But drainage is more than installing pipe and slit drains – understanding how surface water moves both on and below the surface is probably the most important consideration. Water lingering on the surface of heavy clay topsoiled pitches becomes a scourge to footballers. This water must be removed as soon as possible – at least before firm playing surface becomes a soft muddy quagmire.

Rainfall becomes a vital consideration and it is the intensity and duration that is most important. Wallingford research (FEH,1993) rates the occurrence of in the region of 25mm in one hour to be the maximum intensity expected over a 10 year period in central and southern England. It may be startling to some that over 80% of rainfall in the south is less than 10mm per day with up to 50% being less than 2mm per day. Nevertheless, although average rainfall intensity in Britain is in the region of 5mm per hour, short duration heavy downpours in 5 to 10 minutes can far exceed this rate falling at up to 100mm per hour. This is especially significant with the realization that rain can fall on average 12 days in the month during the winter season when for much of the time the surface soil is saturated and evapotranspiration is negligible (Jaaback,2008).

Impact of water moving over and above the soil surface (run-off)

Naturally, on commencement of rainfall, there is water retained in the turfgrass foliage, the micro-depressions within the soil surface and porous surfaces to drain installations. Estimates of the water retained have not been researched but it is expected that they could amount to between 3mm and 5mm. What is certain is the fact that rainfall must exceed the retained amount before run-off commences (Tindell and Kunkel, 1999)

Secondly – and often over-looked – water run-off on to the pitch from higher ground always results in wetter areas. This flow must be diverted above the cut slope if it exists. A practical measure to halt the movement of water over the soil surface is the installation of ditches and swales. The latter can become an integral means in attenuation (CIRIA,2000). In preventing run-off on to the pitch, shallow mowable swales at the base of cut slopes have proved very effective – particularly with pipe drainage installed in the invert of the swale.

Water does not move quickly laterally within the soil surface. On the surface, compaction, the presence of organic matter and thatch reduce infiltration significantly and with persistent rain saturation soon develops. With high intensity downpours far exceeding the consequent infiltration rate of the soil, run-off is inevitable – provided there is a suitable gradient over which to run. The degree of run-off after short sharp showers is underrated – yet it is always evident in depressions in a pitch and has been significant in the swales down the sides of three cambered pitches without installed drainage over the last eighteen months.

What are the gradients necessary for satisfactory surface drainage of a sports pitch? Adams and Gibbs, 1994 are not specific suggesting a diagonal fall between 1:67 and 1:100 but McIntyre, 1998 contends that a cross-gradient should not be flatter than 1:70. In cut to fill construction gradients of 1:40 to 1:50 are well accepted and have been very effective. On level ground the creation of a camber with side slopes of 1:70 is hardly noticeable and has also proved very successful. In fact, both Sport England and the Football Foundation do not state preferred gradients other than to say the maximum gradient across the line of play should not exceed 1:40 to 1:50.

If a gradient is essential to move water laterally over the surface and this water is to be removed as quickly as possible to retain firm topsoil conditions – then it goes without saying that close-spaced slit drains intended to bypass the heavy relatively impermeable topsoil should be as close as possible. Spacing of 1m appears to be most practical and suitable to retain firm conditions. What we do know, is that this method of bypassing the heavy clay soils does work and soft muddy conditions can be prevented if this surface water is removed quickly in this manner.

Contrary to desired normal summer procedures of aerating with the vertidrain and earthquake, any loosening and opening up of the firm clay loam topsoil in the winter months can lead to disaster – surplus water enters and is collected in the upper layers making them wetter and softer. At this time firm surface conditions are vital to sustain play and surplus water should be despatched quickly into the bypass system of slit drains or grooves.

Water moving below the soil surface

Infiltration rate is critical and so dependent on the condition of the soil matrix, the homogeneity of the particle size distribution and the organic matter content. At this point it is worth mentioning the folly of ameliorating the upper rootzone by incorporating relatively small quantities of sand into heavy clay loam soils. Since the objective is to improve resistance to compaction and increase porosity, the particle size distribution of the sand is vital and there must be a dominance of sand in the resultant mixture (Waddington et al, 1974).

Where an improved rootzone is imported it should be fully evaluated in laboratory tests. The depth is determined on assessment of the critical tension. There has been extensive research into the criterion in rootzone design. Awareness of the capillary fringe above a drainage carpet or slow draining base is essential in drainage design – particularly the fact that water is held in the fringe to almost saturation before being released into the lower layers or adjacent drainage (Adams, and Gibbs, 1994, McIntyre, 1998). The nature and condition of the base (subsoil) are often over-looked and often little is done during construction to create optimum transition between the subsoil and topsoil. In most locations 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 be removed. Slits and grooves are only functional so long as entry access at the top of mini-drain is maintained in an open condition allowing the surface water to easily get away. This means It becomes imperative to regularly apply sand dressing to the playing surface (Adams and Gibbs, 1974) In practice at the cost of in the order of £3500 per pitch annually this expenditure generally is out of reach for many schools, clubs and local authorities. This leads to an inevitable conclusion that no surface drainage system employing slit drains or grooves should be installed if regular sand dressings are not going to be undertaken.

