Greater Wellington Flood Modelling FAQs
collection of information (including your photos and memories of historic floods)
determining the patterns of rainfall in the catchment and the resulting river flows
building a computer model to predict where the river is likely to flood
producing maps showing areas that are likely to be flooded.
rainfall and river flow gauges throughout the catchment
cross-section survey data collected over the past summer, which show the capacity of the river channel
LiDAR data collected over the past summer, which shows the shape of the ground around the river
minimise the future damage from flood events to property
identify any potential threat to life
allow evaluation of any impact on the river environment
alert people to any potential flood and erosion risks.
Speak to your local council before you start building.
Avoid the area affected by flood or erosion. No new development should occur beyond the Building Setback Line. We also advise that development avoids flood hazard areas, but if this is not possible, such as for an existing dwelling, we can provide you with site-specific advice.
Raise your building platform or floor levels or build to two storeys. The underside of the floor joists or concrete slab should be clear of the 1 in 100-year return period flood level. Remember that the design flood event could be exceeded.
Consider access issues and provide flood-free evacuation routes. No one wants to be caught in a flood event with no safe escape routes. Elevating access routes is not recommended as these may act as barriers to flood waters.
A Household Emergency Plan that will help you and your household plan for what to do when disaster strikes.
Well stocked emergency survival items such as food, water, clothing and medical supplies for you and your family. You will need enough for at least 3 days.
A Getaway Kit of essential emergency and medical items you need to have if you’re evacuated.
What is the flood hazard modelling standard?
Greater Wellington has developed the Flood Hazard Modelling Standard to outline the process we follow when undertaking any flood hazard mapping project. This standard was developed to ensure all flood hazard modelling projects are undertaken in a consistent and robust way in line with industry best practice. It is designed to be flexible enough to incorporate catchment or community-specific needs. For more information on our Flood Hazard Modelling Standard visit our website Flood Hazard Modelling Standard
How were the flood maps created?
Greater Wellington creates flood hazard maps using computer models. This involves:
Why is this work being done? Why can’t we just use the old maps?
Flood hazard modelling is crucial in estimating flood risks. It helps us to be better informed when making decisions about how best to manage flooding.
We want our flood hazard maps to be as up to date as possible to make sure they provide the best representation of what areas may flood.
We now have more data to add to the models, and advances in computer software that improve the accuracy of our maps.
How are floods measured?
The amount of water flowing in a river is measured by a unit called a cumec (cubic metre per second), which is a measure of how much water flows past a given point every second. Flood frequency is measured by how often a flood of a particular size is likely to happen such as 1% or 2% annual exceedance probability (AEP) flood event.
What do you mean by a very large flood/1% AEP flood?
In New Zealand, and around the world, we use measurements for large floods such as a 1% annual exceedance probability flood. These measurements are based on probability supported by historic flood records. In everyday terms it means that there is a 1% chance of such a flood happening in any given year. Sometimes people refer to this is the 1-in-100 year flood. Of course, floods can occur that are greater and lesser than this size, smaller floods occur more frequently, whereas larger floods are statistically rarer.
Will climate change be considered? If so, how has it been included?
Yes, the project incorporates the latest climate change projections, in line with the advice from the Ministry for the Environment (MfE). MfE’s climate change predictions equate to an approximate 20% increase in rainfall. For more information on how climate change is incorporated into Greater Wellington’s flood modelling process, see the climate change section of the Flood Hazard Modelling Standard. Flood Hazard Modelling Standard
Where does the data for the computer models come from?
Data for the computer models are collected from a variety of different sources. One of the main sources of data is historical records and knowledge. Stories and pictures from the community help us to further improve the pool of data used for modelling. Other data sources include:
Procedure 01 (Gather & Assess Data) of the Flood Hazard Modelling Standard has more information.
How does this information relate to Earth Science New Zealand’s national flood hazard data?
Earth Science New Zealand (formerly NIWA) has recently released the first-ever national model of flood risk across the country. There is a public flood hazard viewer to compare areas of risk across the motu. This viewer does not replace the very detailed hazard maps or Land Information Memorandum (LIM) information that regional and local councils publish.
How does this relate to Wellington Water’s Flood Hazard Maps?
Wellington Water Limited has been working on stormwater flood maps on behalf of Upper Hutt City Council. Greater Wellington modelling relates to the Hutt River, not the stormwater. You can find out more about Wellington Water stormwater modelling on their website:
Why is this information useful?
Potential hazards associated with flooding and the natural evolution of the floodplain should be considered when new development is being considered on the floodplain. This approach is useful as it helps to:
Where does the information go?
Our flood hazard maps show areas in the Hutt Valley that are susceptible to flooding, which includes the effects of inundation and erosion. This information is incorporated into Hutt City Council and Upper Hutt City Council long-term plans.
