In the early 1990s, the existing flood banks along the Ōreti River were upgraded between Wallacetown and Lumsden to reduce the likelihood and effects of flooding similar to the April 1968, October 1978, April 1980, January 1984 and March 1987 events. However, in the Winton area, those floods have return periods of approximately 15–20 years, so the level of service is not high in statistical terms. (A 20-year return-period flood has a 5% chance of occurring in any one year.) These flood-alleviation works are not designed for larger flows. Overtopping increases the likelihood of breach, as overtopping floodwater can scour into the flood banks.
Flood statistics
Ōreti River flood statistics are available for Environment Southland’s monitoring sites at Lumsden Cableway, Centre Bush and Wallacetown. Centre Bush is the closest monitoring site to Winton. The design capacity/level of service of the Ōreti River flood banks at Centre Bush is 1200 cumecs. A 100-year return-period flow on the Ōreti River at Centre Bush is 1695 cumecs.
The significant residual flood risk increases when climate change and the associated projected increases in rainfall are taken into account. The Ministry for the Environment’s climate change projections for Southland include more frequent heavy rainfall and a higher likelihood of extreme rainfall. They project flood return periods could halve by 2040 and reduce fourfold by 2090.
There have been three large floods since the flood-alleviation works were upgraded. November 1999 was the largest, with an approximate 20-year return period in the Winton area. The April 2010 and February 2020 floods were of similar size, each with an approximate 10-year return period in the Winton area. Upstream of Winton, the Ōreti River was generally contained by the flood banks. Downstream of Winton, there was some overtopping of the flood banks—more so in 1999—and mainly towards the Lochiel Bridge.
Recommendations
The Southland Regional Policy Statement 2017 directs Southland councils to reduce risk to communities from natural hazards.
Increasing the density of residential development in this area would set a precedent for further intensification and—given the residual flood risk—would be contrary to that direction. On this basis, Environment Southland would not support subdivision in this area for residential purposes.
For new dwellings in this area, Environment Southland recommends that the finished floor level is at least 600 millimetres above the highest past recorded flood level to mitigate flood risk. For further advice on minimum floor levels, please enquire here.
For farm sheds in this area, Environment Southland recommends that the shed be constructed with its floor level as high as practicable to minimise the potential for flood damage to the shed and its contents. The interpretation of “as high as practicable” should be left to the owner’s discretion.
For dairy sheds in this area, Environment Southland recommends that flood-sensitive electrical plant be elevated as high as practicable to minimise the potential for flood damage.
Environment Southland recommends that owners/occupants develop a flood contingency plan and make themselves familiar with the Ōreti catchment flood-warning system. Please see the attached fact sheet and the following pages on Environment Southland’s website.
