Turf & Rec

Features Agronomy
Basic drainage principles applied to sports field design


June 17, 2009
By Francois Hebert Landscape Architect DSSS ltee.

Topics

By François Hébert
Design & Solutions For Sports Surfaces F.H. Ltd.

Today’s sports fields are subjected to levels of usage that very often far exceed their inherent capacity. These levels most certainly surpass those that older, more basic sports fields were ever designed to be able to handle. User demand has risen extremely fast and many municipalities’ capacity to keep up with this increase in needed playing hours has been stretched to the limit. In many cases, municipalities have had to respond to unanticipated and sustained surges in demand and their playing field inventories are feeling the strain.drainage

One of the primary factors that define a sports field’s usage capacity is its inherent drainage capacity. In fact, if we look at the different sports field construction systems available today, and those that have been developed over the years, we find that in most cases, their defining characteristic is their ability to control and evacuate water. This is achieved through a number of strategies, techniques and systems, but in all cases, drainage is the cornerstone of a sports field’s very essence.

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One eloquent illustration of this is found in the Canadian Sports Turf
Association’s recently published “Athletic Field Construction Manual.”
In it, we find sports field construction systems broken down in five
distinct categories. This classification system is organized in a very
clear diagram. The first defining feature between construction types is
the presence, or not, of a field lighting system, the premise being
that lit fields must be of a higher performance level than non-lit
fields, since they will inevitably be subjected to higher levels of
usage. The second design parameter is drainage. In fact, only the
fifth, the simplest type of construction is described as being without
drainage. Even in this case, the design specifications call for a
minimal surface slope, which should ensure some runoff, which in effect
constitutes a minimal form of surface drainage. And percolation through
the soil constitutes another type of drainage process that even the
most basic sports fields possess, however poorly this performs. So we
see that drainage, even in this last case, is an integral component of
the construction system.

In a sports field, water has essentially three different paths it can
follow in order for it to be evacuated: it can move vertically through
the soil, it can travel laterally towards drainage infrastructures or
it can move over the surface as runoff. One last possible evacuation
strategy is by evaporation, but I think we can all agree that this is
certainly not the most effective approach.

Through drainage

Through drainage is the most common drainage process used in sports
fields. Some even think that in its simplest form, when no drainage
pipe is installed, there is no drainage at all. In fact, this is the
most basic and natural form of drainage available.

When water penetrates a soil profile, it first fills in the biggest
pores and moves downward by gravitational force. Smaller pores restrain
this movement and, as we saw in a previous article, even smaller pores
cause the water to be held captive in the soil. Capillary force will
pull the water along through this network of fine pores, but a very
slow rate. As the water accumulates in the soil, its weight naturally
causes it to move down under the effect of the water column this
creates. Even if the soil is fine and heavy, capillarity will
contribute to its downward movement, just as it pulls it in other
directions.

In most sports field construction, we find that a layer of topsoil is
laid over a profiled subgrade. Certain precautions must be taken so
that through drainage can be the most effective. The first condition is
that every precaution must be taken to avoid compacting the subgrade.
This way, when water percolating through the topsoil reaches this
level, it will continue moving downwards and this will aid in the
sports field’s overall drainage. If the subgrade is allowed to become
compacted, downward water movement will be slowed and water will start
accumulating in the topsoil, eventually reaching the surface. Another
precaution that can be taken is to mix some topsoil into the subgrade,
at the contact level, so that the transition between the two is more
gradual, thus avoiding problems associated with soil layering.

Through drainage used alone is a relatively passive technique and is
solely dependent on the soil’s permeability. Furthermore, once the
water has reached the subsoil, the drainage rate will be limited to
that of this layer, which has often suffered compaction during the
construction phase of the project. Maintaining a deep layer of topsoil
is good practice in this case since the water that will inevitably
accumulate at this transition point can be kept farther from the
playing surface.

The importance of ensuring efficient surface runoff

In situations where no particular drainage process is planned for,
other than the soil’s natural drainage rate, it is wise to plan for
efficient surface drainage. It is of the utmost importance that water
be moved away from the playing surface in order to avoid saturation,
which can cause grave problems. Although it may seem visually
excessive, the final grade of a sports field built with natural topsoil
should have at least a 1½ per cent surface slope from center to the
sides. It is important to keep in mind that turfgrass has a very high
coefficient of friction (it is rough as opposed to a smooth surface
over which water would flow quickly), which slows runoff. This in turn
increases percolation in the soil. By allowing sufficient pitch, this
increases runoff efficiency.

Surface runoff is also important in the spring, when the soil is still
frozen and there is no percolation whatsoever. Runoff is then the only
measure that will ensure some kind of drainage and keep surface water
from accumulating on the turf cover.

