Soil in motion

During the heavy rainfall event in early June, Hawke’s Bay Regional Council’s scientists were out measuring how much soil was carried out to sea – and there was plenty.

Regional Council Land scientist Tim Norris headed to the Tukituki River at Red Bridge (Waimarama Road) during the recent 1-in-5 year flow event.

Tim kept a close eye on the rising river level from the Regional Council’s sophisticated river flow and level system, before retrieving sediment samples collected by a recently installed automatic sampler.

The flow combined with sediment collected during sampling means that about 14,500 tonnes of sediment was flowing past Red Bridge and out to sea every hour at the river’s peak flow.

“With these samples, we can work out how much sediment has come down the river. At its peak we recorded 2 million litres of water per second passing Red Bridge - around 7 billion litres an hour,” says Tim.

“We measure sediment samples during normal flows, but this rain event gave us the opportunity to find out how much soil gets ‘mobilised’ in the river and fed out to the marine environment,” adds Tim.

The Regional Council’s science staff plan to carry out similar sampling in future on the Tukituki and other river systems in the region at high flows, to offer a clearer picture of sediment movement.

27 June 2018

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