Is natural turf losing out?
Like it or not, there is a profound conflict between natural and artificial sports surfaces in the UK and, right now, it seems to me that the artificials are winning. The key sporting bodies have developed a predisposition towards artificial turf which has extended into the culture of sport itself. Even very small clubs or schools leap at the chance of installing, for example, a 3G pitch with fencing and floodlighting, and increasingly view anything involving natural grass as somehow inferior.
I don't wish to go into the many arguments against artificial turf surfaces, though these are many and varied. They include questions about general environmental soundness, their contribution to the increasing 'urbanisation' of our landscape, the diminished flexibility in terms of provision, their long term economic viability, social inclusiveness and overall reach, aesthetic considerations and so on. What I am concerned about is how, in the face of so many negative factors in relation to artificials, natural turf proponents seem unable to stand up for themselves in similar terms.
Considering playing quality in relation to specific individual sports, artificial surfaces are undoubtedly now of an extremely high standard. Interestingly, the motivation behind much of their development has tended to be, for most sports, how best to emulate the properties of the 'best' of natural turf. It would seem appropriate, therefore, to consider how these 'best' examples are actually achieved.
Getting technical
Any discussion about artificial surfaces tends to become very technical very quickly. This may be because it is in the non-technical areas, mentioned above, that the case for artificial sports surfaces is at its weakest. Advocates of artificial surfaces prefer to locate the debate in areas where they are most comfortable. It is also the case that the technical area is where the arguments for natural turf sports surfaces are at their most feeble. This weakness in the natural turf technical argument is what I am concerned with here.
It is in relation to the built environment that the technicalities of artificial sports surface design and construction tends to be examined in greatest detail. This falls under the variably watchful eyes of the planning authorities, whose concerns must be addressed prior to any particular installation. Of particular concern is flood risk and storm water management, and it is in relation to the management and control of rain water that the technical distinctions between natural and artificial turf are perhaps most pronounced.
Put simply, there is a much greater understanding of and, hence, greater capability to design successfully for the behaviour of water in artificial sports surfaces than in natural turf. Criticism of natural turf invariably comes down to its performance during the winter when large amounts of water often cause such surfaces to become unplayable, sometimes for months on end. Solutions put forward, like 'improve drainage', while intuitively obvious, don't always achieve the wished for results. Thus, natural turf is given a bad name due to the generally poor understanding of what actually governs its behaviour and performance.
Compared to the solid, yet technically quite simple, foundations upon which the design and construction of artificial turf sub-base and drainage and attenuation systems are built, the equivalent understanding pertaining to natural turf is woefully inadequate. Circumstances cannot readily be explained to planners and funders, and so mistakes and misdirected investment occur whilst client expectations are continually mismanaged. This has led to a diminution of the overall standing of natural turf in the current sport and recreation industry, a tragedy to say the least.
Infiltration rate
In the case of natural turf, Sport England have indicated tolerance limits of infiltration rate for some sports, including football and rugby. This is assumed to be measured using double ring infiltrometers. Straightaway, we run into an oversimplification of the circumstances, however. Firstly, it will be found that infiltration rates, especially on clay soils, can be very high indeed during the summer months, but fall to near zero on the same soil during the winter. This is because clay soils shrink as they dry and this creates fissures through which water may flow very freely. With the coming of winter, persistent wetting of the soil causes the clay to expand once more, which closes the fissures and reduces the infiltration rate. The time of year at which measurement of infiltration rate takes place is, therefore, highly significant in the interpretation of such data.
Sometimes, a very low infiltration rate may not be correlated with poor drainage performance. This is because no account is made of the influence of surface runoff. Sports pitches with a marked slope can often shed water to the side more rapidly than they can absorb it, so the effect on overall performance is the same as if the pitch had a relatively high infiltration rate. Sport England's maximum tolerance of pitch cross-fall is 1 in 60 and this is certainly sufficient to achieve a significant runoff effect of this kind. In fact, improving the infiltration of pitches that behave like this can sometimes actually diminish their performance. The soil is made capable of becoming saturated more rapidly as it absorbs water that would previously have run off the side. As you see, the situation is more complicated than it first appears.
Void space and total porosity
The materials that make up the sub-base of an artificial sports pitch have a quality known as the 'void space', which is the percentage of the overall volume of the layer that is air and therefore capable of temporarily storing water. What used to be known as MOT Type 3 aggregate, for example, is often selected for its high void space qualities, in addition to providing the structural support for the surface itself. This might have a void space of up to 40% which, for a sub base depth of 250mm, would be sufficient to contain 100mm of water, a heck of a flood.
