May 12, 2009 By Mike Jiggens
By François Hébert
Natural sports turf drainage systems exist in many forms and shapes, but share one common trait. While some water can be evacuated by surface runoff, most of it must percolate through the root zone to reach some form of underground drainage system.
How fast and efficiently this percolation can occur and the amount of water that will remain available for the turf in the actual root zone determine a drainage system’s overall effectiveness, and consequently the playing surface’s performance and resistance to wear. All of this is highly dependent on the interaction between soil particles and water molecules.
Natural sports turf surfaces are subjected to uses and abuses that are totally incompatible with the very nature of the turfgrasses they are made of. Proper drainage is essential for this turf cover to withstand the abuse it is subjected to. Yet, the very factors that affect the turf surface affect the performance of the drainage systems that are built into the sports fields. In order to design proper systems and to ensure their maintenance and prolonged effectiveness, it is important to understand the basic principles and processes that are at play.
Perched water table
Up, down, all around
Water movement in soil can be compared to that of water moving through a sponge. When a dry sponge comes into contact with water, we see a wet front moving through it. This front can move downwards if water is poured from the top, but it will also move sideways, or even up, depending on where the water is coming from. The wet front moves through the sponge, away from its source. Unlike what many people think, this movement is not produced by gravity alone, for if it were, water would not move any other way than down. Other factors must thus be at play.
If the sponge is dunked into water and pulled back out, some water will flow out, but the sponge will also retain a certain amount of it. The forces holding water in the sponge are the same that cause the wet front to move through the dry sponge: cohesion and adsorption. The combination of the two produces what is known as capillarity.
When water moves through a soil profile, water molecules are attracted to individual soil particles by adsorption. This is the force that binds the water molecules to the dry surfaces of the particles that compose the soil. It is the same phenomenon that causes water drops cling to a glass surface even if it is maintained at an angle.
While water molecules are attracted to dry surfaces by adsorptive forces, they are also attracted to each other by cohesive forces. It is the cohesion between water molecules that causes them to bunch together into drops. These cohesive forces can be quite strong. To understand cohesion, let us look at how this affects other materials. The cohesive forces between mercury molecules are so strong that you can drop mercury on a flat surface and you will see it explode into a multitude of differently-sized beads. Then, you will see these beads starting to move towards each other, attracted that they are by huge cohesive forces. On the other hand, you have acetone that will spread out in a thin film when it is spilled, before evaporating before your very eyes. In acetone, the cohesive forces are extremely weak and the molecules will easily drift apart. Strong cohesive forces cause water molecules cling to one another.
Now, once water molecules have wet a particle by attaching themselves to it, other molecules that are pulled along by cohesive forces surround them. These molecules are in turn attracted to other neighbouring surfaces to which they attach themselves. Thus, we see what we perceive as the wet front moving on its own, while it is in effect being pulled along by capillarity.
Capillary forces pull along the water molecules that are bound together by cohesion. As opposed to when water is pushed by positive pressure, capillarity pulls the wet front through the soil profile. This is why, when capillary forces move water, we say that it is under tension, which is a kind of negative pressure. This tension causes the water molecules to move any which way there is a soil particle to cling to, independently of gravity.
The adsorptive forces are exerted not only on the molecules that are directly in contact with the attracting surface but also on the other molecules that surround it. The smaller the distance between the molecule and the attracting surface, the greater will be the attraction.
Individual molecules can also be attracted by more than one surface at a time. For instance, if we take two glass sheets, pour water between them and them join them together, we see that they seem to stick together. Instinctively, we would think that we created a vacuum by squeezing the glass sheets together. In fact, we see that the water molecules caught between the glass sheets find themselves attracted from both sides at the same time. If we increase the pressure on the glass, we reduce the space between the two surfaces, thus increasing the tension that is exerted on the water molecules from both directions. This tension in turn pulls the glass sheets together, which is why it becomes difficult to pry them apart.
In soils, this translates in more capillary force being exerted on water when the pores through which the water must travel are smaller. In bigger pores, water can flow freely. In smaller pores, the capillary forces start to interfere on the flow of water. In the finer soils, such as silt or clay, the pores can be so small that the water finds itself held captive. The fact that the molecules are bunched together by the cohesive forces reduces their ability to squeeze through extremely tight spaces. This is why, for instance, clay can be effectively used to create an impervious layer, such as it does when it is used to line the bottoms of water retention ponds. The pores are so small, and the capillary forces are so strong that water just cannot pour out of it.
As water moves through the soil and more water is supplied, the pores between soil particles gradually fill. Some water will then freely escape the soil profile, having found its way through the bigger pores (>0.06 mm in diameter).
As more water enters the system, all the pores gradually become filled. At this point, the soil is said to be saturated. Additional water has nowhere to go, so it starts to pour out of the soil profile. This is the beginning of the actual drainage of this soil profile. This is also the point where water will start to move laterally towards drainage structures that have been put in place.
When the water supply is cut off, the larger pores empty out and, at a certain point, drainage stops. Capillary forces retain water caught in the smaller pores, which can then be used by the turfgrass root system. Soils, just as a sponge, can retain a specific amount of water.
This accumulation of some water in the soil profile is perfectly normal and desirable if we want to provide the turfgrass with sufficient water to meet its needs without constantly having to irrigate. But it becomes problematic if, for some reason, the soil retains too much water, which can affect plant growth or render the playing surface exceedingly sensitive to player activity.
Compaction and water movement
In sports fields built with any type of soil profile, the root zone layer presents a certain pore structure. Unique to each soil, this combination of small, medium, and large pores determines the field’s initial drainage capability, as well as the amount of water that will be available to be used by the grass.
