Stopbanks, vital for flood defense, safeguard lives and infrastructure. Collective responsibility ensures their effectiveness in averting major floods.
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Stopbanks
Stopbanks are specially designed and maintained mounds of earth that help to limit or prevent the spread of floodwater onto surrounding land. Stopbanks usually include grass cover, smooth surfaces, a consistent height and no bare soil, holes or ruts.
Most stopbanks in Waitaha/Canterbury are owned and maintained by us. These stopbanks may be on private or public land. Some individual landowners may have constructed their own stopbanks or other flood protection measures, however, these do not guarantee protection for wider areas of land.
Flood protection along rivers
Stopbanks are found beside rivers and streams that are at risk of flooding nearby land during heavy or prolonged rainfall.
Some rivers have a single line of stopbank; others, such as the Waimakariri River, have a second line of stopbank that acts as a backup if the first line of defence fails or is overtopped.
While some stopbanks are put to the test regularly, others only prove their true value over time, sometimes up to 50 years or more.
You can find the locations of stopbanks and other flood protection infrastructure on Canterbury Maps.
A stopbank under construction in West Melton
Effectiveness of stopbanks
Where a major river, such as the Waimakariri, is close to a city the size of Christchurch, stopbanks are designed to protect the community from flooding. As such, they need to be well constructed and maintained.
A stopbank is only as strong as the weakest link and damage can result in the bank failing during a flood.
This is primarily due to damage causing weakness and allowing water to seep into the structure, causing it to wash out. Common damage that can weaken stopbanks includes:
- Gouges or ruts: caused by 4WD or recreational vehicles
- Damage to grass cover: from animals grazing or trampling the surface
- Excavations: for buildings or fences
- Planting: unwanted trees or shrubs
- Construction: the building of other structures.
Damage to stopbanks
Where a major river, such as the Waimakariri, is close to a city the size of Christchurch, stopbanks are designed to protect the community from flooding. As such, they need to be well constructed and maintained.
The structural integrity of a stopbank is crucial to its ability to protect communities from flooding. A stopbank is only as strong as the weakest link and damage can result in the bank failing during a flood.
This is primarily due to damage causing weakness and allowing water to seep into the structure, causing it to wash out. Common damage that can weaken stopbanks includes:
- Gouges or ruts: caused by 4WD or recreational vehicles
- Damage to grass cover: from animals grazing or trampling the surface
- Excavations: for buildings or fences
- Planting: unwanted trees or shrubs
- Construction: the building of other structures.
If you or your organisation damage a stopbank, you risk people's lives and livelihoods in the event of a flood. You may be liable for fines of up to $20,000, three years' imprisonment, or both.
If you see a damaged stopbank, please report it to us as soon as possible. You can call our advisory team on 0800 324 636 or report an environmental incident via Snap Send Solve.
Regulating the protection of stopbanks
Stopbanks are protected by the Flood Protection and Drainage Bylaw. This bylaw helps us manage, regulate and protect these assets from inappropriate modification, damage or destruction, and enables maintenance work.
Other flood protection infrastructure also protected by the Bylaw includes rock work, flood protection trees, floodgates, groynes and drainage networks.
Activites around stopbanks
The Bylaw regulates activities within 7.5m of stopbanks or 20 metres at specific mapped parts of the stopbanks on Waimakariri River and Ashley River/Rakahuri.
Under the Bylaw, stopbanks are classed as ‘Defences Against Water’ (Section 5.3) and the following activities are regulated:
- Altering, damaging, or otherwise interfering
- Damaging or allowing damage to occur
- Allowing stock to damage or overgraze
- Planting any shrubs, trees and hedges
- Dumping or depositing any material
- Adjusting or interfering with equipment associated with the stopbank
- Constructing or locating structures
- Carrying out earthworks
- Constructing crossings or roads/tracks in, over, through, along or under
- Damaging any gates, signs, or fences.
If you need to do any of the above activities, you will need to apply for a Bylaw authority from Environment Canterbury. The approval process is free and we are available to help you with your application.
Find out more about the Flood Protection and Drainage Bylaw, view maps and apply for bylaw authorities and/or advice on our managed waterways.
Effectiveness of stopbanks
Stopbanks are an effective way to prevent small to moderate flood flows from breaking out but they are only built to a certain design capacity.
Historically, our reliance on stopbanks has encouraged a higher level of development on floodplains than what might have occurred had the structural protection not been put in place.
So although stopbanks might reduce the frequency of flooding in the short-term, in the long-term they may actually increase the flood hazard posed to many communities.
Stopbanks also rely on the stability of adjacent berms and a clear fairway to perform as designed
With climate change increasing the number and frequency of severe weather events, maintaining and improving flood protection is becoming increasingly expensive and complex and the design capacity of stopbanks will continue to be pushed to their limit.
As part of our suite of flood protection and climate resilience work, we need to look holistically at rivers ki uta ki tai and how we can protect/enhance their natural features to complement existing flood protection.
Our Braided River Revival / Whakahaumanu Ngā Awa ā Pākihi Programme does just that. It signals a shift from river management focused solely on flood protection, to one that takes a holistic approach to our rivers to value, protect and restore them now and into the future.
Read more about Whakahaumanu Ngā Awa ā Pākihi Programme here.