There’s a strong case for including trees in urban streetscapes, thanks to a wide variety of community wellbeing advantages, from providing shade to pollution reduction. In addition, attenuation and water treatment benefits can be maximised by using tree pits to improve tree health, within a SuDS. But is it really worth specifying tree pits? Our past eight years of research answers this question with a resounding ‘yes’. In this article we revisit our Keighley tree pit installation, and examine our latest study on the efficacy of ArborFlow, in particular the system’s ability to filter out road water run-off pollutants.
Executive summary
- SuDS installation at Keighley proves benefits of tree pits thanks to long-term research programme.
- Six Field Maples in ArborFlow tree pits reduce street surface water, minimise flood events, and run-off-associated pollution.
- Positive impact felt by Bradford Metropolitan District Council, Yorkshire Water, Keighley residents and visitors to the area.
- Sap flow and vapour pressure deficit (VPD) testing shows ArborFlow tree pits encourage vigorous growth.
- Tree health is proportional to efficacy in terms of combating surface water and flooding, reducing pollution and noise, lowering street temperatures on hot days, improving biodiversity, and increasing the wellbeing impact.
- Research data from Manchester Metropolitan University shows the ArborFlow tree pit filters out pollutants effectively.
SuDS case study: benefits of tree pits
Those of you with a specific interest in SuDS might remember our 2018 installation of tree pits at Keighley, West Yorkshire, where six Field Maples (Acer Campestre ‘Streetwise’) were planted on North Street, as part of road network alterations to ease traffic congestion.
With several pre-existing mature trees being removed in the course of these works, Bradford Metropolitan District Council was keen to reinstate some green urban landscaping. Using tree pits not only would safeguard the future of the Maples, but also presented the ideal opportunity to design the optimal SuDS. Designing the correct SuDS solution would effectively reduce carriageway surface water, for safer roads and less likelihood of a flood event. In addition, a well-chosen tree pit would help reduce the level of run-off pollutants making it into the combined sewer system.
The installation was carried out in 2018 by SIG Geotechnical. The project was considered an ideal testing ground to demonstrate the tangible long-term benefits of the ArborFlow tree pit system – and that’s exactly what we’ve done over the past six years. We were only too aware of the lack of dependable data previously available within the industry, and wanted to quantify the advantages of tree pits as attenuation and run-off filtering solutions.
Tree pit research project 1: growth and vigour
In 2020, working with IROSARB, a leading arboriculture and urban forestry specialist, we conducted sap flow and vapour pressure deficit (VPD) testing, to investigate how well the trees were establishing themselves thanks to the support of the unique design of ArborFlow, with its ventilation and irrigation systems designed to optimise root health.
Heat pulse sap flow technology was used to make a real-time assessment of moisture uptake from the roots and estimate the annulus of live sapwood within the tree section – in other words, calculate the likely sapwood growth rate. In addition, vapour pressure deficit testing helped ascertain how much transpiration was occurring, and therefore whether the trees were retaining enough moisture for optimal growth.
These tests demonstrated that sap flow in the Keighley project Maples was very high in relation to their cross-sectional area, while VPD was well below average. This indicated a very high level of tree vigour, demonstrating very successful root establishment within the unique design of ArborFlow tree pits.
Why are healthy urban trees so important?
Healthy urban trees are not just a nice to have. Their health is directly proportional to their efficacy – myriad benefits can be maximised through their vigorous growth. These include:
- Reducing air pollution
- Quietening noise
- Keeping streets cooler in summer
- Improving biodiversity
- Increasing the wellbeing of residents and visitors
- Combating surface water flooding
- Slowing the passage of water into sewers
This type of thinking was very much in the minds of Bradford Metropolitan District Council when they specified the ArborFlow system at North Street, Keighley. These tree pits form a key part of the drainage strategy for the area, chosen to help future-proof the specification, ensuring it continues to take pressure off the Yorkshire Water combined sewer system even in the face of increased rainfall and storm events due to global warming. The feedback has been that these healthy trees have constituted an important step in the Authority’s goal of creating a more resilient district that is well-placed to deal with the likely effects of climate change.
Tree pit research project 2: road run-off filtering efficacy
Working in partnership with a team of scientists from Manchester Metropolitan University, our second and most recent research angle for the Keighley tree SuDS project was the ability of tree pits to filter out pollutants. This study would provide evidence whether contaminants were being effectively removed from the road run-off before being able to foul the combined sewer network.
We took this approach because we found that one of the key concerns of planning authorities, resulting in caution over the adoption of SuDS options, was a perceived risk of performance deterioration and uncertainty over post-installation maintenance. One of the key aspects within this was whether surface-derived pollutants might affect the tree, or require significant maintenance expenditure.
This research was designed to demonstrate whether surface-derived pollutants were able to pass through the geotextile layer of the tree pit, and provide dependable data to support these conclusions. Samples were collected from the six North Street tree pits in May 2023 and August 2023, and were analysed by a team of Manchester Metropolitan University environmental scientists in a controlled laboratory setting.
The three elements of the study were:
- An analysis of the physical character of a SuDS tree pit feature, achieved by generating a high-resolution vertical profile of the sediment and soil.
- An analysis of the pollution levels within the tree pit structure, identifying any potential seasonal variation in the chemical character and comparing roadside and kerbside influences.
- A characterisation of the microbiological communities within the tree pit, quantifying the levels of microbial activity and exploring any relationship with the chemical composition.
The resulting data provided us with a comprehensive picture of the physical, chemical and microbiological character of the SuDS tree pit feature at North Street Keighley. This indicates that the SuDS tree pit system is indeed working effectively and contains a healthy soil system, without any significant maintenance regimen.
Key takeaways
- ArborFlow tree pits are proven to encourage tree health and vigour, which results in a more effective SuDS.
- Local authorities, water companies, local residents and visitors feel various benefits from SuDS, including a reduction in street surface water and flood events, reduced run-off-associated pollution, and other advantages of healthier trees. These are maximised by ArborFlow tree pits.
- A study by IROSARB quantified the vigour and root health of trees planted in ArborFlow tree pits.
- Research data from Manchester Metropolitan University shows the ArborFlow tree pit filters out pollutants effectively.
