Stormwater flood hazard maps

The initial data release covers most of Wānaka’s urban catchments. It includes the areas of Wānaka and Albert Town that are centred around Roys Bay, Beacon Point, and the Clutha River to the north and up towards Mount Alpha and Waterfall Creek to the west.

Further modelling of other urban areas in the district is underway and data will be released as modelling is completed.

The maps show potential flood depth and hazard from extreme rainfall events. Flood information is provided for all areas with a flood depth of 5 cm or more.

QLDC has mapped catchment results from a 1% and 2% annual exceedance probability over a 24-hour rainfall event.

• A 1% annual exceedance probability means that the rainfall event has a 1% chance of occurring or being exceeded every year.

• A 2% annual exceedance probability means that the rainfall event has a 2% chance of occurring or being exceeded every year.

While both scenarios would be an extreme natural event, statistically, it may be experienced at least once in a lifetime.   

A note on terminology

A 1% annual exceedance probability (AEP) is also sometimes referred to as a 1-in-100-year flood event, and a 2% AEP is sometimes referred to as a 1-in-50-year flood event.

This terminology can be a little misleading, as a 1-in-100-year event does not mean it will only happen once every 100 years. Instead, it means that, in any given year, there is a 1% chance that it will happen, regardless of when the last similar event was. This is explained here on the NIWA website.

No. These maps show stormwater surface flooding which occurs when normally dry land becomes inundated due to excessive rainfall.

The maps do not show flood hazard from high lake levels. High lake levels can be caused by multiple sources such as high river flows (rainfall in the headwaters) or snowmelt. This data is assessed by Otago Regional Council with results available on QLDC’s mapping services here: Natural Hazards Maps.

Yes. Modelling has been completed in a conservative manner using the climate change scenario RCP8.5 ‘pathway’. This is a worst-case climate change scenario accounting for no greenhouse gas emissions reductions. This is explained further on the NIWA website.

This is also the same scenario that is used in QLDC’s Land Development and Subdivision Code of Practice. The use of this input to the model, as well as other inputs, will be reviewed over time as future climate effects are further understood.

There is a hazard map and a flood map.

• The flood map indicates potential water depth and can be used to understand how a property might be affected.

• The hazard map indicates the hazard to people, property, or vehicles. This is based on a combination of water depth and velocity which once combined can make flood waters hazardous.

Both maps are used when considering investment in stormwater and consenting processes.

The Wānaka stormwater flood model was developed to help QLDC and the community better understand flood risk in the area.

It will inform QLDC’s investment in stormwater infrastructure and assessment of resource consent applications. The models will also be used to inform QLDC’s Catchment Management Plan for the area.

Yes. This information will be included on Land Information Memorandums (LIMs) and Project Information Memorandums (PIMs) to ensure property owners have information about potential hazards on their property. QLDC is legally required to include this information on LIMs and PIMs under section 44A of the Local Government Official Information and Meetings Act.

QLDC cannot advise you about house insurance for your property. If you have any questions on this, we recommend you contact your insurance provider directly.

Information will appear on a Land Information Memorandum (LIM) or Project Information Memorandum if one is requested for your property.

These FAQs relate specifically to Urban Stormwater Flooding Hazard maps. It is important that property owners consider all available data to determine if there is a risk of flooding for specific properties from all potential flooding events. View additional information from Otago Regional Council about lake and river flooding. Your property may be subject to other hazard constraints and District Plan rules. These can be checked in the relevant sections of the QLDC Map Navigator.

If you are not planning on undertaking any development of your property

You do not need to undertake any specific actions at this time. However, if you believe your property is at risk you may want to engage a stormwater expert to help understand the flooding potential on your property and any options you may have to protect your property. The Engineering New Zealand Directory lists all chartered engineers and has a ‘find an engineer’ search tool.  

Please note that any options to protect your property cannot divert flood water from your property onto another private or public property. 

If you are planning on developing your site further

This new flood and hazard information will be used to inform requirements for building consents and resource consents.  

