Council addresses Taieri flooding risk

The vulnerability of the Taieri Plain to flooding, such as near Henley, has been put to the test....
The vulnerability of the Taieri Plain to flooding, such as near Henley, has been put to the test. PHOTO: STEPHEN JAQUIERY
The Otago Regional Council has moved to provide reassurance it is getting on with what needs to be done to maintain flood defences on the Taieri Plain.

The council is collecting more than $1.4 million, excluding GST, in rates in 2023-24 to maintain the lower Taieri flood protection scheme, but concerns have been raised in the community about maintenance levels and trees causing issues for floodbanks.

Parts of the 109km network date back to the early 1900s and a recent risk assessment showed 400m would be exposed to extreme risk in a massive flood.

Dunedin Airport would be one site vulnerable to the impact.

The risk during an extreme event was considered to be very high for 16km of the network and high for 51km.

Such concerns — including a perceived lack of urgency by the regional council — were explored at a Mosgiel-Taieri Community Board meeting last week.

The board has signalled it is likely to host a public meeting.

Staff at the regional council have looked to explain precisely what the modelling for the Tonkin + Taylor consultants’ risk assessment was designed to do.

The scenario was set at a level certain to have some of the network fail — a flood even larger than one that hammered the area in 1980.

"If we chose a lower flood, it just wouldn’t stress it enough to show where the problems are," council operations general manager Gavin Palmer said.

"If you stress it too little, everything looks like it’s fine and there’s nothing to be done; if you stress it too much, everything fails equally and it’s useless as well."

Engineering manager Michelle Mifflin characterised the work as being helpful for "updating our understanding of risk in the area" and determining priorities.

Staff indicated there would be upcoming discussions with the community about levels of risk that could be considered "tolerable".

The flood scheme provides protection of a farming area, as well as Mosgiel, Outram, Momona and Dunedin Airport.

Two sections of the Waipori floodbank were identified by Tonkin + Taylor as at extreme risk which they related to variation in bank height and intrusion of trees.

Ms Mifflin told councillors a tree in question was not "in" the bank, but it did need to be taken out.

"So, instead of rushing in to whip it out, we need to actually look at how we remove it and remediate that area."

One of the oldest floodbanks is at Outram and investigative work is to start there next month.

The council also provided the Otago Daily Times with a rundown of work since 2020.

This included a $5m upgrade on the Taieri near Mosgiel that began in March last year.

It involves reconstruction of about 2.5km of floodbank and two bridge replacements.

The Ministry of Business, Innovation and Employment (MBIE) contributed $3.2m of funding and the project is due to be completed in 2025.

At Outram, a floodbank "weighting blanket" was added last year to curb seepage that threatened the integrity of the floodbank.

MBIE chipped in $1.1m and the council $0.6m to the $1.7m project.

An upgrade of 906m of floodbank at Henley was completed in December last year.

A site investigation would soon get under way into restoring capacity of the Silver Stream, the council said.

"Project scoping" was under way for upgrading the Riverside spillway gates on the Taieri River at Outram.

Routine work included floodbank inspections, repairs to banks and structures, control programmes for rabbits and plant pests, mowing and waterway inspections, the council said. — Additional reporting Hamish MacLean