They have faced mounting problems with saturated or pugged soil, not counting compliance with industry-agreed Good Management Practice for nutrient runoff and stream quality.
Environment Canterbury has offered affected farmers a grace period on the rollout of its rules but improvements are still expected because they are region-wide.
The Kaikoura Plains Recovery Project will share research findings with farmers around the country. A handout for the project said the work started with understanding the unknown and working with it.
Poor drainage might be caused by quake-related factors such as a perched water table, rising water, seepage, a spring or upthrust or surface water flow.
Drainage systems designed to overcome the problems will likely include a combination of open and subsurface drains combined with secondary drainage techniques such as mole ploughing, subsoil ripping and gravel slotting.
At the Kowleigh Farm trial site the aim is to reduce drainage problems caused by underground seepage and point-source water breakout.
In terms of the soil type, it is assumed part of the issue is water perching on the underlaying clay subsoil. The Kowleigh trial targets collection of seepage.
To do that, 315m of 160mm Nexus drainage pipe was installed 1.2m to 1.5m deep on a 1-1.9% grade. Trial managers found it was important to use a laser to maintain the correct trench bottom grades.
The trial team used a Nexus pipe rather than Novaflo to increase internal flow volume and velocity for self-cleaning of the pipe.
The system could also be installed on a shallower grade while maintaining similar self-cleaning velocity and it was found to be a more rigid pipe with less risk of collapse.
All excavated drains were backfilled to the surface with topsoil not gravel. The topsoil provided free-draining and friable backfill connected to the excavated trench wall and surface. It was also free, already on-site and effective.
At the Maghera Farm trial the site was surveyed to determine available fall for a gravity-fed drainage system. Digging an open drain 2m deep provided a close look at the soil profile, which consisted of 300mm of topsoil on top of clay.
It was agreed the wet soils were the result of a combination of historical and new earthquake-related point source or spring outlets where pressurised water in the subsoil breaks out into the soil surface.
Parts of the farm were affected by seepage, where the groundwater breaks onto the soil surface, and a perching water table, where surface water cannot infiltrate quickly enough through the underlaying clay.
The farm also showed the effects of a shallow, meandering watercourse through the paddock, overflowing into low-lying areas before discharging into an open drain.
The farm has implemented a two-stage drainage programme.
Stage one included the installation of a 2m deep open drain extending 190m from the Lukes Creek outlet. The open drain was dug 2m deep to maximise the potential for drain sidewall seepage and to intercept any subsurface groundwater flows. That prevented overflow into the adjacent paddock and low-lying areas.
The second stage planned for this summer will consist of the installation of 150mm slotted PVC or 160mm Nexus drainage pipe. It will allow drainage of spring outlets at a minimum of 1.2m and the installation of gravel with a geotextile overlay.
The open drain was dug using an excavator that, with the help of a laser, excavated the bottom to a grade of 0.5-1%.
Maintaining an open drain depth of 2m was important for future subsurface drainage outlets that could be as deep as 1.5m and to maximise the open drain sidewall seepage. To reduce sediment issues in the receiving waterway, multiple hay bales were put in the base during construction.
The project is funded by the Ministry for Primary Industries’ Earthquake Recovery Fund, supported by Kaikoura District Council, Environment Canterbury, iwi, Fonterra, farmers and DairyNZ. Farmers involved in the trials contributed 30% of the costs.
THINGS TO REMEMBER:
• It is important that drainage works are completed during the dry, summer or autumn months when ground water sources can be easily identified.
• Use a contractor who can cut to grade with laser equipment either installed on the excavator or set up alongside the job. That ensures appropriate trench grade for long-term success.
• Are there other types of trenching equipment available?
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For further information on the Kaikoura Plains Recovery Project, contact project manager Jodie Hoggard on 027 551 5902 or jodie.hoggard@ecan.govt.nz
