Sustainable drainage of sports pitches

The drainage behaviour of sports pitches is not well understood nor has performance been measured in the past. Within planning authorities there is a perceived contribution of pitch water discharge to local flood risk; whereby all the rainfall surface runoff is rapidly channelled through the drainage system to the pitch outfall. However, empirical evidence from industry suggested that this may not be a realistic assumption from observations of low drainage volumes yielded from pitch drainage systems. Furthermore, discharge constraints imposed have in many cases resulted in grossly over-designed off-line drainage attenuation systems for new sports developments through lack of understanding. In contrast, sports pitches indeed have the potential to enhance the attenuation performance of the subsoils and provide localised effective management of surface water runoff, and a significant storage volume if designed appropriately The findings in this thesis confirm that pitch bases demonstrate the key functions that are in fact reflected in the design requirements of Sustainable Urban Drainage Systems (SuDS). This PhD research project was conducted to investigate and document the performance of common pitch construction and drainage systems to better characterise the key drainage mechanisms that occur and control the flow of surface rain water through the pitch to the discharge outfall. The project developed a triangulated approach to the investigations, comprising: field measurements of climate and discharge behaviour at a range of artificial and natural turf pitches in England; laboratory physical model testing of pitch component hydraulics; and predictive mathematical modelling of how a pitch system may be expected to perform hydraulically based on key material and system drainage principles. The field monitoring systems were developed as part of the research, as was bespoke laboratory physical simulation of a pitch construction. It was found that very variable yields (% out versus % in) of water were detected from the monitored field sites. The values varied across a range of <1 to 88%, with the natural turf providing higher yields in general. The antecedent weather patterns did not show a clear relationship with yield as might have been expected. However, it was not always possible to retrieve detailed information on the subsoil conditions or hydraulic capability reducing the conclusiveness of the discharge flow measurements. The scaled laboratory testing of pitch materials established the importance and magnitude of barriers to percolation of surface water through the layers of the pitch constructions, in particular artificial pitch profiles. It was found that a significant proportion of the total rainfall head was required to instigate percolation of surface water through the carpet and into the pitch i.e. breakthrough head. In addition, several constituent pitch materials exhibited water retention characteristics that reduced that rate of free percolation of surface water through the pitch profile. The net impact is to reduce the net available head of water to further drive flow through the layers to the pipe network drainage system. A conceptual hydraulic model, developed from the literature, was further developed into a simple numerical model. The model was informed by parameters determined from the laboratory measurements and key groundwater drainage flow theory to attempt to replicate a pitch drainage system. It was envisaged that the models would be validated by the field data, although this proved challenging as a result of the field data variability and the multivariate nature of the influences on flows measured. A key finding of the modelling was further establishing the likely head of water generated at the interfaces between the bottom of the granular sub-base and the pipe collection drainage system beneath. This resulted in limited pipe infiltration and low total flows to the outfall, further corroborating the project field results and the anecdotal observations from practitioners. The combined unique data sets provide a refined model for sports pitch drainage to both reinforce understanding and inform practical design and operation

Briefing: sustainable drainage for sports pitch developments

Effective management of storm water is of paramount importance in urban development, and drainage design is usually governed by planning constraints. In the development of sports pitches, planning bodies often impose discharge constraints, and frequently class such areas as impermeable surfaces, thus treating their drainage behaviour in a similar fashion to roads and pavements, which may require the provision of separate attenuation. This briefing presents preliminary findings of a project to assess the drainage behaviour of sports pitch developments. The work undertaken to date suggests only a fraction of water falling on a pitch (rain) is discharged to the drains, identifying an apparent attenuation capacity and potential over-design within current sports pitch drainage systems. In addition to the low discharge volumes measured from pitch systems, there has also been a broad range of flow rates experienced. This led to the development of a bespoke flow monitoring device, FloPod. Designed and fabricated at Loughborough University, this device allows a broad range of flow rates to be measured without compromising aspects of data resolution and reliability – key factors that were not found in commercially available devices

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

Drainage behavior of sports pitches–A case study review

The drainage behavior of sports pitches has traditionally been designed from experience with hydraulic performance rarely measured in detail. Within the wider industry and regulatory bodies there is a perception that storm water and increased drainage rates from sports pitches contribute to local flood risk. Empirical observations have suggested that in reality pitch drainage systems may discharge water at low volumes and rates and there is often limited surface run-off. Furthermore it appears that lack of technical guidance on the discharge of water from sport pitch drainage systems may have led to misunderstanding their drainage behavior and possible benefits they could bring to water management as opposed to perceived dis-benefits. This paper summarizes selected results of a case study which included field measurements of weather and discharge behavior on a range of natural turf sports pitches in England. The findings from this study indicate that natural turf sports pitches can provide resistance to flow and hence advantageous attenuation of rainfall and storm water. Additionally sports pitches can store large volumes of water within the pervious materials used in their design. The study has confirmed that sport pitches demonstrate the key functions that are reflected in the design requirements of Sustainable Urban Drainage Systems (SuDs) such as pervious pavements providing source control of surface rain water