Erosion is one of the most common causes of structural failure in critical infrastructure and coastal assets, and regular site inspection is what catches it before it compromises structural integrity. But knowing erosion is a risk isn’t the same as knowing how to identify erosion risk on a given site.
A site assessment checklist closes that gap, turning general awareness into a specific, repeatable list you can run through on your next walkover, whether the goal is preventing water erosion on a bridge, riverbank, weir or wharf.
This guide sets out that checklist including the signs of scour, undercutting, sediment loss and ground movement to check for, how often to inspect, and when to bring in an engineer. It also covers how and where Bluemont Rock Bags™ fit into reducing that risk.
What Drives Erosion Risk On New Zealand Sites
New Zealand loses an estimated 192 million tonnes of soil to erosion every year, with 44 percent of that coming off pastoral land according to the Ministry for the Environment’s Environment Aotearoa 2019 report. That’s the background rate. Storm events accelerate it sharply.
New Zealand’s erosion risk isn’t generic. It’s shaped by geology, rainfall intensity and a coastline that behaves differently region to region, which is why NIWA and the Ministry for the Environment maintain New Zealand-specific guidance rather than importing overseas standards wholesale.
Gisborne and the West Coast carry the country’s highest natural erosion rates, the former from naturally erodible slopes worsened by historical forest clearance, the latter from steep terrain combined with high rainfall, per Manaaki Whenua’s soil monitoring. Coastal sites face a second, slower-moving driver: the Ministry for the Environment’s 2024 Coastal Hazards and Climate Change Guidance, built on updated NZ SeaRise projections, now factors in relative sea-level rise that varies by location once local land movement is accounted for, alongside a national average rise of roughly 3.5mm a year.
Why Early Assessment Is Important
In April 2024, a pier on a rail bridge over the Rangitata River gave way after floodwaters scoured out the riverbed material supporting it, and the Transport Accident Investigation Commission’s report into the collapse, released in 2026, pointed to a lesson that goes well beyond that one bridge: knowing an asset carries scour risk isn’t the same as having a system in place to track how that risk is changing.
The financial argument for early intervention is simple. A site assessed and treated before failure is a maintenance line item. A site assessed after failure is emergency works, and emergency works in New Zealand usually come with urgency premiums, temporary traffic management, and environmental consent conditions triggered by uncontrolled sediment discharge. In the worst cases, it’s a structure or an asset that has to be rebuilt rather than repaired. A checklist that catches a problem six months earlier changes which column the cost lands in.
The Site Assessment Checklist: What to Look For
The erosion control checklist below covers four categories of warning signs: scour, undercutting, sediment loss and turbidity, and vegetation or ground movement, along with how often to check and when a walkover on its own isn’t enough.
None of it requires specialist equipment or training. It requires knowing what to look for, and walking the site at the right time to see it.
When and how often to check
A single inspection only tells you the state of a site on one day. Erosion risk moves with the seasons and with individual storm events, so timing matters as much as knowing what to look for when it comes to preventing water erosion.
Routine walkover
Check most sites twice a year, heading into the wet season and again coming out of it. This picks up gradual change before the next storm puts it under pressure.
After significant rain or high flow
Inspect within a few days. Evidence of scour or bank loss fades quickly once the next rain event moves through.
Ahead of a consent or capital works decision
Carry out a fresh assessment as a standard practice before applying for a resource consent or setting a capital works budget, since decisions need to reflect current site conditions rather than an outdated inspection.
After anything changes upstream
New development, a realigned channel, or a structure built nearby can redirect flow onto ground that was previously stable. That’s worth checking outside the normal schedule.
There’s no single cadence that suits every site. A wharf facing regular vessel traffic and a hill country paddock don’t erode on the same clock, so treat the above as a starting point rather than a replacement for judgement on the specific asset.
Signs of scour
Scour is localised removal of bed or bank material by concentrated flow, most common at structures that disrupt flow: bridge piers, culvert outlets, wharf faces, weir toes.
- Exposed footings, piles or foundations that were previously covered by bed material
- Depressions or holes in the streambed or seabed immediately downstream of a structure, or on the upstream and downstream faces of a wharf where vessel thrusters operate
- Undermined riprap or rock armour, where individual rocks have shifted or rolled away from their original position
- Turbulence or a visible drop in bed level at pinch points, culvert mouths and weir aprons
Signs of undercutting
Undercutting happens below the waterline or below the visible bank face, so it’s often missed until the bank above it collapses.
- An overhanging bank edge, sometimes with vegetation still rooted in soil that has lost its underlying support
- Slumping or a stepped bank profile, where a section has dropped relative to the ground above and below it
- Cracks running parallel to the bank line, a few hundred millimetres to a few metres back from the edge
- Recently exposed tree roots along a bank that previously showed none
Signs of sediment loss and turbidity
Sediment on the move is erosion in progress somewhere upstream, even if the site in front of you looks stable.
- Persistent turbidity in a waterway after flow has otherwise settled following rain
- Sediment fans or deposits at the base of slopes, culvert outlets or drain junctions
- Fine sediment build-up smothering gravel beds, which also signals a water quality issue under the Resource Management Act
- A channel that has visibly widened, straightened or shifted position compared to earlier site records or aerial imagery
Vegetation and ground movement
Vegetation is a slow-moving indicator, which makes it useful for picking up erosion that hasn’t yet produced a dramatic failure.