A final comment – following the difficulties experienced and the effort made in topping up slit drains in the first year of establishment, thoughts have been directed to reducing the spacing of lateral drains to 3m and cutting out the installation of slit drains. Time will tell but the removal of collecting slits at right angles to the laterals reduces the potential for removing surface water – particularly if the laterals are installed down the steepest slope, probably the cross-gradient. However, by installing the close-spaced laterals and following with grooving at right angles this alternative system has much merit. Though grooving is narrower being 20mm in width, the close spacing of 260mm apart makes this alternative a worthwhile consideration. On the downside, the effectiveness of these narrower slits under heavy wear over time is under question. Repeat treatments may well be needed within a few years.

Literature cited
Adams, W.A. and Gibbs, R.J. 1994. Natural turf for sport and amenity. CAB
International.p.102-156.
CIRIA, 2000. Sustainable urban drainage systems design manual. Construction Industry
Research and Information. C522. p. 8, 74-77.
Davison, G. 2005. Personal communication
FEH, 1993. Flood estimation handbook. Centre for Ecology and Hydrology. Wallingford.
Jaaback, G. 2008. The role of water run-off on grassed sports pitches. Proc. 1st
European Turfgrass Society Conference, Italy 19-20 May. P.99-100.
McIntyre, K. and Jacobsen, B. 1998. Drainage for sports turf and horticulture.
Horticulture Agency Consulting. p. 64-69, 110.
Tindall, J.A. and Kunkel, J.R. 1999. Unsaturated zone hydrology for scientists and
engineers. Prentice Hall. p. 367-368.
Waddington,D.V et al,1974. Soil modification for Turfgrass Areas. Progress Report 337.
Pennsylvania State University.
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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.

First and foremost there is the need to prevent any surface water run-off on to playing areas. Provisions can be simple but this vital need is often overlooked. Secondly, irrespective of drainage piping or slit drain installation, the need for a satisfactory even gradient – at least across the direction of play – has become glaringly apparent with the collection of surplus water in depressions or the collection on pitches with insufficient fall in gradient to enable satisfactory water run-off at times of persistent rainfall.

Adequate drainage installation is vital on sports pitches used heavily in the wet winter months – but ensuring from the start that the pitch only needs to cope with the rain falling on it and is able to shed surplus surface water are essential pre-requisites

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 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.

Close spaced lateral drains 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 surface water moves very slowly laterally. 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 to the drains – water flow will simply find its way on to the pitch. A manageable solution is the creation of a shallow mowable swale at the toe of cut slopes with a pipe drain installed in the invert of the swale.

Any local sharp sand covering pea gravel in a pipe drain trench will serve the required need and enable satisfactory water inflow

False. It is vital that the gravel and the sand are chosen with the particle size distribution of each material being such that the acceptable bridging factor is attained. Fine particles in the sand moving down into the gravel soon restrict the drainage performance by occupying the voids necessary to enable satisfactory downward water movement. Of particular concern is the fine material content in the sand (less than 0.25mm) – which ideally should be less than 10%.

Drainage design must be adequate enough to accommodate all water not retained after high intensity rainfall

False. Only sand pitches can accommodate rainfall of 25mm/hour (considered the maximum expected intensity over a 20 year period). Heavy clay loam soils on pitches permit infiltration of between 0 and 5mm/hour. Even the best maintained 1m spaced slit drained surface will not permit a drainage rate of much in excess of 7mm/hour after continual use – even if regular sand dressings are applied. Hence, with rainfall intensity of 25mm in one hour, over half the water volume will be subject to run-off – and this water will simply flow to the lowest areas and off the pitch. In residential areas with limited or no outfall locations the solution is the creation of adequate attenuation – this can be achieved in a number of ways with the creation of swales, wetlands and underground temporary storage

Average daily rainfall in the British Isles varies between 5 and 12mm. Even with the soil at field capacity most of the winter, the surface water retained in the grass foliage and micro-depressions, a low infiltration rate in heavy loam soils and the ability to retain water in the aggregates within the drainage installation, there is very little surplus water to pass through the drainage system.

