Technical Secondary Drainage
Secondary drainage, such as sand slits and gravel bands, can further increase the rate of drainage of surface water from sportsturf. They are installed to supplement a primary piped drainage system.
It is worth stating that, as the name says, slits and bands are ‘secondary’ drainage - they overlay and link to a primary piped drainage system. This needs emphasising because sand slits and gravel bands are often thought to be stand alone drainage schemes. By themselves, they will temporarily alleviate a surface water problem for a short while but, if there is nowhere for the water to go, they will soon cease to function.
Sand slits and gravel bands are installed at right angles to the pipe and intercept the permeable backfill above it to provide a connection to the piped system. Their purpose is to provide a more speedy short cut for surface water to reach the piped drainage system, rather than going through the soil. This is why they are sometimes referred to as by-pass systems because they by-pass the soil, transporting surface water direct to the drainage system and thus to an outfall.
Another form of secondary drainage, which is used mainly in agriculture but has also been used for sportsturf, is mole drainage. It is created by drawing a metal plug through the soil at right angles to the piped system to intercept the permeable fill.
The ‘moles’ can only be formed in clay rich soils. It provides quicker drainage of the upper
soil levels, but is not as effective as slits and bands in providing rapid surface drainage. It offers a lower cost alternative for smaller clubs on a tight budget.
Drainage Layout
The traditional layout for drainage schemes was the ‘herringbone’, with a centre main drain and laterals running off it an angle like the backbone of a fish, which is why it got its name.
Though it is still used, a grid system is now more common, especially in sportsturf drainage. In any drainage scheme wherever there is a junction between a lateral and a main drain, there is more concentration of pipe at that point and more permeable backfill over the pipe and around the junction. Hence, there is a higher rate of drainage at that point which results in uneven plant growth. In some situations this doesn’t matter but, in sportsturf, if you use a herringbone layout with the main drain running down the centre of a pitch, or fairway, this uneven grass growth will be clearly visible in certain conditions. So, aesthetically, it is not pleasing, also it is more expensive in time and materials because there are more junctions and shorter runs of laterals for a given area.
This is why a grid system is more common for sportsturf areas. The main drain can be positioned off the playing surface. For pitches, this can be beyond the side-line; for golf courses, this can be at the side of a fairway or off the green. Laterals then form a grid across the playing surface into the main.
One common misconception about drainage is
that it is supposed that drains should run downhill! Yes, they should have a slight fall of something around 1 in 100, and this is called the ‘grade’ of the drain. But, on sloping ground they should not run downhill to follow the lie of the land. Instead, laterals should run across the slope following the contours of the land.
Effectively, each lateral then becomes an interceptor drain, reducing the amount of water moving down the slope. This is why you can have the strange phenomenon of a main drain appearing to be on the high ground; on the high side of a fairway, for example, where even the main drain can also be an interceptor.
Interceptor or cut off drains, such as ditches, swales and french drains, can have an important role as part of a drainage scheme. Most soils become waterlogged due to groundwater, but there are many cases where land is wet due to surface run off from land at a higher level. The installation of an interceptor drain to catch the water before it runs downhill may, in some cases, alleviate the problem to the extent that no drainage, or much less drainage, is necessary at the lower level.
Drainage Checklist
There are two things to establish before starting to drain - where is the water coming from, i.e. the cause of the problem, and where is it going to go to - the outfall?
SANDBANDER
For excellent secondary drainage. Fast, easy installation of sand slits from 25mm to 40mm (1" to 1.5") to a depth of 250mm (10").
AFT Trenchers Limited T: 01787 311811
E:
info@trenchers.co.uk www.trenchers.co.uk
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