River and Stream Flooding
Source: Trafalgar Street, Bridge, Nelson, August 2022 (NCC)
How do I know if my property is located within an area potentially susceptible to river flooding?
Visit the River Flooding Maps to find out whether the latest information the Council holds indicates your property may be susceptible to river flooding.
The map shows the potential extent and depth of flooding in the present day and for the year 2130.
Please note that the studies on which the mapping is based are of regional scale and are indicative only of natural hazards 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.
Why is my property included in a flooding area when it has never been affected before?
Natural hazard maps change as our environment changes and our knowledge evolves. The map reflects our most up-to-date understanding of flooding, based on the latest information available to us.
The river flooding assessments and maps show where flooding could happen during very heavy rain. By the year 2130, rainfall may be heavier and more frequent, which may means floods are bigger. A flood with a 1% chance of happening in any given year today (sometimes called a 1 in 100 year flood) may happen more often, and the size of a 1 in 100 year flood in 2130 will be larger than it is today.
What if my property is within an area potentially susceptible to river flooding – does this affect what I can do on my property?
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 also has 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 acceptable floor and ground levels, when assessing resource and building consents.
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.
What determines a high flood hazard area?
The table below provides a descriptive overview of how predicted flood levels are likely to impact people and property across all areas in Nelson.
| Flood hazard layer | Floodwater depth (m) | Floodwater speed (m/s) | Effect on people and property |
| North Nelson Flood Hazard Overlay | ≥ 0.1 | any | Shows where flooding might occur. Hazard levels not mapped. |
| No flooding overlay | < 0.05 | any | Generally safe for people, vehicles and buildings. |
| Flood Hazard Overlay | 0.05 to 0.3 | < 2.0 | Generally safe for people, vehicles and buildings. |
| High Flood Hazard Overlay | ≥ 0.3 | ≥ 2.0 | Generally unsafe for vehicles and people. Buildings may be vulnerable to structural damage. Some less robust building types vulnerable to damage or failure. |
Why do the North Nelson flood maps look different?
As North Nelson covers a larger, mostly rural area without much built development, and with relatively similar drainage characteristics, a different model was used.
Rural catchments in North Nelson (Wakapuaka and Whangamoa catchments) generally lack reticulated stormwater networks, so runoff travels overland towards the rivers and their tributaries. The flood models calculate flow depth and speed throughout the flood plain, so the hazard level can be readily established for any existing or new development that may be exposed to flood risk.
Flowpaths are shown in the North Nelson mapping where they are over 0.1m in depth.
What is a 1% AEP (Annual Exceedance Probability)?
Larger floods are 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 and it is very common to use a 1% AEP. This is often called a 1 in 100-year event and it means that there is a 1% chance of a flood of this size or bigger, happening in any year. This doesn’t mean a flood event would happen once every 100 years, it could happen more or less often.
The river flooding map shows two flood events. One is a 1% AEP rainfall event in the present day and the other is a 1% AEP rainfall event in the year 2130. The 2130 event assumes increased rainfall and approximately 1 metre of sea level rise under a high emission climate change scenario (RCP8.5).
