A freshly cut terrace is highly vulnerable to erosion — bare, disturbed soil has no root structure holding it together, and NZ's heavy rainfall events can wash away weeks of earthworks in a single storm. The good news is that with the right combination of drainage, erosion control, and vegetation, you can protect your terrace quickly and effectively.
Why Newly Cut Terraces Erode So Easily
When an earthmover cuts into a slope to create a terrace, the exposed soil loses the natural root network that previously held it in place. Disturbed subsoil is often compacted and low in organic matter, meaning water runs off rather than soaking in. NZ's variable climate — particularly in regions like Northland, the West Coast, and Hawke's Bay — means heavy downpours can arrive before any vegetation has had a chance to establish. Even in drier areas, a single intense summer thunderstorm can cause significant damage.
Step 1: Get Drainage Right First
Poor drainage is the number-one cause of terrace failure. Before anything else, plan where water will go.
- Cut a swale or catch drain along the uphill edge of the terrace to intercept water running down from above. This is a shallow, gently graded channel that directs water to a safe outlet.
- Install subsoil drainage (ag-pipe wrapped in geotextile fabric) if the cut face is in clay-heavy soil that holds water. Saturated soil is far more likely to slip.
- Direct all runoff to a stable outlet — a vegetated area, a stormwater drain, or a soak pit — never let it discharge directly onto the cut face or fill slope.
- Ensure the terrace surface itself has a slight fall (around 1–2%) away from any retaining structure so water doesn't pond against it.
Check with your local council about stormwater discharge rules, as directing runoff onto a neighbour's property or into a waterway may require consent.
Step 2: Stabilise the Cut Face Immediately
The cut face (the vertical or near-vertical slope behind the terrace) is the most exposed area. Act quickly:
- Hydroseed or oversow with a fast-germinating erosion-control mix. Ryegrass and clover establish quickly and are widely available from farm supply stores. Some contractors offer hydroseeding, which sprays seed, fertiliser, and a tackifier in one pass.
- Lay erosion control matting (also called geojute or coir matting) over the cut face and peg it firmly. This biodegradable mesh holds seed in place and protects the surface while plants establish. It's available from Mitre 10 and Bunnings.
- Avoid leaving bare soil over winter if at all possible — this is when NZ's highest rainfall and lowest evaporation rates combine to cause the most erosion damage.
Step 3: Install Appropriate Retaining Structures
For terraces with a significant height difference, a retaining wall is often the most reliable long-term solution. Common options include:
| Structure | Best for | Notes |
|---|---|---|
| Timber sleeper wall | Low-to-medium height (up to ~1 m) | Needs drainage behind it; treated timber only |
| Concrete block wall | Medium-to-high retention | Durable; may need engineering for walls over 1.5 m |
| Gabion baskets | Sloped or irregular sites | Good drainage; suits rural settings |
| Planted rock riprap | Gentle to moderate slopes | Natural look; combines structure with vegetation |
Important: Under the NZ Building Code and most council rules, retaining walls over 1.5 metres in height (or in some cases lower, depending on surcharge and proximity to boundaries) require a building consent — but the exact threshold can vary, so check with your local council. Always check with your local council before building. Schedule 1 of the Building Act lists some exemptions for low retaining walls, but the rules vary — confirm before you start.
Step 4: Establish Permanent Vegetation
Vegetation is your best long-term defence. Roots bind soil particles together and intercept rainfall before it hits bare ground.
- Plant deep-rooted groundcovers such as native grasses (e.g., Carex species), flax (Phormium tenax), or prostrate coprosma on the cut face and terrace edges.
- On steeper faces, native plantings are often more effective than exotic grasses because their root systems are better suited to NZ soil types and they require less ongoing maintenance.
- Water new plantings through any dry spells in the first season — stressed plants won't establish the root depth you need.
- Avoid heavy foot traffic on newly seeded areas until plants are well established.
BRANZ provides useful guidance on slope stability and ground cover in the NZ context.
Step 5: Ongoing Maintenance
- Inspect the terrace and drainage channels after every significant rainfall event, especially in the first year.
- Clear any debris from swales and outlets so water can flow freely.
- Re-sow any bare patches promptly — small bare spots can quickly become large gullies.
- Check that any retaining structure hasn't moved or cracked, and that drainage behind it is still functioning.
When to Call a Professional
Some situations go beyond DIY and need expert input:
- Terraces over 1.5 m high or on unstable soils (e.g., expansive clays, fill material, or areas with a history of slipping) should be assessed by a geotechnical engineer before any earthworks proceed.
- Retaining walls requiring building consent must be designed and built to meet the NZ Building Code — an LBP (Licensed Building Practitioner) is required for restricted building work.
- If you're unsure about the scale of earthworks needed, a qualified earthmover or civil contractor can assess the site and recommend the right approach. You can find experienced local earthmoving contractors on tradedirectory.co.nz.
- Any work near public stormwater or drainage infrastructure may require council approval and should involve a licensed drainlayer — check with the Plumbers Gasfitters & Drainlayers Board for licensing requirements.
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