Risk summary for rural zoned land to the east and south of Winton

In the early 1990’s the existing stopbanks along the Oreti River were upgraded between Wallacetown and Lumsden, and a flood detention dam was built upstream on Winton Stream, 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 ponding area behind the Winton Detention Dam filled to within 200 mm of the spillway in November 1999. It was a lot lower in April 2010 and February 2020.

See planning maps from the Operative Southland District Plan for the mapped floodplains of the Oreti River and Winton Stream. Planning maps 32, 80 and 81.

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. On the east side of Winton the January 1984 flood level was 200 millimetres higher than the floodwater seen in the August 1980 flood photos. 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.