Tidal data

The 2021 report uses an advanced internationally recognised methodology for calculating the probability of high tide and rain events occurring – resulting in a high degree of accuracy for modelling flood risks.

The 2021 report also provides new high tide statistics. The methodology and results have been externally peer-reviewed by the external Multi Hazards Peer Review Panel.

Main points:

• 2021 statistics predict a slightly higher water level for most high tide events. The Pūharakekenui/Styx River flood statistics show slightly lower water levels for most less-frequent flood events.
• The maximum increase in high tide levels is 84mm for a one-in-ten-year flood event at Ferrymead Bridge.
• The differences in high tide levels in the 2021 study compared to the 2018 study are within the margins used for determining floor levels (400mm) as well as for designing our (temporary) stopbanks and bunds (300mm).
• Changes are largely a result of two main factors:
  • The first is the use of a different methodology for determining extreme events compared to the methodology used in 2018. The new methodology allows for multiple events happening each year to be taken into account.
  • The second factor relates to sea-level rise. Historic average sea levels are lower than those of the present day. As a result, the water levels related to historic events are also slightly lower than the level reached, if the same event would happen today. This change has been accounted for in the new high tide statistics.
• As a result of this new study, we have a better understanding of the dependencies between rain events and high tide events. This allows us to more accurately predict water levels for specific extreme events.
• As a result of the new high tide data, whenever a LIM is requested for some properties they will be updated with the new information to say they're now considered to be at risk in a one-in-50 year flood. The number of properties likely to gain a new LIM comment is expected to be very small. That is because most of these properties will already have a comment about potential flooding and the risk of coastal inundation.

Read more on Newsline(external link).

We continually monitor tide levels and use this information in our statistical analysis for high tides. We also look at weather patterns and sea-level rise.

We’ve been recording tide levels at 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.

Successive highest tides have been recorded over the years, culminating in the July 2017 and February 2018 high tides, which were about 300mm higher than the tides recorded in 1974.

Since 2018 we have not seen abnormally high tides in Christchurch.

The 2021 High Tide Statistics report updates the 2018 report.

We use high tide statistics data (including a statistical assessment of the correlation between high tide and rain events) 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.

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 and/or joint high tides and rainfall events.

Read the 2021 joint risks of pluvial and tidal flooding report (including the new statistics).

Using the statistics

The 2021 high tide statistics give us better information about flood risk in coastal areas and areas influenced by tides, to help us make informed planning and infrastructure decisions.

The Council uses high tide statistics data (including a statistical assessment of the correlation between high tide and rain events) 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.

As a result of the new high tide data, whenever a LIM is requested for some properties they will be updated with the new information to say they're now considered to be at risk in a one-in-50 year flood. Most properties in these tidally affected areas will already have a comment about potential flooding, and the risk of coastal inundation. It is expected that very few properties could gain a new LIM comment.

Review of the statistics

The 2021 high tides statistical assessment was carried out by GHD/HKV – a joint Christchurch-based consultancy and a Netherlands-based consultancy.

The work has been reviewed by the Council’s stormwater planning staff and has also been reviewed by the external Multi Hazards Peer Review Panel.

The updated high tide statistics data will be incorporated into the Pūharakekenui/Styx (Brooklands), Ōpāwaho/Heathcote and Ōtākaro/Avon River models to provide the design flood levels in tidally influenced areas. This is expected to take several months to complete.

The revised floor level information will be used for all new building consent applications. In flood management areas it will also be used in providing advice on resource consent applications in High Flood Hazard Management Areas and other areas affected by flooding.

The impact on floor levels

Our floor level viewer(external link) will be updated with the new information. However, a large amount of information in the floor level viewer is based on model results and it will take several months to update the models and information.

The table below shows high tide statistics that are used for planning purposes:

Climate change is the biggest challenge of our time. It is already affecting our weather, health and wellbeing, natural environment, taonga species, mahinga kai, food production, biosecurity, infrastructure, and the economy.

Sea level rise is one of the challenges we face with ongoing climate change. Scientists predict a 30cm sea level rise by 2050, with a 50cm rise by 2075 and 1m by 2115, if global greenhouse gas emissions continue at the current rate.

This will have huge impacts on our district’s low-lying areas, cause significant drainage issues and place a major strain on our infrastructure and communities.

The updated high tide statistics have been derived from data where the effects of sea-level rise have been removed as best as possible. This means the statistics should more accurately show the actual natural variability in the tide and, therefore, provide more stable statistics for the future.

We need to do more

As a district, we need to reduce our carbon emissions and do what we can to mitigate the effects of climate change. We also need to prepare for the changes that are ahead of us and respond to the social, environmental and economic effects of our changing climate.

We’ve created our draft Ōtautahi Christchurch Climate Change Strategy as a blueprint for collective action and part of a wider conversation about how we work together to reduce our greenhouse gas emissions to minimise future harm and plan for the ongoing effects of climate change.

Read the full draft Ōtautahi Christchurch Climate Change Strategy [PDF, 604 KB].