Sports pitches with heavy clay soil can support regular play without becoming a muddy mess. By ensuring adequate sand dressings are applied annually, properly installed slit drains on 1:50 cross gradients have withstood the wettest winter on record to provide sound playing conditions week after week in a north London sports complex..
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It is sad when newly constructed pitches prove unable to cope with rainfall – however excessive – and usage is prevented because of poor design.
In a major complex where reliance was put on inadequate soakaways, indecision on secondary drainage and with no concern of the importance of suitable gradient, the project was doomed from the start.
On predominant heavy soils it is vital that surface water is removed before it ponds and produces soft areas that quickly make a pitch unplayable.
The ground is saturated if there is no sound secondary drainage. Don’t rush in and vertidrain clay loam soils now – it is like pressing holes into cheese and creating reservoirs for water collection. Vertidraining and earthquaking/groundbreaking are only of value when the soil is dry enough to cause fracturing.
Now is the time to take stock of games cancelled and properly assess the extent of the problem. Installing drains into uneven or level ground does not produce the results expected. To get the best results in overcoming surface drainage problems, first check that water running on to the pitch is adequately controlled – the next essentials are a good even grade with a cross gradient of between 1:50 and 1:70. Without these basic requirements the results from installing any measures will be short-lived.
Before jumping and arranging to install drains assess the problem fully and seek professional advice if necessary.
Despite the highest annual rainfall recorded the failure in sports pitch drainage must give concern. Time and again assessments of poorly drained and muddy sites are made – the most common situation being the collection of surface water on pitches that are unable to promote any degree of surface drainage. Strange, after so much has been said and written on drainage installation techniques – particularly related to slit drains and grooving. Standards of installation have reached new levels and current specialised equipment used has become so efficient.
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.
Adams, W.A. and Gibbs, R.J. 1994. Natural turf for sport and amenity. CAB
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.
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.
The incessant rainfall in the last six months of 2012 – now the wettest year on record – has highlighted the need for surface drainage provisions. Besides the failures in urban flood control, sports pitches have shown up the consequences of surface water lingering on the playing field.
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
A 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.
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.
If you are looking to reduce cancelled fixtures due to water-logged conditions the solutions are clear and simple.
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.
Muddy football pitches are common throughout Europe during the wet winter months. In most instances, particularly in the United Kingdom, topsoils are relatively impermeable with high silt and clay content. Infiltration measurements using a double ring infiltrometer are negligible varying between 0.1 and 0.5 mm/hr after they have been subject to compaction in wet conditions and have developed a degree of thatch growth. With winter rainfall there is negligible evapotranspiration and consequently topsoils take in moisture slowly. Having a high water holding capacity they can retain a wet and plastic condition throughout the winter. Hence, the rapid removal of surplus surface water is required soon after rain and before water softens the pitch surface; the object being to make pitches playable within the shortest time thereafter.
Water run-off on the pitch
Water run-off is not easily gauged though it is very evident after heavy rainfall with the development of ponded areas on uneven ground. While the preparation of the subsoil prior to topsoil replacement and the on-going level of maintenance all influence the infiltration capacity of sports pitches, there remains a point at which there is run-off of surplus surface water. Once rainfall begins a proportion is withheld within the grass cover. The height of grass growth on sports pitches varies from 25 to 35 mm for football and 50 to 75 mm for rugby. Though no measurements appear to have been recorded, the top growth including the surface litter and thatch retains a considerable volume of water adsorbed to the plant surface and this could be in the region of 2 to 4 mm over the playing area. Micro-irregularities including foot depressions must further retain water that eventually infiltrates into the soil. This volume could also amount to in the region of 4 to 5mm. Finally some water evaporates, some infiltrates into the soil or is contained in surface slit drains. Referred to as abstractions, the sum of these retentions must be complete before run-off commences (Tindall and Kunkel, 1999) and to summarize, there would probably be at least 8 to 12mm falling in the first hour before any significant water reaches lateral drains and is discharged from site.
Slit drains function as a by-pass system in impermeable topsoils. However, positive surface run-off must occur for surplus water to reach the slits even though they are a short distance apart. Without it water is simply retained on the surface. Calculations employing a modified Hooghoudt’s formula can be made to estimate the rate of removing surface water using close spaced slit drains. Expectedly, the rate of removal in slit drains is much less than would be possible in a permeable sand rootzone and capacities of slit drains are generally estimated over 24 hours with the drainage of not much more than 100 mm in a day (Adams and Gibbs, 1994).
