This Agnote helps you select an appropriate subsurface drainage method for your farm. It explains some practical on-farm tests to help in the selection of the most appropriate system for particular soil types. Other Agnotes in the “Managing Wet Soils” series provide more technical details of each drainage system.
Introduction
It is important to select the right drainage system for particular soil types as their different characteristics require different drainage methods. Surveys have found that some farmers are unsure of the characteristics of their soil types and so cannot determine which method of drainage is best suited to their soils.
Subsurface drainage can be achieved by subsurface pipes, mole drainage or by a combination of both. To decide which subsurface drainage system is most suited to a particular soil type, the soil’s permeability (ability to allow water to pass through), its characteristics and its clay content and type must be investigated. The soil’s permeability can be measured to provide a number, its hydraulic conductivity, which is valuable for selecting a drainage system.
Factors to consider when deciding on a drainage system
The type of subsurface drainage system most suitable for a particular soil type depends on specific soil characteristics such as the soil’s texture (content of clay, sand, stones, loam), type of clay (is it suitable for mole drainage or not) and water flow rate through the soil profile (permeability). The soil’s physical characteristics in the profile section to be drained must also be determined. For example, how deep is an impermeable layer situated?, how much of the profile is very permeable?, will the soil slake (collapse) or disperse (erode) when excessively wet? etc.
Therefore it is vital to know the soil type and profile when deciding which subsurface drainage system is appropriate. Testing involves taking soil cores so you have an understanding of what the soil types are below the surface. Examining the soil profile and measuring the soil permeability will assist in selecting the drainage method. Other simple on-farm tests can help to determine the suitability of the clay type for mole drainage.
Once the soil’s suitability for a particular drainage system is determined, the attached flowchart provides a further refinement for selecting the appropriate system for different soil types.
Knowing the likely location and level of the outfall for the proposed drainage system is also essential. If there is no suitable outfall, then no drainage is possible unless very expensive sumps and pumps are used. The gradient of the land can also influence the drainage type and its design.
Soil class, water flow rate and soil type.
Questions for determining which drainage system to install
Examining the soil using the practical on-farm tests will help choose the best suited subsurface drainage system to install. These tests should be able to answer the following questions to help in making the decision:-
Is the soil suitable for subsurface pipe drainage?
Suitable soils for subsurface pipe drainage will be very permeable, free draining, down to depths greater than 1m. There will be sufficient depth and gradient (fall) to allow a suitable slope for the slotted drain pipe. The location of the outfall must be such to allow this to occur. Although slotted pipe drains are very effective in permeable soils, pipe spacing would have to be so close on impermeable soils that they become uneconomical on their own.
Is the soil suitable for mole drains?
Suitable soils for mole drainage will have a suitable proportion of clay content down to moling depth (400 to 600 mm) and minimal rock or sand at mole draining depth. Many clay mineral types will allow the mole channel to hold its shape after moling. When the mole channel is “wetted up” during drainage it will not slake (collapse), and will not be dispersive and prone to tunnel erosion. However, some clay types are unsuitable for mole drainage and can lead to serious erosion if mole drained.
Is the soil suitable for mole draining over a collector pipe system?
Soils suited to mole draining over a collector pipe system will have a suitable clay content and type partially suited to mole draining down to moling depth but will not sustain long mole drains (greater than 80 m) for a particular reason. Such a reason may be the presence of sand or small rock to varying degrees at or near moling depth. The soil may be marginally suited to mole draining but having closely spaced subsurface pipes to drain water quickly, should extend the mole drain life.
Is the soil suitable for gravel mole drains?
These soils typically are not suited to mole drainage. They may slake after wetting up or may contain sandy or rocky layers or areas. Inserting permeable backfill via a specifically designed gravel mole drainer into the mole slots (actually square channels), will maintain the integrity of the channel slot allowing drain water to be removed.
.