Lateral water movement and subsoil drainage systems

As the name implies, lateral drainage involves water moving sideways
through a soil profile, usually to reach some type of drainage
infrastructure. As soon as a drainpipe is put into the ground in or
around a sports field, many people think that this constitutes a
drainage system and the field automatically gains a few notches in the
performance scale. These people are often quite disappointed when the
anticipated results do not materialize. Let us have a look how this
movement occurs and what can affect it.

Imagine a heavy rain event. The water spreads through the profile and
gradually builds up at the bottom of the topsoil profile. When the
pores become filled with water, the soil becomes saturated. There first
develops a layer that contains the amount of water that the soil’s
capillarity can retain. If we dunk a sponge in water, this is the layer
of water that would accumulate at the bottom once it has stopped
dripping out. Even though this layer of soil is saturated, this water
is tightly held in the soil by the capillary forces. This is called the
capillary fringe. As more water is added and its level in the profile
rises, pressure increases at the bottom of the saturated profile to
levels that exceed the soil’s retention capacity. 

Once this free water level appears in the profile, underground drainage
structures start working because this water can now escape the hold of
the soil capillarity. If this free water layer is set directly above
the drainage infrastructure, it then flows directly down into it and is
carried away. But if it must move laterally in order to reach the
drains, then it must force its way through the maze of pores and soil
particles it finds in its way.
Let’s try to visualise how water moves through soil towards underlying
drains. Imagine yourself walking downtown on a Sunday morning and the
sidewalks are almost deserted. You are rushing to get to where you are
going. Here and there, people are walking slower than you are or are
standing in your way. You just shift right and left and your movement
remains constant and fluid. This could be compared to free water
flowing through a course material such as an evenly graded coarse sand
or clean stone base.

As the material becomes heavier (finer and/or denser – compacted), the
obstacles are set closer together. The space is tighter and it is more
difficult to manoeuvre, just like when you’re walking downtown through
a crowd on a busy summer afternoon. You have to weave around, sometimes
stopping because of people who have stopped in front of you. To cover
the same distance, your path is much more tortuous and your progression
is much slower. The crowd creates a pressure restraining your forward
movement.

Now imagine that you are at a rock concert. You’ve arrived early and
are positioned close to the stage. The crowd around you is compact, the
people are tightly pressed together. For some reason, you have to
leave. You find your way blocked by the pressing crowd. Groups of
people are pressed together. You have to push and squeeze yourself
between small gaps and openings between compact groups. A little
farther, there is an exit with an opening in the crowd. You would think
that this opening would help open the space around you, but this relief
in pressure does not reach you, and despite this opening being close
by, you still must struggle to move forwards.
Although the analogy is a little simplistic, this is somewhat similar
to what happens within compacted soil, as one moves away from a drain.
Directly over it, free water below the capillary fringe will flow into
the drain. Water on either side will need to pass through the soil in
order to be evacuated. Depending on the soil type, the resistance to
this lateral movement will vary. At some point, the resistance exerted
by the soil nullifies the draining effect and the free water between
two drains is held captive.

Drain spacing and sports field drainage design

The pressure of its own weight pressing down powers the water’s lateral
movement. As water is removed over the drain, this pressure acts to
move more water to fill the void created in the soil’s pores so that
the system remains balanced. If there is sufficient space in the soil
profile for enough water to accumulate below the capillary fringe to
develop pressure, this will provide power to maintain water movement
towards the drain. It is important to maintain a deep layer of topsoil
so that this layer can develop while keeping the saturated soil as low
as possible from the surface, where it can affect the sports field’s
stability.

The farther the evacuation point is set from the water’s source, the
less effective the system. In sports field construction systems, we
often see that relatively thin layers of topsoil are put in place,
presumably because this material is so expensive. But this in turn has
a detrimental effect on both drainage performance, as well as turfgrass
root development. So the other design parameter that then comes into
play is drain spacing. In other words, the closer the drains are set,
the better will be the drainage potential of the system.

Soil type (hydraulic conductivity), its depth and drain spacing are the
three main determining factors that affect a sports field’s drainage
performance. The following tables illustrate how manipulating these
factors affects drainage performance. They also illustrate quite
eloquently just how slow water movement can be in soils, even in those
cases we would think as extremely fast draining.

Alternative drainage methods

This description of drainage principles is both simplistic and
incomplete. When we start to explore this subject, we discover a large
number of other phenomena and principles that make it even more
complicated. While a very scientific approach can allow us to fine tune
drainage system designs, in practice, we find that we usually tend to
find a system configuration that works for us, while using the soils we
have available and then we stick to it. 
The most important to remember of all these notions are the fundamental
principles that govern water movement. This allows us to avoid costly
mistakes that inappropriate designs can produce and also to diagnose
the causes of drainage problems we may encounter in existing sports
surfaces.

Besides the most basic drainage approaches described above, there are
other, more elaborate drainage techniques that the turfgrass industry
has come up with over the years and that can be applied to modern
sports field design.

