Field drainage: Renewal and installation

From soil type to outfall availability, there are many technical factors to consider when designing a new drainage system.

Site

Always try to avoid installing drainage near trees and hedges. When this is not possible, use sealed pipes within a tree rooting zone or within 1.5 metres of a hedge.

Drain depth

Install drains at an appropriate depth and constant gradient (fall).

Depth in slow-draining soils

In slowly permeable soils, drains need to be deep enough to avoid damage from soil implements.

Unless there is a specific crop need, lateral drain depths greater than 0.75 metres give no additional benefit.

Depth in permeable soils

In soils where the drains control the depth of the water table, deeper drains allow the spacing between drains to be increased.

Drain depths in such soils are typically 1.2–1.5 metres.

Maximum depth

This is often limited by the depth of the ditches or watercourses into which the drains discharge.

These can be deepened but only to the level of the downstream channel.

Drain spacing

Pipe spacing is dependent on soil type and field slope. Generally, it is between 20 and 100 meters.

In heavy clay soils

The theoretical best drain spacing to optimise drainage is often not economically viable.

Wide-spaced drains with permeable backfill, supplemented with mole drains, are often the best choice (where soil conditions are appropriate).

Spacing for a mole drainage system can be as wide as 80 metres, although 40 metres is more typical.

The main limiting factors are soil stability and landform.

On land not suitable for moling

Modern systems usually have a spacing of 20–25 metres with permeable backfill over the drains.

The effectiveness of this system relies on maintaining good soil structure, sometimes aided by subsoiling.

If permeable backfill is not used

Drain spacing of about 10 metres will be needed, but this is unlikely to be as effective as a scheme with permeable backfill.

In permeable soils with a rising groundwater

The drain spacing will be determined by the depth of the drains and the level at which the groundwater is to be controlled. Permeable backfill is not usually needed.

Drain diameter

In the UK, drain diameters are calculated using the procedures set out in the current guidelines: MAFF/ADAS Reference Book 345 (The design of field drainage systems).

This method takes account of:

• Soil type and slope − speed of water movement
• Land use − the degree of risk that is acceptable depending on the crop value
• Climate − rainfall intensity*
• Type of drainage system − for example, mole drains must not be left submerged for more than 24 hours (excess water must be evacuated rapidly)

*Note: Reference Book 345 uses old rainfall figures that may not match current rainfall and do not account for extreme events, due to climate change.

Outfalls

Most modern outfalls are installed with glass-reinforced concrete headwalls. However, the actual outfall type may vary according to its location.

Position new outfalls sensitively at ditches and ponds to avoid damaging habitat.

Availability and gradient

Install drains at an appropriate depth and constant gradient (fall).

As a comparison, a bath/shower is designed to slope and has a strategically positioned plug hole (outfall) to drain the water.

Lack of available outfall and/or gradient to enable water to drain away affects the efficiency of the field drainage system.

Outfalls and pipe siltation

Where there is an ochre or other pipe siltation risk, systems with separate outfall pipes for each drain are best, as they allow easier access for clearance operations.

Use of permeable backfill

Gravel/stone chippings applied to the trench above the drain, typically to the base of the topsoil, is called permeable backfill. It provides a hydraulic connection between the cultivated layer, subsoiling fissures, mole channels and pipes.

Material should:

• Be hard and durable (when wet and dry)
• Have most material in the range 5–50 mm
• Contain no more than 10% fine material (fines)

Where mole drains are not installed, the use of permeable backfill is a long-debated subject, primarily due to the significant associated cost.

There are many examples of old drainage systems without permeable backfill that still have some function. However on drained clay soils, permeable backfill helps extend the life of the drain. Without it soil permeability of the drain trench will decrease, reducing drainage effectiveness over time.

The performance of drains installed without permeable backfill cannot be rejuvenated by subsoiling.

Note: Permeable backfill cannot control a rising water table in a coarsely textured soil.

Standards, material and quality

There are two fundamental standards:

• Reference Book 345: The design of field drainage pipe systems (MAFF/ADAS, 1982)
• Technical Note on Workmanship and Materials for Land Drainage Schemes (ADAS, 1995)

Within these primary standards, several decisions need to be made about the design specification.

Pipe type

It is essential that a material designed for use in field drainage is used.

Currently, all new drainage schemes are installed using plastic pipes, although many older schemes were installed with clay pipes and may be replaced with the same.

Consideration should be given to the use of twin-wall or ductile iron pipes or gravel pipe surround where there is a risk of pipe crushing.

Sealed/unperforated pipes should be used within a tree rooting zone or within at least 1.5 m of a hedge to prevent blockages.

Filter wrap

Filter wrap is a geotextile barrier around the outside of the pipe to prevent soil particles entering the drain.

It is not commonly used in the UK, as pipe sedimentation is not usually a problem where pipes have been laid and maintained properly.

However, there are some cases with fine, sandy soils when filter wrap can be beneficial.

Filter wrap should never be used where there is a risk of ochre.