Technical
Consultant Gordon Jaaback explodes some of the myths surrounding sports pitch drainage
Exploding the myths...
Poppycock, pish and piffle!
number of factors contribute to the success in a sports pitch drainage programme. Installing and maintaining a system is an expensive undertaking. A complete system, incorporating slit drainage and sand dressing, can amount to in the region of £25000 to £30000. Negligence in just one area can jeopardise the entire investment. The following myths are worth considering:
A
Amelioration of clay loam topsoil by mixing in 30mm to 50mm of sand (400 to 600t) into the top 50mm of the topsoil surface improves the drainage of the pitch.
False. Developing what amounts to a 50/50 mix of heavy loam soil with sand creates a still relatively impermeable layer of sandy loam material, which no longer expands and contracts as clay does. It compacts easily into a hard crust after play in wet conditions.
To achieve satisfactory amelioration in a sand carpet or complete rootzone, the sand content must be at least 80% to create adequate non-capillary porosity, and so permit drainage to lower layers without the fear of compacting. Furthermore, unless the organic matter content is contained in these improved rootzones, the infiltration and consequent surface drainage potential will reduce.
Lateral drains spaced as close as 5m or 3m, installed without slit drains, will overcome
drainage problems.
False. Invariably, drains are installed prior to final levelling and seeding. Besides contamination at the surface with fine particles, even this spacing is too great to remove surplus surface water before it enters the surface and develops localised wet areas. Below the soil, surface water moves very slowly laterally - too slowly to reach drains at this spacing. Irrespective of the gradient of the pitch, low rainfall promotes little run-off, and the distance between laterals is too great for water movement to overcome the resistance of the grass growth and the micro-depressions that will retain surface water.
A pitch can be over-drained.
False. Only surplus water moves from saturated soil into trenched pipe drains. Water is held far more strongly by the clay loam topsoil and clay subsoil than the porous aggregate in the drains. There can never be an attraction/suction of soil water from soil to sand aggregate. The argument that retaining the surplus water makes it available at times of water shortage cannot be supported. Water lost downwards into the subsoil cannot move readily up through the topsoil during times of need.
A well designed slit drain system installed into a level or undulating surface will overcome drainage problems.
False. Surface water must be able to move
laterally over the surface to reach slit drains, and there should be adequate gradient in a single cross-fall or on either side of a camber. Average rainfalls of around 5 to 12mm per 24 hour day hardly promote surface movement over relatively flat surfaces, and water will accumulate and be lost only be evaporation. Undulating pitches will promote run-off to depressions.
Initially, slit drains will take in the surplus water in these depressions, but the wetter conditions created in these locations will make the surface softer. They will be more subject to deformation from play and prone to collecting silt containing water run-off that, in time, caps off the opening in slit drains and nullifies their effectiveness.
French drains or pipe drains, with pea gravel to the surface, will control surface water moving down cut slopes on to the pitch.
False. Water run-off moving down a cut slope will always contain silt passing over the soil surface. This continual silt content will eventually blind open drains, temporarily sealing the surface. It is just a matter of time before grass growth covers the silt covered aggregate. Furthermore, at times of high intensity rainfall, surface run- off will not stop at drains to gain entry - water flow will simply find its way on to the pitch. A manageable solution is the creation of a shallow mowable swale at the
122 PC JUNE/JULY 2012
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