Ōtautau flood risk summary
The Ōtautau township is located on the floodplains of the Aparima River and the Ōtautau Stream.
The town flooded in January 1984 when the Aparima River broke out and entered the Ōtautau Stream. The 1984 event is the worst known flood in Ōtautau.
Since then, new flood-alleviation works have been constructed on the Aparima River and the Ōtautau Stream to reduce the effects of flooding and the likelihood of repeat events. Between Wreys Bush and Ōtautau, the flood banks on the eastern side of the Aparima are 500 millimetres lower than those on the right bank. This is a design feature that provides a higher level of service to the Ōtautau township and a lower level of service to rural areas east of the river.
In floods that exceed the 1984 event, water can escape overland into the Waimatuku catchment, providing some relief to Ōtautau downstream. However, there is always residual flood risk, especially if any of the flood banks that contribute to Ōtautau’s level of service were to fail.
The urban design standard for Ōtautau is to contain a January 1984-equivalent flood discharge with 0.5 metres of freeboard. The flood discharge was assessed at 1530 cumecs at Yellow Bluffs Bridge for what was then a 1-in-100-year return period flood.
Recommendations
Environment Southland recommends that new houses in Ōtautau be erected with a minimum floor level 300 millimetres above the 1984 flood level as a safeguard against residual flood risk. For garages and sheds, we recommend constructing the floor as high as practicable to minimise potential flood damage to buildings and contents. The interpretation of “as high as practicable” should be left to the owner’s discretion.
Understanding return periods
A return period describes the probability of a flow of a specific size occurring in any river or stream; a 100-year return period event will occur on average once every 100 years.
Another way of describing flood likelihood is the annual exceedance probability (AEP). A 1% AEP flood flow has a 1% (1 in 100) chance of occurring in any one year. If a 1% AEP flood flow were recorded this year, the probability of another flow of that size occurring in the following year (or any subsequent year) would still be 1%.
Return period | Annual exceedance probability AEP |
100 year | 1% |
50 year | 2% |
20 year | 5% |
10 year | 10% |
5 year | 20% |
2 year | 50% |
Further information
If you require specific information about an address in Ōtautau, please get in touch by email service@es.govt.nz or phone 0800 76 88 45
Useful links
Environment Southland recommends developing a flood contingency plan and familiarising yourself with the Aparima Catchment flood warning system. This may include removing vehicles to high ground if the flood warning system is activated and storing flood-sensitive items as high as practicable.
https://www.es.govt.nz/environment/flood-warning
https://envdata.es.govt.nz/index.aspx?c=water-level
https://www.es.govt.nz/environment/flood-warning/catchment-sites-and-flood-level-travel-times
