Lumsden flood risk summary

Flood risk

The main street of Lumsden was flooded in 1913 and 1935. After 1935, flood banks of some description were built. In one of the two significant floods in 1978, water came close to overtopping the flood banks. Presumably, water also came close to overtopping them during the 1980 and 1984 floods.

Flooding events in the 1990s

The existing flood banks along the Ōreti River were upgraded between Wallacetown and Lumsden in the early 1990s to reduce the effects of flooding based on past recorded floods, including those in 1978, 1980, 1984 and 1987. The flood-bank system in Lumsden is designed for flood flows up to 1100 cumecs. Using the flood statistics, this falls somewhere between a 50- and 100-year return-period flood. These works are not designed for more significant flows.

The most significant flood since the new flood banks were built was in November 1999. During the 1999 flood, erosion threatened the flood bank towards the top end of Lumsden. The Lumsden flood 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 flood banks.

Future flood risk predictions for Southland

The significant residual flood risk is greater when climate change and its associated projected rainfall increases are considered. The Ministry for the Environment’s climate change projections for Murihiku Southland include an increased frequency of heavy rainfall events and an increased likelihood of extreme rainfall. They also project up to a halving of flood return periods by 2040 and a fourfold reduction by 2090.

Recommendations

Although the level of service in Lumsden is considered high, as with any flood infrastructure system, there is always residual flood risk due to potential flood bank failure or the design capacity of the flood infrastructure network being exceeded.

To further reduce risk, Environment Southland recommends that any dwelling erected in Lumsden have a minimum floor level of 600 mm above the existing ground level, and that the floor level of non-living areas, such as attached garages and laundries, is no more than 300 mm below the floor level of the dwelling.

For detached garages and sheds, Environment Southland recommends constructing the floor level as high as practicable to minimise potential 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 residual flood risk. A more conservative approach could be adopted to further reduce 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; a 100-year return period occurs on average once every 100 years.

Another way of measuring flood risk 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%.

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