Frequently asked questions
applying and communicating lessons learned from past floods
underpinning flood resilience strategies and Long-term Plan programmes with good science and the latest modelling
improving land use planning and floodplain management
to enhance flood forecasting and warnings, and
adopting smarter urban design and integrated catchment management.
- a rain-on-grid approach was applied to low-lying, flat land, particularly where there is the potential for ponding and where interaction with groundwater is likely
- a sub-catchment-based approach was applied to steeper catchments where water flows in channels and there’s insufficient data to simulate fine-scale hydraulics.
- Kāpiti Coast District Council manages the stormwater system and associated urban streams. We lead the emergency response in a local flood event.
- Greater Wellington Regional Council (GW) focuses on the two major rivers (Ōtaki and Waikanae) and rural waterways. It is also updating its flood maps. Find out more about GW flood projects.
- Wellington Emergency Management Office (WREMO) coordinates Civil Defence and Emergency Management Services on behalf of the nine councils in the Wellington Region to protect people and property in a regional emergency flood event.
- Central Government sets national policies, standards, and legislation for managing flooding and natural hazards, provides funding and guidance to local authorities, and coordinates emergency response efforts during major events.
- National Institute for Water and Atmospheric Research (NIWA): NIWA is creating a New Zealand–wide flood map for national policy and emergency planning but acknowledges that local flood models (such as ours) are best for community-level decision-making. Learn more about NIWA flood hazard mapping.
What is flood hazard modelling?
A flood hazard model is a tool used to predict the potential impact of flooding. It is based on factors like rainfall, runoff, infrastructure information, coastal and groundwater inundation, and land topography.
What are flood maps?
Flood hazard maps generated from these models show potential flooding in a specific rainfall event [in this case a 1-in-100-year rainfall event]. The maps show water depth and extent, but the models can be run for different rainfall events and produce different outputs, such as flood risk maps (which represent the potential impact of flooding such as the risk associated with flood depth and velocity) and flood hazard maps (which identifies areas susceptible to flooding i.e. represent the hazard associated with flood extent).
How do these models differ from those released in 2012?
Council has been using models built in 2010-2012 for flood hazard mapping to inform planning decisions.
The updated models show flooding from the district’s urban drainage network [piped and minor watercourses within urban areas] but have been extended to include the Te Kowhai and Mauapoko catchments.
The updated models use the latest modelling processes and technology. They incorporate soil and groundwater conditions for the district, recent development and major roading projects, and updated climate change projections.
The new models are built in such a way as to ensure they remain current and can be easily updated to reflect new information, as it becomes available.
How do the models help us manage flood risk?
We can’t stop floods; there will always be a risk of flooding. But we can manage this risk through a combination of measures including:
Our models help identify areas at risk and inform flood management strategies, such as flood warning, flood adaptation and mitigation planning, asset management, advice during emergency responses, and may inform future district plan changes.
Although we cannot predict exactly the effects of climate change or natural hazards on specific locations, the information from the models will help council and our communities prepare for a range of potential impacts.
Why do the models use a 1-in-100 rainfall event?
Understanding and managing risks associated with 100-year flood events are best practice and are required under the Regional Policy Statement for the Wellington region. A 1-in-100 rainfall event [also referred as a 1% Annual Exceedance Probability (AEP) or Q100 event) has a 1% or 1-in-100 chance of occurring in any 12-month period or a 10% chance of occurring in any 10-year period.
How do we build models?
We start by collecting and verifying data about waterways, land, and climate. This includes topographic and infrastructure information, and groundwater, sea level and rainfall data – refer to Model Build Methodology Report for more information of model build datasets
Hydrological and hydraulic models are then combined to create an accurate picture of flood risks. Hydrological models simulate rainfall and runoff, while hydraulic model simulate water flow and inundation.
Historical flood data, insights, and images from big flood events in May 2015 and December 2021 were used to check the model’s accuracy, ensuring the simulated flood characteristics match real-world observations.
The resulting information was then analysed to identify areas at risk of flooding, estimate flood depths, and determine the extent of potential damage.
We used specialists to peer review the model for one of the catchments (Mazengarb). The Mazengarb catchment has all the elements [such as pump stations, open channels, soak pits, attenuation ponds, piped network and a mix of land uses] found within Council’s urban catchment so this shows us the model’s recommendations can be applied across all other models – refer to appendix A from Mazengarb Model Build Report.
Finally, we check in with the community to ensure that the simulated flood characteristics match their knowledge of the local environment.
