What causes drainage problems – the principles of water movement in soils
Whilst the effect of a functioning drainage scheme can be observed by all, a basic understanding of the physical principles which govern this process enables the observer to both assess the effectiveness of a drainage scheme and also to appreciate why inappropriately or poorly designed drainage schemes fail to work.
The key to all of the following discussion is to understand what makes up a rootzone. The key ingredients are:
- Mineral particles (sand, silt, clay)
- Organic matter
Consider for example the soil rootzone represented in Figure 1, mineral and organic particles are locked together to form the solid fraction of the soil, whilst the pores (the spaces between the solid particles) are occupied by either soil water or soil air. The combination of solids, water and air is critical for not only plant growth, turfgrasses need all three components, but also surface strength. Figure 1 A diagram depicting a microscopic view of soil
Figure 2 The effect of soil moisture content and bulk density on rootzone strength. As moisture content increases, surface strength initially increases but then rapidly decreases. As bulk density increases from Bulk density 1 to Bulk density 2, the rootzone strength increases Sports surface strength and soil moisture content
The resistance of a football pitch to wear by players and the ability of a pitch to provide a stable platform for quality football is highly dependent upon the turfgrass and the soil in which it grows. The grass plant provides a cushioned surface, with natural lubrication to reduce skin abrasion – its root network also provides a core structural function. The soil not only provides an anchorage for the grass plant but it is also the key load bearing component. The two components combined (grass and soil), must be of sufficient strength to resist damage from repeated use – both from running and sliding, but not of excessive strength so as to cause impact or musculoskeletal damage.
The strength of a turfgrass plant is a function of agronomic factors related to plant health and growing environment. The strength of a natural soil or sand rootzone is a question of engineering and is closely related to the bulk density of the soil (how well packed the soil is) and its moisture content (how wet the soil is). Typically, as bulk density increases – soil strength increases and as moisture content increases – soil strength decreases (see Figure 2 ).
The principal target for groundstaff is to prepare surfaces that have a sufficiently high bulk density so as to provide strength to resist wear. The problem is that as bulk density increases, porosity (the amount of pores in a soil) decreases and restricts the amount of air in the soil available for the turfgrass; a balance is required.
Water movement and retention in soils
As is shown in Figure 1, water occupies the pore space in a soil; it also illustrates the range of pore sizes in a soil. Water is held in a pore by capillary action, effectively sucking water into the pore – this is the same way in which a sponge absorbs a liquid. Large pores create a low amount of suction; small pores create high amounts of suction – therefore the size of pores is critical in water movement in soils. Figure 3 Unsaturated (a) and saturated (b) rootzones. In the saturated condition all the pores are full of water
All the water in a soil is being pulled downwards by gravity, large pores will drain because there is not enough suction to hold the water in the pore against gravity, meanwhile small pores, where the suction is greater than the pull of gravity do not drain. This is why sandy soils, where the particles are large, and hence the pores are large, drain easily under gravity, but clay soils, where the particles and pores are very small will hardly drain under gravity and require drainage schemes. In fact in clay soils, it is very difficult to pull water out – it must be pushed out, this happens when the soil is saturated (see Figure 3 ).
The same physical property also means that water is pulled upwards (and sideways) to occupy dry pores – a phenomenon known as capillary rise. Figure 4 Flooding due to high water table; (a) water added to the top of the saturated soil displaces water from the freely draining base of the profile; (b) when the water table is high, water cannot flow out of the soil and accumulates at the surface – flooding the pitch
Saturation in a soil is defined as when all the pores are full of water. Note that any water applied to the top of the soil in Figure 3 b will displace water out of the bottom of the soil by pushing it out as a head of water builds (see Figure 4a). The rate at which the water moves through and out of the soil is known as the saturated hydraulic conductivity. In a coarse grained soil such as a sand rootzone, the hydraulic conductivity is high; in finer clay soils, the hydraulic conductivity is orders of magnitude lower.
If there is no exit for the water below (e.g. the water table lies directly below the soil) then water will accumulate at the surface, water logging and eventually flooding the pitch (Figure 4b). This situation is known as a ground water drainage problem and can be solved by lowering the water table with piped drainage so that the soil has a greater storage capacity for rain water (Figure 5).
Such groundwater problems in sports surfaces are actually quite rare. The majority of waterlogging and flooding of football pitches is actually caused by another problem entirely – requiring a different solution.
