Over the past six or so years, a number of river restoration projects, known as pilefields, have been carried out in on steeply eroded banks of the Mary River, particularly around Kenilworth.

They haven’t been without their sceptics, along the lines of “ dunno why they’re wasting all that money, they’ll just get washed away in the first flood”.

First cab off the rank, the Charles Street Park pilefield and planting, behind the police station in Kenilworth, stood up well to a number of small floods, so the scepticism was modified to “just wait till there’s a decent flood”.

By anyone’s reckoning, the flooding in late February amply qualified as a “decent flood”. By the time it reached Gympie it eclipsed all predecessors bar the one in 1893. The media dubbed it “the biggest in living memory”, yes definitely a decent flood.

After a few false alarms, levels receding, then rising again, the water started going down. The pilefields had faced their biggest test. How had they fared?

It’s important to understand the state of the riverbanks prior to the construction of pilefields. There were long sweeps of steep, eroded banks and each successive flood saw more land collapse into the river and be carried away. At Charles Street Park there was the added concern that the water offtake for Kenilworth’s water supply would be completely compromised. Water was trucked in for the town while works were carried out.

Both upstream and downstream were two much larger, taller and longer, steep banks that have been tackled far more recently. The work involved “battering back” the steep bank then installing a number of pilefields, lines of vertical wooden poles fortified by rock.

They’ve been dubbed “instant trees” for they aim to do the job of original riparian vegetation sadly no longer present. The pilefields, though, are relatively short-term, the key aspect is the planting of trees, shrubs and grasses in the area protected by the piles as well as higher up the bank. A few before and after photographs, before construction and after flood, show how well they’ve stood up.

For some years, the question as to how to deal with the steep banks had been sitting in the too hard basket, not only how to do it, but how to find the funds necessary to carry it out. It was beyond the reach of most landholders.

Realising the impact of flood sediment on the southern portion of the Great Barrier Reef, and knowing that most of the sediment came from bank collapse, brought the Great Barrier Reef Foundation on board, along with a collaboration of the Mary River Catchment Coordinating Committee, the Burnett Mary Regional Group, Alluvium Consulting (to do the design)Seqwater and Sunshine Coast Council.

The pilefields stood up very, very well. There was some damage to trees planted, but nowhere near as dire as the pessimistic predictions.

And given the battering of the flood, both the height of it and the duration, it’s a credit to all involved, particularly in the design (Alluvium), the construction (GWT), the planting and maintenance (Noosa Landcare and the landholders) as well as the partners listed previously.

Article by Ian Mackay