Slit drainage as an alternative to underground drainage

Slit drainage was invented in England more than 40 years ago. It
consists of a series of channels, or slits, that are cut directly into
the topsoil surface in order to intercept surface runoff. The slits are
filled with a coarse drainage material that allows effective water
movement. The water that is intercepted is then either transmitted into
the soil profile, or into a pipe network that is built into the system
and that ensures evacuation.

Slit drainage has evolved tremendously over the years with the latest
developments involving the laser guidance of the slit cutting and pipe
laying. There are also variants of the system described here. In some
systems slits are not excavated, but rather ploughed through the
profile. Different systems propose slits of different widths, some with
pipes while others do without.

While slit drainage was originally designed to intercept and eliminate
surface runoff, these systems can also help wring out the soil profile
once it has become saturated. Referring to the previous illustration,
if the slit is cut below the capillary fringe, it can eliminate the
free water that builds up below it. Because of the narrow spacing that
slit drainage is usually installed at, it greatly improves drainage
performance in most types of soils and, after it has rained or the
sports field has been irrigated, it helps restore sports fields to a
playable state much faster than more conventional drainage systems.

One last word about slit drainage systems:  they can be easily
retrofitted into existing turfgrass surfaces. Modern drainage methods
allow these systems to be installed with very little disturbance to the
existing turfgrass cover so that drainage system upgrades can be done
quickly without ripping up the surface.

Modified and manufactured soil mixes

We have seen that soil properties have a huge impact on a sports
field’s drainage characteristics. They affect water retention, water
movement and water evacuation in many ways. Also, the heavier, less
draining soils are more sensitive to compaction. Compaction increases
soil density which in turn greatly alters a soil’s general drainage
characteristics over time.

In order to reduce these soil related problems, the industry has
developed a different approach by which it can produce rootzone mixes
by combining different materials (usually sand for drainage and
resistance to compaction, finer soils and organic matter for water
retention and increased agronomic performance) to produce mixes with
specific characteristics.

Manufactured soils have been used for decades by the USGA in its golf
green construction system. Sports turf designers started using similar
mixes to build sports fields with high drainage performance and
increased resistance to compaction. 

Such mixes can be a little tricky to use for many reasons but they are
appearing in a growing number of higher end sports field construction
projects. These mixes have also been used in conjunction with slit
drainage systems to produce playing surfaces with amazing usage
resistance qualities.

Perched water table systems

One last type of sports field we see is the perched water table system.
This system is directly derived from the USGA greens construction
method. This is the system the STA qualifies as a Category 1 field.

In this type of construction, the root zone mix is laid over a coarser
granular layer under which the pipe drainage system is put in place. As
we explained previously, the root zone will gradually fill with water
as it rains or the field is irrigated. As the water level rises, the
capillary fringe forms at the bottom, right over the coarse drainage
layer. Capillarity holds the water in the upper layer until a layer of
free water starts forming. In the traditional drainage approach where
the pipe is laid just below the soil, water then needs to move
laterally towards the drain, with all the problems this entails. In the
perched water system, the free water is pushed directly into the lower
drainage layer, without having to move laterally. As the water supply
is cut off, the free water finishes its downward movement and the
capillary fringe then settles at the bottom of the soil profile. This
water then serves to supply the turf’s root system. As the surface
dries, water is pulled up by capillarity. As the supply is gradually
used up, the wet front gradually lowers, which in turn acts to pull the
roots downwards to reach more water.
On paper, such a system is extremely effective. In practice, such a
system requires both a very high level of expertise on the part of the
manager and an intensive level of care and maintenance.

Understanding sports field drainage design

Reading all this, one can get the impression that sports field drainage
design is an exact science reserved for a select few. This is not the
case and this text is not meant to convey this impression. Quite the
contrary.
Drainage follows a few rules and is governed by rather simple
principles. What can be confusing is that some of these principles,
such as the perched water table, are counter intuitive and go against
what some might see as common sense. Also, drainage has long been
discounted as an overly simple proposition that could be addressed
simply by laying a few sections of pipe. 

This is definitely not the case and misunderstanding of the basic
principles is at the root of a great many problems sports field
managers have been faced with over the years. But once the basics are
understood, drainage is a pretty straight forward process that follows
simple rules that, if well applied, produce consistently good results.
With this article, I am trying to share some of the very basic
understanding I have acquired of some of the principles associated with
drainage. Some of these explanations and analogies will certainly seem
simplistic, of not awkward, to some who have a more scientific
knowledge of the principles involved. 

When browsing through the specialised text books consulted while
writing this article and viewing the complicated formulas presented
within these sources, one would think that drainage is as complicated
as rocket science. But in practice, a little knowledge and a good dose
of common sense will get us a long way in increasing the quality of our
sports fields by improving their drainage characteristics.