We periodically review and update the high tide statistics for various locations around our coast. Record high tides have prompted this 2018 review and update of tidal statistics for Christchurch coastal areas.

The 2018 tidal data is captured in the report  Extreme sea levels at Christchurch sites [PDF, 191 KB].

Monitoring and recording tide levels

Tide levels have been recorded Ferrymead Bridge since 1974 and at the Styx tide gates (Brooklands) since 1990, Sumner Head since 1994, and on the Avon at Bridge Street since 1997.  Tide levels are also recorded at Lyttelton.

Historical trends in mean sea level rise

The Ministry for the Environment in their document, Coastal Hazards and Climate Change - Guidance for Local Government, note that records from the four main port gauges indicate “there has been a doubling of mean sea level rise from an average of 1mm/year earlier last century to nearly 2mm/year from 1961”.  The Ministry also note satellite estimates of mean sea level rise of 4.4mm/year around New Zealand in the period 1993 to 2015.

NASA have assessed the rate of sea level rise on a global scale based on 25 years of American and European satellite data. They note the rate of sea level rise has increased from about 2.5mm/year in the 1990s to about 3.4mm/year today.

Using the high tide statistics

Record high tides in 2017 and 2018 in Christchurch (as also seen around the country) have prompted the 2018 review and update of tidal statistics for Christchurch coastal areas. The change in the statistics is partly a result of the longer period of data that is now available and partly due to climate change and sea level rise over the period of record.

Using the statistics

The 2018 High Tide Statistics give us better information about flood risk in coastal areas, to help us make informed planning and infrastructure decisions.

The Council uses high tide statistics as an input into flood modelling to determine minimum floor levels for building, design levels for infrastructure in tidally affected areas, flood management areas in the Christchurch District Plan and other technical work such as coastal hazards.

Most properties in these tidally influenced areas will already have a standard comment about potential flooding and the risk of coastal inundation. Some LIMs will be updated to say the property is now considered to be at risk in a 1/50 year flood.

Review of the statistics

The 2018 analysis of the high tides was undertaken by Dr Derek Goring of Mulgor Consulting Ltd and has been peer reviewed by Charles Pearson of NIWA.

Environment Canterbury and NIWA have further reviewed the updated analysis of the high tides for Christchurch. Technical staff at Environment Canterbury concur with the methodology employed, agree with the updated information and support it being used to inform future planning considerations. NIWA also concluded that the analysis and review process were robust. NIWA noted the increase in the number of significant coastal flooding events around New Zealand, in the last five to ten years, and that they were updating the extreme sea levels (the high tides) for other districts.

Floor levels

The updated tide levels will be incorporated in the Avon, Heathcote and Styx River models to provide updated floor levels to protect new homes from flooding in tidally influenced areas.

The revised floor level information will be used for all new building consent applications in these areas. It will also be used in providing advice on resource consent applications in Flood Management Areas and High Flood Hazard Management Areas affected by tides.

The impact on floor levels

The impact on floor levels varies depending on the location within the city. For example, a floor level for a building consent near Bridge Street would be expected to rise around 150mm, and one near Brooklands Lagoon about 240mm when the new tidal data is taken into account.  The Council floor level viewer website will be updated using the new high tide statistics.

The table below shows planning level increases:

  High flood hazard management areas (1/500yr)
Flood management areas (1/200yr) Building Act hazard notice (1/100yr) Building Act floor level (1/50yr)
Ferrymead 390mm 290mm 260mm 230mm
Bridge Street 360mm 260mm 200mm 150mm
Brooklands 460mm 330mm 280mm 240mm

Proposed District Plan changes

The Council recently approved asking the Minister to make proposed changes to the District Plan through the Section 71 process in relation to the Residential Unit Overlay. This includes changes to a policy and rule in the District Plan to clarify assessment of resource consent applications, and broadening the scope of an existing permitted activity rule for fairness and equity purposes. The Residential Unit Overlay applies to parts of Southshore, South New Brighton, New Brighton and Redcliffs. These areas will be affected by the updated high tide statistics.

The updated tide information does not change the appropriateness of the provisions sought in the section 71 process. The new information will be relevant to floor level heights for permitted activities, and the assessment of resource consent applications, for all relevant provisions in the District Plan including for the new ones in the section 71 process.

If the section 71 proposal is accepted, the new tide data will inform the assessment of individual resource consent applications, as it would without the District Plan changes. 

Updating the District Plan

When the Council is able to update its District Plan, Flood Management Areas, Fixed Minimum Floor Level Overlay and High Flood Hazard Management Areas will be updated to reflect the new tidal information.