Best practice drainage

Best practice drainage

Best practice drainage

Drainage: best practice, improved practices and new technologies


Stronger, more lightweight pipe materials are one of the widely recognised advances in drainage, but there are several other technologies and practices that rural drainage specialists can advise on to improve drainage systems. 

This article outlines technologies and practices that can help achieve better drainage and create the foundation for more productive land. Remember that the best drainage solution for you will depend on your land and usage — so it is a good idea to get advice from your trusted local drainage specialists.


Improved practice: Making mole and tile drains work


Using mole and tile (pipe) drainage is common practice in parts of New Zealand where farmers or growers are working with heavy soils and need to improve the productivity of their pastures and crops. Mole drains are unlined channels formed in clay subsoil (formed by pulling a ripper blade with a cylindrical foot/torpedo attached, through the subsoil), typically at a depth of around 400 to 600cm. Mole drains help prevent the downward movement of groundwater. Rather than draining the groundwater, these drains work by removing water as it enters from the ground surface helping to reduce waterlogging issues.

From our experience, in cases where farmers are still experiencing boggy paddocks or standing water issues the reason is usually less about blocked or broken pipes and more about not having enough pipes in the ground, or having pipes that are too small to handle the volume of water being collected.

For example, in Southland clay soils, drainage engineer John Scandrett recommends approximately 120m of tiles per hectare for a mole-tile system to achieve adequate drainage.

Ideal depth and layout for mole drains including positioning of subsurface drainage pipes running perpendicular below.
Ideal depth and layout for mole drains including positioning of subsurface drainage pipes running perpendicular below.


Drainage experts have learned a lot about improving the success of mole drains and installing these drains over a collector pipe system is recommended in heavy clay type soils. This system requires the installation of slotted subsurface drainage pipes, set at approximately 60m to 100m apart, and with mole drains pulled across the top. The collector pipe needs to be installed using a laser to ensure a constant fall in the pipe to the outfall.

It is also recommended that a permeable backfill — such as washed sand or small diameter ‘pea’ gravel — is placed (backfilled) on top of the slotted pipe in the base of the trench. Depending on the clay content and its depth, this backfill must reach at least 150mm above the moling depth so that the water moves into the backfill via the mole channel.

Apart from improved drainage performance, another major benefit of pairing mole drains with a collector pipe system, is that in cases where soil collapses in on mole drains, the subsurface drainage pipes will continue to reduce standing water and waterlogging issues.


Improved practice: Controlled drainage for better management and water quality


Controlled drainage (CD) is a type of drainage water management commonly used in parts of the United States and Europe. It has been applied to reduce nutrient losses and improve yields for deep rooted crops but in New Zealand concerns have been raised about an increase in surface runoff and soil damage through an increased risk of pugging.

Modifying controlled drainage and closely managing it during weather events and seasonal changes shows potential for delivering the benefits without the downsides.

Controlled drainage is implemented by installing a water control structure at strategic drainage outlets. The outflow of water from a drainage system is then regulated by controlling the level of water at the drainage outlet. Controlled drainage gives farmers the ability to adjust the intensity of the drainage system from full, to partial, to no drainage during different times of the year based on the environmental conditions and agronomic and farming needs. It is important to note that these systems work well with land slopes less than 0.5 percent.

Management of controlled drainage systems during difference times of the year.
Management of controlled drainage systems during difference times of the year. I) Stop logs in place, minimal drainage II) No stop logs, free drainage III) Stop logs hold water at high levels


A study out of Iowa State University has identified crop benefits, irrigation savings and reduce nitrate leaching from a controlled drainage approach. The study looked at a method for controlling ground water in “a simple way that is very effective at allowing crops to grow taller, healthier and faster” by using what they call control boxes strategically placed in the drainage system main lines being able to raise and lower the water table by up to 3 feet (area dependent). Being able to manipulate the water table was also shown to result in savings on irrigation costs and reduced nitrate leaching by keeping the water in the ground for longer, making the nutrients more available for plants to access and resulting in less making it to the waterways. This makes the approach environmentally sustainable and doubles as a water supply and a drainage system at the same time. Iowa State University has demonstrated that by raising the water table up when the plants are young and roots are short making the water table closer to the surface make ground water more readily available to young plants and the lowering the water table as they grow larger to avoid stunted growth due to shorter root systems.


New technology: Using drones to understand the land


Drone mapping and assessments of land can help remove the guesswork from drainage solutions.

Drone assessments can provide a more comprehensive picture of drainage issues, including helping to identify whether surface or subsurface water is a key issue, where water is coming from, and how large the affected areas are.


One of the major advantages of drone assessments is the detailed understanding it provides about the fall of the land across entire areas and farmlands. With the results of the assessment, landowners can make decisions about any drainage issues that require urgent attention and need to be prioritised, and develop a longer term drainage plan that will meet their future needs.

Using drone mapping and consulting expert drainage advisors as the foundation for a drainage plan ensures the right products and design are selected from the outset, and can help achieve more productive, resilient land.


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If you need help or advice on farm drainage, P&F Global offer a free on farm drone mapping service. Future-proof your farm for all weather events and ensure you are getting the best return from your land with a personalised drainage plan for your farm. For more information, contact a rural specialist today on either 0800 99 77 33 or