MEDIA RELEASE
 

Monitoring unusual river mouth changes

Friday 8 May 2009

Environment Bay of Plenty staff are working closely with the Whakatane District Council to monitor the Whakatane river mouth since noticing an unusual modification of the Opihi Spit.

Recent coastal dynamics have removed the build-up of sand and sediment that normally protects the seaward side of the rock confinement on the western side of the river mouth and separated it from the spit.

This spit breakthrough has occurred a number of times in the past as a consequence of substantial flooding, but not by the action of coastal currents and waves alone.

In this new shape, the spit is much safer from a flood passage point of view as the river is better able to erode the spit sideways during the early stages of a flood.

During the 2004 flood this widening of the river mouth was found to dramatically reduce flood levels in the river.

Under a resource consent that is currently up for renewal, a 50 metre wide flood channel must be maintained at a specific point to allow flood waters to escape through the spit.

The changes in the formation of the river mouth however have created difficulties for navigation of the harbour entrance.

A large part of the flushing effect has now been lost as a percentage of the ebb flow now exits the river to the south of the rock confinement which may contribute to additional shallowing of the bar and increased restrictions to navigation.

The Eastern Bay of Plenty Harbour Master has some concerns regarding the efficiency of the rock confinement to direct ebb flows out through the river mouth.

Environment Bay of Plenty’s engineers are working with Whakatane District Council’s operations staff to look at the longer term impacts of these changes. They are working to maintain the efficiency of the managed floodway and also to ensure that the harbour entrance remains accessible and navigable.

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