Technical
To put it into perspective, a 70/30 rootzone material will have an infiltration rate of up to 800mm/hour, a normal sandy loam will be as low as 100, a clay soil only 50, but a compacted clay soil is virtually nil.
Compaction and Infiltration
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 is why we often hear “we put a drainage system in three years ago, but it’s stopped working.” Perhaps it's the maintenance that needs looking at.
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 it is above the surface you have a flood!
Formation of a water table
To support plant growth, the water table must be maintained below root level to give plants aerated soil conditions. The purpose of drainage is to control the level of the water table.
Drainage to lower water table
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.
Primary drainage Drain spacing/depth and water table
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.
Secondary drainage
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 are deep to create a low water table, so that the soil is more stable at a greater depth to support heavy agricultural machinery like tractors hauling heavy 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.
Effect of drain spacing on groundwater level and plant damage
In contrast, though sportsturf has the same requirement to lower the water table to prevent plant damage, it has an additional requirement for very quick surface drainage. This favours shallower, more closely spaced drains. Though drains can be shallower because of the need to support lower ground pressure machinery, they are often put in at a similar depth to cater for secondary drainage, but they will generally be much more closely spaced to as little as say 10 or 5 metres on winter games pitches.
GreenTek roller units on greens triple
Areas where surface conditions are critical, such as golf greens and bowling greens, drains may be even more intensive, with some systems using very shallow, small diameter pipes at 500mm spacing.
The main consideration affecting intensity of drainage on sportsturf is how quickly does the surface need to drain. If it’s parkland where you go to play with the kids or take the dog for walk, though good conditions underfoot are necessary, the need for quick drainage is somewhat less than say a local parish football pitch. Heavily used school playing fields may need rather more to keep them in play. With sports clubs, there begins to be a financial consequence if a surface is unplayable.
You eventually reach the point when, because of fixture timetables, paying members or spectators or television coverage, be it a Premier League football club or a Ryder Cup venue, play has to commence on a certain day at a certain time, regardless of two hours torrential rain beforehand. Clearly, the need for rapid drainage is essential and a much more intensive drainage system is required in these situations. Also a higher budget!
122 I PC JUNE/JULY 2015
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