The Council has commissioned several reports over a period of years to gain a strong understanding of coastal processes and hazards.

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.

The assessment found that the risk to life is low from the stopbank failing or being overtopped by water in 1 in 100-year annual recurrence interval event. The annual risk of one person dying as a result of a 1 in 100-year annual recurrence interval tide is less than 1 in 900,000.

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.

In 2019 Council commissioned a report from Jacobs New Zealand Ltd to understand the asset condition of the rock sea wall from Cave Rock to Scarborough.

This review was undertaken as part of our normal operational asset assessment and repairs process.

The report tasks included:

  • Reporting on the present condition of the rock revetment, in particular, its height, shape, slope and rock volume.
  • Identifying low or weak points and critical areas for rock replenishment or regrading.
  • Reporting on the optimal design slope, volume and size of the rocks of the wall, in its current form, as required to retain the functionality of the sea wall.
  • Providing recommendations to assist Council staff to enable the most effective maintenance practices to restore the effectiveness of the sea wall within its present form.

Findings of the report

The report found that no urgent or immediate works are required on the sea wall in its current condition, it continues to do the job of protecting the esplanade from erosion and inundation.

We know that, in large storms, seawater can overtop the revetment. However, the water is then contained by the setback wall which separates the promenade from the esplanade. The whole structure (revetment and set-back wall) provides two lines of defence which effectively manage existing sea-level conditions.

The report did find that the wall is below the optimal design for people to walk along the promenade safely in a 1 in 100-year storm. However, upgrading it would be a major project requiring significant investment to maintain access when an alternative walking route (the esplanade) is available during extreme weather. We can manage the risk to pedestrians during extreme storm events by restricting access – as we do with other Council assets in extreme weather (for example, the New Brighton Pier).

To assist Council with its maintenance priorities, the report identifies 19 sites along the wall where repairs are required to increase the durability of the existing structure:

  • Three sites with small armour rock size and steep slopes are identified as a priority for maintenance give the consequence of this combination for further rock displacement and damage within the revetment structure.
  • Five sites with small armour rock size and flat slopes are identified as having a slightly lower priority relative to the steep-sloped revetment sites, as they are likely to suffer less rock displacement in storm events.
  • Seven sites where armour rock size is an issue are identified as a third priority to be maintained so that sufficient size armour material is in place to reduce the likelihood of further rock displacement, which may result in slope issues.
  • Another four areas with slope issues are identified as a lower priority for maintenance as displacement of the sufficiently sized existing armour rock is less likely than for small rocks. For these sites, additional armour rock may still be required and existing armour rock repositioned to achieve optimum design slopes.

As there are no urgent or immediate works required, and we don’t currently have any funding allocated to this project, we’ll be using the findings of this report to help prepare a multi-year repairs strategy and work programme. 

The two 2018 reports on the southern Pegasus Bay coastal sand budget commissioned by the Council consider current and future sediment loads from the Waimakariri River and the potential impacts on the city shoreline and the Ihutai/Avon – Heathcote Estuary mouth.  

This information will be used in future coastal hazard assessments and the multi-hazard analysis work.

A peer review [PDF, 1.2 MB] of the reports supported the technical findings of the study.  The comments will be incorporated in the future update of the coastal hazard assessments, so the Part B report is labelled as an ‘interim final report’.

An updated coastal hazards assessment is currently being developed and is due later in 2021. This assessment will incorporate these latest technical findings. 

Since this report was prepared, the Council has new information on extreme water levels(external link) and sediment budgets which will result in changes to some of the locations at risk from flooding and erosion.

An updated coastal hazards assessment is currently being developed and due later in 2021. This assessment will incorporate these latest technical finding

The 2017 assessment provides detailed information on inundation (coastal flooding) and erosion for the main inhabited coastal parts of Christchurch and Banks Peninsula. Four climate change and sea-level rise scenarios have been considered out to 2065 and 2120.

