Land Drains cure waterlogged areas such as agricultural fields, gardens or other landscaped areas including sports fields. Essentially, land drains are pipes that allow water to enter through small perforations and flow away to a suitable disposal point. This could include streams, a storm drain or a soakaway.
In the past, land drains were installed in both clay and peat soils. This process would have involved excavating a trench and then forming a ‘tunnel’ using flat stones. These tunnels would have been put in at a depth of 2-3ft (600mm-900mm) below the surface. This would ensure enough depth to avoid any damage occurring from ploughing on the surface.
In England, short earthenware pipes were first used in 1843. They were then laid edge to edge; the earliest type consisted of a "U" shaped trough onto which a flat lid was placed. Later, extruded clay pipes were developed and are still used today. These developed pipes can be laid into excavated trenches, or a horizontal tunnel is formed in the ground using a mole plough and the pipes are forced in by hand or mechanical press.
Many modern land drains are created using rigid or flexible plastic pipes, These pipes are then pierced with holes and laid on pea gravel (pebbles without sharp points which would damage the pipe) in an excavated trench. Specialised mole ploughs are available that can form the tunnel, insert the perforated pipe (and gravel if required), all in one simultaneous and continuous process.
People often talk of using dowsing to try to locate land-drains which have become ‘lost’ with time. But the results are unpredictable and are by no means totally reliable. A variety of geophysical techniques have also been used one of which we have described below.
While carrying out routine archaeological surveys with magnetometers, a by-product of the survey is that excellent plans of drainage systems can be obtained.
Land drains have differing effects on magnetic datasets. Drainage pipes made from fired clay often result in linear anomalies. Sometimes these are recognisable by the shapes they form. A herringbone pattern is a common shape.
Plastic drains cannot be detected magnetically., Bbut they are often laid on pea gravel or similar, and in these instances, the narrow trenches and changes of density can often be detected.
As with ridge and furrow cultivation (medieval deep ploughing), drains often bring enhanced magnetic material to the surface. This is where they cut through pre-existing archaeological features; so land drains can also help identify historic sites.