- Tilting or leaning fence posts, power poles or trees along a bank or slope, all leaning the same direction
- Bare, scalped patches on pastoral hill country, particularly on north-facing or steeper slopes after heavy rain
- New rill or gully channels forming across paddocks or embankments, even shallow ones
- A track, driveway or access road edge that has narrowed or lost material along one side
When to escalate to a formal assessment
A checklist tells you where to look closer, not whether to act. Consult a geotechnical or coastal engineer, or your regional council’s natural hazards team, when you see active undercutting near a structure carrying load, such as a bridge, building or piece of critical infrastructure, when a bank or slope has moved measurably since the last inspection, or when a site sits inside a mapped hazard zone under your regional or district plan. At that point, a formal assessment is what determines whether the site needs monitoring, remedial work or a change in use.
Quick-reference checklist
Print or save this section for a site walkover.
- Exposed footings, piles or foundations previously covered by bed material
- Holes or depressions in the bed immediately downstream of a structure
- Rocks shifted or rolled out of an existing riprap or rock armour layer
- Overhanging or undercut bank edge
- Slumping, stepped bank profile, or cracks running parallel to the bank
- Recently exposed tree roots along a bank
- Persistent turbidity after flow has otherwise settled
- Sediment fans or deposits at slope bases, culvert outlets or drain junctions
- Leaning fence posts, poles or trees along a bank or slope, all in one direction
- Bare or scalped patches on sloped pastoral land
- New rill or gully channels, even shallow ones
- Narrowed or lost material along a track, driveway or access road edge
Any single item on its own may be minor. Two or more on the same site, or one item worsening between visits, is the signal to escalate.
Preventing Water Erosion at an Early Stage: Where Bluemont Rock Bags™ Fit
Once your erosion control checklist flags active scour, undercutting or bank loss, the next question is which water erosion control method stops it without requiring a full civil rebuild.
This is where Bluemont Rock Bags™ come in. Also known as Kyowa filter units, they are a flexible rock-filled containment system that acts as a barrier, holding the ground beneath it in place under sustained flow. In other words, a bag of rocks to the rescue!
Each Bluemont Rock Bag™ is a heavy-duty netted bag filled with small rock sourced at the site or wherever available.
They can be placed using a crane directly against the eroding surface, whether that’s a riverbank, a weir toe, a bridge pier base or a wharf face. Because the bag conforms to the ground rather than requiring a shaped, engineered surface underneath it, it can be installed on irregular contours where poured concrete or interlocking block systems would need significant site preparation first. The netting holds the rock fill in place as a single unit under wave and current load, which is the specific failure mode that loose riprap doesn’t solve on its own.
That matters for the erosion signs covered above. Undercutting stops once the toe of a bank is armoured against further material loss. Scour at a structure reduces once the concentrated flow around it no longer has loose bed material to remove. Bluemont Rock Bags™ come in different sizes to suit different flow conditions.
They don’t demand heavy formwork, and can be installed by a small crew, which keeps them relevant for both a council-funded emergency stabilisation and a planned asset protection programme.
Proven on New Zealand sites
Bluemont Rock Bags™ have been deployed across a range of applications to control and prevent water erosion, from weir structures to port infrastructure. The cases below are examples of erosion caught early and addressed before it compromised the asset.
Kapuni Weir had been showing clear erosion around its base and the flanking riverbanks, with steady water flow gradually wearing away the structure’s foundation and the banks beside it. Left unaddressed, the risk was undermining the weir itself. Bluemont Rock Bags™, in the 2-tonne Eco-Green format, were installed to conform to the weir’s irregular contours and hold position under continuous flow. Bluemont Rock Bags™ stabilised the eroded sections, protected the toe and flanks from further scour, and did it without significant civil construction.
At New Zealand’s Lyttelton Port, bow thruster turbulence from vessels manoeuvring at low speed was eroding the wharf face and undermining the revetment, a common problem as vessel sizes and thruster power have increased. The continuous currents directed at the shore during berthing were disrupting the wharf’s foundation, the kind of toe scour that, left unmanaged, drives up renovation costs over time. Bluemont Rock Bags™ were deployed against the wharf face to absorb that turbulence without the extended installation time a poured or precast solution would need, allowing the port to keep accommodating larger vessels without an open-ended scour problem building underneath the berth.
Preventing Water Erosion on Your Site: Next Steps
An erosion control checklist is only useful if it turns into action before the next storm event, not after it. If your walkover has flagged active scour, undercutting, or bank movement, get it in front of an engineer and start scoping a fix while it’s still a maintenance job rather than an emergency one.
Bluemont supplies and installs Kyowa filter units across New Zealand and the Pacific, backed by twenty years of erosion control experience in the wider industry. Talk to our team about site conditions, sizing, and deployment timelines, or go through the specifications and application notes directly. Get in touch with us by calling +64 (9) 872 2622 or emailing us at info@bluemont.nz
Sources:
- https://soils.landcareresearch.co.nz/topics/soil-quality/state-of-nz-soils
- https://niwa.co.nz/hazards/sea-level-rise
- https://www.legislation.govt.nz/act/public/1991/69/en/latest/#DLM230272
- https://environment.govt.nz/assets/publications/freshwater-policy/Sediment-ME1663-Final-1.9.pdf