The report is the result of a thorough and independent peer review of the 2015 Coastal Hazards Report. A peer review panel made recommendations for changes to the 2015 report and further investigations. These recommendations have been addressed in the 2017 report.

The report has been independently peer reviewed to ensure it met the panel’s recommendations.

The 2017 Coastal Hazards assessment [PDF, 8.1 MB] for Christchurch and Banks Peninsula is the most up-to-date information on coastal hazards across these areas. The report is a detailed technical document.

We've put together a summary of the report [PDF, 1.9 MB] to help you understand the key terms, the methodologies used to assess coastal inundation and erosion hazards, and an overview of the mapping information across our different coastal environments.

Please note a correction to Figure H-38in the report:  The line type (seawall) shown at the Bridge Street bridge is incorrect. The line shown at the bridge represents the practical ‘upstream’ extent of the coastal hazard inundation modelling where inundation from the sea is likely to dominate flooding derived from catchment flooding.

Further upstream flooding is a combination of Avon River catchment flooding and sea flooding.

You can view coastal erosion hazard maps in appendix H, and coastal inundation maps in Appendix I.

Appendix Title Description
A

Site location plan [PDF, 2.2 MB]

Coastal areas of Christchurch covered by the 2017 Coastal Hazard Assessment Report
B Beach profile output plots [PDF, 1.5 MB] Beach profiles for 26 locations from Waimairi Beach to Southshore from 1990 through to 2016
C SBEACH assessment [PDF, 126 KB] SBEACH is a model that defines storm-cut beach material volumes and the horizontal movement of the toe of dunes. Appendix C gives the SBEACH results, used in the coastal hazard erosion assessment.
D Storm cut matrix [PDF, 389 KB] Inter-survey storm cut distances since 1990 along the open coast (excluding Sumner) in southern Pegasus Bay.
E DSAS maps including beach profile locations [PDF, 17 MB] Digital Shoreline Analysis System (DSAS) is a model used to evaluate long-term shoreline trends.  Presents maps showing the rate of shoreline change at 10 metre intervals along the open coast, and the location of the beach profiles, along with shoreline positions from 1940 to 2010 with respect to the 2011 shoreline.
F Assessment of input parameters for long-term distribution [PDF, 276 KB] Detailed explanation of the methods used to determine the long-term rate of coastline movement.
G Site specific assessments of the Lyttelton and Akaroa Harbour environments [PDF, 3.3 MB]  Site inspection and aerial photograph information specific to four bays in Akaroa Harbour (Wainui, Duvauchelle, Takamatua and Akaroa) and four bays in Lyttelton Harbour (Alandale, Teddington, Charteris Bay and Purau).
H CEHZ result maps [PDF, 209 MB](external link) Coastal erosion hazard zone maps for the open coast and harbour coasts (the Avon-Heathcote Estuary and Brooklands Lagoon, and Banks Peninsula). Please note a correction to Figure H-38in the report:  The line type (seawall) shown at the Bridge Street bridge is incorrect.  The line shown at the bridge represents the practical ‘upstream’ extent of the coastal hazard inundation modelling where inundation from the sea is likely to dominate flooding derived from catchment flooding.  Further upstream flooding is a combination of Avon River catchment flooding and sea flooding.  Flood hazard information for the Avon River catchment flooding in South New Brighton and Southshore is available here
I CIHZ result maps [PDF, 106 MB](external link) Coastal inundation hazard zone maps for the open coast and harbour coasts (the Avon-Heathcote Estuary and Brooklands Lagoon, and Banks Peninsula).
J TUFLOW model description [PDF, 482 KB] Description of the TUFLOW model used to more accurately define potential coastal inundation in low-lying and wide flat areas.
K Open Coast CHEZ probabilistic model outputs [PDF, 772 KB] Full set of  the histogram and cumulative distribution function graphs from the probabilistic assessment output for each site along the open coast for the average sediment budget and reduced sediment budget scenarios

Presentation [PDF, 1.4 MB] from Council meeting on 9 November 2017 where the 2017 Report was received.