Stopping the big sediment slide

Stopping the big sediment slide

Bala Tikkisetty

Soil is a farmer’s most precious resource. But, in New Zealand, we lose it to the ocean about 10 times faster than the rest of the world, with between 200 million and 300 million tonnes sliding into the sea every year.

That equates to an astonishing 1.1 to1.7 per cent of total global soil loss to oceans despite us having only 0.1 per cent of total land area.

Those statistics are a reflection of the erosion that results from our mountainous and hilly landscape, solid rainfall events and land use practices.

And, as we all know, the resulting sedimentation of our waterways contributes to poor water quality and interferes with aquatic life, as well as affecting the utility of our farming soils.

So preventing current and potential erosion is a key to both the economic and environmental sustainability of our farming.

When considering the best way to tackle erosion, it’s important for farmers and others to look at the various types such as splash, sheet, rill, gully, tunnel, channel and mass movement erosion.

The latter is one of the most common and involves the erosion of soil or rock by gravity-induced collapse. Mass movement erosion is usually triggered by ground water pressure after heavy rain. But it can also have other causes, notably streams undercutting the base of a slope or earthworks. Movement can be either rapid and near instantaneous (landslides, avalanches, debris flows) or slow and intermittent (earth flows and slumps).

Earth and soil slip movement are also often noted after the removal of vegetation from critical slopes associated with soil disturbing activities. These sorts of slopes need to be identified before development starts and should be avoided wherever practicable.

Tackling the source of the problem in this type of way is generally most effective when it comes to preventing actual sedimentation of waterways. The main emphasis should be on erosion control, rather than controls stopping eroded sediment from entering waterways.

There are two main approaches to erosion control: mechanical and biological.

Mechanical methods such as terracing, debris dams, detention dams, retaining walls and other engineering structures can have an immediate benefit by removing excess water and artificially strengthening slopes or by capturing sediment.

But biological methods – the use of live vegetation – are the more economic means of rehabilitation of eroded land.

Planting vegetation works by roots “holding” the soil and providing ground cover so that the elements don’t wear away directly on the soil.

Planting poplar and willow poles and stakes is a good way to restore physical strength to slopes and minimise slipping and slumping.

Large three metre poles should be planted at least 60 centimetres deep to obtain satisfactory establishment and stability. Lighter two metre poles in sheep-only situations and retired areas should be planted at 50 centimetres deep. One metre stakes can be planted at 40 centimetres deep, depending on the dryness of the site.

Using suitable plant species and lower stocking rates on steep land are other practices that can reduce erosion. In cases where severe erosion is present it may be best to retire land from grazing and, if possible, change land use to plantation forestry.

Waikato Regional Council staff are happy to provide advice on what’s best practice on individual sites. The council also has funding (up to 70 per cent of costs) available to help farmers in susceptible west coast and Waipa catchments carry out erosion control and other land management activities.

Activities the funding covers are;
· tree planting, including pole planting and native plant species

· fencing off marginal land or bush from active use

· riparian management (fencing, planting and stock water reticulation)

· farm plans to identify soils, land use capability and environmental projects (100 per cent of costs).

To provide more information on hill country management the council is organising a stock wintering and soil health field day on Thursday 28 April 2016. It will run from 10am to 1pm at Shaun and Kate Carter’s property, 418 Mangaotaki Road, RD1, Piopio.

· Bala Tikkisetty is a sustainable agriculture co-ordinator at Waikato Regional Council.

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