For effective run-off from a pitch to be initiated after abstractions are complete, there must naturally be an even grade to specified tolerances and a positive gradient in the playing area. A minimum gradient of 1:70 is needed for satisfactory run-off (McIntyre and Jacobsen, 1998). Adams and Gibbs further maintain a diagonal fall of between 1:67 and 1:100 is advisable. Slit drain systems must remove surplus water from the surface soon after rain has fallen. Installed at 1 m spacing they have proved very effective though the introduction of 260 mm spaced sand-injected mini-slits offer an even greater potential for rapid water removal. In this way the relatively impermeable topsoil can be maintained in a firm and playable condition. Otherwise, isolated muddy areas soon develop. Also vital is the importance of sand dressings to promote surface water flow laterally to slit drains and prevent these drains from being smeared with adjacent clay loam soil dislodged during play (Adams and Gibbs,1994).
Run-off outside the playing area
In many instances water run-off is initiated from higher ground passing down on to the pitches. The long-term performance of cut-off drains or those at the toe of the cut slope depend on the degree of maintenance as silting at the surface is bound to accumulate. The installation of shallow grassed swales or diversion ditches with appropriate gradient is an approved alternative solution (CIRIA, 2000). Controlling surface water movement moving on to the pitches must be the first priority.
The potential for surplus water run-off from sports pitches is of greater consequence after prolonged heavy rain. In a study of the rainfall in southern England the data from the recognized meteorological station at the Wisely Horticultural Research site was chosen. Over 11 years during the wet winter months of October to April, the average number of rain days per month was 16.5. Of these 15.1 days had rainfall totals less than 10 mm in a day and on 8.7 days the daily rainfall measured less than 2 mm. It was estimated earlier that at least the first 8 to 12 mm of rain is retained by abstraction. However, the 1 in 2 year or 1 in 10 year storm in southern England can produce rainfall of 25 to 35 mm in one hour (FEH,1993). With this extreme rainfall in excess of the amount that can be abstracted, surface run-off must occur. Naturally, the immediate surroundings below the playing area now become a vital factor. On graded sports pitches water run-off is generally not significantly more than would occur if the land had not been developed. With the use of diversion ditches, shallow swales, shorter grass cover, a gradient levelled to be suitable for sport and the installation of lateral and slit drains, there is a significant control in surface water flow and a degree of attenuation is achieved.
With run-off into a stream, pond, watercourse or open pasture there is generally no concern. However, where outfall locations are restricted or do not exist in the case of built up areas, provision must be made for the attenuation of this surplus water to prevent peak flow and distribute the surplus water more evenly over the boundary. Furthermore, with a diagonal gradient over a graded area there will be concentrated flow of water run-off after heavy storms. Swales become a vital provision and temporary storage structures incorporating large stone of 150 to 200 mm dimension, large diameter twin wall plastic piping and reinforced plastic cells are used for this purpose. The latter materials have maximum potential for storage and are used extensively under car park areas.
Water run-off is a vital factor in sports pitch design. The control of surplus water flow on to pitches remains the first priority. Acceptable gradient and grade are vital to create surface movement of surplus water on the pitch at times of high intensity rainfall and prevent the development of water-logged areas. Notwithstanding surface drainage provisions in the form of gradient, swales and slit drains, the pitch surroundings will dictate the extent to which attenuation is taken to accommodate water run-off off the pitch at times of heavy rainfall. However, it therefore becomes essential to define the rainfall risk to be catered for in terms of the maximum rainfall intensity allowed for over a nominated return period.
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.
McIntyre, K. and Jacobsen, B. 1998. Drainage for sports turf and horticulture. Horticulture Agency
Consulting. p. 110.
FEH, 1993. Flood estimation handbook. Centre for Ecology and Hydrology. Wallingford.
Tindall, J.A. and Kunkel, J.R.. 1999. Unsaturated zone hydrology for scientists and engineers. Prentice Hall. p. 367-368.
Today there is increasing need to control stormwater surface flow over levelled grassed sites. Suitable outfalls do not always exist in residential areas and so efforts are needed to attenuate this flow with the use of swales and temporary storage areas.
Reference to www.turfandgrass.com/s_articles/Restricting Drainage.php can be made for a more detailed account of attenuation in a local authority playing field complex.