Flood hazard modelling and mapping

An overland flowpath is the route taken by water from a rainfall event, making its way downhill towards streams, harbour and coast. Overland flowpaths are a natural occurrence and flow through roads, stormwater reserves and private property. Overland flowpaths are part of the stormwater system and need to be protected in order to reduce the impacts of flooding on people and property. If an overland flowpath is altered or obstructed, the depth and velocity (speed of flow) and volume of the water on site, and on properties upstream and downstream increases.

Overland flowpath in Mount Maunganui

To maintain the function of overland flowpaths, the plan change proposes:

• Obstruction of an overland flowpath, including building a fence or wall will require a resource consent.
• Additions and new buildings in an overland flowpath will require a resource consent.
• Minor earthworks are permitted, but earthworks above 10m³ and 300mm will require a resource consent.
• Alteration of a minor overland flowpath on site is permitted as long as water conveyance capacity and entry and exit points from the site are maintained.
• Avoid new social and cultural and critical buildings in an overland flowpath and restrict additions to these buildings within an overland flowpath.

Floodplains are areas of land situated near a river or stream which are inundated by water during a flood event. They are part of the stormwater system and need to be protected to reduce impacts on people and property. If a floodplain is altered or reduced, the depth and velocity (speed of flow) and volume of the water on site and on properties upstream and downstream increases.

Water storage capacity of a floodplain

To maintain the function of floodplains, the plan change proposes:

• Obstruction of water, including building a fence or wall will require a resource consent.
• Additions to existing buildings of up to 20m² in total are permitted.
• New residential, industrial or business activities within a floodplain require a resource consent.
• Minor earthworks are permitted, but earthworks above 10m³ and 300mm in height require a resource consent.
• Subdivision partially or wholly within a floodplain will consider the protection of these areas as part of a resource consent.
• Avoid building new social and cultural (e.g. church, schools) and critical buildings (e.g. hospitals) in a floodplain and restrict additions to these buildings within a floodplain. These buildings tend to house more vulnerable people and/or must continue to provide essential services during and after a flood and therefore must be protected from flooding.
   

Flood prone areas are locations where ponding of water occurs in an intense rainfall event. The flow of the water in flood prone areas is much slower than in floodplains and overland flowpaths. Flood prone areas generally occur in areas where there is a depression in the land. Managing activities in flood prone areas includes ensuring safe evacuation from the building in case of a flood, the location of the building in relation to the level of flooding and the type of activity on the land prone to flooding.

To manage activities in a flood prone area, the plan change proposes:

• Minor additions and new builds where the water depth is below 300mm are permitted, subject to floor levels being set above flood level.
• Additions or new developments where the water depth is 300mm or more will require a resource consent. Water levels above 300mm can be dangerous specially to children and vulnerable adults and can cause trafficability issues (e.g. cause cars to float). New developments need to be designed to ensure the safety of people and damage to neighbouring properties in a flood event is mitigated.
• Minor earthworks are permitted, but earthworks above 10m³ and 300mm will require a resource consent.
• Social and cultural and critical buildings should be located outside flood prone areas. Any additions of more than 20m² will require a resource consent.

Pervious surfaces absorb and detain stormwater, reducing runoff from a site. Natural pervious surfaces include grass, landscaped and planted areas. Constructed pervious surfaces generally consist of a layered construction to enable rainwater filtration into the ground, such as living roofs and slatted decks.

As Tauranga continues to grow and develop land, the potential increase in impervious surfaces can lead to increased flooding on downstream properties and put additional pressure on the existing stormwater network.

To manage activities that increase flooding, the plan change proposes:

• Impervious surfaces of up to 70% are permitted within the Suburban Residential and Large Lot Residential zones. Developments proposing to exceed the 70% threshold require a resource consent.
• The operative City Plan already controls impervious surfaces in the City Living Zone, permitting a maximum 75% of total site coverage. The existing site coverage rule in the City Living zone is being amended: Where the development proposes to exceed 70% impervious surfaces, it will require a resource consent.

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.

If you lease or tenant your property to a third party, we strongly recommend you inform your tenant(s) or lessee(s)of the updated flood mapping, so that they too are aware of the updated data. 

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 these with an insurance provider. For more information read the letter provided by the Insurance Council of New Zealand.

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 a 100-year storm.

Over the next 100 years it is predicted Tauranga, along with the rest of New Zealand, 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 our 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.

These stormwater flood hazard maps are based on a worst-case scenario – intense rainfall from a 1-in-a-100-year storm as in the year 2130 with climate change. Building infrastructure, or pipes, large enough to accommodate the amount of rainwater generated during this type of event below ground, is simply not affordable or realistic.

Instead, council has adopted a risk to life approach – which focuses on mitigating flood risks in areas where the depth and speed of floodwater could pose a risk to people.

You can request a review of the flooding risk that has been identified on your property, by emailing city.plan@tauranga.govt.nz with all relevant property information and a waters engineer will review the situation. Please note, the maps provided are non-statutory and can be reviewed to take account of any property-specific information, outside of the plan change submission process.

Flood hazard maps are generated from computer models which use land contours and infrastructure information to represent flooding in an intense rainfall event. Each model calculates how, when and where the rainwater flows and tells us which areas are likely to be covered by water 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. These changes to the landform necessitate an update to the flood models. This update can only occur when all earthworks are completed and the last house has been built.

Tauranga’s stormwater network includes over 460km of stormwater pipes, 9800 manholes, 75km of open drains and more than 800 outlets that release stormwater into the harbour, rivers and ocean.

