Big flood leaves huge loss of soil

Scientists say the February storm in the lower North Island produced such severe and widespread landslides that nearly 20,000ha of pasture has been lost from farms.

Erosion scars cover about 25 per cent of the damaged farmland and will take up to 10 years to regain productivity, Landcare Research said yesterday.

Staff at the Palmerston North office have used satellite remote sensing equipment to map the hill-country landslides and river valley silt deposits that did the damage.

The worst-affected district was the Rangitikei (6300ha), followed by Manawatu (4500ha) and Wanganui (4100ha).

Landcare soil scientist Troy Baisden said Manawatu-Wanganui regional council records for the February 16 flood showed the Manawatu River peaked at 8916mm.

On the Rangitikei River, the flood was the third-largest since the biggest in 1897, and the Whangaehu River experienced its largest flood in living memory.

"The floodwaters moved huge amounts of sediment, leaving widespread and serious damage," Dr Baisden said.

He spent much of the flood on Fitzherbert Bridge in Palmerston North with colleague Roger Parfitt, sampling the raging floodwaters as trees, hay bales and even livestock were swept past.

"We calculated that around the peak of the flood about 4pm, the amount of sediment moving under the Fitzherbert Bridge was 28 tonnes every second," said Dr Parfitt.

The equivalent of three truckloads of sediment passing every second translated to almost 1700 tonnes of sediment per minute.

"This lasted for eight hours, with the amount decreasing to 1000 tonnes per minute over time," he said.

The scientists analysed the carbon, nitrogen and phosphate in the samples and found 25 per cent of the sediment was made up of valuable topsoil.

Dr Baisden said this indicated a need for farmers to appropriately manage farmland on steep hill-country.

"The loss of topsoil and the nutrients it contains can take years to recover from."

He also said there was limited immediate benefit for pastures where silt had been deposited by the floodwaters.

Floodplains were fertile because of the nutrients in sediment deposited over the long term. But large, sudden deposits of sediment could have a choking effect.

It was difficult to get grass growing on thick layers of silt, and because the silt retained water, farm machinery would bog down on the paddocks.

Dr Baisden said the measurements of organic matter in flood sediment had also provided information on carbon losses that contributed to New Zealand's greenhouse gas emissions.

"More carbon can move in a day during this kind of flood than would ordinarily move in a decade."

- NZPA