Please read through this FAQ section to see if these answer your question(s).
The maps were created using sophisticated modelling based on rainfall, the lie of the land and the Council’s stormwater network.
The models have been set up using recorded rainfall and flood levels from past events.
The model uses forecasted changes to our local climate and sea level. Flood events will be more severe in the future as sea levels rise, and we experience more damaging and disruptive weather patterns.
Other evidence such as flood records, gauges, photos, videos and eyewitness accounts has also been used to develop the maps.
A method called a dynamic freeboard allowance has been incorporated into the maps to represent known contributors of flooding risk that are difficult to represent in the models, such as network blockages, sedimentation or vehicle generated waves.
Typically, the freeboard allowance added is 200mm of water to every measurement.
The maps show the areas of inundation greater than 50mm, known stream corridors, and hydraulically significant flow paths that push flooding overland.
Wellington Water use best-practice flood modelling standards and take into account the predicted impacts of climate change to 2120. This includes a sea-level rise of 1m and predicted increase in rainfall intensity and volume of 20%.
Porirua has experienced significant weather events over the past few years, reminding us of the flooding risks that we live with now and highlighting the need to plan for future climate disruption.
To help us plan for the future and understand these risks, people at council are working with Wellington Water to complete computer models of the likely impact of extreme rainfall on most of Porirua's urban areas.
The updated flood modelling has the best evidence we have at the moment, so we can make informed decisions at the local, neighbourhood and city level.
Final flood depth maps are used to inform flood hazard maps in the Council’s District Plan.
The Proposed District Plan tells us what we can and can’t do with our land. It makes rules about where we can subdivide, what activities we can do and where we can do them.
The Proposed District Plan takes a risk-based approach to natural hazards. The Plan balances enabling people and communities to use their property and undertake activities, while also ensuring that lives or property are not harmed or lost as result of a natural hazard event. Flooding is a natural hazard managed in the Proposed District Plan.
You can view the Proposed District Plan for more information.
Wellington Water produced the flood models, using this information to also produce the flood maps. This work was done on behalf of Porirua City Council.
Wellington Water has a comprehensive programme of modelling and mapping flood risk across the entire Wellington metropolitan region.
These maps were done according to best-practice modelling standards and have been peer reviewed by external experts.
Greater Wellington Regional Council also produce separate flood maps for the Porirua River, which do not affect the areas we are currently engaging on.
Council manages flood risk in a variety of ways.
The stormwater network includes a network of pipes and pumps designed to drain regular rainfall away from properties and into waterways and to the coast.
There are also a number of detention basins and wetlands in strategic locations to catch a certain amount of floodwater when the system is under pressure.
Stormwater pipe networks historically were designed to carry away water during only the low-to-medium intensity rainfall events. When the storm intensity exceeds this pipe design capacity then water flows overland and residences and businesses can be at risk of flooding.
When this occurs, Council often responds through our emergency management response processes to help residents and landowners, alongside agencies such as the Fire and Emergency New Zealand.
House prices vary with changes in the real estate market, and are based on a range of criteria, including location, schools, views and house type.
Many areas in New Zealand are subject to natural hazards, and this does have some influence on people’s decisions on where they want to live.
It is important to note that these maps do not create this natural hazard risk, they just illustrate where flood events are already likely to occur.
These maps will help ensure that future developments avoid flooding, this includes ensuring development on other nearby properties does not increase the risk on your property.
House prices vary with changes in the real estate market, and are based on a range of criteria, including location, schools, views and house type.
Many areas in New Zealand are subject to natural hazards, and this does have some influence on people’s decisions on where they want to live.
It is important to note that these maps do not create this natural hazard risk, they just illustrate where flood events are already likely to occur.
These maps will help ensure that future developments avoid flooding, this includes ensuring development on other nearby properties does not increase the risk on your property.
To help us plan for the future and understand these risks, people at Council are working with Wellington Water to complete computer models of the likely impact of extreme rainfall on most of Porirua's urban areas.
Wellington Water is responsible for modelling the flood depths and producing flood hazard maps across Porirua city. These flood hazard maps are included in Council’s District Plan as flood hazard risk overlays. This is to ensure that we’re using the best information available to us to make responsible decisions about flood risk to surrounding properties before any future developments are undertaken.