Vertidraining or deep aeration into heavy subsoils will improve the drainage of a sports pitch

False. When clay loam topsoil and clay subsoil is loosened by aerating and creating holes, this will only create more water retention which will develop into waterlogged areas. The main aim in maintaining heavy loam soils in a condition in which to play football must hinge on retaining a firm surface with adequate removal of surplus water by means of a suitable by-pass system. Allowing the topsoil to become loose at the onset of winter promotes the development of soft areas that are prone to displacement and the formation of puddles after rain.

A slit drainage system without annual sand dressings is still better than no drainage installation

False. The system is entirely dependent on the slit drains being ‘open’ at the surface and the sand topping remaining uncontaminated with surrounding clay topsoil spread with play activity. The only way this can be achieved is by applying sand dressings to create a sandy medium to at least 25mm thickness over the slit drains in the shortest period of time – at least with the application of annual dressings for five years or more. Without these dressings the drains soon become capped with clay loam topsoil and the installation cost of has become a waste of money.

1. There must be an adequate cross-gradient to promote surface drainage

2. There must be an even grade with no depressions.

3. Water run-off from surrounding areas on to the pitch must be prevented.

4. Secondary drainage installation must prevent any water collection on the pitch.

5. Pitches with secondary drainage must be sand-dressed every year.

At a time when the vigour and density of the grass cover is being encouraged in preparation for the winter season ahead, getting it wrong can lead to a major setback. After the unprecedented rains, grass is growing rapidly and the main failing is to bring down the height too much in a single cut. With the present temperatures this can lead to disaster. The shock can result in severe set-back to the cover – possibly losing cover in parts.

The choice of mower is not as important as the frequency that mowing is undertaken and the need to alter the direction of mowing regularly. The general tendency is to mow weekly or two-weekly. With rapid growth after rains or fertilisation, mowing weekly is just not enough. A single mowing should not remove more than one third of the height of the grass plant and if this is ensured there should not be significant cuttings left after the operation. Generating surplus grass cuttings on the surface can promote fungal disease and die-back leaving troublesome bare patches. Allowing excess cuttings to accumulate and be dispersed by mowing only builds up the organic matter in the surface increasing surface drainage problems and restricting healthy growth.

The cutting height desired varies for different sports. The preferable height for football is now between 25mm and 35mm – the same as desired for cricket. Height for rugby is nearer 40mm to 50mm instead of earlier preferences of 75mm. However, at this time with possibly six weeks of hot weather to come it is wise to raise the height of the cut to overcome the stress arising out of the high temperatures.

• the accumulation of surface water with no immediate and adequate route of escape. It is confined to low lying areas and can be far more destructive. The water level rises relative to the capacity of the unplanned collection area that has inadequate outfall.

Water run-off is always localized with water collecting or finding the easiest route of exit. At times of continual rain there can be temporary restriction in outfall locations or there is just not the provision to cope with excessive water volumes. Where these situations arise, swales and ditches designed to very shallow gradients dispatch surface water and also offer the opportunity for temporary storage, water loss by evaporation and the slow percolation into the soil over time.

Where low areas make the potential for the rise of flood water levels inevitable due to inadequate outfall, consideration should be given where feasible to some remodelling of landform to provide the eventual exit of the surplus water.

The utilisation of flood plains for sport and recreation is cost effective where damage inflicted is not a major concern and human life is not threatened. However, within these areas it is expedient to plan water courses with a ditch reticulation and in the case of recreational areas such as golf courses, sports and leisure sites to elevate playing features where possible and create significant surface gradients. Wherever rising water levels can be diverted away from features and water run-off can be channelled out of harms way, there is merit in undertaking the earthworks necessary to achieve this.

Though contamination may be minimal on recreation sites, land locally exploited industrially and commercially warrants special attention with regard to coping with storm water run-off or flooding.

1. After heavy winter use the pitch should be de-compacted with the vertidrain and/or the earthquake to promote good growth.

2. It is essential to level out significant depressions using compatible topsoil – this should have been done before any seeding is undertaken.

3. Over-seeding is essential if weeds are to be prevented from encroaching and dense grass cover is to be restored for the season ahead.

4. Special attention must be given to badly worn areas. These could need thorough loosening, harrowing and possibly levelling before being seeded.

5. Weed control is necessary when grass cover is restricted. However, special care is needed in the choice of chemical and interval applied after seeding.

6. A good balanced fertiliser should be applied – preferably a coated control release formulation to promote active growth into the playing season.

7. If slit drains are installed the annual sand dressing is needed at the rate of at least 120t/ha in one or two visits – ensuring good penetration into the sward.

8. Ensure the mowing regime is in place – with frequency depending on rate of growth to avoid clippings collecting and lifting the height of cut in hot dry weather.