do well is stay put. A flood peak or a storm surge only has to shift a handful of stones before a gap opens at the toe. Once water finds that gap, it starts working under the structure the rock was meant to protect. A repair rarely stays small from there. Enter Bluemont Rock Bags™ as a riprap alternative.
These limitations matter more today than they once did. Extreme weather is intensifying across New Zealand, and its rivers have shown how fast that can happen. Cyclone Gabrielle hit Hawke’s Bay in February 2023, and a subsequent NIWA study found the Ūawa/Hikuwai river alone lost 220,000 cubic metres of sediment along just 9.1 kilometres of riverbank, an area equivalent to roughly 270 tennis courts. It’s the kind of event that can undo years of erosion protection in a matter of days.
Events like this demand protection that holds under storm-scale conditions, and that’s where Bluemont Rock Bags™ come in. They form a flexible, netted structure that resists displacement instead of scattering under load. This guide breaks riprap and Bluemont Rock Bags™ down side by side, including performance, cost, and sustainability, so you can weigh these riprap alternatives against your own site.
Why We Need Better Erosion Control Solutions
Regional councils already run ongoing bank stabilisation programmes due to the increasing scale and severity of events due to climate change.
The Cyclone Gabrielle data above is one storm on the East Coast, but it isn’t an isolated risk. New Zealand loses around 192 million tonnes of soil to erosion every year, and despite covering just 0.2% of the world’s land area, it contributes roughly 1.7% of all sediment reaching the world’s oceans, among the highest erosion rates for its size anywhere.
Across the wider Pacific, the same cyclone systems that pass over New Zealand do far more damage on the way through. UNESCAP projects that under continued global warming, Pacific small island developing states face economic losses of up to 9 percent of GDP a year. It’s more than double the average loss rate for the rest of the Asia-Pacific region.
None of this is limited to rural rivers. Bridge piles, port wharves and rail corridors take the same load, often in tighter, harder-to-reach spaces than an open riverbank.
Earthworks near a stream, riverbank or wetland can trigger resource consent conditions under the Resource Management Act, covering sediment control and habitat disturbance. Whether they do comes down to the scale of the work and the proximity thresholds set by the relevant regional council. It means the material and method chosen for erosion protection is a consent-relevant decision as much as a cost one. That combination, frequent, high-cost events and a regulatory process that scrutinises how protection is built, is exactly why the choice between riprap and an alternative like Bluemont Rock Bags™ is worth getting right before the next storm, not after it.
What Is Riprap?
Riprap is loose, graded rock placed over a filter layer to absorb the energy of moving water before it reaches the soil beneath. It relies on the weight and mutual interlock of individual stones to resist movement, not on any structure holding them together. Rock size is calculated for the site, based on the flow velocity or wave energy it has to withstand. Faster, heavier water needs bigger stone.
Waterway crossings on New Zealand’s road and rail network are assessed under the Waka Kotahi NZ Transport Agency Bridge Manual, which sets design requirements for bridge scour and waterway hydraulics. Riprap sized and placed to that standard has been used on New Zealand riverbanks, bridge abutments and coastlines for decades, and where correctly graded stone is available close to site, it remains a familiar, relatively affordable material.
Issues with Using Riprap for Erosion Control in New Zealand
Riprap’s problems tend to show up after it’s installed. Sourcing the right stone can be a headache, the layer doesn’t always hold up under load the way you’d expect, and once a section moves, fixing it gets expensive.
High cost
Correctly graded armour rock has to be quarried, sized and freighted to site. It involves a significant cost even before installation. Placing riprap to the design profile, by hand or machine, takes time and labour. That’s a cost a pre-assembled unit doesn’t carry, and it sits on top of the material cost itself.
Sourcing and site constraints
Not every quarry stocks rock in the size and durability class a given flow condition calls for, and a remote river crossing, a rural district council job, or a site off the main freight routes can face a long haul to find one that does. Installation is labour-intensive and needs the right machinery on site, which is a real constraint on a steep bank or a tight working platform beneath a structure. In addition, riprap is not an ideal option for erosion control on steep slopes.
How riprap fails under load
Well-designed riprap holds for years. The problem starts when one rock in the layer moves, whether it was undersized, hit by debris, or simply caught the full force of a flood peak. Once that happens, water gets underneath the adjacent stones, and the ground beneath begins washing out. Left alone, that undermining can spread and threaten the foundation of the structure.
Maintenance and repair costs
Routine upkeep after a storm event usually starts with checking for displaced stone. Any larger repair typically needs an excavator or loader back on site to reposition or replace lost material, rather than a quick patch.
Ecological Disruption
Deploying riprap involves clearing, grading and compacting ground that might otherwise stay undisturbed. There’s a lot of foundation work that goes into it. On an already-fragile site, that disturbance can feed the very erosion problem riprap is meant to fix. Once it’s down, plants can take root in the gaps between stones, which sounds like a positive, but as those roots grow and spread, they can prise individual rocks out of position.
Introducing Bluemont Rock Bags™: A Stronger Alternative
Bluemont Rock Bags™, also known as Kyowa Filter Units, address the same problem differently. Rather than counting on each stone to hold its own ground, the fill sits inside a flexible net that keeps the whole unit together under load. It’s one of the best riprap alternatives for erosion control in New Zealand and the Pacific Islands.
What Are Bluemont Rock Bags™?
A Bluemont Rock Bag™ is a durable, netted unit filled with rock, gravel or crushed material. The synthetic net holds the fill together as one connected mass rather than a scatter of loose stones, so the unit behaves more like a flexible mat than a pile of rock.
How Are They Better Than Riprap?
