Technical GLOSSARY
Drainage design rate - This is the rate at which drainage system installation removes water from a given site and the measurement is given in mm over the drained area.
Infiltration rate - This is the measurement on site of the rate at which water enters the soil surface
Double ring infiltrometer - Two steel concentric rings are knocked into the grass surface and both rings are filled with water to the topmost surface of the rings. The infiltration rate is measured by noting the time taken for the measured full depth of water to drop in inside ring once field capacity is reached or after 30minutes of initial infiltration.
Hydraulic conductivity (or the percolation rate) - This is the laboratory measurement of the movement of water downwards over a period of 24 hours through a saturated compacted soil sample subject to a negative tension of 30cm and under a permanent head of water.
Particle size distribution - This refers to the laboratory assessment of the proportional contents of clay, silt and the different sand fractions in the soil or aggregate.
Saturation - This condition in the soil is reached when all the voids are filled with water.
Field capacity - Once all the water in the soil that drains due to gravity alone is lost to lower layers and only water remaining is held by the soil particles, field capacity is reached.
Surface gradient - This describes the fall in elevation over the length, the width or both in the final grading of the sports pitch.
Porosity - This is a general measurement of the percentage of voids or spaces in the soil and comprises larger air-filled voids (non-capillary porosity) and smaller water-filled voids (capillary porosity). The relative proportion of both these voids determines the porosity.
Critical tension - This is the depth of a root zone material of specific particle size distribution needed to enable drainage downwards and the opening of pore spaces at the surface sufficient to support satisfactory growth.
Pore continuity - When the pore spaces between soil particles of a growing medium conform with the pore spaces of a material directly below it there can be downward movement of drainage water. A coarse material below a finer material does not have pore continuity and drainage water is withheld in the material above until saturation is reached.
Suspended water (capillary fringe) - This water is suspended in the rootzone in a saturated condition above the blinding layer or above a free water zone. This suspended water is not able to move to drains and its depth will depend on the particle size of the sand and the rate of free water removed sideways to drains or downwards into the base material. Where a finer material exists below the rootzone, this water will only move downwards as the base begins to drain.
Free water - This water collects at the bottom of a rootzone below the capillary fringe over a slowly draining base material or penetrates through a blinding layer after saturation in the rootzone is reached. Accumulating below suspended (or perched) water, this free water is not held with any force and it can move readily sideways to drains and downwards into a slowly draining base material.
Slit drains - These are narrower excavated drains without pipes and normally 50mm wide, backfilled with stone aggregate and topped with coarse sand to the surface. The depth is 250 to 300mm.
Sand injected grooves - These are narrow slit drains installed with vibrating rotating tines and simultaneously filled with a coarse sand/grit. The slits are 20mm wide, not more than 170mm deep and are installed at 260mm spacing.
Swales - These are constructed linear depressions to divert surface water flow. They are installed with suitable gradient and side slopes of around 1:8 - and with a depth of around 200mm are easily mowed. Preferably pipe drainage is installed in the invert of the swale.
Attenuation - Slowing down the rate of flow to prevent flooding and erosion, with consequent increase in the duration of flow.
122 PC FEBRUARY/MARCH 2013
“It is worth mentioning the folly of ameliorating the upper rootzone by incorporating relatively small quantities of sand into heavy clay loam soils”
1998). The nature and condition of the base (subsoil) are often overlooked and little is done, during construction, to create optimum transition between the subsoil and topsoil. Generally, compacted subsoil adequately ripped contains a finer combination of soil particles than indigenous topsoil above it. In this instance, pore continuity is maintained and there will be downward movement of water without suspension in the capillary fringe.
On the matter of water flow in slit drains and grooves, there can be misunderstanding. With lateral piped drains, often installed in the steepest gradient in order to despatch drainage
water to collector drains and on to the outfall, the installation of slit drains and grooves at right angles serves to check and collect the surface water, permitting nothing more than - in the words of Geoffrey Davison - ‘seeping’ of water towards the nearest lateral drain. This fact is hard to appreciate given the fact that surface water must be removed as quickly as possible. It is only at times of sustained heavy rain, when all pores are saturated in the slits and grooves, that water flow may be more rapid. The ironic fact is that successfully slit drained or grooved pitches depend on the speed with which surface water can
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