The stopbank between Pages Road and Bridge Street was built to last 20 years, allowing time to plan, design and build long-term stopbanks.
A risk assessment on the current stopbank was done in 2016, prior to approving its design. The risk assessment confirmed that the risk to life during its 20-year life was tolerable, and the stopbank would be designed to contain a tide with a 100-year annual recurrence interval (ARI) with 300mm freeboard.
As part of the earthquake legacy issues project, and in response to Council resolution CNCL/2019/00196(external link), engineering consultancy GHD was commissioned to complete an updated risk assessment of the stopbank from Pages Road to Bridge Street.
The assessment follows the ISO 31000-2018 risk management guidelines and uses the Australian National Committee on Large Dams (ANCOLD) Guidelines on Risk Assessment (2003). We do not have any New Zealand guidance on stopbank risk-to-life assessments, so while stopbanks are not dams, the ANCOLD methodology can be applied.
- Ōtākaro Avon River Stopbank Assessment - Pages Road to Bridge Street (summary report) [PDF, 1.9 MB]
- Ōtākaro Avon River Stopbank Assessment - Pages Road to Bridge Street (full report) [PDF, 14 MB]
The report also assessed the risk for events greater than the stopbank design level of a 100-year annual recurrence interval tide, all the way up to a very rare 50,000-year annual recurrence interval tide. The greatest risk to life for events greater than the current design level is overtopping, rather than stopbank failure. Overtopping is when water flows over the stopbanks, and the risk to life arises if the water flows into populated areas.
When all these events are put together, the assessment found that the societal risk to life goes beyond what ANCOLD guidelines suggest could be tolerable. This is not a surprising finding and could be the same for many stopbanks across the city, as there is always some risk when water levels are higher than the design level of the stopbanks. While the societal risk may be greater, the individual risk to life from all events is still lower (better) than the ANCOLD guideline.
The assessment looks at some of the key factors that impact the risk to life. These include loss-of-life rates, flood levels and floor levels. Some of the assumptions made during the study may be conservative and so over-estimate the risk because it gets increasingly difficult to have certainty when you are dealing with estimates, assumptions and projections. However, it is better to take a more cautious approach and over-estimate the risk, rather than underestimate it.
In summary, the assessment found that the risk is tolerable for the events that the stopbanks are designed for, but in events greater than the design level of service, the risk is greater than the guideline suggests is tolerable because the stopbanks would be overtopped. All stopbanks are designed to hold water back up to a point and do have a risk of overtopping in events that are much bigger than their design level.
Residual risk is managed through stopbank maintenance - we check the stopbanks every year and also after flooding or a significant seismic event. Together with the Civil Defence and Emergency Management Team, we've also put in place some measures should we ever need to evacuate the area. This includes a proposed evacuation area along with a pre-prepared emergency mobile alert.
Longer-term, we’ll be assessing adaptable flood mitigation options in the Ōtākaro Avon River corridor, as part of implementing the Regeneration Plan. However, implementing the Plan will likely take decades so the current stopbanks will need to be maintained and managed for years to come.