This article is a continuation of “Subsurface Pipe Design considerations” please read that section first.
Other Points to Consider
Inspection pits (sediment traps)
Install these at specific pipe junctions. They assist in locating blockages in pipe lines and provide access for removing blockages.
Drain markers
Draw up a farm plan and accurately mark drain locations on paper and distances from known permanent points on the ground, as after a year or two there will be no sign where the drains are installed. An aerial photograph of the drained area immediately after installation is valuable. Recording the location of existing pipes and their sizes are essential for future extensions.
There are many examples where existing drains have been dug up by not having them properly marked. Drainage installations should add value to the land and is a good selling point as long as their location is known.
Outlets
Drain outlets are the most important part of the system. If an outlet becomes blocked, the drainage system fails, becoming a huge needless expense. Outlets must be well marked (particularly when cleaning open drains) and protected from stock trampling. Self-closing flaps will prevent vermin from making a home in the pipes and blocking the outlet.
Drainage water
The water may contain nutrients such as nitrogen and phosphorus and may have high salt levels if saline areas are drained. Good management practices such as not applying fertiliser within four days of predicted rain should reduce nutrient runoff to a minimum. Well-drained soils are less likely to waste nitrogen fertiliser through denitrification and plants are able to access the nitrogen readily. Phosphorus can leach via drainage water but the total amounts lost are less than through surface runoff of saturated soils.
Some pockets of saline soils have the potential to be restored over time as rain water flushes out much of the salt via the drainage system. This will cause saline water to enter streams initially but eventually will actually improve the quality of the stream water as salt is leached from the soil profile.
Tree roots
Fine tree roots will very quickly gain entry through the slots so avoid laying slotted pipe where these may be a problem. Use un-slotted pipe for several metres to bypass tree roots.
Grazing management of drained paddocks
It is absolutely essential that, after heavy rain, drained paddocks are not grazed for at least 24 hours after the last rain. This allows the soil to drain and regain strength to withstand cattle and wheel traffic without damage.
A common fault by many farmers is to put cattle into drained paddocks during or soon after rain. This results in a compacted soil surface and reduces the drainage capacity of the soil. Animals should be kept off drained soils for at least 24 to 48 hours to allow the drainage system to drain the excess moisture to about 30 cm below the ground surface. This will also retain the structural strength of the soil. A waterlogged soil has greatly reduced strength so is easily pugged by hooves and damaged by machinery.
The Water Act
The Water Act (1989) provides guidance for the management of waterways and swamps. Before considering draining a wet area you should contact your local Catchment Management Authority and Regional Water authority for advice, as a permit may be required.
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