Finding solutions to drainage issues in Kaikoura

Drainage has been a nightmare for Kaikoura plains farmers since the November 2016 earthquake. Photos: Supplied

Drainage solutions are being demonstrated to help Kaikoura plains farmers cope with earthquake-related water problems.

Farm drainage has been a major issue for farmers and other landowners since the 2016 earthquake and the Kaikoura Plains Recovery Project has been conducting trials to find solutions.

Agricultural engineer Ian Howatson outlined the trials at a drainage discussion day in Kaikoura in June.

He said poor drainage could lead to land management and environmental issues, such as nutrient run-off, poor stream water quality, saturated and/or pugged soils, poor soil health and stock health problems.

Engineering solutions are being trialled to help Kaikoura plains farmers overcome their drainage challenges.

Drainage problems could be caused by a rising water table, seepage, spring sources and overland water flows, as well as earthquake damage.

''The cause of the drainage problems will largely define what the possible solution is. So before developing a solution, it's important that you identify exactly what the cause is,'' he said.

''Drainage systems designed to overcome these problems may include a combination of open and subsurface drains, combined with secondary drainage techniques, such as mole-ploughing, subsoil ripping and gravel slotting.''

Under Mr Howatson's guidance, drainage trials were conducted at Kowleigh and Maghera farms.

At Kowleigh farm, issues had arisen from water perching in the clay subsoil, which ''results in seepage breaking out on to the paddock surface in low-lying areas'', he said.

''Paddock drainage problems are complicated further with the point source breakouts of subsurface water flows, on to the surface.

''We can also assume that part of the resulting issue on-farm is due to earthquake damage underground.''

To resolve the problem, Mr Howatson advised targeting the collection of seepage with 315 metres of 160mm Nexus drainage pipe, which was installed at a depth of 1.2 to 1.5 metres, on a 1% to 1.9% grade.

Laser equipment was used to ensure the correct trench bottom grades were maintained.

At Maghera farm, wet soils had resulted from a combination of historical and new earthquake issues ''where pressurised water in the subsoil breaks out into the soil surface'', seepage, a perching water table and ''the effects of a shallow meandering water course'' results in overflows in low-lying areas.

A two-stage drainage programme was being trialled, with a 2m-deep open drain installed, extending 190m from the Lukes Creek outlet.

Stage two, this summer, would see a pipe installed to enable the drainage of spring outlets, the installation of gravel and a geotextile overlaying the gravel.

-By David Hill

Central Rural Life