Floodplain management

Flooding issues

Along with many other parts of New Zealand, Canterbury has developed on river floodplains.

These locations are desirable due to the availability of flat land, fertile soils, proximity to water for irrigation and for recreation, transport and access. While there are benefits to living in these areas the result is that many of Canterbury’s communities have settled in areas exposed to flood risk.

The flat nature of the Canterbury plains means any floods are able to spread over comparatively larger areas than in many other New Zealand regions.

Widespread but typically shallow flooding enables us, Canterbury district councils and other organisations to use a range of flood mitigation measures to help sustainably manage our floodplains.

Structural flood protection

Historically we have relied on structural protection works, such as stopbanks, to protect floodplain communities.

Structural protection is an effective means of reducing the frequency of flood overflows. Generally, they do this by preventing small to moderate flood events from breaking onto the floodplain from a defined river channel area.

However, construction costs, the availability of space, the unpredictable nature of Canterbury’s braided rivers, limited hydrological records and other factors mean that structural works are only constructed to a certain design capacity.

Historic reliance on structural protection works has created a false sense of security and encouraged a much higher level of development on floodplains than otherwise would have occurred had the structural protection not been put in place.

A high river flow is not in itself a flood hazard. It is the interaction between the high flow and human development that creates the hazard.

Therefore, the magnitude of a flood disaster is related to both the amount of water that breaks out of a river and the value of assets located in the way of that water, or the number of people exposed.

Inevitably structural protection measures are overwhelmed by greater than design flood events but also smaller floods due to breaching.

Breaches in stopbanks can commonly occur due to lateral erosion or internal erosion of the stopbank's core – a process known as piping. The risk of erosion is particularly high in Canterbury due to the braided nature of our rivers.

In the absence of other flood mitigation measures, structural protection works might reduce the frequency of flooding in the short-term but in the long term may actually increase the flood hazard posed to many communities.

The question for floodplain management is not "will we flood or how do we stop flooding?". It is more the recognition that we will be flooded one day so how can we reduce the resulting effects and losses?

Measures that keep floodwaters away

The emphasis now is to find a balance between measures that keep floodwaters away from people and those that keep people away from floodwaters.

We carry out or advocate for a range of measures as part of a long-term strategy to reduce the effects and costs of future flooding. The goal is to achieve sustainable development of Canterbury’s floodplains without imposing unacceptable limitations or costs on future generations.

Measures to modify the flood event:

• Maintenance and improvement of existing stopbanks and other structural protection works.
• Vegetation clearance from the beds and main channels of many of Canterbury’s rivers.
• Planting vegetation in berms to protect the stopbanks from erosion.
• Targeted removal of gravel to maintain channel capacity.

Measures to modify the damage or loss caused by a flood event:

• Advocating for land-use controls to avoid creating new developments in areas prone to a higher risk of river or coastal flooding.
• Advocating for land-use controls that ensure new development does not occur within 100m of an existing stopbank. This is to ensure development is clear of the area over which high velocity, debris-laden floodwater may burst-out if a stopbank is breached or overtopped.
• Advocating for floor level controls in areas prone to a lesser risk of flooding i.e. ensuring new buildings in floodplain areas are built to an appropriate level that minimises the potential for flood damages.
• Carrying out floodplain investigations to determine the extent and depth of flooding as a result of breakouts from our rivers. For more information, see floodplain management strategies.
• Installing and maintaining systems for flood warnings to key emergency response authorities and infrastructure managers.
• Developing flood forecasting systems.
• Public education and awareness initiatives.
• You can also request a flood hazard assessment.