Bridge vs Culvert: What Is The Right Choice For You?

9 February 2021
Bridge vs Culvert: What Is The Right Choice For You?

Bridges and culverts both allow you to cross waterways unimpeded but their upfront cost, ongoing maintenance, environmental impact and long-term suitability can be vastly different from one another.

So how do you decide which is best?

A basic circular culvert is typically cheaper to build than a single span bridge but that’s not necessarily the case for a larger box culvert. To help you weigh up the pros and cons, we’ve put together this useful summary.

Culvert – PROS

  • Easy and fast to install
  • Can be purchased ‘off the shelf’
  • Low cost compared to other crossing structures
  • Suitable for crossing small to medium-sized rivers
  • Allows for dry passage of vehicles under normal flow conditions
  • Useful when fording a riverbed would generate a fine sediment
  • Can be made from a wide range of material e.g. plastic, concrete, galvanised steel or aluminium

Culvert – CONS

  • Can easily block with debris. Regular maintenance is needed to keep them clear. If a culvert blocks and causes upstream or downstream flooding, you could be personally liable for the damage.
  • Water can flow over the top of the crossing in flood events, forcing road to close
  • Can blow out during a flood, damaging the stream and the road
  • Significant fill could be required if the riverbanks are high
  • Unsuitable near dwellings or other infrastructure that could be damaged by a flood
  • Protection is required to prevent scouring/erosion of riverbanks (e.g. geofabric, rocks) which adds to the installation cost
  • Can impede fish passage
  • Stream diversion might be required during installation
  • Resource consent may be required to comply with the National Environmental Standards for Freshwater Regulations 2020
  • Native fish spawning seasons can restrict when, and how, culvert works are undertaken in the watercourse

Bridge – PROS

  • Ideal for rivers that have a large and variable water flow
  • Can cope with steep riverbed gradients and fast moving water
  • Allows full channel capacity and debris clearance in extreme flood events
  • Causes minimal disturbance so good for ecologically-sensitive riverbeds and banks
  • No barrier to fish passage
  • Low ongoing maintenance
  • Provides all weather access
  • Reduces erosion risk as it doesn’t change the channel capacity
  • Little or no modifications are required to riverbeds or banks

Bridge – CONS

  • Costs more to build upfront
  • Design benefits are not always required for low-flow waterways

On the surface culverts look like the cheaper option – but this is not always the case when you consider the risk of them blocking up or blowing out over time. Councils strongly prefer bridges rather than culverts because they’re a more reliable, environmentally-friendly option.

If your water flows are low, then a culvert could be all you need. Just remember to check what your anticipated water flows would be during a 1/20 year or 1/50 year flood event.

If your water flow is high some or all of the time, then a bridge is a much safer bet.

Is A Bridge Your Preferred Choice?

If ‘yes’, then the team at Bridge It NZ is ready to help guide you every step of the way. Get in touch today by phoning 0800 222 189 for more helpful information.