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, the 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.
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.
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 new techniques for forecasting floods using computer models of atmospheric conditions and river catchments.
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 tend to 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.
Managing flood-prone areas
As even major river works may not prevent flooding, there are now measures to alter the way flood-prone areas are developed. 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. The public can obtain a Land Information Memorandum (LIM) from Councils which includes information on the potential for any property to be flooded.
In many regions, riverside areas are reserved as parks, sports fields and parking areas, so flooding will cause minimal damage.