An investigation of possible flood mitigation options for the Wainui Te Whara Stream in Whakatāne has indicated that channel re-grading and deepening is likely to be the fastest and most cost-effective way to reduce the risk of future stopbank overtopping.

The joint Bay of Plenty Regional Council/Whakatāne District Council study was undertaken utilising the Bay of Plenty Flood Risk Framework and involved an extensive analysis of the stream’s behaviour during extreme weather events and an ‘all-of-catchment’ approach to risk reduction.

The outcomes of the study are summarised in a report to be received by the District Council’s Projects and Services Committee on Thursday, which recommends that stream channel re-grading – which involves deepening the channel by about one metre along much of the urban section – be supported, in principle. Council Infrastructure General Manager, Tomasz Krawczyk, says that subject the Council’s adoption of the recommendation, detailed engineering investigations will be undertaken to confirm the viability of deepening and re-grading the channel.

“An assessment of this option indicates that it would be possible to achieve a capacity flow of close to 30 cubic metres per second, at an estimated cost of between $1.5 million and $1.8 million,” he says. “With additional mitigation measures in the upper catchment to complement the increased downstream capacity, we believe that channel re-grading offers the best solution for containing flood flows such as that experienced during last Easter’s extreme rainfall event.”

Mr Krawczyk says the channel re-grading option should allow the work involved to be started and completed relatively quickly.

The proposed work would see the channel grade improved between the Valley Road bridge and the western side of the King Street bridge – a distance of about 1.3 kilometres – and the channel deepened by about one metre in places. That would require the removal of some 4,000 cubic metres of soil and rock.

Before a final decision to proceed is made, a technical feasibility assessment will be completed, focusing on the effect of the work on sedimentation and stream morphology (flora and fauna); the potential for and solutions to stopbank undercutting; and the associated upgrades that would be required to existing bridges, pump station outfalls and infrastructure service crossings.

Members of the Wainui Te Whara Flood Affected Residents group are expected to provide feedback on the proposal at Thursday’s meeting. That input, and any feedback from planned public consultation, will be taken into consideration in the final project design, which Mr Krawczyk hopes to bring back to the Council before the end of March.