The presence and arrangement of larger pores determines the soil’s ability to drain freely. A soil composed of mainly fine particles will have few large pores. Water will be held captive and poor drainage will result. On the other hand, a soil composed exclusively of large, coarse particles will drain freely, but will be incapable of retaining water necessary for plant growth.
Play and regular maintenance practices apply incessant pressure to the surface, which results in localized compaction. Compaction patterns are specific to each sport, but they are usually similar to the surface’s wear patterns. Pressure that is repeatedly applied over the same surface will gradually pack the soil particles together. This decreases the number of larger draining pores, and consequently decreases drainage performance.
Sports field compaction is an insidious process that develops over time. The constant pounding of feet from the players combined with the repeated passage of maintenance equipment inevitably results in an increase in soil compaction. As the soil becomes denser, the pressure is transmitted deeper and deeper. Because of this, a sports field built with a perfectly good soil can gradually lose its drainage properties, which inevitably results in a loss of surface quality. If this compaction is allowed to reach below the working depth of the tools that are used for aeration and decompaction, we find ourselves faced with problems such as hard pans that can become very difficult and expensive to solve. Regular decompaction with the simplest tools can keep this from happening.
Modern sports field design and construction increasingly integrates manufactured, compaction resistant soil mixes. Combining medium to fine grade sands with organic matter and other materials, these soil mixes can withstand compaction while ensuring sufficient water and nutrient retention compatible with turfgrass growth.
The perched water table and water retention
Up to this point, the principles of water movement in homogenous soils are fairly easy to understand and visualize. Things get a little more complicated when we start laying one soil type over another, as is common in many sports field construction schemes.
Let us get back to our sponge. After free-flowing water has stopped pouring out of the larger pores, you can pick up the sponge and it still holds water. Put it down on a bed of gravel, coarse sand, or another coarse material, and the water will remain in the sponge. Contact with a free-draining material will not induce water to flow out of the sponge.
The same is true with soils. Negative forces applied by the combination of adsorption and cohesion compete with the positive pressure of the mass of the accumulated water to hold it captive. As particle and pore size decrease, the effects of these combined forces on the water molecules increase. As more water is added, it spreads through the profile and accumulates to the point of saturation.
If the soil profile overlies another that is coarser, water will accumulate in the finer soil until the retentive forces cannot contain the weight of the accumulated water and it starts flowing. This is called a perched water table.
This very common phenomenon is widely misunderstood because it is so counterintuitive. It is only normal to assume that by placing a free-draining layer below a heavy soil, drainage will be induced from one layer to the other. In effect, the exact opposite occurs. The greater the difference in particle size distribution between the two soils (granular discontinuity), the harder it is for water to cross over from one to the other.
This phenomenon will also occur when water tries to flow through a coarse material imbedded in a fine soil. The retentive forces keep water molecules captive in the fine soil, and perfectly dry, coarse spots can be found in moist or wet soils. This can cause problems for drainage systems when water is supposed to flow from a fine soil into gravel or other material surrounding drainage pipe.
Water movement and soil stratification
Layers of fine material embedded in an otherwise well-draining soil can also have very disruptive effects on water movement. Water will have no difficulty crossing from coarse to fine material, but water movement is much slower in the fine soil.
Such a barrier affects the whole potential of a drainage system. Once the fine material is saturated and water flows through into the coarser soil, its percolation rate has been reduced to that of the finer layer. This common situation may seem inconsequential, but it can greatly affect a sports field’s performance and it can be very difficult to correct.
The same disrupting effect occurs when, for some reason, a layer of coarse soil finds itself embedded in a finer soil profile. We are then faced with the same situation described above, where a perched water table develops in the soil profile.
Stratification can have many causes, but the most common are related to inappropriate maintenance practices. Topdressing and turfgrass repairs can sometimes spread layers of fine materials, which over time develop into severe stratification problems. A one-eighth-inch layer of fine soil is enough to block water flow in an otherwise perfect soil profile.
We sometimes see a succession of such layers, each one further slowing the percolation process to the point where it can seem to stop.
Stratification is difficult to correct. Aeration and sand topdressing can help but they must be done repeatedly to effectively correct the problem. Prevention is much easier and economical.
Sports turf drainage and its challenge to the turf grass manager
The fact that we refer to sports surfaces when describing these principles and processes does not mean to imply that they are specific to this type of turf grass surface. Adsorption and cohesion are natural processes that occur any time water travels through confined spaces. Natural soils normally maintain a certain pore structure that provides passages for the water to move through. Sports turf is different from other turf surfaces in that by its very nature, it is subjected to uses that directly affect the soil structure and the pore networks. In other less utilitarian turfgrass surfaces, water may have time to move through and out before circulation is allowed to resume. Sports turf surfaces are rarely allowed the luxury of prolonged periods of inactivity after rain or irrigation occurs. This is why in this case drainage performance is so critical.
The challenges facing sports field managers are numerous. One of the greatest of these is that they very rarely have control over how and when the playing surfaces are used. Their job is often limited to trying to grow the grass, and to repairing the damages that are caused by others who do not share their concern for the surface’s well being because they are faced with different but equally tough challenges.
Understanding the basic principles that govern drainage is a very important first step in achieving better quality sports fields. This allows for better designs, more careful construction and more appropriate maintenance and repair practices. But more importantly, once we begin to understand that natural sports fields are more than just turfgrass growing in soil, but complex, fragile and expensive systems, this may help develop new management approaches more in tune with the sustainability and conservation-based values that Canadians are embracing more and more.
Inquiries for the purchase of the illustrations contained in this article and a video can be addressed to: Crop and Soil Sciences Club, c/o Dr. Bruce Frazier, Washington State University, P.O. Box 646420, Pullman, WA 99164-6420. http://www.css.wsu.edu/
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