What is Drainage?
Drainage is the removal of excess water from the soil surface or soil profile either by gravity or artificial means.
Why is drainage important in Pitches?
Good drainage is a key requirement of a well maintained grass or turf field and a naturally well-drained site will form a sound basis for a playing surface. Some fields may have adequate drainage because the field was constructed properly. However, poor drainage is one of the most common problems of high use sports fields. Poor drainage and wet weather increase the susceptibility to damage affecting the quality of the natural or artificial turf pitch and will generally cause the grounds to be closed rather than risk widespread damage that cannot be easily repaired. When water remains on the field surface and does not drain many problems occur including slippery and unsafe conditions. Games are cancelled and maintenance practices are delayed.
On land where the soil tends to be impervious, adequate provision for supplementary drainage by artificial means must be provided to meet the needs of the intended use.
How to ensure good Drainage?
Many sites require major re-grading of levels. The first stage is to scrape off the existing topsoil cover and retain for eventual re-spreading. Levels are adjusted in the subsoil, using the surplus ‘cut’ from high parts to fill lower areas. In some situations imported fill material may be required to build up low areas. Filling must be done in consecutive layers not exceeding 250mm. Each layer must be adequately consolidated to avoid subsequent settlement. Consolidation can be done by means of a road roller weighing a minimum of 10 tons. The final formation must be trimmed smooth to the required level before replacing the topsoil.
Improving Surface Drainage
The raised portion of a sports field that provides a slope to promote runoff of surface water is called the field “crown”. Crowns are the most effective way to remove surface water because they move water the shortest distance possible.
If the centre crown is not provided then the whole slope can be towards one direction too but with more gradient. Football fields should have at least a 1-2% slope. The highest point is usually about 10-18” above the lowest point on the field. Soccer fields made of native soil typically are built with a minimum of 1½ % slope and should never be flat. If the fields have underground drainage the slope should be at least 1%.
The water that is directed to the sidelines will need to be collected by interceptor drains and move away from the field. Underground lateral drains can be provided which connects to the main drain.
3. Improving Sub-surface Drainage
Sub-surface drainage can be effective at removing excess water from the rootzone during high rainfall events and for reducing the water table. These systems assist in moving water out of the soil profile by providing a pathway for “excessive” water to leave the soil.
The above diagram shows a section of an artificial turf pitch and how it is constructed with different layers and sub soil drainage connecting to the main drain.
There are a few commonly used systems to help improve sub-surface drainage including pipe drains, interceptor drains (also known as French drains), trackside drains, strip drains and sand-slit drains.
Currently this is the most frequently used drainage system in pitches. Plastic pipe drains are commonly installed with laterals at spacing between 5m and 10m centres at a depth between 450mm and 600mm below ground level. The plastic pipes can be wrapped with a porous material to prevent fine dust to enter it. All drains must be laid with a steady fall, no less than 1:150. When the drains have been laid, trenches must be backfilled with porous material to ensure the high percolation rates necessary for effective drainage. Predominantly single-sized, hard, angular gravel or broken stone within the range 6–10mm must be used as backfill. This should be brought to within 150mm–200mm of the surface. The trench backfill is then ‘capped’ with a 50mm ‘blinding layer’ in the form of coarse sand or fine grit to prevent infiltration of topsoil. The trench is backfilled to ground level with sand.