Figure 5 The effect on a groundwater table of the installation of piped drainage. Note that at the drain locality, the water table is lowered, increasing the capacity of the soil to store rainwater and reducing the frequency of flooding. Note also, how the water table rises in-between the drains – the height to which it rises is a function of the depth of drains and their spacing. If the drains are too shallow or too far apart, they can actually cause flooding – a common indication of poor drainage design
Infiltration and surface drainage problems Flow of water in and out of the soil in Figure 4a assumes that water can get into the top of the soil in the first place. The entry of water into the top of a soil is termed infiltration. Typical infiltration rates for different soils and rootzones used in sports surfaces are shown in Figure 6. The difference between a high sand content rootzone, such as a 70/30 mix, and a compacted clay is of several orders of magnitude. Figure 6 Typical infiltration rates for different soil media
The infiltration rate (how quickly water enters the soil) is a function of many factors but is governed by the pore space in the soil – if the soil has an open structure at the surface water can flow into the soil easily; if the soil is capped or has small pores, the infiltration rate is reduced (Figure 7).
Low infiltration rate in sports surfaces is caused by:
- small pore sizes (eg. clay soils)
- compaction (wheeled traffic)
- capping (silts or sliding feet)
- some types of organic matter, including thatch
- or a combination of all of these.
The inability to get water into a sports surface is one of the most common drainage issues and is known as a surface drainage problem. Figure 7 The effect of particle size on infiltration rate: (a) coarse particles with wide pores allows water to infiltrate easily; (b) smaller particles with smaller pores restrict (ê) or prevent (r) infiltration
Consideration of the above factors shows why clay soils are so susceptible to this problem – they have very small pores and the surface is easily smeared – providing a water tight seal over the soil. Installation of groundwater drains such as in Figure 5 will not solve this problem, the water cannot even get into the soil, let alone exit through the drains. If waterlogged and flooded pitches are to be avoided then it is necessary to bypass this impermeable surface completely using a surface drainage (or bypass) system. A surface drainage system uses bands of higher hydraulic conductivity / infiltration rate material to allow precipitation to bypass the low conductivity soil, straight from the surface to a piped drainage system. Such a scheme is detailed in Figure 8; the low conductivity soil (such as a heavy clay) has a very low infiltration rate and low hydraulic conductivity. Figure 8 A cross section and plan view of a sand slit system as an example of a surface or bypass drainage system for a low conductivity soil football pitch
In this system, some of the water from precipitation will pass slowly into the low conductivity soil – providing water to the grass plant. The majority of water, however, will flow across the surface and through the topdressing layer, into the vertical sand slits and then down into the collector drain and out to an outfall – completely bypassing the slowly permeable soil.
The system requires two key components for it to be effective:
- A high hydraulic conductivity connection to the surface – if a vertical slit becomes capped with fine textured soil, water cannot flow into the system and it will be redundant. For this reason it is necessary to provide frequent topdressing and careful devoting of the surface.
- An adequate collector drain network and outfall – if the water is not taken away, the material will become saturated rapidly and the system will not function.
Review of the key principles in selecting drainage design
For the design of effective drainage in natural turf pitches the following points must be considered:
- An investigation to determine whether the problem is caused by a water table (groundwater drainage) or a low infiltration rate (surface drainage).
- An investigation to determine the physical properties of the native soil.
- Determination of the correct depth and spacing for any piped drainage infrastructure.
- Calculation of the correct capacity for any infrastructure.
- Selection of hydraulically compatible and appropriate materials.
- Connection to free flowing outfall.
- Provision for hydraulic connection to the surface for bypass systems
Too many drainage schemes fail because these points have not been addressed. The system must be designed to solve the problem in the field. As discussed above, the most common drainage problem is that of surface drainage in fine textured (clay) soils.
The following article is an extract from the final report of a Football Foundation funded research project entitled The physical and financial benefits of mole drainage as an alternative to sand slitting in slowly permeable soils.
The research was conducted by:
Contact and principal author:
Dr Iain James
Cranfield Centre for Sports Surfaces
School of Applied Sciences,
Bedford MK43 0AL
Tel. No: +44 (0)1234 750111 ext 2736
Dr Richard Earl
Cranfield Innovation Centre
Tel No +44 (0)1234 756040