Flooding

Flooding is one of the region’s best understood natural hazards. Local rivers are prone to flooding, especially when heavy rain coincides with a big tide. Flood areas are currently identified in the Nelson Resource Management Plan (NRMP), using information that was current at the time the plan was developed in the 1990s.  This is currently being updated in order to inform the Draft Nelson Plan.

Why do we need to produce flood models?

It is the Council’s role to help you understand where floods occur. The Council is responsible for controlling building and the effects of land use to reduce flood risk.

Under the Resource Management Act, the Council sets rules about where buildings can be located, where subdivision can take place, and the effects of land use. Managing flood risk and other natural hazards is one factor the Council must take into account amongst many other factors when making decisions.

The Council is committed to providing the most up to date flooding information to reduce the risk of flooding to you and your property.

What do we know so far?

In 2013, Council consulted with owners of land that may be subject to potential food events from the Matai River. At this time interim statements were placed on property files noting the flood hazard potential.

From April to June 2017, council consulted with landowners and the wider community on new flood models for all the major flood catchment areas.  These models predict where flooding is likely to occur now, and in various scenarios out to the year 2100.

It is important to get an understanding of where flooding is predicted to occur in the future, as the effects of climate change mean that flooding is likely to become more frequent.  As a coastal region, Nelson is also subject to the effects of sea level rise, which means that the severity of floods are likely to increase in these parts of the city that are subject to tide flows.

 Flood Modelling Reports

Council received feedback on the flood modelling information during our natural hazards community engagement between April and June 2017.

The flood modelling reports which informed this engagement can be downloaded below.

North Nelson (Wakapuaka / Whangamoa)

Wakapuaka Flats (Hillwood/Todd)

Maitai, Brook, York

Stoke Streams

Flood mapping

Council has used the information from each flood model to produce GIS maps for affected areas and these maps can be viewed in the flood model reports above.

Additional technical information and information received as part of the April to June 2017 consultation has enabled further refinement of the flooding maps.

In December 2017, the Ministry for the Environment published guidance for Local Government on Coastal Hazards and Climate Change.  As a result, the Council will now re-run the flood models to account for additional future Climate Change scenarios.

Updated flood maps will be available in late 2018 once the refined flood modelling is complete.  In the meantime, the existing maps are the most up to date information that Council holds on flood risk in Nelson. 

You can search for flood hazards on a particular property by using our natural hazards map. You will be able to see an interactive map of the flood hazard for your property and view two flood scenarios – the “Flood Model Present Day” and “Flood Models Year 2100” on the map.  The We have mapped the extent of two flood scenarios – a 1% AEP (annual exceedance probability) rainfall event for both the present day and the year 2100. A 1% AEP event is a flood that has a 1% chance of occurring or being exceeded every year and is sometimes known as the 1 in 100-year flood.

Land Information Memorandum (LIM) and Project Information Memorandum (PIM) interim notices

A Project Information Memorandum (PIM) is specific to a proposed project and tells you what Council knows about the proposed site, requirements of the Resource Management Act (RMA) and other Acts that might affect your proposal and require separate approvals. Whereas a Land Information Memorandum (LIM) report contains contemporary and historical information that is held by the Council regarding the property.

Properties mapped using the 2013 Maitai flood modelling data have had interim LIM notices put on their property files stating that the property may be at risk of flooding in a flood event.  These interim notices will remain while work on finalising the flood information continues 2018.

The interim LIM notice reads:

“The Council holds new and updated modelling information showing that this property is in an area that may be at risk of flooding in the event of a flood event. That information is in a series of flood modelling reports one of which includes this particular catchment, and are available online at nelson.govt.nz/environment/nelson-plan/natural-hazards/flooding. The modelling reports were not done to a property-specific level of detail and further site-specific investigation may be required should this property be further developed or have a change in use. 

Feedback is being sought from landowners before the new flood modelling information is able to be confirmed as identifying/further identifying a special feature or characteristic of the land within this area under section s44A(2)(a) of the Local Government Official Information and Meetings Act 1987, or s35 of the Building Act 2004.

Note: This is an interim LIM notice only, and may change once all new information has been evaluated.

For more information please contact Customer Services at Nelson City Council specifying the address of the property concerned.”


Frequently Asked Questions

Who prepared the flood modelling?

