Flaxmill Bridge Options

Update - September 2020

The section of road between Flaxmill Bridge and Wanstead Bridge has been closed previously for extended periods due to flooding. Porangahau Road has been identified as a ‘Lifeline’ route and is therefore critical that increased resilience along this route is achieved through flood mitigation.

Hydraulic modelling identifies that Porangahau Rd has a <2year LOS, with the lowest levels being along a 1300m length adjacent to Wanstead Swamp wetland.

An engineering options assessment indicates that a 10 year LOS is achievable within the funding and timeframe constraints of the PGF, and a further lift to a 75 year LOS is considered feasible if a longer timeframe is available and funding outside the PGF is obtained.

Therefore, the preferred option strategy is:

• Online road raising: raise the road embankment level between Wanstead Bridge and Flaxmill Bridge to a 10 year LOS, with associated improvements to Flaxmill Bridge and Ugly Hill Corner culverts, using PGF funding and taking advantage of a scope of work that is online within the road reserve and likely to have a straightforward consenting process. The cost estimate for the works is $8.2M.
• Future online and offline works: begin planning (including consultation) for associated future online and offline works that would further raise the LOS to 75 years capacity for an additional budget cost of $3.3M. This offline work is mainly stream and channel widening through private land and provides improvement works to offset future projected climate change impacts.

You can read our report on the Flaxmill Inundation project here.

Update - June 2020

A key focus of the programme of work is the bridge at Flaxmill.

Flaxmill Bridge has a history of flooding. Rainfall events in February 2004 and March 2018 cut both gas and electricity supplies and compromised community safety. In 2018 the road either side of Flaxmill Bridge was closed in excess of 20 days due to flooding.