Riversdale flood risk summary
Flood Risk
Riversdale is located on the floodplains of the Mataura River and McKellar Stream.
Riversdale has no flood protection infrastructure.
Recommendations
Environment Southland also recommends that new dwellings in Riversdale are erected with a minimum floor level no less than 600 millimetres above the October 1978 and November 1999 flood levels.
By including a 600 millimetre freeboard over and above the October 1978 and November 1999 flood levels, the floor levels derived are approximately 80 to 120 millimetres lower than those which are derived from an alternative approach based on overbank spills from the Mataura River flowing down the true right floodplain and into both the eastern and western sides of Riversdale in a 0.5% Annual Exceedance Probability (AEP) event at Parawa.
The peak flood discharges used in this latter method have been scaled up by 10% to allow for possible increases in runoff arising from climate change effects through to 2063.
In addition to that, a 300 millimetre freeboard has been included to cover local effects, transient short term effects (e.g. surges, wave action generated by the passage of vehicles through the floodwaters ) and uncertainty inherent in the calculation methodology.
The October 1978 and November 1999 floods were approximately 2% and 1% AEP events respectively at Parawa.
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 Riversdale, 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 Mataura 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
