Nelson climate adaptation

Coastal flooding is the flooding of low-lying coastal land with seawater. This can happen when extreme weather events lead to an elevated sea level and waves or swells. Coastal flooding can pose risks in the form of damage to property and dwellings, infrastructure and transport networks, risk to life, environmental degradation, erosion or threats to other things valued.

The present-day coastal flooding maps show areas that might be affected by flooding at present day sea-level in an extreme coastal flooding or 1% AEP event as a result of storm surge combined with high tide. The maps assume that no defensive or protective measures have been taken beyond those that already exist.

What is a 1% annual exceedance probability event? A 1% annual exceedance probability event is an ‘extreme’ event. Without consideration of climate change, a 1% AEP event has a 1% chance of occurring in any year, which is similar to saying an event happens once every 100 years. Because there is the same chance every year of this event happening, a few such events might occur within a few years of each other.

The future maps show how a 1% AEP-coastal flooding event is expected to change for a range of sea-level rise scenarios (0. 3m, 0.5m, 1.0m, 1.5m and 2.0m). These maps also assume that no further defensive or protective measures have been implemented, other than those that already exist. We know that the sea level is rising, but we don’t know yet how quickly. The amount of future sea-level rise we experience depends on the amount of greenhouse gas emissions we continue to emit and offset globally, both now and in the future.

The Ministry for the Environment recommends a risk-based approach, including applying a range of future sea-level rise scenarios for the assessment of coastal hazards and putting together an adaptation strategy. These scenarios are based on IPCC modelling of climate change and consider varying future emissions scenarios called ‘Representative Concentration Pathways (RCPs)’, ranging from low to continuing high emissions.

The map below shows the difference between a present-day 1% AEP coastal flooding event vs. a similar event after 1.5 metre sea-level rise.

As the sea-level rises, the depth of flooding for a particular severity or frequency of flood will increase. For example, an extreme flood (i.e. a 1% AEP event – 1% chance of happening in any given year) in an area might increase from a depth of 0.5 m at present to a depth of 1 m or more in future. Similarly, how often a particular depth of flooding happens will increase. For example, flooding deeper than 0.5 m in an area at present might change from happening from once in 100 years (on average) at present to once in 10 years, or even annually, in future. These maps give us a general indication of the scale and extent of exposure to hazards across our neighbourhoods. They do not provide an assessment of risk to individual properties. However, they will help us make decisions in consultation with those that live there.

Local rivers, including the Maitai River, are prone to flooding, especially when heavy rain occurs over a long period of time, and when peak flow coincides with a big tide. We are considering Maitai River flooding currently as it is a priority area for river flood mitigation. The overland flows that start in this area contribute to flood issues in the downstream basin areas of The Wood, and the city centre.

Maitai River flood levels will be increasingly influenced by sea-level rise as well as increases in rainfall intensity. We have mapped the extent of a range of 1% AEP flood scenarios for the Maitai River – between the present day and the year 2130. This means an event that has a 1% chance of happening in any given year. The mapped future events take account of sea-level rise, and higher temperatures which will result in increased rainfall intensity.

River flooding occurs when water spills over or breaks out from a river channel onto land that is normally dry. In assessing flood hazard for Nelson catchments, consideration has also been given to the potential for a high tide to coincide with a stream or river flood event.

The Maitai River flood map relates to river flooding only, including mapping of flow paths resulting from channel overflow. This flood map may not represent flooding caused by surface ponding, overflow from stormwater networks, or runoff of local stormwater prior to it reaching a stream or river.

Nelson City Council last engaged with the public about river flood hazards in 2021. Flooding of the lower Maitai River, downstream of Hanby Park, is being considered as part of the Nelson climate adaptation project, so that we can understand the values and objectives of the community affected by this river flooding.

 Click here to learn more about river flooding in the Nelson Region. 

The maps are based on technical assessments undertaken by NIWA and the engineering consultancy Tonkin and Taylor Ltd, who do these hazard assessments for councils around the country.

Tonkin and Taylor calculated extreme coastal water levels for different areas across the whole Nelson coastline. By mapping the extreme coastal water levels against land elevation, they are able to predict areas that may be affected by coastal flooding. This is called a ‘bathtub approach’ – imagine a bathtub that fills to equal levels with water across the tub. The method for developing the model can be found at  shape.nelson.govt.nz  or via the map viewer.

The methods used by Tonkin and Taylor Ltd to assess and map coastal flooding have been peer reviewed by coastal expert Jim Dahm of Eco Nomos Ltd to make sure they conform to best practice. The reports associated with the mapping assessment and peer review can be viewed on Council’s website  https://shape.nelson.govt.nz/coastal-hazards .

Council is also updating the coastal flooding mapping to reflect the latest topography survey (LiDAR) that was completed in 2021.

We continue to review our models as new information becomes available. The NZ SeaRise: Te Tai Pari o Aotearoa programme recently released location-specific sea-level rise projections out to the year 2300 for every 2km of the coast of New Zealand. More information and maps are available at:  https://www.searise.nz/ . We are working on reviewing our models to see how this new data may affect results, and council staff are looking at how the new information affects our adaptation response planning.

The coastal flooding maps have been produced in line with current, best-practice techniques, using aerial LiDAR. They provide a reasonably accurate indication of the areas that are currently vulnerable and the areas that could potentially be exposed to risk over the next 100 years and beyond. These maps give us a general indication of the scale and extent of hazards across our neighbourhoods. They do not provide an assessment of risk to individual properties. The maps cannot be relied upon for detailed property development purposes. However, they will help us make decisions in consultation with those that live there.

What is LiDAR? “LiDAR” is an acronym for “light detection and ranging.” It uses light pulses from a laser to accurately map the contours of the earth’s surface. Council's previous LiDAR survey was undertaken in 2015, and updates to modelling with 2021 LiDAR is being undertaken in stages, starting with the coastal flooding maps.

This phase of engagement on Nelson Climate Adaptation ended on14 August 2022. However, you can share your views on adaptation or find out more about adaptation work at any time by emailing  adaptation@ncc.govt.nz .

This is a multiyear project, and Council will be coming back to the community to seek feedback on specific adaptation options for different locations around the city.

We will ensure that any future events are widely advertised, so you have an opportunity to participate and to share your views.