The information is also available on the Greater Wellington website and enables people to make informed decisions about land use and purchases and planning for development in the future.
I’ve lived at the same address for a long time, there’s never been any flooding here before. Why am I in a flood hazard area?
Even if you do not remember flooding at your property, flooding is a reality in and around Upper Hutt. The town is built in a river valley. Works have been done in the past to reduce the risk of flooding, however floods of greater size than these works can occur.
How deep could the flooding be at my property?
Flooding depth will always vary depending on where you are located and may also vary across your property. Contact us to talk about possible flood levels at your property. To find out specific details for your property contact Greater Wellington’s Knowledge Water team on 06 378 2484 or email Flood.Advisory@gw.govt.nz.
How much warning of a flood will I have?
Times of very high rainfall can be followed by flooding. Listening to weather warnings on TV and radio can alert you to any unusually high rainfall. The MetService updates its web page on the internet and offers an alert service, as well as information on Facebook and Twitter.
Severe weather watch or warnings can usually be found on Greater Wellington and Wellington Regional Emergency Management Office websites.
What is happening right now to provide flood protection?
Greater Wellington currently manages the risks of flood and erosion for major rivers and streams in the Wellington region. In Upper Hutt this is carried out through river management schemes that maintain stable river channels, a range of stopbanks and erosion protection structures and the management of gravel affects riverbed levels.
In Upper Hutt the current stopbanks provide protection from smaller more frequent floods. However, a small number of areas are still vulnerable in larger events. Greater Wellington monitors storm warnings and flood levels and provides warnings to some at risk communities.
What is being done to manage the flood risk from the Hutt River?
In 2001, the Hutt River Floodplain Management Plan (HRFMP) was completed by Greater Wellington, Upper Hutt City and Hutt City councils and iwi. The communities in the Hutt Valley also played a big part in the process.
Collectively it was decided that the level of protection preferred is one that will handle 2300 cumec flood or a 1 in 440 return period flood event. The HRFMP provides a comprehensive strategy to manage flooding hazards in the Hutt Valley over the next 40 years through a variety of structural and non-structural measures.
How do I know you are right about the flood risk to my property?
We have been thorough in our work to assess flood risk before sharing this information with you. Greater Wellington has gathered high-quality information from ground and aerial surveys and used a range of well-established analysis techniques to improve on flood risk estimates. Improvements to the technology we have available and the additional twenty years of data, have allowed us to draw a more accurate picture of where flooding may happen.
The flood risk estimate has been checked and reviewed by independent experts who have nationwide and worldwide experience in this work.
What should I do if I intend to develop, build or renovate?
If you are building or renovating in a flood or erosion-prone area, consider the following actions:
Will my house insurance premiums rise?
Insurance premiums consider a range of risk factors. The level of flood risk at your property may change your insurance premium if your insurer believes your property is at a greater risk of damage. Talk to us about the depth of possible flooding at your property and discuss this with your insurer. Your insurer may decide not to change your premiums, charge more, offer you a higher excess or require policy exclusions. Read our Flood Hazard and Insurance factsheet. (PDF 211 KB)
If this information goes on a Land Information Management (LIM) Report, will it lower the value of properties?
It is not possible to categorically say that published information on risks to property won’t influence buyers. However, we have been advised that there is no evidence that data on flood risk has any long-term bearing on property values. This is based on work carried out in a similar situation in the Mangaroa Valley, near Upper Hutt.
Insurance companies regularly ask district councils for updates to LIM data and for current hazard information. Councils have a duty to the community to use the most accurate information available. This information will be attached to your property’s LIM report.
What do I need to do if I live in this area?
Know your risk: Find your property on the flood and erosion hazard map and find out what the predicted depth of water will be.
WREMO’s website provides information about how to reduce the effects of flooding. This information covers evacuation plans, how to protect items in your home by raising them above floor level, and how you can reduce the risk of future flooding to your home.
Be prepared: You will need to have:
What do I do before the next flood?
It’s important to be personally prepared for a flood. Greater Wellington and the Wellington Regional Emergency Management Office (WREMO) support this with up-to-date information on current flood risk and being prepared at home and in your community.
The WREMO website offers a range of information on how you can develop home emergency plans, mitigate your flood risk and what to do in a flood.
Why is this information being made public?
Greater Wellington is responsible for identifying flood risk for the region. We are legally required to identify and share this information with district councils and the community to help them make informed decisions about future developments and flood risk management.
This information is public and available to anyone who has an interest in property in the area. It will be included on LIM (Land Information Memorandum) reports held by district councils. Insurers can request it.
What if I have information about past floods?
If you think you have additional technical information or observations/photos to help us improve our flood maps please share this with us. We will take information we receive into consideration during future model updates and reviews.
You can contact us through our website or by emailing floods@gw.govt.nz