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Land Drainage - A farmer’s practical guide to draining grassland in Ireland
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Date
30/07/2013
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Land Drainage Manual.pdf
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Tuohy, Patrick et al. Land Drainage – a farmer’s practical guide to draining grassland in Ireland: Moorepark Dairy Levy Research Update Series 20. Teagasc – Agricultural & Food Development Authority, July 2013
Abstract
No drainage work should be carried out before the drainage characteristics of the
soil are established by a site and soil test pit investigation.
• Two types of drainage system exist: a groundwater drainage system and a shallow
drainage system. The design of the system depends entirely on the drainage
characteristics of the soil.
• Distinguishing between the two types of drainage systems essentially comes
down to whether or not a permeable layer is present (at a workable depth) that
will allow the flow of water with relative ease. If such a layer is evident, a piped
drain system at that depth is likely to be effective. If no such layer is found during
soil test pit investigations, it will be necessary to improve the drainage capacity
of the soil. This involves a disruption technique such as moling, gravel moling or
subsoiling in tandem with collector drains.
• Drains are not effective unless they are placed in a free draining soil layer or
complimentary measures (mole drainage, subsoiling) are used to improve soil
drainage capacity. If water is not moving through the soil in one or other of these
two ways, the water table will not be lowered.
• Outfall level must not dictate the drainage system depth. If a free draining layer
is present, it must be utilised.
• Drain pipes should always be used for drains longer than 30 m. If these get blocked
it is a drainage stone and not a drainage pipe issue.
• Drainage stone should not be filled to the top of the field trench except for very
limited conditions (the bottom of an obvious hollow). Otherwise it is an extremely
expensive way of collecting little water.
• Most of the stone being used for land drainage today is too big. Clean aggregate
in the 10–40 mm (0.4 to 1.5 inch approx) grading band should be used. Generally
you get what you pay for.
• Subsoiling is not effective unless a shallow impermeable layer is being broken or
field drains have been installed prior to the operation. Otherwise it will not have
any long-term effect and may do more harm than good.
• Most land drainage systems are poorly maintained. Open drains should be clean
and as deep as possible and field drains feeding into them should be regularly
rodded or jetted.