Drainage problems can best be avoided if grounds managers and greenkeepers understand their soil conditions, and know how to diagnose and treat any problems that occur through excess water.
Too often, pipe drainage systems are installed automatically, without enough thought being given to both the soil physics of water movement and alternative methods.
Soil is made of minerals originally derived from rock and organic matter taken from vegetation, water and air. Most soils in the UK have formed on top of glacial or alluvial secondary deposits, which is frequently boulder clay derived from glacial erosion and deposition. However, the original derivation of all this material is from rock.
The mineral fraction is divided into a very wide size range of particles. These are divided into the fractions:
- Sand: 0.05mm-2.0mm
- Silt: 0.002mm-0.05mm
- Clay: <0.002mm
This is a measure of the relative proportions of sand, silt and clay in a soil. Soils are often classified according to their texture, and the texture of a soil often provides a fair indication of how it will drain. The soil texture triangle is used to classify a soil based on the relative percentages of sand, silt and clay.
Structure is a term that describes the way in which the individual soil particles join together to form larger aggregates. A well structured soil is usually characterised by an intensive system of small aggregates or crumbs, gradually leading into larger blocks further down the soil profile.
Well structured soils are established on sites that have been left undisturbed for many years. The smaller particles in a soil form aggregates under the influence of clay, organic matter, earthworm activity and grass root activity. Poorly draining soils have often been badly over used and compacted.
How do these factors affect drainage?
Water moves through soil in the spaces between the soil particles. These are called the pores. In a soil with a high clay and/or silt content, the pore spaces are very small; the pore spaces are usually much larger in a sand dominated soil. A well structured soil contains a system of inter-connected very large pores that enables efficient water movement.
Downward water movement is faster in a soil where the pore spaces are large, hence sandy soils drain faster than clay/silt soils, and well structured soils drain faster than poorly structured soils.
Compaction reduces pore size and causes deterioration in structure.
Breaking up compaction is one of the keys to keeping surfaces playable, as well as a good drainage system in place. Surfaces that have become compacted will not let water through. Compacted soils have had the pore spaces squeezed out of their structure and are a complete barrier to the downward movement of water.
The key to good drainage is to know the sub-soil conditions to allow for the most appropriate drainage system to be applied.
When all the pores of the soil are filled with water, the soil is considered to be saturated. The force of gravity pulls water downward through the soil and the rate at which the soil drains at this point is called the saturated hydraulic conductivity; this rate is proportional to the square of the pore diameter. Hence coarse textured soils will drain much faster than fine textured ones.
As the soil drains, the largest pores are emptied and water remains in the smaller pores. A balance, or equilibrium is reached when the force of gravity can no longer move the water against the resisting forces of adhesion and cohesion. At this point the soil is at field capacity.
The amount of water held in the soil at field capacity varies enormously according to the texture of the soil. Typically, a loam soil will hold around 25 percent moisture at field capacity compared to only 5-7 percent for USGA sand.
Water can now only be moved through the soil by the action of the grass roots. Once all of the water available to the grass roots has been removed, the soil is at wilting point.
How do these principles affect drainage?
A typical sequence of events during rainfall is:
1. Rain falls onto dry soil. The infiltration of the water into the soil is relatively fast, as it is driven by gravity and the suction created by adhesion to the soil particles. The infiltration rate is determined by the texture of the soil - the larger the pores the faster the infiltration rate. Rainfall onto dry clay soil just runs off the surface.
2. As the pores fill with water, a zone of saturated soil is created that moves down through the soil under the influence of gravity. The speed of water movement through the soil at this point is determined by the saturated hydraulic conductivity.
3. If rainfall continues, the infiltration of surface water will be determined by the saturated hydraulic conductivity of the soil. Again, this is related to the texture of the soil.
Many sports surfaces are characterised by a shallow depth of topsoil, frequently overlying very slow draining clay-dominated subsoil.
In these situations, the topsoil accepts water very rapidly and, as a result, reaches saturation point very quickly. The drainage of the site is then completely dependent on the infiltration rate of the subsoil.
It is at this point that pipe drainage systems are typically installed - the aim being to increase the removal of water from the subsoil to allow the topsoil to drain at its optimum rate.
The sideways movement of water is virtually non-existent in most situations, so the drain will only accept water from the area directly above it and slightly to the side.
Pipe drainage systems are only effective when there is a zone of 'free-water' at the depth that they are installed. In this situation, the drains will lower the local water table enabling water to flow due to the hydraulic head of water above the drain.
Here, the produced fractures of deep aeration come into play. Water passes down these fractures, which all lead to the bottom of the probe hole, where water can accumulate and spread along the cracks. Creating a series of probe holes with a network of interconnecting fissures ensures the water can find an easy pathway, both horizontally and vertically to the drain runs.
Planning a drainage system means calling in the experts.
Sportsturf consultant, Gordon Jaaback, says that a detailed professional investigation of the project beforehand should identify most of the problems. A comprehensive work plan and specification prepared by a consultant gives the contractor a clear idea of the scope of work. Technical decisions reached early can prevent unnecessary expense.
Nothing is more costly to a contractor than correcting an item of work that should have been done properly in the first place.
Sand slit drainage systems
These systems are effective at increasing the drainage rate of a site, as they effectively by-pass the soil and the sub-soil by providing a connection between the surface and the gravel over the pipe drain line.
Sand slit drainage systems can often become sealed at the surface after play. Heavy sand topdressing will reduce this problem, but the potential for capping the coarse grit-sand in a drainage slit with the finer sand used for topdressing is sometimes a problem encountered by consultants.
Perched water table constructions
The majority of higher profile pitches and golf greens are constructed using this principle. The construction profile usually consists of a medium sand dominated rootzone over a gravel layer, with drains below. A blinding layer of grit sand is often included between the rootzone and the gravel.
Water will only move downward through the rootzone when the gravitational pull on the water exceeds the adhesive and cohesive forces in the rootzone. Water movement will only occur when the height of saturated water reaches a critical depth, unless special measures are taken to increase pore size. The depth of saturated water required to produce gravitational movement is dependent upon the texture of the rootzone. The finer the rootzone, the deeper it needs to be to obtain gravitational water movement.
Most bowling greens are built with rootzones of 150-200mm in depth, and sometimes less. Theoretically, these greens will be permanently saturated during wet times of the year.
The movement of water between the rootzone and the lower coarser layer is also influenced by the relationship between the particle size distribution of the materials. This is called the 'bridging factor'. If there is too much variation in the particle sizes, water will not move from one to the other.
Careful analysis of soil conditions and specific area problems dictates what products and machine are required.