Coastal Flooding

Visit the Coastal Flooding Map to find out whether the latest information the Council holds indicates your property may be susceptible to coastal flooding.

The maps show areas that may be susceptible to coastal flooding in the present day and for future times (2050, 2080 and 2130) under different climate projections.

Please note that the studies on which the mapping is based are of regional scale and are indicative only of coastal flooding affecting any individual site or property. If detailed information about a specific property is required, a qualified professional will need to carry out a site-specific assessment.

Natural hazard maps change as our environment changes and our knowledge evolves. The maps reflect our most up-to-date understanding of coastal flooding, based on the latest information available to us.

The coastal flooding maps identify areas that are potentially susceptible to coastal flooding and wave runup. These maps are based on a range of factors like natural land topography and subsidence and scenarios for our changing climate, such as sea level rise and storm surges.

Land not directly connected to the coast may appear on the maps due to the low elevation of the land. Though these areas are not expected to be subject to seawater flooding from overland flow they may be subject to ponding from inflow of seawater through stormwater pipes, elevated groundwater levels or river flooding that may become more prevalent as sea levels rise.

The maps are prepared at a regional scale and are only indicative of potential coastal flooding for any individual site or property. They provide projections based on different climate change scenarios and the best information currently available, but they do not guarantee outcomes.

It is possible that larger events than those mapped (such as beyond the 1% AEP scenario) could occur, or that future changes alter the likelihood of coastal flooding.

Over time coastal flooding and storm events may have increasing impacts on everyday life in coastal communities. This could include road access to individual properties, delivery of essential services such as telecommunications, and access to coastal recreation reserves.

Natural hazard information is used to inform planning, building, and resource consent processes.

Before starting any development, check the Nelson Resource Management Plan (NRMP), National Environmental Standards developed under the Resource Management Act 1991, and the Building Act 2004 to confirm whether a consent is needed. If a consent is required, natural hazard information will be considered through the resource and building consent processes.

Council has also adopted a guidance document called an Induction Practice Note. The Inundation Practice Note explains and sets out a standard approach for technical professionals such as surveyors, engineers and the Council, to calculate floor and ground levels when applying for and assessing resource and building consents.

Councils must consider natural hazards and the effects of climate change when making decisions about land use and development.

The New Zealand Coastal Policy Statement 2010 (NZCPS) requires councils to identify areas that are potentially affected by coastal hazard over at least the next 100 years and to avoid or reduce the risk of harm from those hazards.

The natural hazards webmaps show the latest information we have on a range of hazards. This information can support planning and raise awareness of natural hazards in the region. As new information becomes available, our understanding of coastal hazards in Nelson continues to improve and this information can be used to inform updates to our website.

Meanwhile, maps/overlays and related planning provisions for natural hazards exist in the NRMP. The NRMP is a statutory plan under the Resource Management Act 1991 and can only be updated through a formal plan change process. A future plan change may introduce provisions relating to coastal flooding or this could be included in a new plan that replaces the NRMP.

Records show that sea levels are rising in many areas. One of the key influences on future sea level rise and coastal flooding will be changes to the temperature and volume of the earth’s oceans, which are strongly influenced by global greenhouse gas emissions.

Scientists predict that sea level will continue to rise in the future. They can’t predict exactly when and how natural hazard events will occur, how severe they will be, and what effect they will have on our community, environment and landscapes. Because of this uncertainty, any future planning has to account for a range of possibilities.

The maps reflect different sea level rise scenarios and probabilities of occurrence.

Larger hazard events can be classified using what is known as an Annual Exceedance Probability (AEP). An AEP indicates how often a flood of a certain size is likely to occur in any year. It is usually shown as a percentage. A ‘1% AEP’ is often called a 1 in 100-year event and it means that there is a 1% chance of an event happening or being exceeded every year. This doesn’t mean a flood event of this size, or larger, would happen once every 100 years, it could happen more or less often.

If sea levels rise due to climate change, coastal flooding is expected to happen more and affect larger areas. A flood that used to have a 1% chance of happening in a year might have a 5%, or even 10% chance of happening in any year in the future.

These projections are provided on NZ Sea Rise Our Changing Coast and relate to sea level rise projections.

P50 means that there is a fifty percent (50%) chance that the sea level rise projection under the chosen climate scenario will be exceeded, and a fifty percent chance that sea level rise will be less than projected.

P83 (83rd percentile) means that there is an eighty-three percent (83%) chance that the sea level rise projection under the chosen climate scenario will be equal to or less than this, and a seventeen percent (17%) chance that the projected sea level rise will be exceeded. A P83 projection represents an unlikely though still possible, projection.

The use of these scenarios is recommended in the National Adaptation Plan.

LiDAR or Light Detection and Ranging, is laser scanned mapping of the earth’s surface. It provides elevation levels or height information. Landform changes that have occurred since the most recent mapping and LiDAR survey are unlikely to be included in models. These changes could be for example, land raised or lowered by earthworks, natural subsidence or land slips.

Tidal amplification happens when the shape of the coastline, river channels, or estuaries causes the incoming tide to become higher than it would be in open water. This can occur when the tide moves into a narrow or shallow area, like a river mouth or estuary, and gets “squeezed,” making the water level rise more than usual.

MHWS-6 (Mean High Water Springs-6) is a high tide level that only a small number of tides (about 6%) exceed. Most tides are lower than this level.