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.
- 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.