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.