Technical


AFTWhizz Wheel in action “ 118 I PC APRIL/MAY 2014


Colchester United FC’s training ground after primary drainage installed


Water movement is affected by the forces


Water molecules are attracted to each other more strongly than to the air and this causes the effect known as surface tension which is caused by the cohesive forces of the water molecules


of adhesion and cohesion. The forces of adhesion and cohesion are both a result of the electronic charges on the water molecules and soil particles. Water molecules are attracted to each other more strongly than to the air and this causes the effect known as surface tension which is caused by the cohesive forces of the water molecules. Molecules are also attracted to the soil


particles by the forces of adhesion. Water that is adjacent to the particle is subject to the strongest forces. Hence, water is held very tightly in soils with small pores. Water will only move down through soil when the force of gravity is greater than the forces of adhesion and cohesion. Water can move sideways or upwards through a soil from an area of saturation to an area with lower moisture status. However, the speed of water movement under the forces of adhesion and cohesion is very slow and only occurs over small distances. When all the pores of the soil are filled


with water, the soil is considered to be saturated. The force of gravity pulls water downwards through the soil. The rate at which the soil drains at this point is called


the saturated hydraulic conductivity, and 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 a USGA sand. Water can now only be moved through


the soil by the pumping pressure 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


Sandslitting with a Shelton Super Trencher


Drainage work at Haverhill Golf Club


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