A Guide to
Drainage
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COMPACTION and INFILTRATION
DRAINAGE to LOWER WATER TABLE
Compaction combined with smearing of the
surface particularly on winter games pitches,
without proper maintenance will render even the
best drainage system pretty ineffective.
That’s why we often hear ‘we put a drainage
system in three years ago and it’s stopped
working’! Perhaps it’s the maintenance that needs
looking at.
In drained land the level of the water table is
controlled by the depth and spacing of the drains.
They can either be deep and widely spaced, or
shallower and more closely spaced to achieve a
similar result.
DRAIN SPACING / DEPTH and
WATER TABLE
WATER TABLE
It can be seen that a water table is formed in soil
at the top of the saturated zone where all the pores
are filled with water. Above this is the unsaturated
zone of soil where the pores contain air as well as
water. The level of the water table can vary. If it is
at the soil surface you have a waterlogged soil, if
is above the surface you have a flood!
The actual depth and spacing can be determined
by calculation but the principle of which route you
take will depend upon how quickly water needs to
be drained, and how stable the soil needs to be
at depth. There is clearly a cost implication too as
more intensive systems will cost more.
For example agricultural drainage generally
favours deep and wide spacing. Drainage is slower
but provided the land dries out within a day or two
after rainfall, i.e. the water table is lowered below
the rootzone, plants like corn and grassland will
not suffer too much in 24 to 48 hours. The drains
To support plant growth the water table must are deep to create a low water table so that the soil
be maintained below root level to give plants is more stable at a greater depth to support heavy
aerated soil conditions. The purpose of drainage agricultural machinery like tractors hauling heavy
is to control the level of the water table.
trailers and harvesting machinery like combines
which can compact the soil and damage soil
structure in wet conditions, and damage drains
if they are too shallow. So agricultural drains will
often be about 1 metre deep and 20, 30, or even
40 metres apart.
14 Issue 67