Not for sale

A ground-breaking flood control system could also help improve NZ’s rivers and streams.

A technique originally developed in the United States to reduce the impact of flooding on rural land, may help combat environmental impact in flood-prone areas caused by livestock farming in New Zealand.

Two-stage channels - artificially created floodplains designed to lower the power of water by dissipating its energy during flooding - have also been shown to reduce nitrate run-off from pasture by up to 70 per cent during floods.

The channels have been in use on farms in the US mid-west for 12 years and their unexpected environmental benefits - they also result in significant reductions of phosphorous run-off and curb the build-up of sediment in streams and rivers - have New Zealand scientists excited.

Plans are being finalised to trial the concept in New Zealand from mid-2019 in a project to be run by the Canterbury Waterway Rehabilitation Experiment (CAREX) in partnership with DairyNZ.

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Professor Jon Harding of the School of Biological Services at the University of Canterbury is heading up the CAREX team with Dr Catherine Febria and says two-stage channels have real potential to combat the effects of nutrient run off from dairy farms.

"Although I don't think it is a silver bullet, it certainly appears to be something we should have in our toolbox," he says. "These (nitrate and phosphorous run-off and sediment build-up) are three of the big water quality problems in New Zealand."

The concept of the two-stage channel is simple. It incorporates floodplain zones, called benches, within modified drainage ditches. The ditches are reconfigured by cutting out their banks a metre above the bottom to a width of about three metres on each side.

This allows more area for flood water to spread out and overflow within the ditch itself. It has the effect of decreasing the velocity or energy of the water and helps absorb nutrients like phosphorous and nitrate and trap fine sediment.

Harding says the use of two-stage channels overseas and in the US have produced real benefits: "These include reductions in turbidity (the presence of sediment in water) of between 15 and 82 per cent in floods, increases in denitrification (the natural conversion of nitrate into gas) of between 35 and 49 per cent and the removal of nitrate by up to 70 per cent more than from unmodified drains.

"Tests are still to be carried out in New Zealand but indications are that many landscapes here would likely gain similar benefits."

The Nature Conservancy, a charitable environmental organisation headquartered in Virginia in the US, says two-stage channels are a win-win for agriculture and conservation.

In 2015 research funded by the United States Environmental Protection Agency (USEPA), the organisation says the channels not only improve ditch stability by reducing water flow and the need for maintenance, they also have the potential to create and maintain better habitat conditions.

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"This is done by minimising the amount of sediment and nutrients that are transported from ditch to stream to river to sea," USEPA says. "This is because the design allows the sorting of sediment, with finer silt depositing on the benches and courser material forming the bed."

Harding says the two-stage channel was developed about 20 years ago when farmers in the US mid-west were looking for solutions to severe flooding and riverbank erosion. The existing drainage systems feeding into the Mississippi River and the sea in the Gulf of Mexico were unable to adequately cope with flooding.

In 2014 Harding went to the US to talk to some of the two-stage channel pioneers - they included Professor Andy Ward and Dr Jessica D'Ambrosio from Ohio State University and Professor Jennifer Tank from Notre Dame University - and came back convinced the technique could help with similar problems in New Zealand.

Working with DairyNZ water quality team, the CAREX researchers plan to construct and monitor the performance of a range of two-stage channels. Once they have been built Harding expects it will be two years before they know whether the experiment is successful or not.

"There is no point in restricting the channels to 50 to 100m," he says. "In the US most are between 600 and 700m, so we will be looking at a good chunk of drain.

"Due to the excavation required, the upfront costs may be high, but overwhelmingly the data suggests two-stage channels offer an affordable, low maintenance, long-term solution in the US. This is supported by evidence from The Nature Conservancy which suggests the payback period for excavation costs is about 14 years."

Harding says while there may be some concern among farmers they will have to surrender productive land to provide space to create the floodplain benches, the US experience shows that little or no additional land has been given up.

He says other environmental benefits are possible including improving nutrient uptake through planting on the floodplains, trapping faecal microbes and reducing nutrients added to the land.