Private properties play an important role in ensuring water can run off surfaces such as houses and driveways when it rains and into the city’s stormwater network which includes drains, swales and overland flowpaths on both public and private land. Changing weather patterns and the effects of climate change are leading to longer and heavier bursts of rain that can overwhelm systems and increase the risk of flooding.

Council has three 35-year consents for stormwater structures and managing discharges from across the city. Council has developed city-wide catchment management plans to deal with stormwater and stormwater quality issues as they arise. These also provide a programme of renewals and maintenance of existing stormwater infrastructure, to ensure the system is fit for purpose. Plan Change 27 is part of council’s response to managing flood risks.

Find out more about how council manages stormwater

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
• Integrated Stormwater Project – 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 was considered unacceptable as it placed an unreasonable property damage and health and safety burden on property owners and was projected to worsen over time as a result of climate change factors.

The infrastructure-led solution was found to place an unreasonable financial burden on the community due to the high capital cost of building the infrastructure and then the additional high maintenance cost. 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 LTP stormwater projects across the city over a number of years, with future investment proposed in the draft LTP for consultation.
2. Education (information provision 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).
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 (proposed Plan Change 27).

The model that was used for the 2020 flood hazard maps was built in 2013. The new model, built in 2022, is more detailed and includes more of the minor overland flow paths for example.

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).

Since flooding in the area in 2005, council did extensive work to mitigate flood risks which has made a big difference lowering the occurrence of flooding. The storm event used to produce this dataset is based on an extreme event which would produce an extraordinary amount of stormwater in a very short space of time. This would not be absorbed in the stormwater infrastructure, as per the question and answer above.

The reduced flood risk is a result of extensive stormwater improvements completed as part of transportation upgrades around Bayfair and Baypark.

Stormwater in Tauranga is managed by areas called catchments. If your property is situated directly on the boundary of two catchments, you may receive or have received another letter for flood hazard mapping for the Mount Maunganui North/Mount Industrial catchment.

The plan change provides rules that would mean that new developments and redevelopments would be above or outside of the flood level in a 1-in-100-year storm event, with climate change and sea level rise included.

Minor alterations and works (20m2 addition to a house, small shed etc.) on an existing property will not need a resource consent.

An example of where a resource consent maybe required is for a new solid fence that would restrict an overland flowpath, which could cause flooding effects on neighbouring properties. The need for resource consent enables council to assess the effects to ensure they are acceptable.

Another example, if someone builds a house, the proposed rules require the floor level to be above the flood level to protect the house and its occupants. This may also need to be an activity that requires a consent, depending on the flood depth. Where there is significant flood depth (over 300mm), a resource consent is required to assess the effects on neighbouring properties.

The plan change also proposes rules around earthworks. Where someone is undertaking earthworks to raise their land above the flood level, this could cause flooding effects for neighbouring properties. When a body of water is altered or reduced, the depth and velocity (speed of flow) and volume of the water on site and on properties upstream and downstream can increase.

Find out more in our guidance document, assisting you in understanding what the proposed rules mean.

Guideline document (2.1mb pdf)

If works, such as a fence or small shed, are outside the flooding area identified on the property, there is no resource consent required. Resource consent may only be required if the activity is located in the flood area. This does not apply to gardening. Where a development does not meet the permitted activity standards within the City Plan, a resource consent will be required. Consent is required for a fence where it is a solid fence that would restrict an overland flowpath, which could cause flooding effects on neighbouring properties. As an example, a resource consent for a fence under 2m height, that is pervious (water is able to flow through it) doesn’t require a resource. The proposed changes only apply to new fences in an overland flowpath or floodplain, and a consent is not required for an existing fence. Find out more in our guidance document, assisting you in understanding what the proposed rules mean.

Guideline document (2.1mb pdf)

Tauranga City Council has been mapping and releasing flood hazard information for many years to meet its obligations under various national and regional legislation.

This is why we have a programme to periodically update flood maps. Each catchment area of the city will be updated every couple of years to take into account new land levels, building and infrastructure information. Because large scale earthworks can make significant changes to landform (such as raising building platforms and lowering road levels over a large area), we need to make sure we have final information for the subdivision so that areas are properly updated to reflect changes in floodable areas.

Tauranga continues to grow and there are areas where landform and infrastructure is changing, such as new large subdivisions. In these areas, earthworks can change the contours, changing how or where the water flows. In larger subdivisions earthworks can occur over many years, and changes ‘on the ground’ continue as houses are being built. These changes to the landform necessitate an update to the flood models.

Council maps identify ‘Future Reassessment Areas’ to acknowledge the changing landform and that flood maps will need to be updated following development in these areas. This information will be included in Land Information Memorandum (LIM) reports and used when reviewing building and resource consents.

In between map updates, any resource consent or building consent assessments will take into account the latest available site information. If you would like to undertake an activity in a mapped floodable area and have information (such as new site survey levels) to demonstrate that flooding no longer applies, then that can be presented and discussed with council.

Climate change refers to the changes to the climate associated with the effects of global warming. Climate change is projected to have a significant impact on the land near the coast and harbour, mostly due to sea level rise and the intensification of extreme storm events. 

Information about the global impact of climate change is provided in the Fifth Assessment Report of the United Nation’s Intergovernmental Panel on Climate Change (IPCC). National and regional analysis of climate change impacts have also been carried out by the National Institute of Water and Atmospheric Research (NIWA).