Culverts 101 – the Benefits for Your Property

I never really gave much thought to the culvert under our driveway until the winter of 2012, the first on our block. Ewan was working overseas, I was looking after the farm by myself and we’d had a massive amount of rainfall.

With hills on our northern and western boundaries, it seemed as if all the runoff was concentrated through our property.

The culvert couldn’t cope. It became blocked with debris and overflowed. The resulting stream began to cut into our unsealed road.

Haunted by the prospect of future potholes, I called our local plumber/drainlayer/man-of-all-trades. I spent two hours helping him dig out gravel and assorted vegetation because we’d allowed the culvert to be choked by weeds.

Never again, I swore, would I take our culvert for granted. But I did, because culverts are like septic tanks, car batteries and other conveniences of 21st century life: you only think about them when they don’t work.

If you’re considering installing a culvert on your block, here’s a beginners’ guide that might help:

Q: What is a culvert?

A: A culvert is a structure installed under a road, driveway or track that allows the flow of water. Culverts are short in length and open at both ends. Waterways wider than 3 m generally require bridges rather than culverts.

Q: What purpose do culverts serve?

A: Culverts allow the passage of traffic while protecting waterways and the life they support from damage and pollution. Many of New Zealand’s native aquatic life, including whitebait and eels, need unimpeded access from rivers and streams to the sea to complete their life cycles. Culverts, when properly installed, can support this.

Q: How many different types of culvert are there?

A: Culverts come in a variety of sizes and shapes, including round, flat-bottomed, elliptical and square. They can be made from concrete, aluminium, galvanised steel or plastic.

RX Plastics stocks both PVC-U (lead-free) and polyethylene pipes suitable for culverts.

Q: What do I need to consider when installing a culvert?

A: You should think about these factors:

  1. Average rainfall in your area, including storm flow from a one-in-five-year event
  2. Topography (i.e. steep hill gradients)
  3. The type and degree of debris likely to be carried in the waterway
  4. The size of the culvert and whether this is suitable for the water volume it will have to carry. Smaller culverts are more prone to blockage and bank erosion.
  5. The best crossing location, e.g. the narrowest point of a stream
  6. Whether other properties are at risk of flooding if the culvert overtops
  7. How much traffic load the culvert will have to support (including stock trucks, etc.)
  8. Possible environmental impacts

If you’re not sure about anything, consult your local council and/or a qualified engineer.

Q: What causes culverts to fail?

A: The three main causes of culvert failure are:

  • Corrosion of the material they’re made from, e.g. soil, sand and gravel can corrode the galvanising of a steel culvert
  • Erosion of the soil on supporting banks
  • Flood damage. Plastic pipes are sometimes inserted as liners to support corroding steel or failing concrete structures, with the gaps between them filled with grout.

Q: Will I need a resource consent to install a culvert?

A: Probably. Different local bodies in New Zealand have different regulations concerning culverts, so check with your local regional council.

Q: How can I find out more?

A: The Ministry for the Environment has a useful set of guidelines on its website. These include culvert size, materials, construction and rainfall bands across New Zealand.