Once you have some details about what you are planning to subdivide or build, our building and resource consent officers will be able to provide advice for your specific situation.

An application for building and/or resource consent may require you to provide a suitable assessment from a stormwater expert to demonstrate that potential flood hazards are avoided or mitigated.

QLDC used specialised software to build a computer model of the catchment.

The basis of the catchment model is the topography of the land as well as the existing infrastructure (inlets, pipes, etc). The topography element is generated by a system called LiDAR (light detection and ranging) which accurately measures the ground levels of the land using laser pulses from the air. LiDAR is a very reliable technology used by most councils in New Zealand. More information about LiDAR can be found here.

The model of the land is then expanded to include the stormwater drainage system: pipes, basins, creeks, bridges/culverts, and soak pits, as well as information about the environment (e.g., soil characteristics) and urban zoning.

Where data is unavailable it is standard practice that model inputs are estimated, and assumptions are made to represent the processes and interactions between overland flow, streamflow, and underground network. 

The model calculates how, when and where the rainwater flows. The water can flow over the land, enter the pipe system or a stream, spill out of the stream banks and out of the pipe inlets if the pipes are full or blocked. The model also considers the effects of the lake or river levels on stormwater outfalls. The model can then tell us which areas are likely to be covered by water, to what depth and how fast it may be flowing. 

Some of the variables considered include:

• How hard is it raining, and how long has it rained?

• What is the contour (sloping nature) of the ground?

• Where will rainwater move slowly or soak into the ground (e.g., grass), and where will it flow relatively fast over hard surfaces like rocky ground or pavement?

• How long will it take for rainwater to flow from one part of the catchment to another based on all the factors above?

• What stormwater systems are already in place?

We also look at historical floods in the catchment to validate the model. Adjustments are made during this process to increase the agreement between the model-predicted and observed flooding.

When the modelling is complete, it is peer reviewed.  

No model is a perfect representation of the natural environment. The large number of different inputs and complex interactions inevitably carry uncertainty in model predictions however the process to produce these flood maps followed established best practice. 

The modelling was undertaken to inform QLDC’s stormwater investment programme for the 2021-31 Ten Year Plan and beyond.

There is an ongoing programme of work to update models as development occurs or new information is received such as updated rainfall patterns, improved topography data, and data from new stormwater infrastructure or investigations. 

Underway:

• Stormwater upgrade at Aubrey Road and Rata Street intersection. This includes intake upgrades, creation of a stormwater detention facility, as well as pipe upgrades.

• Stormwater upgrade at the Anderson Road and Aubrey Road intersection, including intake upgrades and construction of new soakage pits.

• Stormwater upgrade for Stone Street area and Bullock Creek, including stormwater flow diversion and treatment from the Stone Street area to reduce the potential flood risk and to improve water quality impacts on Bullock Creek.

There is also funding available in the 2021-31 Ten Year Plan to further investigate stormwater issues highlighted through our modelling.

This information is complimentary to Otago Regional Council’s (ORC) current data on rainfall inundation and natural hazards.

ORC is responsible for lakes and rivers, as well as other hazards, and QLDC manages the urban stormwater system.

Otago Civil Defence Emergency Management provides advice on preparing and responding to a flood event.

• Otago CDEM: Further information on floods.

It is recommended that you keep your valuables up high, as well as any items you may need in an emergency, such as a getaway kit. Store weedkillers, insecticides, and other chemicals off the ground. Make a plan with your family and ensure you keep your insurance up to date.  

If you notice a blocked inlet or drain, please contact QLDC. This will help ensure we can run our stormwater system smoothly.

You can submit information on QLDC’s Snap, Send, Solve app, or go online at www.qldc.govt.nz/do-it-online/contact-us-fix-it or phone 03 441 0499 (Queenstown) or 03 443 0024 (Wānaka).

Stay up to date by checking out QLDC’s website www.qldc.govt.nz and Facebook page. We’ll also share new information via other channels such as our bi-monthly magazine Scuttlebutt and via the media.