This will further refine our understanding of flooding patterns and ensure we have captured or eliminated areas as accurately as possible.
What climate change scenarios have been used?
Incorporating climate change projections into flood hazard modelling provides an estimate of its potential impact on future flood risk. This includes considering changes in rainfall intensity, river flows and/or sea and groundwater levels.
Models use climate projections based on different emission scenarios. It is important to consider different time as well as size scales when assessing the impacts of climate change, so the models use two scenarios: current climate (2030), and SSP5-8.5M (a “high carbon” scenario) for 2070- and 2130-time horizons. The two modelled timeframes: 2070 and 2130 correspond to approximately 1.8°C and 3.7°C of global warming.
Why have we used these particular scenarios?
The presentation of a range of climate scenarios gives an envelope of impact. In reality, where we land with respect to climate change will likely fall somewhere between current climate and a worst-case scenario of 3.7°C of warming.
Different projections can be used for different purposes, i.e. when considering a major highway or new development – where the consequences and/or likelihood of an extreme flood event is very high – the costs of adapting to SSP5-8.5 may be considered worthwhile.
In other circumstances, where the consequences and/or likelihood are low, adapting to SSP5-8.5 may not be necessary or economically viable, e.g. for insurance purposes where companies are more concerned about the short- or medium-term risk rather than one projected over the next 100 years.
Using temperature-based descriptions allows the community to compare new outputs with the current district plan maps. As a comparison, in our earlier models these used a predicted increase in rainfall of 16% or a 2°C temperature increase for the 1-in-100 rainfall event for 2090-time horizon.
How much risk are we willing to take?
The question of ‘how much risk the community is comfortable taking’ is a complex one and will be the subject of on-going conversations with the community through annual and long-term planning and District Plan change processes.
For more information on the climate change scenarios used in the models visit NIWA.
How do rainfall and tide interact in the model?
A hybrid approach was adopted for the hydrological model:
The proximity of Kāpiti to the coast, together with its low-lying topography, makes flooding prone to tidal influence. The greatest risk of coastal flooding arises when all conditions (wind, rain, tide, groundwater levels etc) peak together to produce unusually high-water levels. To take this possibility into account, the models simulate tidal conditions during a storm event.
We also undertake ‘joint probability analysis’. This gives us the statistical likelihood that multiple events that are independent of one another would happen at the same time. Joint probability analysis ensures that an appropriate level of conservatism is adopted when considering the likelihood of coastal-catchment flooding, rather than simply assuming the worst-case scenario across the two domains.
Refer to Model Build Methodology Report for more information.
Why do you want our views on this?
Community input, including details of past flooding events or observations of drainage in and around properties, is an important part of reviewing and testing our flood maps before we finalise the modelling behind them.
We want to hear from you if you don’t believe the maps accurately reflect local flood behaviour, ground features, and built infrastructure on your property or local areas of interest
Feedback closes at 11.59pm Sunday 8 June 2025.
How will this affect my house insurance?
Insurance companies independently assess flood risk when offering policies and determining premiums. While insurers consider local flood information, many factors influence their decisions, including individual property characteristics and historical claims. The new flood maps, by providing more accurate and detailed information, will help insurers better understand local risk at a more granular level.
While understanding future climate risks is important to insurance companies (including understanding councils’ response to those risks), understanding current flood risks is of more immediate concern in terms of insurability and setting premiums.
Mitigation and adaptation measures, such as flood infrastructure improvements and land-use planning, help reduce flood risks over time. Proactive strategies such as these can positively influence insurers’ risk evaluations. This supports continued insurability and helps keep premiums down over the longer term.
Will the flood maps affect my property value?
Property values depend on many factors, including location, local amenities, demand, and market conditions. Flood risk can influence buyer perceptions, but clear and accurate flood risk information often supports informed decision-making and can reassure prospective buyers.
Experience shows that property values tend to be more resilient in areas where risks are openly managed and where planned flood mitigation measures are clearly communicated. Our goal is to manage and reduce flood risks effectively over time, which should minimise potential negative impacts on property values.
If you would like specific information on how this may affect your property value, we recommend you seek professional advice from a property valuer.
Flooding: who manages what?
How does the flood mapping relate to ‘local water done well’?
Under Local Water Done Well, Councils will be able to choose whether to retain all, some, or none of its stormwater management; or transfer all assets and operations to the new water services organisation. The updated flood modelling and mapping will help the future water services entity (either in-house or a new organisation with other council) manage the current and future impact of stormwater flooding across the district.