Natural soils or sports turf rootzones have an equivalent property to void space which is known as total porosity. Near the surface of a sports pitch, this property may vary between about 30 and 40% and tends to decline with depth (in natural soils). In recent years, we have consistently been measuring total porosity on natural turf sports field sites with the aim of achieving greater understanding of how this quality of soil is related to performance as a sports surface. Ultimately, we hope to identify actual tolerance limits for total porosity which may be related directly to the overall quality of a particular surface, existing or yet to be made. Such tolerance limits might ultimately find their way into a practical set of performance quality standards.
Hydraulic conductivity
Another crucial factor to consider is hydraulic conductivity. This is a measure of how quickly water is able actually to flow through a particular soil. In the case of our Type 3 aggregate, the hydraulic conductivity will be very high indeed. In fact, in flood risk assessment models, water is assumed to be capable of draining out of such materials virtually instantaneously, provided of course it has somewhere to go, for example into a pipe drainage system.
In the case of natural soils or manufactured rootzones, hydraulic conductivity figures are usually very much less than those of extremely coarse aggregates. They also tend to be very variable. Water flows through the spaces within the soil, the overall volume of which is the total porosity, but the actual sizes of the spaces is also of crucial importance in determining hydraulic conductivity. Is the total porosity made up of lots of very small spaces through which water may flow only very slowly, or of fewer larger spaces through which water may flow much more rapidly? Different soils have differing pore size distributions and, hence, differing hydraulic conductivity readings.
Measurement of hydraulic conductivity and of the distribution of void space within an intact sample of soil needs to be undertaken in a laboratory and requires considerable expertise. Again, we have collected some data on this during the course of our work. In addition, methods exist that can derive, albeit theoretically, hydraulic conductivity from particle size distribution data (how much sand, silt and clay is present). This is something which can readily be established, although we have yet to test the practicality of this predictive approach.
Nevertheless, bearing in mind that the qualities of a topsoil are different in all these respects from those of the subsoil, it will be clear by now how un-enlightening a simple measure of infiltration rate, for example, can actually be. It is the product of so many factors, any one of which may be the primary determinant of the measured value and which may be effective at any depth within the soil profile. The upshot is that, because of the complexity of the real situation, the single measure of infiltration rate does not provide sufficient information to make a reliable judgement on the way forward. The interpretation of infiltration data and, more widely, the establishment and measurement of performance standards need to be accomplished in a much more nuanced and considered way.
It will be becoming clear that the development of a predictive model that will describe the behaviour of water in natural turf systems, with sufficient accuracy to allow a fully supported design to be put before a planning authority or funding body, with the equivalent degree of confidence with which designs for artificial sports surfaces are presented, is some way away yet. However, I am convinced that it is possible and that it is a very worthwhile objective.
One possible way in to all this, and that has impressed me very much, is that of the HOST (Hydrology of Soil Types ) system. Published in 1995, this is now included in the methodology behind the estimation of greenfield runoff as established by HR Wallingford working with the Environment Agency and which, since 2013, is an approach increasingly required of developers in relation to planning and flood prevention .
HOST classifies the soils of the United Kingdom into twenty-nine categories and the Wallingford procedure uses this information to derive the greenfield runoff rate that can be anticipated for a particular site. The twenty-nine categories are based on a series of conceptual models that simulate the hydrological behaviour of the soils and interpret soil physical properties and their effects on the behaviour of soil water.
Each of the soil associations defined by the Soil Survey of England and Wales are allocated to a particular HOST category, and it seems reasonable to me to suppose that natural turf sports surfaces in their various forms may be incorporated into the overall model in exactly the same way.
We are currently testing real data from sports grounds on total porosity, particle size distribution, hydraulic conductivity and infiltration rates etc. within models based on this approach. The aim is ultimately to enable the optimum natural turf drainage and attenuation system to be designed for any given situation and for the performance of the completed system to be predicted and described accurately.
This would give planners a much clearer indication of the likely consequences of any development in terms of flood prevention, and would also indicate to the end users of the facility what level of performance they can look forward to with their new or improved installation.
Natural turf needs to fight back
I think that, from both a social and environmental point of view, it is vital that natural turf is restored to its status as the standard form of sport and amenity surface. The restricted accessibility and limited flexibility of most artificial surfaces may ultimately be to the detriment of sport itself. The casual 'kicking about with friends' approach, by which most of us were introduced to at least our national sports of football and cricket, are denied to young people in relation to the almost invariably fenced off artificial surfaces.
If we can convince the authorities and sporting bodies that natural turf facilities of the highest standard can consistently be achieved, with designs supported by accurate data and sound calculations, what seems to be a relentless march of artificial surfaces may be halted. In order to achieve this, we must develop for natural turf behaviour models equivalent to those used to derive artificial pitch drainage, attenuation and storm water management systems. This must be achieved in order to deliver the optimum performance of sporting surfaces capable of being measured by appropriate and realistic standards. I believe all this is entirely possible, but it is by no means simple.
Dr Tim Lodge
Agrostis Turf Consultancy Ltd
www.agrostis.co.uk
01359 259361