Drainage behaviour of sport pitches - findings from a research study

The drainage design of sports pitches has traditionally been based on experience and can be considered an inexact science. Whilst the sport surface can be adequately drained to meet specific criteria, estimating outflows at the discharge point is more challenging. The hydraulic performance of sports pitches has not previously been measured in detail prior to this study. Within the wider industry and regulatory bodies there is a perceived contribution to local flood risk of the storm water and run off from sport pitches. It is also apparent that artificial pitches have in some cases been treated in planning consents as impermeable. Observations from industry have suggested that in reality the pitch drainage systems discharge low volumes of water and low peak flow rates, with limited surface runoff (especially from porous artificial pitches). However, in some cases, for artificial pitches in particular, at planning stage the drainage design has required to include off-line tanks to provide storm water storage and attenuation. A lack of technical guidance on sport pitch design and drainage benefits may be leading to overdesign, and prompted this study. This 3 year study comprised field measurements of weather and discharge behaviour at a range of artificial and natural turf pitches in England; laboratory physical model testing of pitch component hydraulic behaviour; and mathematical modelling to predict how a pitch system may be expected to perform hydraulically. Bespoke field monitoring apparatus was developed as part of the research to measure across a large range of flow rates and volumes. The experimental work in this study has provided the evidence to demonstrate that the porous pitch designs provide high attenuation of peak rainfall events and large capacity for water storage, similar to the requirements of SuDs based ‘source control’ designs required in new urban developments. The field monitoring observations suggest that in reality the drainage system behaviour is not as consistent or predictable as might be expected from assumptions made in design software and that in all cases the measured outflow water volume was far less than that estimated from rainfall as the total water volume flowing into the pitch drainage system. The experimental work, combined with the mathematical modelling, has highlighted the key mechanisms that provide resistance to flow and explain the attenuation behaviour observed. It is considered that in most cases insufficient head is created in the sub-surface layers to drive water to the lateral drainage pipes, and that the high frictional resistance to flow in the corrugated collector pipes provide large ‘head’ losses under the low hydraulic gradients. The research findings support the claims by many in the industry that in some cases planning approvals, where a lack of understanding or evidence on how pitches can attenuate and store water exists, may be causing the over-design of pitch drainage systems requiring unnecessary offline storage tanks