Flood Mapping

Working towards resilience – rainfall flood risks in Porirua

We want to manage the risk from flooding and improve the resilience of our communities, now and into the future. 

Extreme weather events in Porirua over the last few years have been a reminder of the flooding risks that we live with. To help us understand these risks and plan for the future, Wellington Water has completed computer models of the likely impact of extreme rainfall on much of the urban areas of Porirua. These flood maps can be viewed here.  

These models show areas where there is at least a 1% possibility that substantial flooding may occur on any given year (known as a one-in-100 year flood risk). The modelling shows localised flooding issues, overland flow paths and stream corridors. 

They are based on 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%. 

These maps should help reduce the impact of flooding in the future by giving us a better understanding of flood events, so we can improve our infrastructure and our planning of how respond to emergency's.  

This information has been incorporated into Porirua City Council’s District Plan Review to ensure that any future developments are undertaken in an informed manner and do not increase the flood risk to surrounding properties

All Councils are required to make this information public and any property with a history of flooding will have this information added to their Land Information Memorandum (LIM).

If you would like more information about this process in the first instance please email [email protected].

What are we doing to manage the risk of flooding in Porirua?

We are carrying out a full review of our District Plan. The 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. For more information visit Proposed District Plan.

As part of this review, the government needs us to address a lot of things to keep our communities safe and to protect the local environment. This includes a need to better understand areas that could be at risk of flooding and to properly manage those risks.

Wellington Water have completed flood modelling computer exercises to understand areas that are likely to be affected by a level of flood inundation for an extreme event defined as a 'one-in-a-100 year storm'. 

Plimmerton, Pauatahanui, Porirua East, Titahi Bay and the City Centre were prioritised because of known flooding events in these areas in the past. Wellington Water is continuing to model the remainder of the City and the results of this are expected in the next two years.

How we consulted with the communities in Porirua?

We jointly held five publicly advertised drop in session to discuss flooding in Porirua and to share the draft flood maps. The drop in sessions were held on:

  • 22 May 2018 - Pauatahanui Flood Maps
  • 12 July 2018 - Titahi Bay Flood Maps
  • 24 July 2018 - Porirua CBD Flood Maps
  • 31 July 2018 - Plimmerton West Flood Maps
  • 8 August 2018 - Porirua East Flood Maps

Nearly all the feedback received confirmed that the hazards represented in the maps corresponded well with the local resident’s experience of the recent floods. 

What are the different flooding categories?

The maps show flooding hazard in the following categories:

  • stream corridors.
  • overland flow paths. 
  • inundation areas.

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. 

To view the Wellington Water Flood Mapping App click here

By using this app, you accept all terms and conditions outlined below.

Data has been compiled from a variety of sources and its accuracy may vary (sources include: Porirua City Council, Greater Wellington Regional Council, and Land Information New Zealand). Flood modelling data is supplied to the best of Wellington Water's knowledge, and must not be used for detailed engineering design. Use of this data requires Porirua City Council, Greater Wellington Regional Council and Wellington Water Ltd to be acknowledged as the data custodian and data owners respectively. Please contact Wellington Water at [email protected] if you require more detailed information. Wellington Water does not accept any responsibility for the use of or reliance on the data and shall not be liable to any person on any ground for any loss, damage or expense arising from such use or reliance. These terms and conditions apply to the use of this WebApp as well as any and all maps captured and/or printed from this application or shared by any other means.

Flood Mapping FAQs

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 an more damaging and disruptive weather patterns.

Other evidence such as flood records, gauges, photos, videos and eyewitness accounts have 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.

Flooding is one of the major natural hazards in our region and our city has had some recent experiences of extreme weather.

Damage caused by flooding events has a major impact on property owners and the community. The impact is not only economic, but social, emotional and in some locations life threatening.

Council’s main objective in producing these maps is to reduce these impacts of flooding by better understanding flood events.

Council has a responsibility to ensure that future developments avoid flood prone areas and do not make downstream flooding risk worse.

Wellington Water Limited modelled and mapped the catchments for Porirua District Council. 

Wellington Water has a comprehensive programme of modelling and mapping flood risk across the entire Wellington metropolitan region.

These maps were done to best practice modelling standards, and have been peer reviewed by external experts.

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 flood water 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 landowners, alongside agencies such as the Fire and Emergency New Zealand.

Insurance companies also have their own individual methods of determining what prices they charge and how they will provide flood insurance. 

Some of the larger insurance companies are carrying out their own modelling of flood risk. 

Typically insurance companies are more concerned about regular flooding rather than rare events. Speak to your individual provider to find out more.

House prices vary continuously with changes in the real estate market, and are based on a large 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 yours.

Please read through this FAQ section to see if these answer your question.

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 the District Plan please contact the Environment and City Planning team at [email protected] or 04 237 5089.

The Resource Management Act 1991 sets out the process for submitting on proposed district plans, including support or opposition to any particular chapters and the supporting evidence base, including flood maps.  You can view and/or make a submission on the Proposed District Plan.

The stream corridor typically consists of a buffer of 5m either side of the stream centreline. 5m 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.

These maps have been incorporated into the Proposed District Plan – which will protect people and new property from flood risks.

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. 

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.

Wellington Water is mapping the entire Wellington metropolitan regions, including eventually the whole of Porirua.

It is only in the last years that the knowledge of the rainfall data and computer technology has been available to better understand and calculate it on this scale.

There are a number of existing flood maps in the Porirua area, but these are progressively being replaced through this updated comprehensive modelling programme.

A 100 year flood has a probability of 1% of occurring in any given year. If we had long enough rainfall records it would be the flood that will occur or be exceeded on average once every 100 years.