The Council commissioned engineering and environmental consultancy firm Tonkin and Taylor Ltd. to carry out a coastal hazards assessment report for Christchurch and Banks Peninsula.

In December 2015, the Council asked for a second peer review of the Coastal Hazard Assessment Report (Tonkin & Taylor 2015), in response to community concerns about the science and findings in the report.

The Peer Review Report, undertaken by a panel of local and international experts, was received by the Council on 25 August 2016.

The Coastal Hazard Assessment Report 2015 focused on areas of urban development and assessed the following areas:

Christchurch

Akaroa Harbour Lyttelton Harbour
Brooklands Duvauchelle Charteris Bay
Avon Heathcote Estuary Takamatua Teddington
Sumner and Taylors Mistake Akaroa Allandale
Waimairi Beach to Southshore Wainui Purau

The report defines coastal erosion and coastal inundation zones over a 50-year and 100-year timeframe. These time periods are consistent with the New Zealand Coast Policy Statement which requires local authorities to identify areas potentially affected by coastal hazards over at least 100 years.

View the reports

Hard copies are also available at Council libraries and service centres and the Civic Offices at 53 Hereford Street.

Technical questions related to the report

What does a 1 per cent AEP and 2 per cent AEP mean?

AEP stands for ‘Annual exceedance probability’, and means the chance or probability of a natural hazard event (usually rainfall or flooding) occurring or being exceeded in any one year. AEP is usually expressed as a percentage.

Example

  • A 2 per cent AEP flood has a 2 per cent chance of occurring in any one year year, so a probability of occurring once in every 50 years. You will often heard this called a “1 in 50-year-flood"
  • A 20 per cent AEP rainfall has a 20 per cent chance of occurring or being exceeded in any one year year, so a probability of occurring once in every five years. This is often called a “1 in 5-year-flood”.

This does not mean that a 1 in 100 year flood will happen regularly every 100 years, or only once in 100 years. In any given 100 year period, a 100 year event may occur once, twice, more, or not at all.

What is a 0.4m freeboard?

Freeboard is a factor of safety used in relation to the setting of floor levels and designing levels of stop banks. Freeboard compensates for factors that may increase flood levels beyond the calculated level, such as wave action, localised effects, such as blockages in the drainage network, and uncertainties in determining land levels (LIDAR) and in modelling.

What size of storm are we talking about – your average winter sou’wester or a major storm?

The report looked to identify areas of land potentially susceptible to flooding in the 2% and 1% AEP storm surge, so significant storm events, taking predicted sea-level rise into account.

When will my home start flooding, what changes will homes in the 50 year category see in 20 or 30 years' time?

This is difficult to predict as the flooding is the end result of an interaction of a number of natural processes which are subject to variation. However it is expected with climate change and the increasing probability of extreme rainfall events, that flooding will occur more often in years to come. Around the world, coastal inundation is expected to increase in frequency as sea-levels continue to rise, and may become worse with extreme storm events becoming more frequent.

Previous report

The report is the latest in a series commissioned by the Council over a period of years to gain a comprehensive understanding of sea level rise and its potential effects. The 2013 report [PDF 13.41 MB] provides additional background on sea level rise that is relevant to the latest report.