People working at Council are now providing this information to you in draft form on your Land Information Memorandum (LIM). This is to ensure you have early and easy access to all the information available about natural hazards that may impact you. If needed, people working at the council can help you navigate this new information, including from draft reports.
When the flood maps are finalised and proposed to be included in the Council’s District Plan, we’ll be in contact again to keep you updated.
Making it easier for people to access the latest information about flooding risk by including it on their LIM is a responsible and practical way for council to communicate their important work of looking ahead and planning for our collective future.
Please read through this FAQ section to see if these answer your question(s).
If you want to discuss the modelling and mapping, or the maintenance of the stormwater network in your area, please contact Wellington Water on [email protected]
If you want to discuss how flood risk will be managed through Council's plan-making process please contact the Climate and Environmental Planning team at [email protected] or 04 237 5089.
The maps show flooding hazard in the following categories:
The maps show areas where there is at least a 1% probability that substantial flooding could occur in any given year. This type of event is often referred to as a "one-in-100-year storm". Freeboard is a variable flood hazard that is known to occur during heavy rainfall, with its impact sometimes varying between each storm. Freeboard includes factors such as culvert blockage, debris build-up, wave action or changes in the stream bed.
The stream corridor consists of a buffer of five metres either side of the centre of the stream. Open water courses in built up areas were picked to be included in the stream corridor layer alongside the upper reaches of stormwater catchments.
The five metre buffer either side of the stream includes most significant paths of flooding and allows for erosion of the stream banks caused by flooding, as well as access for maintenance.
Overland flow paths carry water from the catchment during heavy rain when there is too much water for the pipe network or it is blocked. All stormwater networks are designed to include overland flowpaths.
Overland flowpaths were identified and mapped using the model results and flood records taking into account depth and speed to spot the overland flowpaths that carry flood water.
Inundation areas are mapped for all flooding modelled above a depth of 50mm.
The stream corridor typically consists of a buffer of 5 metres either side of the stream centreline. The 5 metre distance was selected as this was found to incorporate much of the hydraulically significant stream flows during a flood as well as allow for natural variations to the streams bed.
Open water courses in urban areas were selected to be included in the stream corridor layer alongside significant contributing branches in the upper reaches of stormwater catchments.
Overland flow paths are mapped using model results and flood records taking into account depth and velocity to identify the hydraulically significant flow paths between inundation areas.
The inundation layer shows the areas where flooding is likely to exceed 50mm in the mapped flood event.
The Proposed District Plan takes a risk-based approach to all natural hazards, which means ensuring development that is sensitive to the impacts of hazards occurs outside the most hazardous areas.
For example, we are looking at ensuring houses are not built too close to stream corridors as these pose high risk to people and property; but in lower risk areas such as inundation areas, all we are likely to require is for residential floor levels to be raised to be above the 100-year flood level.
If you have a flood risk on your property, and you want to build or subdivide, we recommend to speak to Council and they will help to plan your work.
To view the Wellington Water Flood Mapping App click here
By using this app, you accept all terms and conditions outlined here
Information on flooding will be provided in LIMs if the information is available at the time the LIM report was issued. The process of developing flood maps based on stormwater flood models is complex. Wellington Water is in the process of producing and updating flood maps for Porirua which will be included in LIMs once they are finalised.
The catchments mapped have a history of flooding and while it may not have previously flooded, the modelling shows it is potentially prone to flood risk.
The model incorporates forecasted changes to our local climate and sea level. Flood events will be more severe in the future as sea levels rise, and we experience an increased frequency of damaging and disruptive weather patterns.
Recent technology improvements, particularly around data collection and modelling, has enabled Wellington Water to produce more detailed and complex flood models than ever before. It means we’re able to better understand and map out flood risk at a regional level.
Wellington Water is in the process of producing flood maps for the entire Wellington metropolitan region.
These flood maps will eventually inform Council’s District Plan. These maps are a useful tool for all communities of Porirua to learn about information about the places we love, and support our communities to have the tools to make informed and good decisions regarding flood risk at the local, neighbourhood and city level.
As the name suggests, a one-in-10-year event had a 10% probability of happening on any given year over a 10-year period. We map this type of flood risk because it is an indication of frequent flooding that a primary stormwater network should manage. A one-in-100-year event has a 1% probability of happening in any given year over a 100-year period. We create 100-year flood maps as a standard practice because they are shown in the District Plan to inform planning processes.
Once the models have been confirmed the maps will inform the District Plan through a future plan change process.
