Flood risk summary for zoned rural land to the west and north

In the early 1990’s the existing stopbanks along the Oreti River were upgraded between Wallacetown and Lumsden to prevent the type of flooding that occurred in the April 1968, October 1978, April 1980, January 1984 and March 1987 floods. However, in the Winton area those floods only have return periods of approximately 15 to 20 years so the standard of protection is not high in statistical terms. A 20 year return period flood has a 5% chance of occurring in any one year. The flood alleviation works are not designed to protect against larger flows. It is more likely for stopbank systems to be breached when they are overtopped as the overtopping floodwater can scour into the stopbanks.

Flood statistics

Oreti River Flood Statistics for Environment Southland’s monitoring sites at Lumsden Cableway, Centre Bush and Wallacetown. Centre Bush is the closest monitoring site to Winton. The design standard of the Oreti River stopbanks at Centre Bush is a flow of 1200 cumecs. A 100 year return period flow on the Oreti River at Centre Bush is 1695 cumecs.

The significant residual flood risk is even greater when one takes into account climate change and the associated predicted increases in rainfall. The Ministry for the Environment’s climate change predictions for Southland include an increased frequency of heavy rainfall events and an increased likelihood of extreme rainfall. It also predicts up to a halving of flood return periods by 2040 and up to a fourfold reduction by 2090.

There have been three large floods since the flood alleviation works were upgraded. The November 1999 was the largest of the three, having a return period of approximately 20 years in the Winton area. The April 2010 and February 2020 floods were a similar size having a return periods of approximately 10 years in the Winton area. Upstream of Winton, the Oreti River was generally contained by the stopbanks. Downstream of Winton, there was some overtopping of the stopbanks, more so in 1999, and mainly towards the Lochiel Bridge.

Recommendations

The Southland Regional Policy Statement 2017 directs Southland Councils to reduce risk to its communities from natural hazards.

Increasing the density of residential development in this area would set a precedent for further intensification and, given the significant residual flood risk, would be contrary to the direction to reduce risk to communities from natural hazards. On that basis, Environment Southland would not support subdivision of in this area for residential purposes.

For new dwellings in this area, Environment Southland would recommend that the floor level of the dwelling was no less than 600 millimetres above the highest past recorded flood level to mitigate the flood risk. For further advice on minimum floor levels please enquire here.

For farm sheds in this area Environment Southland recommends that the shed is constructed with its floor level as high as is practicable to minimise the potential for flood damage to the shed and its contents. The interpretation of as high as is practicable should be left to the owner’s discretion.

For dairy sheds in this area Environment Southland recommends that flood sensitive electrical plant is elevated as high as is 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 Oreti Catchment flood warning system. Please see the attached fact sheet and the following pages on Environment Southland’s website.