Regional and district councils have the primary responsibility for managing flood hazards. They monitor rainfall, river flows and lake levels, and maintain flood protection works.
The earliest indications of potential flooding are the heavy rain warnings issued by weather forecasters. In addition, councils independently operate networks of automated instruments that measure rainfall and river levels. Data from these instruments, and high rainfall rates or rising river levels, may trigger automatic warnings to staff. During extreme events such as the February 2023 Esk Valley flood, automated instruments may stop transmitting or be destroyed.
Council staff also use computer models of rainfall and river flow to determine likely rises in river and lake levels downstream, and supply warning information to communities. Among other operations, they organise evacuations, build sandbag barriers, and close roads. Most councils maintain websites and telephone services that inform the public about rainfall and river levels. These too may be vulnerable to disruption during extreme events.
Information on lake and river water levels, river flows and sediment loads also goes into a national Water Resources Database, managed by the National Institute of Water and Atmospheric Research (NIWA).
In collaboration with regional councils, NIWA has developed techniques for forecasting floods using computer models of atmospheric conditions and river catchments. These enable councils to build flood resilience into their planning, and allow people to see where floods are more likely to occur when they are considering purchasing a home.
From a drop to a deluge
Frederick W. Furkert, a New Zealand public works engineer in the 1920s, was experienced in river problems. He recalled a statement that the first engineer he worked under used to make: ‘You have never seen it rain so hard that you could not imagine it raining a little harder or a little longer; only one of those conditions is necessary to make a bigger flood than you have ever seen.’ 1
Researching climate change
NIWA also carries out research on climate change and long-term climate cycles, such as the El Niño–Southern Oscillation that affects New Zealand’s weather patterns. In El Niño years, stronger westerly winds bring more rain to southern and western regions. By contrast, the La Niña pattern brings rainy conditions to the north-east of the North Island.
NIWA climate modelling has found that climate change is likely to increase both the frequency and the intensity of extreme rainfall events. The changes will be different in different parts of the country. Floods may also be made worse by changes to other parts of the environment, such as increased amounts of sediment in rivers and changing shapes of river beds. Also, as temperatures rise, water falling on mountains such as the Southern Alps is less likely to turn to snow. This may mean rivers will flow at higher levels in winter rather than remaining low until the spring snowmelt.
Managing flood-prone areas
As even major river works may not prevent flooding, measures to alter the way flood-prone areas are developed were put in place. Under the Resource Management Act 1991, territorial and regional authorities can regulate land use and construction on high-risk flood plains. Local authorities use instrument records and historical accounts of rainfall, river levels and floods to determine hazard zones, based on the probability of land being inundated. Members of the public can obtain a Land Information Memorandum (LIM) on any property from a councils; this includes information on the risk of the property being flooded.
In many regions, riverside zones in urban areas are reserved as parks, sports fields and parking areas. This allows flood water to pond where it will cause minimal damage. In some areas, developers are required to allow overflow spaces in new subdivisions which will allow flood water to peak and recede without causing damage. New buildings are also often built with stormwater detention tanks which store water during intense rainfall events and then slowly release it, easing the pressure on rivers that are already full.
Central government and councils are also under pressure to help some communities retreat from areas impacted by floods, sea-level rise and coastal flooding. This will allow people to move to safer areas, and give more room for flood waters. They can do this by purchasing at-risk properties where the risk to life is high and it is not feasible to make them safe. After Cyclone Gabrielle in 2023, many hundreds of residential properties in Auckland were deemed to be in this category. This strategy is expected to become a costly burden on the authorities in the future. It is a particularly difficult issue for Māori communities, who have generations-long connections to their (often low-lying) land, and may have nowhere else to go as a result of significant land loss in the past.