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Handy hints: Renewing and installing field drains

As the rain lays rest in many areas of the country, now may be a good time to address grassland drains. Laura Bowyer speaks to Kirk Hill to find out more.

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Getting field drainage right #grasslandmanagement

Locating existing drainage

With grassland fairly dry around the country, fields and drains may be more easily accessed by machinery to tackle drainage problems.
Most fields with soils which can be improved by drainage have had field drains installed at some point in the last 200 years or so.

Kirk Hill, senior soil and water engineer at ADAS, says: “Many fields will have had two, three or even more schemes installed over the years and even the older drains can still provide important benefits, without which the area may not be farmable.

“If you are not satisfied with your field drainage, firstly find your drains and then determine if they are serviceable or in need of replacement.




If you do not know where your outfalls are, try walking the lowest boundaries of each field after a period of wet weather and you might be able to see or hear water running out of an outfall or seeping from a ditch bank. Failing this, old maps can be used and aerial photos to help locate possible drain positions, or you may need to get the digger out.”

Keep outfalls clear

Without doubt, the simplest and most important operation in drainage is to keep the outfalls clear, says Mr Hill. This can take just a few minutes with a spade but longer sections of ditches sometimes need to be cleaned out.

Mr Hill says: “Without this modest bit of work, drainage systems can become almost useless and start to deteriorate beyond serviceability, to the extent an expensive complete replacement may then be required in subsequent years.”

Drain depth

The appropriate depth to install a new drain depends on soil type and drainage need. In most slowly permeable soils the laterals should be about 70-80cm (27.5-31.5in), and the mains typically 90-100cm (35.5-39.5in).

Mr Hill says: “Drain depth also depends on whether you need to intercept existing drains or springs which are deeper or whether you are limited, for example, by the depth of a downstream watercourse or culvert.

“Sandy soils with water tables would typically have drains 1.2-1.5 metres deep and the wider the spacing, the deeper drains need to be.”

Drain spacing and secondary treatments

Historically, drains have been spaced according to local custom but in recent years, with the advent of subsoiling and mole drainage, this has become more standardised.

Mr Hill says: “The correct spacing can be calculated using equations but this is often not done in practice. In heavy clay soils the theoretical correct drain spacing would be so small it would not be economically viable. In such soils mole drainage is a fantastic solution.



“Pipe drain spacing for a mole drainage system can be as wide as 80m, although 40m is more typical as soil stability and landform can limit the distance.


In less heavy clay and medium textured soils, subsoiling may form an essential regular operation to move water sideways and into the gravel backfill over drains, which are typically 20-25m apart. If it is not possible to use stone backfill then the spacing between lateral drains may need to be halved to achieve a similar benefit, although this is likely to have less longevity and would not be suitable for mole drainage.”

Drain spacing on sandy soils can vary greatly, commonly from 10-40m. It depends on available ditch depth and the precise permeability of soil. Drains can be positioned further apart with increased soil permeability but drain depth would also need to increase to control the water table at the same level in soil between drains.

Use of permeable backfill

Permeable backfill refers to gravel or stone chippings applied to the trench above the drain, typically to the base of the topsoil.
Mr Hill says: “The use of permeable backfill has been a long-debated subject due to the associated cost.




Although there are still some old field drains without permeable backfill which still work, research on drained clay soils without backfill shows the permeability of soil in the drain trench decreases with time.

“In slowly permeable soils I would almost always recommend the use of permeable fill as this means drains with wider spacings are just as effective as closely spaced drains with no stone backfill. They stay effective for far longer and facilitate subsoiling and moling operations which maintain soil drainage and mitigate damaged soil structure.

“Best practice is installing sufficient permeable backfill so a connection exists between the drain trench and the cultivated layer. At the very least, the permeable backfill should connect with the mole drains or any channels caused by sub-soiling.”

If mole drains are installed over the pipes, the use of permeable fill is essential to provide a hydraulic connection between the mole channels and the drain. Also, the ability of subsoiling to rejuvenate soil structure and drainage can be greatly limited if drains are installed without permeable backfill, and will deteriorate progressively over time.

“Permeable backfill would not usually be required in these soil,” he says.

Outfall availability and gradient

A lack of outfall availability and gradient can affect the efficiency of drainage systems, says Mr Hill.

“In reality, your drain gradient may be limited by the slope of the land. But careful design is necessary in some areas, as too steep a pipe can lead to erosion around it, and too slack a gradient can lead to siltation of the pipe. Try to positon your laterals to intercept slopes, rather than head straight downhill, but keeping a gradient of at least 1:1,500 wherever possible would be good advice.”

Drain diameter

Field drain diameter calculations take into account rainfall, soil type, drain gradient catchment and crop value. The acceptable degree of risk depending on crop value and ability to access, stock or work a field should be key considerations.

Mr Hill says: “The aim is to remove excess soil water rapidly enough, for most seasons, so the crop does not suffer and soil structure remains undamaged. For mole drainage schemes, the aim is to remove excess water within 24 hours so the channels do not prematurely collapse.

“In the UK, drain diameters are calculated as outlined in MAFF/ADAS reference book 345. While the rainfall figures are now somewhat historic, this is still the accepted guidance and simple adjustments can be applied in the method to account for any newer rainfall data or any change in crop value.




But, as with everything else in the design, this may boil down to available funds as well. Where detailed design is needed, it may be simpler and safer to use a consultant or contractor.”

When replacing or repairing a drain, make sure a pipe is used with at least the same internal diameter as the original pipe, and do not connect a larger diameter pipe into a smaller pipe to discharge.

Using contractors

Mr Hill says: “If considering installing a comprehensive scheme, I would recommend getting a specialist contractor. The Land Drainage Contractors Association have a members list on their website and most good contractors should be on there. It is always worth asking your neighbours for references as well though.”



Good soil drainage is fundamental for modern productive farming, says Mr Hill. It leads to better soil health and improved plant nutrient uptake, reducing pests and increasing field access periods. Underground piped drainage systems are commonly a vital part of the system.

He says: “Effectiveness of drains will rely fundamentally on maintaining good soil structure, to which regular subsoiling and moling may play an important part, as does keeping outfall pipes clear.”

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