Te āwhā me te waipuke Storms and floods

A storm is a weather system with strong winds and often heavy rain, which can lead to flooding. A flood is when water covers normally dry land.

Floods are usually caused by heavy or prolonged rainfall and may be caused by rain some distance away. Many of New Zealand’s rivers are long, and heavy rain in the upper reaches of their catchment can result in flooding much further downstream.

Floods can also occur due to storms (including heavy swells and storm surges), high sea levels at river mouths, land height changes due to earthquakes, hydrothermal activity, and volcanic activity, and failure of dams and stop-banks (levees).

You may often come across the term Annual Exceedance Probability (AEP) for describing the likelihood of storms and floods. AEP describes how often a particular type of may occur within a year. A 1% AEP flood has a 1% chance of happening in any given year. It is equivalent to a one-in-100-year return period. It is important to recognise that this does not guarantee that a storm will happen within this predicted time period.

Storms and flooding can occur almost anywhere in the New Zealand and are one of our most frequent and foreseeable hazards.

In 2023 New Zealand experienced two of the most damaging storm and flood events in recent years.

Cyclone Gabrielle impacted the North Island of New Zealand in February 2023 with extreme rainfall and gale force winds causing widespread flooding, landslides, infrastructure outages, and, tragically, caused 11 deaths.

This event followed a historic storm on 27 January 2023 that severely impacted Auckland. This event claimed 4 lives, damaged homes and infrastructure, and broke rainfall records for the city.

Heavy rain can cause stream and river levels to rise, leading to dangerous, fast-flowing currents. Rain can cause landslides or flooding, and high river levels with strong currents can create floods and destroy bridges.

Flooding

• River flooding generally happens during heavy rain, when rivers overflow their banks onto a floodplain. A floodplain is the flat section next to a river, and can flood quite regularly. Normal rainfall soaks into the soil, is taken up by trees and plants, and runs off the land to form our streams and rivers. Floods happen when there is too much water and the run-off can't be contained by the river's banks and floodbanks.
• Surface flooding can happen when heavy rain falls either in a small area, or in an urban area with many hard surfaces that prevent rainwater from soaking into the ground. Usually, surface flooding starts quickly but generally only lasts minutes to hours. It is often associated with thunderstorms and short, intense rainfall.
• Groundwater flooding can happen during periods of unusually high rainfall, when the rising water table causes water to rise out of the ground. Groundwater flooding can bubble up, start flowing along the surface, and also directly rise up into homes.
• Flash floods occur rapidly. They are usually the result of intense rainfall that overwhelms natural or urban drainage systems. Flash floods can appear as a torrent, carry rocks, mud, and other debris, and sweep away many things in their path.

Coastal flooding and hazards

• Storm surges can be created from a combination of high tides, low pressure from storms and high winds. They can cause beach erosion and threaten life and property. Storm surges are most common on our coasts, where severe weather can be exacerbated by extreme tides. Storm surges can also happen in large lakes.
• Heavy swells and large waves during storms can damage Aotearoa New Zealand’s low-lying coastal areas, especially when they occur alongside storm surges.
• Sea-level rise due to climate change may contribute to more frequent coastal inundation and wave damage.

Other storm hazards

• Hail can cause injuries to people, break glass and damage cars and roofs. Crops can also be ruined and livestock killed. In large quantities, small hailstones can build up to be centimetres deep, making driving dangerous.
• Lightning can occur often in New Zealand with more than 50,000 lightning strikes per year. Lightning can be fatal, with one death on average reported every five to ten years in New Zealand.
• Strong winds and tornadoes can fell trees and poles, tear off roofs, and cause objects to fly through the air. Sometimes there is significant damage and threat to public safety if tornadoes pass through built-up areas.

Storms and floods can cause considerable damage to buildings and infrastructure, as well as injuries and loss of life.

Homes in low-lying areas near waterways are generally more at risk of floods. Buildings with low floors are more likely to be flooded in these areas.

Storms and flooding can lead to:

• structural damage, such as loose or buckled floors, and roof, floor, or foundation cracking
• undermined or scoured foundations
• electrical and appliance damage
• mould or mildew, if the property is not ventilated, dried, and cleaned after flooding
• deposited water, silt, mud, or debris

• Learn how to make your home saferopen_in_new
• Search your address in the Natural Hazards Portal to find out which hazards may affect your property.
• Read more about floodsopen_in_new and how they can affect people and places.
• Check that you have the right insurance in placeopen_in_new, including home insurance that may qualify you for natural hazards cover.
• Check your local council website for information about local hazards and emergency planning in your area.