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.

Flood modelling

Recently, Council has updated its research and modelling to get a better understanding of the scale, nature and frequency of flooding. This research is useful for planning new subdivisions and buildings, or upgrading Council infrastructure with the aim of reducing the possibility of damage in flood prone areas.

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

Since then, Council has continued to work on better defining the potential for flood hazard across the region and now has models for all the major catchments, which includes: Orphanage, Orchard, Jenkins/Arapiki/Poorman Valley, Maitai/Brook/York, Oldham, Wakapuaka/Hillwood and Wakapuaka / Whangamoa. These models predict where flooding is likely to occur now, and in various scenarios out to the year 2100.

Between April and June 2017, Council consulted with landowners and the wider community on the new flood models and how we should be responding to flood hazard in the review of our resource management plan, the Nelson Plan.

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 those parts of the city that are subject to tide flows.

Flood Model Reports

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

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

Since then Council has been refining the technical information with a view to mapping this in the Nelson Plan and developing associated planning rules. Work on our flooding information is ongoing and we will be in touch with affected landowners in early 2018 once the refined modelling is complete. These are largely minor amendments in some localised areas and the extent of flood risk identified and mapped in the reports generally remains the same.

North Nelson (Wakapuaka / Whangamoa)

Wakapuaka Flats (Hillwood/Todd)

Maitai, Brook, York

Stoke Streams

Flood mapping

Council has used the information from each model to produce GIS mapping which shows the affected areas. We will update the mapping in early 2018 once the refined flood modelling is complete.  In the meantime, it remains 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 and view two flood scenarios – a 1% annual exceedance probability (AEP) event for both the present day and the year 2100.

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

Properties subject to the 2013 Maitai flood model have had interim notes on their property files.

Because we’ve received new information for other catchments throughout the region, we've had to add an interim note to the property files that we hold for newly affected properties. These interim notations will remain while work on finalising this information continues throughout 2017 and early 2018.

The interim LIM notation 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 notification 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 models?

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 present 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 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.

Why is my property included in the flood model 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 models which are mostly minor amendments in some localised areas of Nelson. Council will be in touch with affected landowners in early 2018 once the modelling has been refined.

Some feedback has been passed on to our infrastructure team as the issues raised were operational matters.

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 note 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 note’ on the property file mean?

The interim note 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 note be removed from the property file?

The Council could only remove a note 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 within the flood model, only a small area of my property. Why is there still an interim note 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 relevant land and property information files. 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. 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.

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

Council’s Long Term Plan 2015-25 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 detail of the risk-based approach to flood protection will be developed by July 2019 to inform the Long Term Plan 2021-31.

In the meantime, the Long Term Plan 2015-25 also sets out the stormwater and flood protection projects that are currently underway including Saxton Creek, Orphanage Stream, York Stream, Little-Go Stream and the Maitai River.

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 modelling information we will be in touch with affected landowners in early 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 August 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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