Flood hazard modelling – extreme rainfall

No. The recently released flood maps are an update to previously published natural hazard and land information. As with previous flood map updates, we’ve informed affected property owners directly to let them know this information has been updated and provided an opportunity to answer any questions.

Flood hazard maps are generated by computer models that use the contours of the land, flow paths and infrastructure information to represent flood risks during intense rainfall.

Land contours are mapped by what’s called a LIDAR (Light Detection and Ranging) system – this measures the ground levels of the land using laser pulses. This generates an accurate contour map which is used as an input into the stormwater computer model. LIDAR is very reliable technology and used by most councils in New Zealand.

A range of criteria are used to create the model and maps. The sort of things considered are:

• How hard is it raining?
• How long has it rained for?
• What is the contour of the ground?
• Where will rain water soak into the ground (e.g. grass)?
• Where will rain water flow over hard surfaces (e.g. roofs, concrete)?
• How long will it take for rain water to flow from one part of the catchment to another?
• What stormwater systems are already in place?

Each model calculates how, when and where the rainwater flows and tells us which areas are likely to be flooded and to what depth.

Tauranga continues to grow and there are areas, such as new subdivisions, where landform and infrastructure is changing. In these areas, earthworks can change the contours, changing how or where the water flows. Any changes to the landform require an update to the flood models. This update can only occur when all earthworks are completed and the last house has been built.

Council has an obligation to make hazard information it holds available to the public under the Local Government Official Information and Meetings Act 1987. This includes adding the information on each property’s land information memorandum (LIM) report. If you would like information on how this may affect your property value or insurance, we recommend you seek professional advice from a property valuer or insurance expert.

Insurance premiums take into account a range of risk factors. All insurance cover is assessed on a case-by-case basis and insurers consider the likelihood and impact of all risks to a property, including natural hazards.

Council cannot advise on how flood information affects insurance cover. If you have questions, we recommend discussing this with an insurance provider. For more information read the letter provided by the Insurance Council of New Zealand.

Water will naturally flow downhill towards low-lying areas, waterways and the harbour. In an extreme rainfall event, when infrastructure and the ground can’t cope with all the rainfall, excess water will flow overland. Flood maps may indicate a flow path in elevated areas as the water makes its way to lower lying areas, or temporarily collects in localised low points.

Climate change is expected to cause sea level rise as well as bring much heavier rainfall than what we are currently used to. Therefore, whether you live near the coast, or further inland, or up on the hills, climate change could worsen flooding on your property.

Increased intensity of rainfall (in low lying and elevated areas):

• More intense rainfall means a greater amount of rainwater going into ponding areas, overland flow paths, rivers, streams, the harbour and estuaries.
• More intense rainfall would cause land to saturate faster and any drainage systems to reach capacity earlier (meaning flooding would begin and worsen more quickly).

We have three 35-year consents for stormwater structures and managing discharges from across the city. We have also developed city-wide catchment management plans to deal with stormwater and stormwater quality issues as they arise. These provide a programme of renewals and maintenance to existing stormwater infrastructure, to ensure the system is fit for purpose.

After the 2005 flood event, council reconsidered its role in flood risk management, taking into account the following options:

• Status quo, which means people and property would remain at risk from flooding;
• Infrastructure-led solution, that is the upgrading of the existing stormwater network to protect property from damage; and
• An integrated Stormwater approach – a project aimed at improving the level of service, educating the community and understanding the implications of flood management over the long-term.

The status quo and infrastructure-led solutions were considered unacceptable as they placed an unreasonable burden, in terms of property damage, Health & Safety and financial risks, on property owners and the community.

Therefore, the Integrated Stormwater Project was formed and adopted in 2013 by Council to mitigate and reduce stormwater damage and impacts on properties and lives by:

1. A safety-focused level of service - areas where flood water flows fast and deep enough to pose a danger to people’s safety are given the highest priority (The flood water safety threshold is calculated using depth x velocity). This includes stormwater projects across the city over a number of years, with future investment proposed in the draft LTP.
2. Education (providing information on risk reduction and technical advice, including modelling and releasing flood risk information, for a 1-in-100-year event which, at the time, did not consider the effects of climate change but does so now).
3. Residual risk and emergency management (integrating hard and soft infrastructure solutions in the roading, reserves and stormwater asset areas).
4. Reactive response capacity (Stormwater Reactive Reserve Fund Policy).
5. Regulation and policy amendment (such as the proposed Plan Change 27).

Find out more about council’s stormwater improvements

An intense rainfall event is a 1-in-100-year rainfall event, taking into account sea level rise and climate change based on the median scenario (IPCC RCP 8.5) for the year 2130.

For more information on these scenarios visit the Niwa website.

What is a 1% AEP rainfall event?

A 1% AEP (annual exceedance probability) rainfall event has a 1% or 1 in 100 chance of occurring in any one year and a 10% chance of occurring in any 10-year period. Similarly, a 1% AEP event can be considered equivalent to the 100-year storm.

Over the next 100 years it is predicted Tauranga, along with the rest of NZ, will experience the effects of climate change, including more intense rainfall events and sea level rise. This will increase the flooding risk within Tauranga which council needs to address.

The Regional Council’s Regional Policy Statement (RPS) sets out the requirements for managing natural hazards in the Bay of Plenty, and every council within the region is required to give effect to it.

The RPS requires Tauranga City Council to plan for flood events of a scale that occur, on average, once every 100 years, considering the effects of projected sea level rise and climate change by 2130. This is considered best planning practice and more and more councils across New Zealand are now planning to this level.