Downer New Zealand

Downer raises the mark

taking flood protection to a new level

If news reports are to be believed, one-in-100-year floods are becoming increasingly common in New Zealand. It’s lucky then that the iconic St David Street to Union Street stretch of Dunedin’s Water of Leith has been re-engineered to cope with a one-in-200-year event!

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PROJECT: Leith Flood Protection Scheme, Dunedin CONTRACTOR: Downer New Zealand For Otago Regional Council, the need to improve flood protection in central Dunedun was essential. The combined flows of the Water of Leith and its Lindsay Creek tributary have flooded the local area and central business district on several occasions, causing millions of dollars of damage.

So, after years of negotiations and planning, a tender was issued for a flood protection scheme that would see the existing river channel demolished and new embankments constructed.

The historic St David Street Bridge was to be lengthened, and an attractive network of retaining walls, stairways and concrete paths built to link grassed terraces.

The winner of the tender was Downer New Zealand. Led by Downer project manager Braydon Kelly, considerable people power was assigned to the job. Many balls had to be juggled. There were no less than 290 individual line items on the build programme. Important, for example, was the need to be sensitive to the heritage character of adjacent Otago University buildings; not to mention keeping a legion of university and local authority stakeholders happy. Top of mind also was the wellbeing of the river – a body of water

supporting a tremendous variety of aquatic life.

With up to 10,000 students passing through the site at peak periods, one of Downer’s first steps was to build a temporary bridge to help free flow of movement. This was important says Downer’s bid manager Joe Connolly, as it meant work could go ahead relatively unimpeded.

To meet deadlines, Downer ran multiple work activities concurrently – bridge extension, in-stream works and embankment works. Excavation, drilling and laying of the precast wall sections proved especially hard and time-consuming, says Joe. But with the help of rock breakers and a 70 tonne crane this was neatly completed.

Ten micro piles were installed under the new bridge pier. Built off site as a precast unit, the new section of bridge was lowered into place in a delicate operation. It was a perfect fit!

A key consideration with the bridge work was maintaining the integrity of 1500 pairs of copper data cables, fibre optic cables, HV power and a 450mm watermain. The cables control everything from building access to IT data, phone and emergency alarms on the Otago University campus. Again, thanks to Downer’s care and attention, this was not a problem and essential operations continued to run smoothly.

Some of the works involved excavating and breaking up the river banks and pouring concrete underwater in stream flows. To aid in this, sections of the river were temporarily dammed and de-watered, with a piped passage installed to downstream levels. No detail was left unchecked, with the pipe even having a ‘fish ladder’ for salmon to swim upstream.

Key to the project was the engagement and management of several key subcontractors including Hall Brothers for earthworks, Wainwright Stonemasons for bluestone works and United Scaffolding for the temporary bridge.

Eagle eyes were focused on weather conditions during the project. As Joe notes, “While the Leith can be a babbling brook one day, within 12 hours the flow can increase over 100-fold.” This actually happened midway through the project, with the remnants of ex-tropical cyclone Ita tearing through the site.

This put a dampener on things for a short while, but planning allowed it to stop major work from being washed away.

Finished in December 2014, it took a calendar year for the Leith flood protection scheme to be completed. The project was done on schedule and, thanks to some innovative construction, came in $300,000 under budget at $4.1 million.

Otago Regional Council’s director of engineering, hazards and science Gavin Palmer acknowledges the skill and experience demonstrated by Downer.

“We’re highly satisfied with the effectiveness and the results of Downer’s construction management processes. The end result is that the university and the general public have a beautifully landscaped area that adds to the attractiveness and versatility of the campus and benefits from enhanced flood protection.” l