Otautau flood risk summary
The Otautau township is located on the floodplains of the Aparima River and the Otautau Stream.
The town was flooded in January 1984 when the Aparima River broke out and got into the Otautau Stream. The 1984 flood is the worst known flood in Otautau.
Since that time new flood alleviation works have been constructed on the Aparima River and the Otautau Stream to protect the town from a repeat of past floods. In the reach of the Aparima River between Wreys Bush and Otautau, the stop banks on the eastern side of the river are 500 millimetres lower than those on the right. This is a design feature that provides a high standard of protection to Otautau township and a lesser standard of protection to the rural areas east of the river.
In floods that exceed that of 1984 water can to escape overland into the Waimatuku catchment, thereby providing some relief to Otautau further downstream. However, there is still a residual risk of flooding, especially if any of the many stop banks that contribute to Otautau’s protection fail.
The urban protection standard design, for Otautau, is to contain a January 1984 flood discharge with 0.5 metres of freeboard. The flood discharge was assessed as 1530 cumecs at the Yellow Bluffs Bridge for a then 1 in 100 year return period flood.
Recommendations
Environment Southland recommends that new houses in Otautau are erected with a minimum floor level 300 millimetres above the 1984 flood level as a safeguard against any residual flood risk. For garages and sheds we recommend that their floor level is constructed as high as is practicable to minimise the risk of the building and its contents being damaged by floodwater. The interpretation of ‘as high as practicable’ should be left up to the owners’ discretion.
Understanding return periods
A return period describes the probability of a flow of a specific size occurring in any river or stream, meaning a 100-year return period will occur on average once every 100 years.
Another way of measuring flood risk is using the annual exceedance probability or AEP. AEP is the probability of a specific size of flood flow occurring in a single year. A 1% AEP flood flow has a 1% or 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 being recorded 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 Otautau, 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
