We need to stop los­ing pre­cious as­set

Coastal News - - Front Page - By BALA TIKKISETTY

We’re los­ing it. Soil that is. And we need to do more to stop the slide of this pre­cious as­set into wa­ter­ways and, ul­ti­mately, the ocean.

It makes eco­nomic sense to do so and also helps bet­ter pro­tect our wa­ter­ways and aquatic life from the ef­fects of sed­i­men­ta­tion. The scale of this loss of a farmer’s most pre­cious re­source is huge in this coun­try.

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 the ocean de­spite us hav­ing only 0.1 per cent of to­tal land area.

It’s a re­flec­tion of the ero­sion that re­sults from our moun­tain­ous and hilly land­scape, ad­verse weather events and land use prac­tices.

The re­sult­ing sed­i­men­ta­tion of our wa­ter­ways con­trib­utes to poor wa­ter qual­ity and in­ter­feres with aquatic flora and fauna, as well as pro­duc­tive ca­pac­ity of our land. There­fore, pre­vent­ing or at least min­imis­ing cur­rent and po­ten­tial ero­sion is a key to both the eco­nomic and en­vi­ron­men­tal sus­tain­abil­ity.

To ef­fec­tively 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. It’s 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 like this is gen­er­ally most ef­fec­tive when it comes to pre­vent­ing ac­tual sed­i­men­ta­tion of wa­ter­ways. The main em­pha­sis should be on ero­sion con­trol it­self rather than con­trols stop­ping eroded sed­i­ment from en­ter­ing wa­ter­ways.

The two main ap­proaches to ero­sion con­trol are 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.

Bi­o­log­i­cal meth­ods — the use of live veg­e­ta­tion — are the more eco­nomic means of re­ha­bil­i­ta­tion of eroded land.

Plant­ing helps “hold” soil and pro­vides 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 3m poles should be planted at least 60cm deep. Lighter 2m poles in sheep­only sit­u­a­tions and re­tired ar­eas should be planted 50cm deep. Also, 1m stakes at 40cm deep can be ef­fec­tive, de­pend­ing on the dry­ness of the site, but only where stock have no ac­cess.

En­sur­ing suit­able plant species are used 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 can ad­vise on best prac­tice at in­di­vid­ual sites. The coun­cil also has fund­ing (up to 70 per cent of costs) avail­able to help farm­ers in pri­or­ity sus­cep­ti­ble west coast and Waipa¯ catch­ments to carry out ero­sion con­trol.

Fund­ing cov­ers: ■ 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.

Bala Tikkisetty is a sus­tain­able agri­cul­ture ad­vi­sor at Waikato Re­gional Coun­cil avail­able at bala.tikkisetty@waika­tore­gion. govt.nz or on 0800 800 401.

SOIL SLIPS: Lots of smaller slips on farm land get washed into wa­ter­ways and down to the sea, caus­ing silt­ing.

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