The models for the Matai/Brook/York, Hillwood Stream/Todd Valley/Wakapuaka Flats catchments were developed by Tonkin & Taylor and peer-reviewed by MWH Ltd.  

The models for the Oldham Stream, Jenkins/Arapiki/Poormans, Orchard Stream and Orphanage Stream catchments were developed by MWH Ltd and peer-reviewed by Tonkin & Taylor.

The model for the Wakapuaka River and Whangamoa River catchment was developed by Opus and reviewed by MWH Ltd. This is a ‘rapid flood hazard model’ and provides a conservative indication of areas which are potentially at risk from flooding in North Nelson.

Council’s engineering staff have also reviewed the models and outputs throughout the process.

What do the models show?

The models show the maximum predicted flood extents and depths for a range of rainfall events up to the 1% AEP (annual exceedance probability) rainfall event.  A 1% AEP flood is the flood that has a 1% chance of occurring or being exceeded every year and is sometimes known as the 1 in 100-year flood. 

Modelled scenarios include both presents and predicted future rainfall intensities and different amounts of sea level rise between 0 and 1m.

The flood hazard models assume that the peak river flows in each catchment coincide with the peak level of the annually recurring storm tide for each event.

The report for each model explains how the scenarios were developed.

How accurate is the research?

The models are generated using computational hydraulic modelling, best practice assumptions and LiDAR data from 2015 (LiDAR data is used to prepare models of the ground surface).  Where possible the results have been checked against historical records from real flood events.

The Maitai, Brook, York and Orphanage Stream models have been calibrated with respect to historic flood events captured by rainfall and river or stream flow gauges.  Knowledge of catchment response in these catchments has been used to inform our understanding of catchment response in other ungauged catchments.  The modelling takes into account general ground levels and the obstructive effects of individual buildings within each floodplain but does not account for features such as fences, walls and vegetation. The modelling reports were not done to a property-specific level of detail and further site-specific investigation may be required should the property be further developed or have a change in use.

The inputs into the models reflect current guidance produced by NIWA and the Ministry for the Environment with respect to climate change effects on rainfall and sea levels to the Year 2100.  However, the updated flood modelling we are currently working on will include the latest December 2017 Ministry for the Environment guidance on Coastal Hazards and Climate Change that set new sea level rise and rainfall intensity values.

Why is my property included in the flood modelling when it has never flooded at this property?

Council has mapped the extent of two flood scenarios – a 1% AEP (annual exceedance probability) rainfall event for both the present day and the year 2100.  This is an extreme flood event and not all areas of Nelson have experienced such a flood event to date. Many councils across New Zealand model and map a 1% AEP flood event to help inform decisions regarding future development and land uses.

Why does the flood risk look worse in the model for the year 2100?

The flood risks are likely to increase over time due to projections for increased rainfall intensity during storm events, and a rising sea level (up to 1m by 2100), based on guidance from the Ministry for the Environment.

How is Council responding to the issues I raised during the April-June 2017 natural hazards community engagement?

Council received nearly 450 responses to the community engagement held between 1 April and 16 June 2017. Much of the feedback was on the new flood information. In response to the feedback, Council is in the process of refining the flood modelling which are mostly minor amendments in some localised areas of Nelson. Council will be in touch with affected landowners in late 2018 once the modelling has been refined.

Council staff are also considering how the flood information is represented in the draft Nelson Plan and developing associated planning rules. The risk of flood hazard to people and property varies across Nelson depending on a combination of water depth and the force of moving water (velocity) and the level of development occurring in these areas. Community feedback was supportive of identifying ‘high flood hazard areas’ where there is a greater risk to people and property.

How will people know about the flood hazard information?

Council has an obligation to make flood hazard information available to the public, under the Local Government Official Information and Meetings Act 1987 and the Building Act 2004.

That means people can access information held by the Council about their property, and about any property, they are considering buying. The Council will place an interim notice on relevant land and property information files for newly affected properties to inform potential buyers about the information in the models.

What does an ‘interim notice’ on the property file mean?

The interim notice is based on the Council’s current information. This ‘interim status’ means that there may be some changes in response to further information.

Can a LIM notice be removed from the property file?

The Council could only remove a notice from your property file if a flood risk assessment was provided by a suitably qualified person, showing that flooding was not a risk for your property. This information would be assessed by the Council’s engineering experts, who would advise the Council whether or not to remove the flood risk information from your property.