Bluemont Rock Bags™ act as a defence layer that holds the soil layer beneath in place. The netted bag holds every stone against the ones around it, so water can’t work a single stone loose the way it can with loose 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 built to withstand sustained hydraulic pressure without shifting or losing shape. While a 2-tonne bag can withstand flow velocities up to 4.7 metres per second, a 4-tonne bag resists up to 5.3 metres per second. The bags can also be stacked on top of one another where a site faces higher flow or wave loading, building a taller defence without changing the underlying mechanism.
What Are They Made Of?
The net is manufactured from recycled material, and the fill is typically rock, gravel or crushed materials sourced locally to the site rather than trucked in from a distant quarry. That local sourcing is where most of the environmental difference between riprap and Bluemont Rock Bags™ shows up. There’s no long-haul freight and no requirement for a single quarry stocking perfectly graded armour stone.
How Are They Installed?
With a standard production frame and an excavator, a team can fill, prepare, and stockpile a Bluemont Rock Bag™ 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?
Bluemont Rock Bags™ carry a stated service lifespan of up to 50 years. It’s another reason why they are one of the solid riprap alternatives to tackle erosion. Because the whole unit moves as one mass rather than stone by stone, maintenance after a flow event is typically a visual check. If one bag gets damaged, only that bag needs replacing, not the section either side of it.
Riprap vs. Bluemont Rock Bags™ at a Glance
| Riprap | Bluemont Rock Bags™ | |
| Material source | Quarried rock, freighted to site | Local rock, gravel or crushed fill, sourced near site |
| Install method | Placed and graded stone by stone | Filled on or off site, craned or excavated into place as complete units |
| Foundation prep | Often needs a shaped, prepared base | Conforms to uneven or already-eroded ground |
| What happens under flow | Individual stones can dislodge; the layer can undermine at the toe | Fill 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; a damaged unit is replaced individually |
| 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 across New Zealand and the Pacific, 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. It can disturb the bank, seabed or riverbed it’s deployed to protect. And once it’s down, the gaps between stones let vegetation take root. Small grasses can help bind the soil. But larger roots 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.
Durable recycled material makes up the mesh net, holding 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 shelter marine life. You can fill the bags off site and lower them into place by crane or helicopter, which suits the steep banks, weir faces and wharf toes common on New Zealand and Pacific Island sites. The deployment itself disturbs less of the surrounding site than placing and grading riprap by hand.
Deployed on New Zealand Weirs, Wharves and Waterways
Bluemont Rock Bags™ have been used as riprap alternatives on a wide range of scour protection and erosion control projects across New Zealand and Pacific Islands.
At Kapuni Weir, the base and flanking banks had been eroding under sustained water flow, with visible foundation damage before intervention. The team placed two-tonne Eco-Green Bluemont Rock Bags™ around the base and banks. They conformed to the weir’s irregular contours and held position under continuous flow, stabilising the eroded sections without a full civil rebuild.
At Lyttelton Port, bow thruster wash and propeller turbulence from vessels manoeuvring at low speed had begun eroding the wharf toe, a problem that scales with vessel size as ships calling at New Zealand ports have grown. The team in charge deployed Bluemont Rock Bags™ at the toe to absorb that repeated turbulence. The goal was to let the port accommodate larger vessels without the revetment failing at the point of greatest current impact.
Protecting the Pilbara rail bridge is one case among the many deployments of Bluemont Rock Bags™ in Australia. The road-rail bridge servicing mine sites there faced pile scour from episodic, high-intensity storm flow through normally dry channels. The crew placed four-tonne Bluemont Rock Bags™ around the piles as a preventive measure, chosen for their ability to conform to pile geometry. The deployment process suited the remote site with limited access.
When Riprap Still Makes Sense
Riprap is not a failed mechanism for erosion control, and it’s still a solid option under the right conditions. Where a quarry sits close to site, correctly graded riprap works, and installed properly, it’s less likely to fail. On high-energy coastal structures, individual armour units far larger than a standard Bluemont Rock Bags™ unit still have a place, since sheer mass is what those sites need. Given time, an open riprap batter also lets vegetation establish through the gaps between stones. What it comes down to is correct sizing, a properly built base layer, and a nearby quarry keeping freight costs down, all worth weighing carefully on the more remote stretches of New Zealand and Pacific Island coastline.
Combining Riprap and Bluemont Rock Bags™
On a lot of sites, the answer isn’t one material or the other. Some terrain conditions demand a combination of erosion control measures. Riprap can armour the open batter or slope where mass and a natural finish matter. Bluemont Rock Bags™ handle the pinch points where scour risk runs highest and a connected, netted mass is needed 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 beside an active rail corridor, close enough to threaten the track formation. The project used large basalt boulders for the lower layers to absorb direct wave impact, then stacked Bluemont Rock Bags™ above them to reinstate the section around a culvert headwall, with the boulders taking the heaviest impact lower down and the Bluemont Rock Bags™ reinforcing the defence against ongoing wave action further up the slope.
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 option. If your site is remote, your timeline is tight, or your bank or wharf toe has already shown the stone displacement and 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 operated in the erosion and scour protection industry for twenty years. We operate in Australia, New Zealand, and the Pacific. For specifications, sizing or a site-specific recommendation, contact Bluemont on +64 (9) 872 2622 or info@bluemont.nz.
Sources:
- https://www.unescap.org/blog/enabling-partnership-asian-and-pacific-small-island-developing-states
- https://niwa.co.nz/news/new-study-reveals-colossal-scale-riverbank-erosion-during-cyclone-gabrielle
- https://www.nzta.govt.nz/assets/resources/bridge-manual/docs-3rd-edition/Bridge-manual-pdf-new-pages-v3.4.pdf
- https://www.gw.govt.nz/your-region/emergency-and-hazard-management/flood-protection/our-work/rivers-and-streams/