By Daniel Collins
As we have seen in Northland in recent days and in Christchurch in March, severe floods pose a significant threat to rural and urban lives and livelihoods. One person drowned in the Waitangi River on Saturday, and the cost of the flooding will likely be in the millions. As a point of comparison, the Southland floods of 1984 cost insurers $140 million (inflation-adjusted to 2014).
Protecting against flood hazards is a vital part of local government responsibilities, as mandated under the Resource Management Act 1991 and the Soil Conservation and Rivers Control Act 1941. It is also important for land owners to know what level of flood hazard they are exposed to as they decide how to develop their property. To calculate this flood hazard we turn to historical records of river flow.
At present, the only national maps of flood likelihood date back to 1989, which used river flow data up to 1986. A 1989 contour map of average annual flood magnitudes in the North Island, scaled by catchment area, is shown below. Floods are larger the further downstream you are, and particularly in certain regional pockets across the island, such as Northland, the Coromandel, and the East Cape. Also shown below is a map of the ratio of the 100-year flood to the annual flood.
With 30 years having now passed since developing these maps, we have about twice as much additional data to work with, and it is time to revise our national flood statistics.
To this end, NIWA is carrying out a two-year programme to develop an up-to-date regional flood estimation tool for New Zealand. We are drawing together data from over 600 monitoring sites across the country, combining additional systematic data with more ad hoc historical observations, and following international best practice in analysing the data. We are currently half-way through the work and will present our progress at the hydrological community’s conference in November. Once completed, the results will be available to regional councils, consultants, and other interested parties as they identify methods to avoid or mitigate flood inundation risk.
Funding for this work comes from NIWA core funding and EnviroLink. Support for this work comes from all Regional Councils and Unitary Authorities.
Dr Daniel Collins is a hydrologist and water resources scientist at NIWA.
McKerchar AI, Pearson CP (1989). Flood Frequency in New Zealand. Publication No 20 of the Hydrology Centre. Christchurch: Department of Scientific and Industrial Research.