My house is not included in the flood modelling, only a small area of my property. Why is there still an interim notice on my property file?

Even in circumstances where the flood hazard is only identified on a small portion of a property (e.g. driveway, garden, farm paddocks), Council is still required to note this on the property file. A Land Information Memorandum (LIM) report contains a map which will show where on the property the potential flood hazard is located.

I’m currently in the process of selling the property – do I need to tell the new owners?

If you are asked a specific question about it, you must provide the answer. It is important that purchasers do their own due diligence on any property they wish to buy. You should also check what obligations arise under any agreement for sale and purchase you enter into. 

How does the flood hazard information affect insurance costs?

The Council cannot advise property owners about the effect this flood hazard information may have on their ability to obtain insurance or on insurance premiums. Different insurance providers will have different policies, and we suggest you contact your insurer directly to discuss your specific policy.  Council is not aware of any instances where the interim LIM notices for the Maitai study has impacted on insurance costs.

What should I do if I am considering building within one of the flood model areas?

Under the Building Act, the flood risk would be classed as a natural hazard. Before undertaking any work, a Project Information Memorandum (PIM) should be obtained. In some cases, design can be undertaken that may mitigate the impact of the hazard. Council is still able to grant a building consent if it is satisfied that adequate provision has or will be made to protect land from natural hazard damage. A notice may be placed on the Certificate of Title for the property that identifies a building consent has been granted under Section 72 of the Building Act 2004 and includes details on the nature of the natural hazard. You should also check with your insurance company and the EQC about any insurance implications.

Can I subdivide the property if it’s included in a flood overlay in the NRMP?

For properties in a flood overlay, any subdivision needs resource consent under the Nelson Resource Management Plan (NRMP). The purpose of this rule is to ensure that any allotments created are usable and that the work done on the subdivision doesn’t increase the flood hazard or expose additional people or property to risk.

What impact does this new information have on the existing flood overlay in the NRMP?

None. The rules associated with the existing overlays remain current for now. The flood path overlay shown on the planning maps indicates areas where land may be subject to occasional flooding, including the force of moving flood water. In these areas, new building extensions or earthworks are a discretionary activity and require resource consent.

A similar flood overlay applies in the Rural Zone and the Conservation Zone, on an advisory basis. That means there isn’t a specific flood overlay rule, but flood risk is taken into account when assessing resource consents for earthworks and when assessing a building consent in these areas.

Requiring resource consent for development in the flood overlay areas allows Council to evaluate the degree of risk to life and property associated with the proposed development and ensure that this risk is avoided or mitigated.

Given that we hold the new flood information, if you were to seek pre-application advice from Council for a resource consent application we will provide you with the updated flood hazard information.

Will the Flood Overlay in the NRMP be changed to reflect the new information?

The NRMP is currently being reviewed and Council will need to consider how to use the new flood hazard information. As part of the development of the Nelson Plan Council is considering whether any changes to the flood overlay and rules are required.

Community input will help guide what level of risk the community considers acceptable, and this will, in turn, guide the management of land use and development.

Council will be seeking public feedback on the Draft Nelson Plan in December 2018 and you will have a further opportunity then to make comments on the new Flood Overlay Maps and associated draft plan provisions.

What is Council doing to reduce risk from flood hazards now that there is updated information available?

Council’s Long Term Plan 2018-28 confirms that Council is proposing to adopt a risk-based approach to flood protection. A risk-based approach seeks to ensure the response to flood protection recognises the difficulty and cost of constructing networks that can cope with the full range of flood events. It also requires a method of identifying the optimum level of protection in sections of streams and rivers is required. The decision making process and its outcomes will be outlined in more detail in Nelson’s 2021 Infrastructure Strategy, and will provide the direction for future investment in stormwater infrastructure.

Where do I find information about what to do during a flood event?

Civil defence information and advice on getting prepared can be found on the Nelson Tasman Civil Defence Emergency Management website.

How do I provide feedback to Council about the new information?

At this time, we are not seeking further feedback from the community. Once we have refined the flood models we will be in touch with affected landowners in late 2018.

The flood modelling information will be used to inform the Nelson Plan. It is anticipated that Council will release a Draft Nelson Plan in December 2018 and this will be the community’s opportunity to provide feedback on the draft natural hazards provisions, prior to notification of a Proposed Nelson Plan in 2019.

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