Waiwhetu Stream flows naturally again

After 18 months spent excavating 56,000 tonnes of contaminated silt, a cleaner, deeper and wider Waiwhetu Stream is about to start flowing naturally again.

Contractors began removing the sheet piles which dammed the stream this morning (Friday) after the last of 4400 truckloads of “Waiwhetu ooze” had been carted away to the Silverstream Landfill.

A further 32,000 tonnes of cleanfill were removed to widen and deepen the stream channel to allow it to better cope with flooding.

To do this work the water above the dam had been pumped out of its bed and piped along the bank for 200 hundred metres at a time before flowing back into the stream. A second dam stopped the water flowing back up and created a dry enough stream bed for diggers to excavate the ooze.

By starting at the top of the stream and creating a series of dry cells and removing the ooze within them, the lower Waiwhetu Stream, the most contaminated in the country, was cleaned.

From here there will be extensive planting and landscape enhancement programme to help protect the banks, provide wildlife habitat and make it a more natural and enjoyable area to be in.

The $21M project has been jointly funded by Greater Wellington, Hutt City Council and the Ministry of the Environment.



Background information

The lower reaches of the stream were heavily contaminated with heavy metals, copper, zinc and pesticides that were hazardous to human and ecological health.

Right through to the 1970s Waiwhetu Stream was a de facto sewer for trade waste from industry in Gracefield because there wasn’t anywhere else for it to go. Things improved in the late 1970s when a trade waste system was set up.

The flooding in the February 2004 storm is what originally drove the need to widen and deepen the channel. When the stream overtopped its banks during that storm about 70 homes and businesses, mainly at the south of Riverside Drive were flooded, causing several million dollars damage.

The widening and deepening was not able to be done until the contamination was cleaned up and hence the combined project of an environmental clean and flood protection work.

The next phase of the flood plan management planning process will be working with the community on how best to manage the flood risk that still remains in the area. There is still a significant flood risk in some areas and a sustainable approach needs to be taken to managing this risk.