Sports Field design – perched water table
The perched water table is by far the most commonly used and successful designs for professional grade sports fields in Australia and around the world. Sports fields that utilise the perched water table design include: golf greens; bowling greens; and race courses, as well as the elite football and cricket grounds.
But, as Keith McIntrye explains, it is also one of the most difficult to understand.
The perched water table design concept uses a finer textured material (root zone sand) uniformly placed over a coarser material (drainage gravel), with the coarse material having an influence over the drainage of the material above. The materials must be compatible – they must meet the bridging criteria where the largest 15 % of the root zone sand particles form bridges with the smallest 15 % of the gravel particles to prevent fines migration. Where materials cannot be matched for compatibility, an intermediate layer is used.
A zone of saturation, the “perched water table”, forms above the coarser material, in the zone called the capillary fringe. The height of the capillary fringe, and hence the height of the perched water table, is determined by the inherent characteristics of the rootzone material – the particle size grading and the subsequent porosity distribution.
It is important to understand that water held in the perched water table does not move sideways. The height of the perched water table forms an equilibrium once the system is in place. Once equilibrium is achieved in the perched water table, correctly constructed profiles will drain at rates of 100 – 300 mm per hour in the field, as McIntrye describes it – a one drop in, one drop out phenomena occurring.
The phenomena occurs because of two reasons:
- The materials selected for the construction of a perched water table are generally free draining in their own right, enabling the profile to be generally free draining; and
- The water in the perched water table is held in the pores of the root zone material by capillary tension and pressure or suction is required to remove it. The suction from the coarser gravel material below is not sufficient to remove the water, but pressure from water added to the system from above is, therefore as water is added to the system, the equivalent volume will be “pushed” from the capillary fringe into the free-draining gravel layer below, where it is removed via the systematic drainage in place.
The suction from turf water uptake is also sufficient to remove water from the perched water table, therefore rainfall and irrigation act to “top up” the perched water table to maintain the turf uptake requirements.
While the materials selected for the construction of a perched water table are generally free draining in their own right, in order for the root zone material to facilitate rapid surface infiltration during heavy rainfall events, sufficient height of rootzone material above the capillary fringe is required. This allows the the surface to return to a desirable state as rapidly as possible to minimise return to play time. However, should the root zone material horizon be too high, the surface will be prone to drying out and possible instability. This is offset by reducing the total height of the root zone material horizon, using a material with a finer (but still compatible) distribution, or high-quality organic amendments such as peat moss or coco fibre to improve the water holding capacity at the surface. High-quality organics are required and favored over composted soil conditioners and the like as they maintain their particle size characteristics longer and have a lower risk of impacting the particle size distribution at the surface. A small change in the particle size distribution at the surface or throughout the root zone material horizon can have dramatic negative impacts to the infiltration rate, compaction risk and performance of the field, particularly after heavy rain.
SESL Australia are an independent team of soil scientists, consultants and laboratory technicians that specialise in sports field: performance specifications and design; material compliance analysis and interpretation; in-field and laboratory investigations; construction quality assurance/quality control (QAQC) programs and supervision.
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McIntyre, K. (2004). Problem Solving for Golf Courses, The Landscape Sports Grounds & Race Courses. Kambah, Australia: Horticultural Engineering Consultancy.
McIntyre, K. & Jakobsen, B. (1998). Drainage for Sportsturf and Horticulture. Kambah, Australia: Horticultural Engineering Consultancy.
United States Golf Association. (USGA). Green Section Staff (2004); USGA Recommendations for a Method of Putting Green Construction.