Lumsden Hazard Summary
Flood Risk
The main street of Lumsden was flooded in 1913 and 1935. After 1935 stop banks of some description were built. In one of the two significant floods in 1978, water came close to overtopping the stop banks. Presumably, water also came close to overtopping them during the 1980 and 1984 floods.
Flooding events in the 1990s
The existing stop banks along the Oreti River were upgraded between Wallacetown and Lumsden in the early 1990s to protect against all past recorded floods, including those in 1978, 1980, 1984 and 1987. The stop bank system in Lumsden is designed to protect the area from flood flows up to 1100 cumecs. Using the flood statistics, this falls somewhere between a 50 and 100-year return period flood. The flood alleviation works are not designed to protect against more significant flows.
The most significant flood since the new stop banks were built was in November 1999. During the 1999 flood, erosion threatened the stop bank towards the top end of Lumsden. The Lumsden stop banks are close to the river and more vulnerable to erosion or scour during flood events than other areas. After the 1999 flood, rock protection had to be placed along the river bank near the upstream end of the Lumsden stop banks.
Future flood risk predictions for Southland
The significant residual flood risk is even greater when considering climate change and the associated predicted increases in rainfall. The Ministry for the Environment’s climate change predictions for Southland includes an increased frequency of heavy rainfall events and an increased likelihood of extreme rainfall. It also predicts up to a halving of flood return periods by 2040 and a fourfold reduction by 2090.
Recommendations
Although the standard of flood protection in Lumsden is considered high, as with any flood protection system, there is always a residual risk of flooding due to stop bank failure or the design capacity of the scheme being exceeded.
To further reduce the risk, Environment Southland recommends that any dwelling erected in Lumsden have a minimum floor level of no less than 600 millimetres above the existing ground level, and the floor level of non-living areas such as attached garages and laundries is to be no more than 300 mm below the floor level of the dwelling.
For detached garages and sheds, Environment Southland recommends that the garage or shed is constructed with its floor level as high as is practicable to minimise the potential for flood damage to the building and its contents. The interpretation of as high as practicable should be left to the applicant’s discretion.
Please note that this recommendation is not based on flood modelling and is a practical approach to mitigating the residual flood risk. You could adopt a more conservative approach to further reduce the residual flood risk.
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 Lumsden, 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 Ōreti 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
