Riversdale flood risk summary
Flood Risk
Riversdale is located on the floodplains of the Mataura River and McKellar Stream.
Riversdale has no flood infrastructure works (e.g., flood banks) in place.
Recommendations
Environment Southland recommends that new dwellings in Riversdale be erected with a minimum floor level no less than 600 millimetres above the October 1978 and November 1999 flood levels.
Including 600 mm freeboard above the October 1978 and November 1999 flood levels results in floor heights that are approximately 80–120 mm lower than those 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.
For that alternative method, the peak flood discharges were scaled up by 10% to allow for projected increases in runoff associated with climate change through to 2063.
In addition, a 300 mm freeboard has been included to account for local effects, transient short-term effects (e.g., surges and wave action generated by vehicles passing through floodwaters), and the uncertainty inherent in the calculation methodology.
The October 1978 and November 1999 floods were approximately 2% AEP 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; a 100-year return period event will occur on average once every 100 years.
Another way of measuring 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 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
